the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Oct 2021
Intercomparison of Vaisala RS92 and RS41 radiosonde temperature sensors under controlled laboratory conditions
Abstract. Radiosounding profiles are essential for weather and climate applications, as well as for the calibration and validation of remote sensing measurements. Vaisala RS92 radiosondes have been widely used on a global scale until 2016, although in the fall of 2013 Vaisala introduced the RS41 model to progressively replace the RS92. To ensure homogeneity and the highest quality of data records following the transition from RS92 to RS41, intercomparisons of the two radiosonde models are needed. An intercomparison experiment has been performed where, for the first time and independently of the manufacturer, RS92 and RS41 radiosondes have been simultaneously tested and compared inside climatic chambers in order to characterize the noise, the calibration accuracy and the bias of their temperature measurements. A pair of RS41 and RS92 radiosondes has been tested at ambient pressure under very different temperature and humidity conditions. The results reveal that the temperature sensor of RS41 is less affected by noise and more accurate than that of RS92, with noise values less than 0.06 °C for RS41 and less than 0.1 °C for RS92. The error corrected by means of calibration, evaluated as the deviation from a reference value and referred as calibration error, is within ±0.1 °C for RS41 and the related uncertainty (hereafter with coverage factor k = 1) is less than 0.06 °C, while RS92 is affected by a cold bias in the calibration, which ranges from 0.1 °C up to a few tenths of a degree, with a calibration uncertainty less than 0.1 °C. Under conditions similar to those that radiosondes meet at the ground in nighttime radiosoundings, the temperature bias between RS41 and RS92 is within ±0.1 °C, while its uncertainty is less than 0.1 °C. The radiosondes have also been tested before and after fast (within ≈ 10 s) temperature changes of about ±20 °C, simulating a scenario similar to steep thermal changes that radiosondes may meet when passing from indoor to outdoor environment during the pre-launch phase. The results reveal that such thermal changes may increase the noise of temperature sensors during radiosoundings, up to 0.1 °C for the RS41 and up to 0.3 °C for the RS92, with a similar increase in the calibration uncertainty of temperature sensors, as well as an increase in the uncertainty of their bias up to 0.3 °C. However, the thermal changes do not appear to affect sensors’ calibration error and temperature bias.
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RC1: 'Comment on amt-2021-337', Anonymous Referee #1, 24 Nov 2021
The manuscript by Rosoldi et al. evaluates the calibration of the temperature sensors on an RS92 radiosonde and an RS41 radiosonde. The latter model is the successor of the former and plays an important role in the global radiosonde observing network. An understanding of the performance of the temperature sensors on these radiosonde models is important to evaluate their effect on numerical weather forecasting and climate records.
Brief summary of my main concerns
The authors have not prepared the ground check of the RS92 correctly and used an improper data table for the RS92 generated by the sounding software in their analysis. They missed the influence of the RS92 humidity sensor heater, which contributes to the temperature signal measured by the RS92 and which requires a different setup to reduce its effect. They have used only one sonde of each, making it difficult to transfer the results to an average production run. I am very doubtful that the current setup is suitable to address the temperature calibration of these two radiosondes to the level the authors try to achieve so that others may build upon.
Major comments
The Vaisala RS41 radiosonde uses a platinum resistance thermometer as temperature sensor, which does not require re-calibration between production and use of the radiosonde in normal operations. A simple comparison with the temperature sensor of the humidity sensor is sufficient to justify the calibration stability at the time of launch and within preset limits.
In contrast, the Vaisala RS92 radiosonde uses a capacitive sensing element, which has small inherent calibration drifts. To account for these drifts, Vaisala uses a ground check device (GC-25) to compare the measurement of the radiosonde against a platinum reference thermometer, built into the ground check unit. The processing software uses this measurement to correct the calibration drift between production and use of the radiosonde. A correction to the raw measurements is then applied to produce finalized data. Any study evaluating the calibration of the RS92 temperature sensor must use the processed data, not the raw data as the authors have done in their study. Although the authors describe that this re-calibration is occurring in the Vaisala software, it is ignored.
The authors point out that an improper ground check can make the observations worse. The conclusion should have been to do a proper ground check and to evaluate the calibration of the GC25 reference thermometer in order to evaluate the calibration of the processed RS92 temperature data. The accuracy of the RS92 temperature measurements depends on it. In operational use the GC25 reference temperature sensors should have been recalibrated in regular intervals of every one to two years, which high quality radiosonde stations typically did. Without an evaluation of the GC25 reference thermometer, an evaluation of the RS92 temperature sensor calibration is not very useful.
The authors did not pick up on the fact, that the RS92 temperature shows a periodic signal of about 140 s or so. This disturbance is most likely caused by the heaters of the two humidity sensors, which cycle at about that period. One of these two sensors is located closer to the temperature sensor than the other. The humidity sensors of the RS92 are heated much more strongly than on the RS41 and this heat source clearly affects the temperature reading of the RS92 temperature sensor. In normal sounding operations, this is not expected to be an issue due to the much stronger ventilation passing first over the temperature sensor. In the configuration shown here, multiple heat transfer paths are possible. The authors interpret this signal as additional noise, where in fact it is most likely due to artificial heating by one of the two humidity sensors. It is possible that the heating of the humidity sensor is also responsible for the temperature dependence that the authors observe in the calibration accuracy of the RS92, depending on the details of the heating of the humidity sensors.
Just using two radiosondes for this evaluation is not sufficient, since there is some production variability of this mass-produced radiosonde. To understand the calibration stability of these sondes for the global network requires some statistical analysis of more than one sonde of each. Without that evaluation, the results are specific for the two tested radiosondes, but not applicable to any other. For the RS92, using sondes from different production batches would be useful, in particular, since the sensor did undergo some substantial changes during the lifetime of this sonde model.
Minor comments:
The dynamic tests are more or less meaningless. The time response of the sensors during a sounding profile play a very important role for the ability to resolve vertical structures. As the authors point out the balloon ascent provides a reasonably well defined ventilation speed of at least the balloon ascent velocity. The tests done have an undefined ventilation and are not representative for atmospheric observations. As the authors note, these dynamic tests may simulate taking a radiosonde from the preparation office to the outside. This transition is completely irrelevant for soundings.
There is also some concern that placing two transmitting radiosondes in close proximity in an environmental chamber may cause some radio frequency interference effects that possibly do not occur in a normal sounding environment. Given the level of confidence the authors try to achieve (<0.1 K), evaluating whether RFI effects occur would be paramount. In particular the capacitive sensor of the RS92 may possibly be more susceptible to this effect under these conditions.
With all that criticism, I would like to point out that evaluating the calibration stability is only a minor factor for the measurement of atmospheric temperature. Much more significant is their behavior in real world conditions, i.e. evaluating the radiation correction, which is up to an order of magnitude larger than the calibration uncertainties discussed here. Additionally, the behavior in clouds under condensation conditions, is another essential challenge for in situ temperature measurements, where these two sondes may show significant differences. However, these factors were not addressed.
Citation: https://doi.org/10.5194/amt-2021-337-RC1 -
AC1: 'Reply on RC1', Marco Rosoldi, 16 Dec 2021
The authors thank the reviewer 1 for his time and comments. Please see below the authors' replies for each specific comment.
Reviewer 1 comment:
Major comments
The Vaisala RS41 radiosonde uses a platinum resistance thermometer as temperature sensor, which does not require re-calibration between production and use of the radiosonde in normal operations. A simple comparison with the temperature sensor of the humidity sensor is sufficient to justify the calibration stability at the time of launch and within preset limits.
Authors’ reply:
What reported in the reviewer’s comment is in line with the manufacturer’s specifications, while the main goal of this work is to independently assess the reliability and the performance of the RS92 and RS41 temperature sensors. The calibration drift and uncertainty of the RS41 temperature sensor have been quantified independently of the manufacturer, showing a very good agreement with the manufacturer's specifications and that this sensor does not require re-calibration before the radiosonde launch. Moreover, the fact that a platinum resistance thermometer does not require periodical recalibration is not supported by facts. The sensing platinum element is a delicate piece of equipment, and mechanical and thermal shocks can significantly alter their nominal resistance at a given temperature, see for instance Kowal et al 2020.
