Measurement characteristics of an airborne microwave temperature profiler (MTP)

Heckl, Mareike; Fix, Andreas; Jirousek, Matthias; Schreier, Franz; Xu, Jian; Rapp, Markus

doi:https://doi.org/10.5194/amt-14-1689-2021

Articles | Volume 14, issue 2

https://doi.org/10.5194/amt-14-1689-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/amt-14-1689-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 14, issue 2

Research article

|

01 Mar 2021

Research article |

| 01 Mar 2021

Measurement characteristics of an airborne microwave temperature profiler (MTP)

Mareike Heckl, Andreas Fix, Matthias Jirousek, Franz Schreier, Jian Xu, and Markus Rapp

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Final revised paper (published on 01 Mar 2021)
Preprint (discussion started on 08 Jan 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'review kenntner', Anonymous Referee #1, 01 Mar 2020
- AC1: 'Author reply to referee #1 comments', Mareike Kenntner, 26 May 2020
RC2: 'Comments on "Measurement Characteristics of an Airborne Microwave Temperature Profiler (MTP)"', Anonymous Referee #2, 31 Mar 2020
- AC2: 'Author reply to referee #2 comments', Mareike Kenntner, 26 May 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Mareike Heckl on behalf of the Authors (02 Jun 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (27 Jun 2020) by Karin Kreher

RR by Anonymous Referee #2 (11 Jul 2020)

RR by Anonymous Referee #1 (26 Jul 2020)

Suggestions for revision or reasons for rejection

The manuscript has improved significantly, however, there are some flaws that need to be corrected. As a major issue a clear definition and subsequent wording of the terms how the measurement uncertainty characterized in terms of systematic and random components needs to be made (see also comments below). In this respect it might also be important to mention the uncertainty characteristics of the in-situ HALO-TS measurement which is given as 0.5 K. However, the standard deviation between HALO-TS and TB is ≤ 0.38 K which implies – together with the random noise characteristics of TB (around 0.3 K) that the precision (noise) of HALO-TS is much lower than that.

The term “noise figure” is used frequently in the text at several instances when a general characterization of the noise is meant. However, the term “noise figure” is a typical specification for microwave elements (mixers, amplifiers) expressing the noise factor between in and output in decibels (given in dB). Therefore, please use this term with care. Most of the time it will be sufficient to just remove the word “figure” or replace it with “characterisation”.

Line 22. Another important application of HAMP is the liquid water path (see Jacob et al., AMT, 2020). In fact later on it would be good to mention that also non-resonant interaction of microwave radiation with atmospheric hydrometeors influences TB.

P2,l2: Shorten to “Based on these mesoscale temperature fluctuation analyses, a number of modelling studies aimed at improving the understanding and numerical description of atmospheric gravity waves”

P3l15: I would always been careful to make such a general statement as:
“For the first time, this study presents all relevant instrument characteristics of the HALO-MTP instrument.” Be aware that there might be always surprises ahead of the road. For example, I think that there is currently no chance to specify the absolute accuracy of the MTP as there is no absolute truth available – HALO TS is only an approximation as the authors discuss later on. Furthermore, in terms of spectral analysis investigations the current investigation is not all-encompassing. The Allan variance could still provide additional information especially in terms of the maximum time between two subsequent calibrations.

P4l12: Please correct to “thermal radiation mainly emitted by oxygen ..” Though the impact is low some influence of N2 and water vapor continuum occurrs especially when flying low. Similarly, hydrometeors can contribute to the signal.

P4, Eq. 2.1: Please mention that in this case the measured radiance I (usual notation) is equal to the Brightness B of a blackbody described by Plank’s law.
Also note that the omission of the higher order terms in the Rayleigh-Jeans approximation can be more than 0.5 K at your frequencies, check table 1 in Liu et al., 2008 https://doi.org/10.1016/j.jqsrt.2008.03.001

P4, l26: The horn is no receiver. Please make also the connection to the antenna response function: “..a horn antenna guides the incoming atmospheric radiation and determines the spatial response function”. What about the rotating mirror – is it just flat or does it focus the antenna beam?

P4, l28. Why don’t you use the common term interim frequency (IF) instead of base-band? Further you could also provide the information that this is around 100 MHz which makes it much clearer for the reader in the following. Suggest the following text:
“.. the incoming signal is converted to the interim frequency (IF) around 100 MHz. Both difference frequencies below and above the LO frequency are down-converted to the IF in the double side band receiver. Low pass filtering supresses any incoming radiation outside the IF bandwidth of 200 MHz such that the symmetric spectrum around the current LO frequency is measured with only a minor gap of approx. 20 MHz at the LO frequency. The IF signal is converted to a voltage..” In this way the information on Page 5 last line is not necessary. I think this information fits much better here – or is the IF bandwidth different for the wing-canister instrument than for other MTP?