Reviewer 1 comment:
In contrast, the Vaisala RS92 radiosonde uses a capacitive sensing element, which has small inherent calibration drifts. To account for these drifts, Vaisala uses a ground check device (GC-25) to compare the measurement of the radiosonde against a platinum reference thermometer, built into the ground check unit. The processing software uses this measurement to correct the calibration drift between production and use of the radiosonde. A correction to the raw measurements is then applied to produce finalized data. Any study evaluating the calibration of the RS92 temperature sensor must use the processed data, not the raw data as the authors have done in their study. Although the authors describe that this re-calibration is occurring in the Vaisala software, it is ignored.
Authors’ reply:
As for the RS41, the goal is the manufacturer (and GC25)-independent assessment of the calibration drift and uncertainty of the temperature sensor, in order to compare them, respectively, with the calibration drifts resulting from the GC25 and the calibration uncertainty declared by the manufacturer. Such an independent assessment is necessary to develop a transparent, reproducible and manufacturer-independent data processing starting from the same radiosonde’s raw data, following the approach of GCOS Upper-Air Reference Network (GRUAN) for providing reference measurements. Therefore, the RS92 raw temperature measurements instead of the manufacturer-processed measurements, corrected by the GC25 results, were compared with the measurements of the reference thermometer calibrated at INRIM. This independent comparison resulted in a cold bias in the calibration, with a correction factor ranging from 0.1 °C up to 0.3 °C, as well as a calibration uncertainty (k=1) less than 0.1 °C and 0.025 °C larger than that reported in the manufacturer specifications.
Reviewer 1 comment:
The authors point out that an improper ground check can make the observations worse. The conclusion should have been to do a proper ground check and to evaluate the calibration of the GC25 reference thermometer in order to evaluate the calibration of the processed RS92 temperature data. The accuracy of the RS92 temperature measurements depends on it. In operational use the GC25 reference temperature sensors should have been recalibrated in regular intervals of every one to two years, which high quality radiosonde stations typically did. Without an evaluation of the GC25 reference thermometer, an evaluation of the RS92 temperature sensor calibration is not very useful.
Authors’ reply:
The calibration drifts measured with the GC25 resulted in a warm bias compared to the platinum resistance thermometer inside the GC25 unit, with a correction factor ranging from 0.15 °C up to 0.27 °C. As a result, the application of this correction to RS92 raw measurements leads to an increase (up to 0.6 °C) of the cold bias compared to the reference thermometer calibrated at INRIM, worsening the accuracy of RS92 measurements rather than correcting them. This is clearly due to a not reliable correction of the GC25, presumably caused by not having recalibrated in the last two years the Pt100 thermometer inside the GC25 unit. Thus, the conclusion is that the RS92 temperature sensor requires both a pre-launch calibration correction with the GC25 and regular (at least every 2 years) quality assurance checks (recalibrations) of the Pt100 thermometer inside the GC25 unit to avoid significant biases (up to 0.6 °C) in radiosounding measurements. This clearly shows the usefulness of an evaluation of the RS92 temperature sensor calibration independent of the GC25, for example in order to estimate and possibly correct any biases that may affect the data records of not high quality (reference) radiosonde stations on a global scale, that may not always have performed the regular recalibrations mentioned above. Ultimately, our methodology and results confirm, independently of the manufacturer, the better performance of RS41 compared to RS92, in terms of both accuracy in pre-launch temperature measurements and less demanding procedures for the quality assurance of the ground check device.
Reviewer 1 comment:
The authors did not pick up on the fact, that the RS92 temperature shows a periodic signal of about 140 s or so. This disturbance is most likely caused by the heaters of the two humidity sensors, which cycle at about that period. One of these two sensors is located closer to the temperature sensor than the other. The humidity sensors of the RS92 are heated much more strongly than on the RS41 and this heat source clearly affects the temperature reading of the RS92 temperature sensor. In normal sounding operations, this is not expected to be an issue due to the much stronger ventilation passing first over the temperature sensor. In the configuration shown here, multiple heat transfer paths are possible. The authors interpret this signal as additional noise, where in fact it is most likely due to artificial heating by one of the two humidity sensors. It is possible that the heating of the humidity sensor is also responsible for the temperature dependence that the authors observe in the calibration accuracy of the RS92, depending on the details of the heating of the humidity sensors.
Authors’ reply:
Actually, the periodic structure of the RS92 temperature signal immediately appeared to the authors, somehow related to the periodic switching on and off of the two humidity sensors and their heaters. However, although the temperature signal is certainty affected by the swapping cycle of humidity sensors and their heaters, this effect is challenging to be properly quantified, also considering the irregular duration and intensity of the signal maxima. Therefore, the RS92 temperature signal has been considered characteristic of the simultaneous operation of the radiosonde’s temperature and humidity sensors and appropriate to characterize the calibration accuracy. This is also in view to provide the results according to fairness criteria.
Certainly, it is very reasonable to assume that simulating conditions more similar to those of a real radiosounding with a stronger ventilation in the chamber can reduce the effects of the heating of the humidity sensors on the temperature signal. Therefore, in agreement with the reviewer’s consideration, the text of the manuscript will be amended as follows:
- Mentioning the disturbance to the RS92 temperature signal due to the RS92 sensors’ architecture and periodic switching on and off of the two humidity sensors and their heaters, in particular those closer to the temperature sensor.
- Mentioning in the results and conclusions that the noise and the calibration uncertainty obtained for the RS92 are probably to some extent overestimated compared to the conditions of a real radiosounding due to the above disturbance, which, in real soundings, is mitigated by a stronger ventilation on the sensors.
It’s also useful to point out that, to our knowledge, it does not exist a publicly available documentation, from the manufacturer or independent, showing how and to what extent the signal and the calibration accuracy of the RS92 temperature sensor change with respect to those reported in this work under ventilation conditions similar to those of radiosoundings. Moreover, in the documentation provided by the manufacturer the ventilation and pressure conditions to which the calibration of RS92 temperature sensor refers are not reported. On the other hand, in the manuscript conclusions it will be added that further experiments in climatic chambers are recommended by using a measurement configuration suitable for simulating conditions more similar to those of a real radiosounding, with decreasing pressure levels and different ventilations on the sensors (although we are aware there are issues in controlling the air flow around the sensors and reproducing the real ventilation on the sensors in radiosoundings, which results from the complex combination of the balloon lifting vertical speed (typically 5m/s), the horizontal wind, and radiosonde rotations and pendulum motions).
Reviewer 1 comment:
Just using two radiosondes for this evaluation is not sufficient, since there is some production variability of this mass-produced radiosonde. To understand the calibration stability of these sondes for the global network requires some statistical analysis of more than one sonde of each. Without that evaluation, the results are specific for the two tested radiosondes, but not applicable to any other. For the RS92, using sondes from different production batches would be useful, in particular, since the sensor did undergo some substantial changes during the lifetime of this sonde model.
Authors’ reply:
As clearly stated in the conclusions of the manuscript (see lines 668-669), the authors are aware that the results of their evaluation are specific for the two tested radiosondes and these results need to be consolidated by further tests with multiple pairs of radiosondes, in order to obtain results applicable to the different production batches. Nevertheless, in our opinion, the main contribution of this work consists in the introduction of a methodology to simultaneously and independently test the temperature sensors of two different radiosonde models within climatic chambers, in terms of noise, calibration accuracy and bias of sensors’ measurements. The latter have also been quantified, for the first time independently of the manufacturer, for the temperature sensors of Vaisala RS92 and RS41 radiosondes, although referred to a single pair of radiosondes. However, further tests with multiple pairs and production batches of radiosondes and under ventilation/pressure conditions more similar to those of radiosoundings are expected to be performed in the future.
Reviewer 1 comment:
Minor comments:
The dynamic tests are more or less meaningless. The time response of the sensors during a sounding profile play a very important role for the ability to resolve vertical structures. As the authors point out the balloon ascent provides a reasonably well defined ventilation speed of at least the balloon ascent velocity. The tests done have an undefined ventilation and are not representative for atmospheric observations. As the authors note, these dynamic tests may simulate taking a radiosonde from the preparation office to the outside. This transition is completely irrelevant for soundings.
Authors’ reply:
The time response of the sensors is not discussed in this work. Similarly to the tests performed at the first stage of the experiment, the subsequent tests with two climatic chambers were performed under the ventilation conditions (not well defined) generated by the chambers to homogenize the temperature field inside. However, the supposed invalidity or irrelevance for radiosoundings of the outcome of these tests needs to be demonstrated by means of similar tests performed under conditions more similar to those of a real radiosounding. To our knowledge, similar tests have never been carried out by the manufacturer or independently. More specifically, the dynamic tests of this work aim to investigate potential effects on the radiosondes’ temperature sensors of fast and steep thermal changes (in the order of about 20 °C) that radiosondes may meet when passing from indoor to outdoor environment before launch. These thermal changes are simulated by quickly moving the measurement frame equipped with the two radiosondes between the two climatic chambers. The test results reveal that such thermal changes may increase the noise and the calibration uncertainty of temperature sensors, at least during the first part of a radiosoundings. In our opinion, this result can be of great interest for metrology and meteorology and climate communities, as it indicates a possible underestimation of the above uncertainty contributions in the algorithms currently used to process the raw measurements of both radiosonde models. Surely, this result needs to be confirmed by further tests with multiple pairs and production batches of radiosondes and under conditions more similar to those of a real radiosounding.
Reviewer 1 comment:
There is also some concern that placing two transmitting radiosondes in close proximity in an environmental chamber may cause some radio frequency interference effects that possibly do not occur in a normal sounding environment. Given the level of confidence the authors try to achieve (<0.1 K), evaluating whether RFI effects occur would be paramount. In particular the capacitive sensor of the RS92 may possibly be more susceptible to this effect under these conditions.
Authors’ reply:
As reported in the description of the experimental setup (see lines 161-168), the two Vaisala sounding systems used were configured to separately receive and process the signals transmitted by the two radiosonde models at two different frequencies, 402 MHz for the RS92 and 405 MHz for the RS41. The bandwidth of the telemetry signals (5 - 20 KHz) and the distance between the two selected frequencies ensures there is no interference between the signals received from the two radiosondes. More specifically, before the tests, each sounding processing subsystem (SPS311) was set to receive the signal from a single radiosonde model at the selected frequency and to communicate with a single ground-check device type. During the ground check procedure, the SPS311 recognizes the sonde, enables it to transmit at the selected frequency and then selectively receives and processes the signal transmitted at that frequency. On the other hand, no interference was reported for receiving systems similar to that of this experiment, used for comparing the same radiosonde models via dual or multiple soundings (e.g.: Nash et al., 2011; Jensen et al., Atmos. Meas. Tech, 2016 ), even with the two transmitting radiosondes in a closer proximity than in the present experiment (Kawai et al., Atmos. Meas. Tech, 2017). Finally, no interference from other sources was identified during the tests in the climatic chambers.
Reviewer 1 comment:
With all that criticism, I would like to point out that evaluating the calibration stability is only a minor factor for the measurement of atmospheric temperature. Much more significant is their behavior in real world conditions, i.e. evaluating the radiation correction, which is up to an order of magnitude larger than the calibration uncertainties discussed here. Additionally, the behavior in clouds under condensation conditions, is another essential challenge for in situ temperature measurements, where these two sondes may show significant differences. However, thesefactorswerenotaddressed.
Authors’ reply:
The authors would like to stress that to provide reference measurements, all known systematic errors, as well as the uncertainty contributions related to these errors, should be properly quantified. Moreover, although the radiation uncertainty is dominant for daytime radiosoundings through the upper troposphere and stratosphere, the calibration uncertainty is a major contribution for night time radiosoundings (through the whole atmosphere) and for daytime radiosoundings through the lower troposphere. On the other hand, the radiation error and uncertainty for the temperature sensors of RS92 e RS41 have been characterized in other dedicated laboratory experiments (Dirksen et al., Atmos. Meas. Tech, 2014; von Rohden et al., Atmos. Meas. Tech, 2021). Finally, regarding the sensors’ behavior under condensation conditions, the tests performed in this work include the comparison of radiosondes’ temperature sensors under conditions very close to condensation, with RH values of 98% and 95% for temperature values of 20 °C and 40 °C respectively. Although further investigation is needed, by extending the tests above to lower temperatures and simulating ventilation and pressure conditions more similar to those of real radiosoundings, to our knowledge, no dedicated laboratory experiments have so far been performed on this specific topic.
Citation: https://doi.org/10.5194/amt-2021-337-AC1
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AC1: 'Reply on RC1', Marco Rosoldi, 16 Dec 2021
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RC2: 'Comment on amt-2021-337', Anonymous Referee #2, 24 Jan 2022
In the manuscript, Intercomparison of Vaisala RS92 and RS41 radiosonde temperature sensors under controlled laboratory conditions, the authors describe a laboratory setup to characterise the temperature sensors of the Vaisala RS92 and RS41 radiosondes. The concept to characterize radiosonde's sensors in a dedicated setup under well-controlled laboratory conditions is employed to achieve measurement traceability for reference observations. This could answer the long-standing need in the research-community for independent and traceable assessment of measurement- and calibration errors of radiosondes.
However the experiment and the methodology described in this manuscript have several severe shortcomings that make it hard to draw reliable conclusions from the measurement data. In its current shape manuscript does not meet the quality standards for publication in AMT, and I don't think this can be remedied by a major revision, since the shortcomings in the experimental setup are of a more fundamental nature, which would require a modification of the setup and a complete repeat of the experimental work. On this basis, I would recommend to reject the manuscript for publication in AMT.
The main issue concerns the lack of ventilation in the experiment:
The authors do not provide an exact number for the ventilation speed in the chamber at the location of the radiosondes' temperature sensors, it is stated in line 499 that "the ventilation is weak". This is an important difference with the way radiosondes are normally operated, namely with a ventilation of 3-7 m/s (WMO-CIMO recommendation) provided by the lift of the balloon. This ascent-driven ventilation makes sure that the sensors, that are located near the tip of the sensor boom, do not sample air that is contaminated by the casing of the radiosonde. This lack of ventilation is the likely cause for the periodic peaks in for the RS92 in Figure 3 (that are not addressed by the authors). The RS92 is equipped with two humidity sensors that are alternately heated to remove contamination. It appears that during the experiment, this heating of the RH sensors leaks through to the temperature sensor by conduction. In case of proper ventilation, these periodic peaks in the temperature signal would have been suppressed.For future reference: this heating can be switched off by subjecting the RS92 to T < -60C or to pressures lower than 100 hPa. The heating function is reactivated by initialisation in the GC25 unit.
The lack of ventilation also affects the results of the temperature-change experiment, an example of which is presented in Figure 6. Due to the thermal mass of the table on which the radiosondes is mounted, it takes several minutes for the setup to stabilize. During this stabilisation phase, the temperature recordings appear noisy. However, this is not to be interpreted as as noise from the sensor (an instrument property) but rather the result from thermal gradients and other inhomogeneities in the setup which cause for example small-scale turbulences. Proper ventilation in the setup would reduce this transient noise.
Furthermore, I don't think there is much added value in investigating the behavior of the radiosonde when subjected to a temperature change associated with e.g. leaving a building. This pre-flight situation (with very limited ventilation) does not represent the actual operational mode of the radiosonde during an ascent for which it is devised.
Citation: https://doi.org/10.5194/amt-2021-337-RC2 -
AC2: 'Reply on RC2', Marco Rosoldi, 28 Feb 2022
Reviewer 2 comment:
In the manuscript, Intercomparison of Vaisala RS92 and RS41 radiosonde temperature sensors under controlled laboratory conditions, the authors describe a laboratory setup to characterise the temperature sensors of the Vaisala RS92 and RS41 radiosondes. The concept to characterize radiosonde's sensors in a dedicated setup under well-controlled laboratory conditions is employed to achieve measurement traceability for reference observations. This could answer the long-standing need in the research-community for independent and traceable assessment of measurement- and calibration errors of radiosondes.
However the experiment and the methodology described in this manuscript have several severe shortcomings that make it hard to draw reliable conclusions from the measurement data. In its current shape manuscript does not meet the quality standards for publication in AMT, and I don't think this can be remedied by a major revision, since the shortcomings in the experimental setup are of a more fundamental nature, which would require a modification of the setup and a complete repeat of the experimental work. On this basis, I would recommend to reject the manuscript for publication in AMT.