P5, l2: “The PHYSICAL temperatures..”

P5, l3: Also mention the amplifier which becomes important later on.

P6, l2: “..doubled twice, POTENTIALLY allowing …Here only the standard set… is used.” In the beginning I was confused here so this might also help others.

P6l25: I find “half-hemisphere” =quarter sphere rather confusing. It is not necessary to mention it but rather it is important to say what “Both functions” in the next sentence means.

P6/5: I don’t think that the logic for the second part of the sentence is correct “despite the fundamental assumption in MTP calibration that this relation is always linear “. I suggest to change to “Amplifiers might change their characteristics and thus the relation between the recorded signal and the source temperature, i.e. the calibration parameters, change.” You still assume linearity. For nonlinearity either higher order terms or an exponential relationship would be need to be considered in the calibration equation and determined by introducing a third reference point.

P7,l4: noise diode signal strength

P7,l24: “The gap in the centre is due to the use of a double-side-band receiver as explained in Sec?”

P8,l3: Anntenna pattern: “It is actually mainly defined by the shape of the rotating mirror at the front of the instrument.” The text did not provide any information on the shape and I can not see something in Fig. 1. If it is a flat mirror then only the horn is important. If it is used for beam forming, e.g. an offset parabolic mirror, it is important to mention that? Has the antenna beamwidth not been calculated using gaussian beam optics?

P9,l11: “itself IS monitored”

P9,l11: Over this large temperature range it is difficult to see by eye if a linear relation ship fits the data better than a non linear/exponential one? This comment is maybe a bit picky but one could easily check the significance of the statement. What I further notice is that the slope for the dark blue load seams to be lower than the one of the light blue load for the lower two channels and vice versa for the upper channel. As this is shown in counts rather than in brightness temperatures it is difficult to judge if this is significant. Why don’t the authors use a simple/standard calibration that would help the reader to judge the impact – this could just be plotted on the second y-axis for illustration – of course an offset needs to be introduced to separate the channels? This comment also holds for the following two figures as nobody is interested in counts,

P9,l30 – same comment relate counts to Tb

P10, l11: In addition to the term “noise figure” the use of “long-term stability” is unclear. Suggest: “This is strong evidence that the HALO-MTP noise characteristics do not change between flights, and the laboratory assessment can be used to characterize measurements of different campaigns and serve as information for the retrieval development.”

P12, l20: In fact if you are flying high (thus having an optical thin atmosphere above you) and assume horizontal homogeneity you could do the tipping curve calibration from the different elevation angle measurements pointing upwards (probing different opacities). For sure, I don’t expect that you do that but it is not the instrument design limiting you.

P14 l7: Please again relate counts to TB

P15, l18 – suggest to say absolute accuracy

P15, l29: “The accuracy of the temperatures “ – be more specific. Which T, which type of accuracy?

P16, l17: Eq. 4.5 give the noise equivalent temperature NeDT not the measurement uncertainty. This is not the variance (rather square root of variance) of the measurement noise but the radiometric resolution (see Eq. 6.13 by Woodhouse, 2017) and can be estimated as standard deviation from time series, e.g. from Fig.6 I would estimate something like 0.3 K. This already roughly corresponds to the values in Fig. 11. Note, the effect of the passband is negligible compared to other deviations from an ideal radiometer and should not be mentioned.

Maybe it is good to clarify that in its simplest concept measurement uncertainty includes systematic errors (bias, absolute accuracy) and random errors (noise, precision).

P18, l14: can you clarify whether this is random or systematic?

P18, l26: It is likely only relevant if hydrometeors or WV variations occur,,

P20, l7: please transfer to Tb and discus in respect to the later results and the term precision. It is especially important here as you want to give an idea which amplitude of gravity waves (be clear on that) can be detected.

P20l20: “To achieve this accuracy, the necessity of an offset-correction relative to HALO TS” Here you talked about precision not about the bias. Just delete the first part “To achieve this accuracy”

P20, l30. “clearly dominated by the contribution from measurement noise.” Is this really true if I look at Fig. 10.

P21, l12: “all necessary instrument parameters” I wouldn’ be so confident!

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ED: Reconsider after major revisions (11 Aug 2020) by Karin Kreher

AR by Svenja Lange on behalf of the Authors (04 Nov 2020) Author's response

ED: Referee Nomination & Report Request started (25 Nov 2020) by Karin Kreher

RR by Anonymous Referee #1 (12 Dec 2020)

ED: Publish as is (13 Dec 2020) by Karin Kreher