Authors’ reply:
The authors thank the reviewer 2 for his time and comments. The authors believe that the experimental setup and methodology described in the manuscript are suitable to reliably characterize the temperature sensors of the radiosondes tested, compatibly with the current state of the art for such a characterization based on laboratory tests with climatic chambers. Please see below the authors' replies for each specific comment
Reviewer 2 comment:
The main issue concerns the lack of ventilation in the experiment:
The authors do not provide an exact number for the ventilation speed in the chamber at the location of the radiosondes' temperature sensors, it is stated in line 499 that "the ventilation is weak". This is an important difference with the way radiosondes are normally operated, namely with a ventilation of 3-7 m/s (WMO-CIMO recommendation) provided by the lift of the balloon. This ascent-driven ventilation makes sure that the sensors, that are located near the tip of the sensor boom, do not sample air that is contaminated by the casing of the radiosonde. This lack of ventilation is the likely cause for the periodic peaks in for the RS92 in Figure 3 (that are not addressed by the authors). The RS92 is equipped with two humidity sensors that are alternately heated to remove contamination. It appears that during the experiment, this heating of the RH sensors leaks through to the temperature sensor by conduction. In case of proper ventilation, these periodic peaks in the temperature signal would have been suppressed.
For future reference: this heating can be switched off by subjecting the RS92 to T < -60C or to pressures lower than 100 hPa. The heating function is reactivated by initialisation in the GC25 unit.Authors’ reply:
To address the reviewers' comments, the ventilation speed in the chamber at the location of radiosondes’ temperature sensors has been estimated with a portable digital anemometer (wind speed range and uncertainty 0.3-30m/s and 5%, respectively) and resulted in 2m/s. This ventilation speed is not so far from the assumed ventilation of 3-7m/s on the sensors during radiosoundings, and it does not seem to be weaker than that produced during the ground check with the GC25, whose results have so far been used to characterize the calibration accuracy of the RS92 temperature sensor. Indeed, it is very hard to reproduce under controlled laboratory conditions the real ventilation on the sensors in radiosoundings, which results from the complex combination of the balloon lifting vertical speed, the horizontal wind, as well as rotations and pendulum motions of the radiosonde. The text of the manuscript will be amended, including the above estimate of the ventilation speed on the sensors inside the climatic chamber.
As for the periodic peaks of the RS92 temperature signal in Fig. 5 (I suppose the reviewer 2 refers to Fig.5 instead of Fig.3), these peaks immediately appeared to the authors, somehow related to the periodic switching on and off of the two humidity sensors and their heaters. However, although the temperature signal is certainty affected by the swapping cycle of humidity sensors and their heaters, this effect is challenging to be properly quantified, also considering the irregular duration and intensity of the peaks. Therefore, the RS92 temperature signal has been considered characteristic of the simultaneous operation of the radiosonde’s temperature and humidity sensors and appropriate to characterize the calibration accuracy. Certainly, it is very reasonable to assume that simulating conditions more similar to those of a radiosounding, with a stronger ventilation in the chamber, can reduce the effects of the heating of the humidity sensors on the temperature signal. Therefore, taking into account the reviewer’s comment, the text of the manuscript will be amended as follows:1) Mentioning the disturbance to the RS92 temperature signal due to the RS92 sensors’ architecture and periodic switching on and off of the two humidity sensors and their heaters, in particular those closer to the temperature sensor.
2) Mentioning in the results and conclusions that:
- The potential effects of a stronger ventilation on RS92 sensor have been estimated, by removing the peaks in RS92 temperature signals and recalculating the statistical quantities used to characterize the noise and calibration accuracy (the authors performed this estimate assuming that, in case of proper ventilation, the periodic peaks in RS92 temperature signals would have been suppressed, as suggested by the reviewer)
- The results of the above estimate indicate noise and calibration uncertainty values for the RS92 up to 0.03 °C lower than those obtained in our experiment, which are presumably overestimated up to 0.03 °C compared to those with a stronger ventilation, under conditions more similar to those of a radiosounding.
On the other hand, it’s useful to point out that, to authors’ knowledge, it does not exist a publicly available documentation, from the manufacturer or independent, showing how and to what extent the signal and the calibration accuracy of the RS92 temperature sensor change with respect to those reported in this work under ventilation conditions more similar to those of radiosoundings. Moreover, in the documentation provided by the manufacturer, the ventilation and pressure conditions to which the calibration of RS92 temperature sensor refers are not reported. However, the results on the calibration accuracy obtained from the laboratory tests performed in this work are in very good agreement with those provided by the manufacturer.
Therefore, arguing that the experimental setup and methodology described in this work do not allow to obtain reliable results seems not supported by the necessary documentation and laboratory tests. Instead, further laboratory experiments should be recommended by using a measurement configuration suitable for simulating conditions more similar to those of a radiosounding, with decreasing pressure levels and stronger ventilations on the sensors, in order to assess if and to what extent the results of this work may change under those conditions. The recommendation to carry out the above experiments will be added in the manuscript conclusions.Reviewer 2 comment:
The lack of ventilation also affects the results of the temperature-change experiment, an example of which is presented in Figure 6. Due to the thermal mass of the table on which the radiosondes is mounted, it takes several minutes for the setup to stabilize. During this stabilisation phase, the temperature recordings appear noisy. However, this is not to be interpreted as as noise from the sensor (an instrument property) but rather the result from thermal gradients and other inhomogeneities in the setup which cause for example small-scale turbulences. Proper ventilation in the setup would reduce this transient noise.
Authors’ reply:
If the authors understood correctly, in the previous comment the reviewer argues that the noise increase in radiosondes’ temperature readings, observed after each temperature change and reported in Table 3, is not be interpreted as noise from radiosondes’ sensors (sensors’ property), but rather the result from the thermal instability of the experimental setup, which takes several minutes to stabilize. Such an argument does not seem plausible because, as reported in the description of the methodology ( lines 319-321), the acquisition period considered after each temperature change was started as soon as the thermal stability was achieved in the chamber, typically about 15 min after the change. For the example shown in Fig.6, the acquisition period considered after the temperature change started 17 min after the change. This time interval between each temperature change and the start of the acquisition period was sufficient to achieve the thermal stability of the setup, which is demonstrated by the readings of reference thermometers used to identify the stability conditions and whose standard deviation measures the setup stability. Indeed, the values of the setup stability before and after each change reported in Table 3 indicate that the stability after each change is similar to that before that change, and even higher for the change #3, corresponding to the measurements shown in Fig. 6. Now, if the noise increase in radiosondes’ temperature readings resulted from the setup instability, the values of this instability after each change should have been much higher than those observed before the change, also considering the higher response time of the reference thermometers compared to radiosondes’ temperature sensors. On the contrary, the values of the setup stability during the acquisition periods before and after each change are very similar. Therefore, the higher noise values measured after the changes in radiosondes’ temperature readings are due to radiosondes’ sensors (sensors’ property) and not to the instability of the measurement setup. As for the thermal mass of the frame on which the radiosondes are mounted, this should affect the time interval required to achieve the stability, but not the noise level once the stability was achieved.
Reviewer 2 comment:
Furthermore, I don't think there is much added value in investigating the behavior of the radiosonde when subjected to a temperature change associated with e.g. leaving a building. This pre-flight situation (with very limited ventilation) does not represent the actual operational mode of the radiosonde during an ascent for which it is devised.
Authors’ reply:
The tests of the experiment performed with two climatic chambers aim to investigate potential effects on the radiosondes’ temperature sensors of fast and steep thermal changes (in the order of about 20 °C) that radiosondes may meet when passing from indoor to outdoor environment before launch (no matter the ventilation conditions during the pre-launch phase). These thermal changes are simulated by quickly moving the measurement frame equipped with the two radiosondes between the two climatic chambers. The test results reveal that such thermal changes may increase the noise and the calibration uncertainty of temperature sensors, at least during the first part of a radiosoundings. In our opinion, this result can be of great interest for metrology, meteorology and climate communities, as it indicates a possible underestimation of the above uncertainty contributions in the algorithms currently used to process the raw measurements of both radiosonde models.
Similarly to the tests performed at the first stage of the experiment, the tests with two climatic chambers were performed under the ventilation conditions described above and generated by the chambers to homogenize the temperature field inside. The supposed invalidity or irrelevance for radiosoundings of the outcome of these tests due to the limited ventilation in the chambers (if the reviewer’s comment refers to this) needs to be demonstrated by means of similar tests performed under conditions more similar to those of a real radiosounding, with decreasing pressure levels and stronger ventilations on the sensors. To our knowledge, similar tests have never been carried out by the manufacturer or independently.
Furthermore, the potential effects of a stronger ventilation, estimated as mentioned in the authors’ second reply, should only affect the results obtained for the RS92, and not significantly enough to compromise their plausibility.
Surely, the outcome of our tests needs to be confirmed by further tests with multiple pairs and production batches of radiosondes and under conditions more similar to those of radiosoundings.Citation: https://doi.org/10.5194/amt-2021-337-AC2
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AC2: 'Reply on RC2', Marco Rosoldi, 28 Feb 2022
Interactive discussion
Status: closed
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RC1: 'Comment on amt-2021-337', Anonymous Referee #1, 24 Nov 2021
The manuscript by Rosoldi et al. evaluates the calibration of the temperature sensors on an RS92 radiosonde and an RS41 radiosonde. The latter model is the successor of the former and plays an important role in the global radiosonde observing network. An understanding of the performance of the temperature sensors on these radiosonde models is important to evaluate their effect on numerical weather forecasting and climate records.
Brief summary of my main concerns
The authors have not prepared the ground check of the RS92 correctly and used an improper data table for the RS92 generated by the sounding software in their analysis. They missed the influence of the RS92 humidity sensor heater, which contributes to the temperature signal measured by the RS92 and which requires a different setup to reduce its effect. They have used only one sonde of each, making it difficult to transfer the results to an average production run. I am very doubtful that the current setup is suitable to address the temperature calibration of these two radiosondes to the level the authors try to achieve so that others may build upon.
Major comments
The Vaisala RS41 radiosonde uses a platinum resistance thermometer as temperature sensor, which does not require re-calibration between production and use of the radiosonde in normal operations. A simple comparison with the temperature sensor of the humidity sensor is sufficient to justify the calibration stability at the time of launch and within preset limits.
In contrast, the Vaisala RS92 radiosonde uses a capacitive sensing element, which has small inherent calibration drifts. To account for these drifts, Vaisala uses a ground check device (GC-25) to compare the measurement of the radiosonde against a platinum reference thermometer, built into the ground check unit. The processing software uses this measurement to correct the calibration drift between production and use of the radiosonde. A correction to the raw measurements is then applied to produce finalized data. Any study evaluating the calibration of the RS92 temperature sensor must use the processed data, not the raw data as the authors have done in their study. Although the authors describe that this re-calibration is occurring in the Vaisala software, it is ignored.
The authors point out that an improper ground check can make the observations worse. The conclusion should have been to do a proper ground check and to evaluate the calibration of the GC25 reference thermometer in order to evaluate the calibration of the processed RS92 temperature data. The accuracy of the RS92 temperature measurements depends on it. In operational use the GC25 reference temperature sensors should have been recalibrated in regular intervals of every one to two years, which high quality radiosonde stations typically did. Without an evaluation of the GC25 reference thermometer, an evaluation of the RS92 temperature sensor calibration is not very useful.
The authors did not pick up on the fact, that the RS92 temperature shows a periodic signal of about 140 s or so. This disturbance is most likely caused by the heaters of the two humidity sensors, which cycle at about that period. One of these two sensors is located closer to the temperature sensor than the other. The humidity sensors of the RS92 are heated much more strongly than on the RS41 and this heat source clearly affects the temperature reading of the RS92 temperature sensor. In normal sounding operations, this is not expected to be an issue due to the much stronger ventilation passing first over the temperature sensor. In the configuration shown here, multiple heat transfer paths are possible. The authors interpret this signal as additional noise, where in fact it is most likely due to artificial heating by one of the two humidity sensors. It is possible that the heating of the humidity sensor is also responsible for the temperature dependence that the authors observe in the calibration accuracy of the RS92, depending on the details of the heating of the humidity sensors.
Just using two radiosondes for this evaluation is not sufficient, since there is some production variability of this mass-produced radiosonde. To understand the calibration stability of these sondes for the global network requires some statistical analysis of more than one sonde of each. Without that evaluation, the results are specific for the two tested radiosondes, but not applicable to any other. For the RS92, using sondes from different production batches would be useful, in particular, since the sensor did undergo some substantial changes during the lifetime of this sonde model.
Minor comments:
The dynamic tests are more or less meaningless. The time response of the sensors during a sounding profile play a very important role for the ability to resolve vertical structures. As the authors point out the balloon ascent provides a reasonably well defined ventilation speed of at least the balloon ascent velocity. The tests done have an undefined ventilation and are not representative for atmospheric observations. As the authors note, these dynamic tests may simulate taking a radiosonde from the preparation office to the outside. This transition is completely irrelevant for soundings.
There is also some concern that placing two transmitting radiosondes in close proximity in an environmental chamber may cause some radio frequency interference effects that possibly do not occur in a normal sounding environment. Given the level of confidence the authors try to achieve (<0.1 K), evaluating whether RFI effects occur would be paramount. In particular the capacitive sensor of the RS92 may possibly be more susceptible to this effect under these conditions.
With all that criticism, I would like to point out that evaluating the calibration stability is only a minor factor for the measurement of atmospheric temperature. Much more significant is their behavior in real world conditions, i.e. evaluating the radiation correction, which is up to an order of magnitude larger than the calibration uncertainties discussed here. Additionally, the behavior in clouds under condensation conditions, is another essential challenge for in situ temperature measurements, where these two sondes may show significant differences. However, these factors were not addressed.
Citation: https://doi.org/10.5194/amt-2021-337-RC1 -
AC1: 'Reply on RC1', Marco Rosoldi, 16 Dec 2021
The authors thank the reviewer 1 for his time and comments. Please see below the authors' replies for each specific comment.
Reviewer 1 comment:
Major comments
The Vaisala RS41 radiosonde uses a platinum resistance thermometer as temperature sensor, which does not require re-calibration between production and use of the radiosonde in normal operations. A simple comparison with the temperature sensor of the humidity sensor is sufficient to justify the calibration stability at the time of launch and within preset limits.
Authors’ reply:
What reported in the reviewer’s comment is in line with the manufacturer’s specifications, while the main goal of this work is to independently assess the reliability and the performance of the RS92 and RS41 temperature sensors. The calibration drift and uncertainty of the RS41 temperature sensor have been quantified independently of the manufacturer, showing a very good agreement with the manufacturer's specifications and that this sensor does not require re-calibration before the radiosonde launch. Moreover, the fact that a platinum resistance thermometer does not require periodical recalibration is not supported by facts. The sensing platinum element is a delicate piece of equipment, and mechanical and thermal shocks can significantly alter their nominal resistance at a given temperature, see for instance Kowal et al 2020.
Reviewer 1 comment:
In contrast, the Vaisala RS92 radiosonde uses a capacitive sensing element, which has small inherent calibration drifts. To account for these drifts, Vaisala uses a ground check device (GC-25) to compare the measurement of the radiosonde against a platinum reference thermometer, built into the ground check unit. The processing software uses this measurement to correct the calibration drift between production and use of the radiosonde. A correction to the raw measurements is then applied to produce finalized data. Any study evaluating the calibration of the RS92 temperature sensor must use the processed data, not the raw data as the authors have done in their study. Although the authors describe that this re-calibration is occurring in the Vaisala software, it is ignored.
Authors’ reply:
As for the RS41, the goal is the manufacturer (and GC25)-independent assessment of the calibration drift and uncertainty of the temperature sensor, in order to compare them, respectively, with the calibration drifts resulting from the GC25 and the calibration uncertainty declared by the manufacturer. Such an independent assessment is necessary to develop a transparent, reproducible and manufacturer-independent data processing starting from the same radiosonde’s raw data, following the approach of GCOS Upper-Air Reference Network (GRUAN) for providing reference measurements. Therefore, the RS92 raw temperature measurements instead of the manufacturer-processed measurements, corrected by the GC25 results, were compared with the measurements of the reference thermometer calibrated at INRIM. This independent comparison resulted in a cold bias in the calibration, with a correction factor ranging from 0.1 °C up to 0.3 °C, as well as a calibration uncertainty (k=1) less than 0.1 °C and 0.025 °C larger than that reported in the manufacturer specifications.
Reviewer 1 comment:
The authors point out that an improper ground check can make the observations worse. The conclusion should have been to do a proper ground check and to evaluate the calibration of the GC25 reference thermometer in order to evaluate the calibration of the processed RS92 temperature data. The accuracy of the RS92 temperature measurements depends on it. In operational use the GC25 reference temperature sensors should have been recalibrated in regular intervals of every one to two years, which high quality radiosonde stations typically did. Without an evaluation of the GC25 reference thermometer, an evaluation of the RS92 temperature sensor calibration is not very useful.
Authors’ reply:
The calibration drifts measured with the GC25 resulted in a warm bias compared to the platinum resistance thermometer inside the GC25 unit, with a correction factor ranging from 0.15 °C up to 0.27 °C. As a result, the application of this correction to RS92 raw measurements leads to an increase (up to 0.6 °C) of the cold bias compared to the reference thermometer calibrated at INRIM, worsening the accuracy of RS92 measurements rather than correcting them. This is clearly due to a not reliable correction of the GC25, presumably caused by not having recalibrated in the last two years the Pt100 thermometer inside the GC25 unit. Thus, the conclusion is that the RS92 temperature sensor requires both a pre-launch calibration correction with the GC25 and regular (at least every 2 years) quality assurance checks (recalibrations) of the Pt100 thermometer inside the GC25 unit to avoid significant biases (up to 0.6 °C) in radiosounding measurements. This clearly shows the usefulness of an evaluation of the RS92 temperature sensor calibration independent of the GC25, for example in order to estimate and possibly correct any biases that may affect the data records of not high quality (reference) radiosonde stations on a global scale, that may not always have performed the regular recalibrations mentioned above. Ultimately, our methodology and results confirm, independently of the manufacturer, the better performance of RS41 compared to RS92, in terms of both accuracy in pre-launch temperature measurements and less demanding procedures for the quality assurance of the ground check device.
Reviewer 1 comment:
The authors did not pick up on the fact, that the RS92 temperature shows a periodic signal of about 140 s or so. This disturbance is most likely caused by the heaters of the two humidity sensors, which cycle at about that period. One of these two sensors is located closer to the temperature sensor than the other. The humidity sensors of the RS92 are heated much more strongly than on the RS41 and this heat source clearly affects the temperature reading of the RS92 temperature sensor. In normal sounding operations, this is not expected to be an issue due to the much stronger ventilation passing first over the temperature sensor. In the configuration shown here, multiple heat transfer paths are possible. The authors interpret this signal as additional noise, where in fact it is most likely due to artificial heating by one of the two humidity sensors. It is possible that the heating of the humidity sensor is also responsible for the temperature dependence that the authors observe in the calibration accuracy of the RS92, depending on the details of the heating of the humidity sensors.
Authors’ reply:
Actually, the periodic structure of the RS92 temperature signal immediately appeared to the authors, somehow related to the periodic switching on and off of the two humidity sensors and their heaters. However, although the temperature signal is certainty affected by the swapping cycle of humidity sensors and their heaters, this effect is challenging to be properly quantified, also considering the irregular duration and intensity of the signal maxima. Therefore, the RS92 temperature signal has been considered characteristic of the simultaneous operation of the radiosonde’s temperature and humidity sensors and appropriate to characterize the calibration accuracy. This is also in view to provide the results according to fairness criteria.
Certainly, it is very reasonable to assume that simulating conditions more similar to those of a real radiosounding with a stronger ventilation in the chamber can reduce the effects of the heating of the humidity sensors on the temperature signal. Therefore, in agreement with the reviewer’s consideration, the text of the manuscript will be amended as follows:
- Mentioning the disturbance to the RS92 temperature signal due to the RS92 sensors’ architecture and periodic switching on and off of the two humidity sensors and their heaters, in particular those closer to the temperature sensor.
- Mentioning in the results and conclusions that the noise and the calibration uncertainty obtained for the RS92 are probably to some extent overestimated compared to the conditions of a real radiosounding due to the above disturbance, which, in real soundings, is mitigated by a stronger ventilation on the sensors.
It’s also useful to point out that, to our knowledge, it does not exist a publicly available documentation, from the manufacturer or independent, showing how and to what extent the signal and the calibration accuracy of the RS92 temperature sensor change with respect to those reported in this work under ventilation conditions similar to those of radiosoundings. Moreover, in the documentation provided by the manufacturer the ventilation and pressure conditions to which the calibration of RS92 temperature sensor refers are not reported. On the other hand, in the manuscript conclusions it will be added that further experiments in climatic chambers are recommended by using a measurement configuration suitable for simulating conditions more similar to those of a real radiosounding, with decreasing pressure levels and different ventilations on the sensors (although we are aware there are issues in controlling the air flow around the sensors and reproducing the real ventilation on the sensors in radiosoundings, which results from the complex combination of the balloon lifting vertical speed (typically 5m/s), the horizontal wind, and radiosonde rotations and pendulum motions).
Reviewer 1 comment:
Just using two radiosondes for this evaluation is not sufficient, since there is some production variability of this mass-produced radiosonde. To understand the calibration stability of these sondes for the global network requires some statistical analysis of more than one sonde of each. Without that evaluation, the results are specific for the two tested radiosondes, but not applicable to any other. For the RS92, using sondes from different production batches would be useful, in particular, since the sensor did undergo some substantial changes during the lifetime of this sonde model.
Authors’ reply:
As clearly stated in the conclusions of the manuscript (see lines 668-669), the authors are aware that the results of their evaluation are specific for the two tested radiosondes and these results need to be consolidated by further tests with multiple pairs of radiosondes, in order to obtain results applicable to the different production batches. Nevertheless, in our opinion, the main contribution of this work consists in the introduction of a methodology to simultaneously and independently test the temperature sensors of two different radiosonde models within climatic chambers, in terms of noise, calibration accuracy and bias of sensors’ measurements. The latter have also been quantified, for the first time independently of the manufacturer, for the temperature sensors of Vaisala RS92 and RS41 radiosondes, although referred to a single pair of radiosondes. However, further tests with multiple pairs and production batches of radiosondes and under ventilation/pressure conditions more similar to those of radiosoundings are expected to be performed in the future.
Reviewer 1 comment:
Minor comments:
The dynamic tests are more or less meaningless. The time response of the sensors during a sounding profile play a very important role for the ability to resolve vertical structures. As the authors point out the balloon ascent provides a reasonably well defined ventilation speed of at least the balloon ascent velocity. The tests done have an undefined ventilation and are not representative for atmospheric observations. As the authors note, these dynamic tests may simulate taking a radiosonde from the preparation office to the outside. This transition is completely irrelevant for soundings.
Authors’ reply:
The time response of the sensors is not discussed in this work. Similarly to the tests performed at the first stage of the experiment, the subsequent tests with two climatic chambers were performed under the ventilation conditions (not well defined) generated by the chambers to homogenize the temperature field inside. However, the supposed invalidity or irrelevance for radiosoundings of the outcome of these tests needs to be demonstrated by means of similar tests performed under conditions more similar to those of a real radiosounding. To our knowledge, similar tests have never been carried out by the manufacturer or independently. More specifically, the dynamic tests of this work aim to investigate potential effects on the radiosondes’ temperature sensors of fast and steep thermal changes (in the order of about 20 °C) that radiosondes may meet when passing from indoor to outdoor environment before launch. These thermal changes are simulated by quickly moving the measurement frame equipped with the two radiosondes between the two climatic chambers. The test results reveal that such thermal changes may increase the noise and the calibration uncertainty of temperature sensors, at least during the first part of a radiosoundings. In our opinion, this result can be of great interest for metrology and meteorology and climate communities, as it indicates a possible underestimation of the above uncertainty contributions in the algorithms currently used to process the raw measurements of both radiosonde models. Surely, this result needs to be confirmed by further tests with multiple pairs and production batches of radiosondes and under conditions more similar to those of a real radiosounding.
Reviewer 1 comment:
There is also some concern that placing two transmitting radiosondes in close proximity in an environmental chamber may cause some radio frequency interference effects that possibly do not occur in a normal sounding environment. Given the level of confidence the authors try to achieve (<0.1 K), evaluating whether RFI effects occur would be paramount. In particular the capacitive sensor of the RS92 may possibly be more susceptible to this effect under these conditions.
Authors’ reply:
As reported in the description of the experimental setup (see lines 161-168), the two Vaisala sounding systems used were configured to separately receive and process the signals transmitted by the two radiosonde models at two different frequencies, 402 MHz for the RS92 and 405 MHz for the RS41. The bandwidth of the telemetry signals (5 - 20 KHz) and the distance between the two selected frequencies ensures there is no interference between the signals received from the two radiosondes. More specifically, before the tests, each sounding processing subsystem (SPS311) was set to receive the signal from a single radiosonde model at the selected frequency and to communicate with a single ground-check device type. During the ground check procedure, the SPS311 recognizes the sonde, enables it to transmit at the selected frequency and then selectively receives and processes the signal transmitted at that frequency. On the other hand, no interference was reported for receiving systems similar to that of this experiment, used for comparing the same radiosonde models via dual or multiple soundings (e.g.: Nash et al., 2011; Jensen et al., Atmos. Meas. Tech, 2016 ), even with the two transmitting radiosondes in a closer proximity than in the present experiment (Kawai et al., Atmos. Meas. Tech, 2017). Finally, no interference from other sources was identified during the tests in the climatic chambers.
Reviewer 1 comment:
With all that criticism, I would like to point out that evaluating the calibration stability is only a minor factor for the measurement of atmospheric temperature. Much more significant is their behavior in real world conditions, i.e. evaluating the radiation correction, which is up to an order of magnitude larger than the calibration uncertainties discussed here. Additionally, the behavior in clouds under condensation conditions, is another essential challenge for in situ temperature measurements, where these two sondes may show significant differences. However, thesefactorswerenotaddressed.
Authors’ reply:
The authors would like to stress that to provide reference measurements, all known systematic errors, as well as the uncertainty contributions related to these errors, should be properly quantified. Moreover, although the radiation uncertainty is dominant for daytime radiosoundings through the upper troposphere and stratosphere, the calibration uncertainty is a major contribution for night time radiosoundings (through the whole atmosphere) and for daytime radiosoundings through the lower troposphere. On the other hand, the radiation error and uncertainty for the temperature sensors of RS92 e RS41 have been characterized in other dedicated laboratory experiments (Dirksen et al., Atmos. Meas. Tech, 2014; von Rohden et al., Atmos. Meas. Tech, 2021). Finally, regarding the sensors’ behavior under condensation conditions, the tests performed in this work include the comparison of radiosondes’ temperature sensors under conditions very close to condensation, with RH values of 98% and 95% for temperature values of 20 °C and 40 °C respectively. Although further investigation is needed, by extending the tests above to lower temperatures and simulating ventilation and pressure conditions more similar to those of real radiosoundings, to our knowledge, no dedicated laboratory experiments have so far been performed on this specific topic.
Citation: https://doi.org/10.5194/amt-2021-337-AC1
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AC1: 'Reply on RC1', Marco Rosoldi, 16 Dec 2021
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RC2: 'Comment on amt-2021-337', Anonymous Referee #2, 24 Jan 2022
In the manuscript, Intercomparison of Vaisala RS92 and RS41 radiosonde temperature sensors under controlled laboratory conditions, the authors describe a laboratory setup to characterise the temperature sensors of the Vaisala RS92 and RS41 radiosondes. The concept to characterize radiosonde's sensors in a dedicated setup under well-controlled laboratory conditions is employed to achieve measurement traceability for reference observations. This could answer the long-standing need in the research-community for independent and traceable assessment of measurement- and calibration errors of radiosondes.
However the experiment and the methodology described in this manuscript have several severe shortcomings that make it hard to draw reliable conclusions from the measurement data. In its current shape manuscript does not meet the quality standards for publication in AMT, and I don't think this can be remedied by a major revision, since the shortcomings in the experimental setup are of a more fundamental nature, which would require a modification of the setup and a complete repeat of the experimental work. On this basis, I would recommend to reject the manuscript for publication in AMT.
The main issue concerns the lack of ventilation in the experiment:
The authors do not provide an exact number for the ventilation speed in the chamber at the location of the radiosondes' temperature sensors, it is stated in line 499 that "the ventilation is weak". This is an important difference with the way radiosondes are normally operated, namely with a ventilation of 3-7 m/s (WMO-CIMO recommendation) provided by the lift of the balloon. This ascent-driven ventilation makes sure that the sensors, that are located near the tip of the sensor boom, do not sample air that is contaminated by the casing of the radiosonde. This lack of ventilation is the likely cause for the periodic peaks in for the RS92 in Figure 3 (that are not addressed by the authors). The RS92 is equipped with two humidity sensors that are alternately heated to remove contamination. It appears that during the experiment, this heating of the RH sensors leaks through to the temperature sensor by conduction. In case of proper ventilation, these periodic peaks in the temperature signal would have been suppressed.For future reference: this heating can be switched off by subjecting the RS92 to T < -60C or to pressures lower than 100 hPa. The heating function is reactivated by initialisation in the GC25 unit.
The lack of ventilation also affects the results of the temperature-change experiment, an example of which is presented in Figure 6. Due to the thermal mass of the table on which the radiosondes is mounted, it takes several minutes for the setup to stabilize. During this stabilisation phase, the temperature recordings appear noisy. However, this is not to be interpreted as as noise from the sensor (an instrument property) but rather the result from thermal gradients and other inhomogeneities in the setup which cause for example small-scale turbulences. Proper ventilation in the setup would reduce this transient noise.
Furthermore, I don't think there is much added value in investigating the behavior of the radiosonde when subjected to a temperature change associated with e.g. leaving a building. This pre-flight situation (with very limited ventilation) does not represent the actual operational mode of the radiosonde during an ascent for which it is devised.
Citation: https://doi.org/10.5194/amt-2021-337-RC2 -
AC2: 'Reply on RC2', Marco Rosoldi, 28 Feb 2022
Reviewer 2 comment:
In the manuscript, Intercomparison of Vaisala RS92 and RS41 radiosonde temperature sensors under controlled laboratory conditions, the authors describe a laboratory setup to characterise the temperature sensors of the Vaisala RS92 and RS41 radiosondes. The concept to characterize radiosonde's sensors in a dedicated setup under well-controlled laboratory conditions is employed to achieve measurement traceability for reference observations. This could answer the long-standing need in the research-community for independent and traceable assessment of measurement- and calibration errors of radiosondes.
However the experiment and the methodology described in this manuscript have several severe shortcomings that make it hard to draw reliable conclusions from the measurement data. In its current shape manuscript does not meet the quality standards for publication in AMT, and I don't think this can be remedied by a major revision, since the shortcomings in the experimental setup are of a more fundamental nature, which would require a modification of the setup and a complete repeat of the experimental work. On this basis, I would recommend to reject the manuscript for publication in AMT.
Authors’ reply:
The authors thank the reviewer 2 for his time and comments. The authors believe that the experimental setup and methodology described in the manuscript are suitable to reliably characterize the temperature sensors of the radiosondes tested, compatibly with the current state of the art for such a characterization based on laboratory tests with climatic chambers. Please see below the authors' replies for each specific comment
Reviewer 2 comment:
The main issue concerns the lack of ventilation in the experiment:
The authors do not provide an exact number for the ventilation speed in the chamber at the location of the radiosondes' temperature sensors, it is stated in line 499 that "the ventilation is weak". This is an important difference with the way radiosondes are normally operated, namely with a ventilation of 3-7 m/s (WMO-CIMO recommendation) provided by the lift of the balloon. This ascent-driven ventilation makes sure that the sensors, that are located near the tip of the sensor boom, do not sample air that is contaminated by the casing of the radiosonde. This lack of ventilation is the likely cause for the periodic peaks in for the RS92 in Figure 3 (that are not addressed by the authors). The RS92 is equipped with two humidity sensors that are alternately heated to remove contamination. It appears that during the experiment, this heating of the RH sensors leaks through to the temperature sensor by conduction. In case of proper ventilation, these periodic peaks in the temperature signal would have been suppressed.
For future reference: this heating can be switched off by subjecting the RS92 to T < -60C or to pressures lower than 100 hPa. The heating function is reactivated by initialisation in the GC25 unit.Authors’ reply:
To address the reviewers' comments, the ventilation speed in the chamber at the location of radiosondes’ temperature sensors has been estimated with a portable digital anemometer (wind speed range and uncertainty 0.3-30m/s and 5%, respectively) and resulted in 2m/s. This ventilation speed is not so far from the assumed ventilation of 3-7m/s on the sensors during radiosoundings, and it does not seem to be weaker than that produced during the ground check with the GC25, whose results have so far been used to characterize the calibration accuracy of the RS92 temperature sensor. Indeed, it is very hard to reproduce under controlled laboratory conditions the real ventilation on the sensors in radiosoundings, which results from the complex combination of the balloon lifting vertical speed, the horizontal wind, as well as rotations and pendulum motions of the radiosonde. The text of the manuscript will be amended, including the above estimate of the ventilation speed on the sensors inside the climatic chamber.
As for the periodic peaks of the RS92 temperature signal in Fig. 5 (I suppose the reviewer 2 refers to Fig.5 instead of Fig.3), these peaks immediately appeared to the authors, somehow related to the periodic switching on and off of the two humidity sensors and their heaters. However, although the temperature signal is certainty affected by the swapping cycle of humidity sensors and their heaters, this effect is challenging to be properly quantified, also considering the irregular duration and intensity of the peaks. Therefore, the RS92 temperature signal has been considered characteristic of the simultaneous operation of the radiosonde’s temperature and humidity sensors and appropriate to characterize the calibration accuracy. Certainly, it is very reasonable to assume that simulating conditions more similar to those of a radiosounding, with a stronger ventilation in the chamber, can reduce the effects of the heating of the humidity sensors on the temperature signal. Therefore, taking into account the reviewer’s comment, the text of the manuscript will be amended as follows:1) Mentioning the disturbance to the RS92 temperature signal due to the RS92 sensors’ architecture and periodic switching on and off of the two humidity sensors and their heaters, in particular those closer to the temperature sensor.
2) Mentioning in the results and conclusions that:
- The potential effects of a stronger ventilation on RS92 sensor have been estimated, by removing the peaks in RS92 temperature signals and recalculating the statistical quantities used to characterize the noise and calibration accuracy (the authors performed this estimate assuming that, in case of proper ventilation, the periodic peaks in RS92 temperature signals would have been suppressed, as suggested by the reviewer)
- The results of the above estimate indicate noise and calibration uncertainty values for the RS92 up to 0.03 °C lower than those obtained in our experiment, which are presumably overestimated up to 0.03 °C compared to those with a stronger ventilation, under conditions more similar to those of a radiosounding.
On the other hand, it’s useful to point out that, to authors’ knowledge, it does not exist a publicly available documentation, from the manufacturer or independent, showing how and to what extent the signal and the calibration accuracy of the RS92 temperature sensor change with respect to those reported in this work under ventilation conditions more similar to those of radiosoundings. Moreover, in the documentation provided by the manufacturer, the ventilation and pressure conditions to which the calibration of RS92 temperature sensor refers are not reported. However, the results on the calibration accuracy obtained from the laboratory tests performed in this work are in very good agreement with those provided by the manufacturer.
Therefore, arguing that the experimental setup and methodology described in this work do not allow to obtain reliable results seems not supported by the necessary documentation and laboratory tests. Instead, further laboratory experiments should be recommended by using a measurement configuration suitable for simulating conditions more similar to those of a radiosounding, with decreasing pressure levels and stronger ventilations on the sensors, in order to assess if and to what extent the results of this work may change under those conditions. The recommendation to carry out the above experiments will be added in the manuscript conclusions.Reviewer 2 comment:
The lack of ventilation also affects the results of the temperature-change experiment, an example of which is presented in Figure 6. Due to the thermal mass of the table on which the radiosondes is mounted, it takes several minutes for the setup to stabilize. During this stabilisation phase, the temperature recordings appear noisy. However, this is not to be interpreted as as noise from the sensor (an instrument property) but rather the result from thermal gradients and other inhomogeneities in the setup which cause for example small-scale turbulences. Proper ventilation in the setup would reduce this transient noise.
Authors’ reply:
If the authors understood correctly, in the previous comment the reviewer argues that the noise increase in radiosondes’ temperature readings, observed after each temperature change and reported in Table 3, is not be interpreted as noise from radiosondes’ sensors (sensors’ property), but rather the result from the thermal instability of the experimental setup, which takes several minutes to stabilize. Such an argument does not seem plausible because, as reported in the description of the methodology ( lines 319-321), the acquisition period considered after each temperature change was started as soon as the thermal stability was achieved in the chamber, typically about 15 min after the change. For the example shown in Fig.6, the acquisition period considered after the temperature change started 17 min after the change. This time interval between each temperature change and the start of the acquisition period was sufficient to achieve the thermal stability of the setup, which is demonstrated by the readings of reference thermometers used to identify the stability conditions and whose standard deviation measures the setup stability. Indeed, the values of the setup stability before and after each change reported in Table 3 indicate that the stability after each change is similar to that before that change, and even higher for the change #3, corresponding to the measurements shown in Fig. 6. Now, if the noise increase in radiosondes’ temperature readings resulted from the setup instability, the values of this instability after each change should have been much higher than those observed before the change, also considering the higher response time of the reference thermometers compared to radiosondes’ temperature sensors. On the contrary, the values of the setup stability during the acquisition periods before and after each change are very similar. Therefore, the higher noise values measured after the changes in radiosondes’ temperature readings are due to radiosondes’ sensors (sensors’ property) and not to the instability of the measurement setup. As for the thermal mass of the frame on which the radiosondes are mounted, this should affect the time interval required to achieve the stability, but not the noise level once the stability was achieved.
Reviewer 2 comment:
Furthermore, I don't think there is much added value in investigating the behavior of the radiosonde when subjected to a temperature change associated with e.g. leaving a building. This pre-flight situation (with very limited ventilation) does not represent the actual operational mode of the radiosonde during an ascent for which it is devised.
Authors’ reply:
The tests of the experiment performed with two climatic chambers aim to investigate potential effects on the radiosondes’ temperature sensors of fast and steep thermal changes (in the order of about 20 °C) that radiosondes may meet when passing from indoor to outdoor environment before launch (no matter the ventilation conditions during the pre-launch phase). These thermal changes are simulated by quickly moving the measurement frame equipped with the two radiosondes between the two climatic chambers. The test results reveal that such thermal changes may increase the noise and the calibration uncertainty of temperature sensors, at least during the first part of a radiosoundings. In our opinion, this result can be of great interest for metrology, meteorology and climate communities, as it indicates a possible underestimation of the above uncertainty contributions in the algorithms currently used to process the raw measurements of both radiosonde models.
Similarly to the tests performed at the first stage of the experiment, the tests with two climatic chambers were performed under the ventilation conditions described above and generated by the chambers to homogenize the temperature field inside. The supposed invalidity or irrelevance for radiosoundings of the outcome of these tests due to the limited ventilation in the chambers (if the reviewer’s comment refers to this) needs to be demonstrated by means of similar tests performed under conditions more similar to those of a real radiosounding, with decreasing pressure levels and stronger ventilations on the sensors. To our knowledge, similar tests have never been carried out by the manufacturer or independently.
Furthermore, the potential effects of a stronger ventilation, estimated as mentioned in the authors’ second reply, should only affect the results obtained for the RS92, and not significantly enough to compromise their plausibility.
Surely, the outcome of our tests needs to be confirmed by further tests with multiple pairs and production batches of radiosondes and under conditions more similar to those of radiosoundings.Citation: https://doi.org/10.5194/amt-2021-337-AC2
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AC2: 'Reply on RC2', Marco Rosoldi, 28 Feb 2022
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