the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Apr 2022
Performance evaluation of an integrated path differential absorption LIDAR model for surface pressure from low-Earth orbit
Abstract. Remote sensing of surface pressure from space is critical; differential absorption LIDAR and differential absorption radar are only two kinds of remote sensing instruments with this potential. The differential absorption LIDAR works in integral path mode from the satellite in low-Earth orbit. It measures the differential optical depth of the Oxygen A-band, and the surface pressure is thereafter obtained by performing circle-iterative calculation. Performance evaluation of the differential absorption LIDAR model was conducted with respect to the advanced system parameters of the space instrument, Low echo pulse energy at ocean surface and the challenging calculation of repetitive cumulative average of echo on uneven land surface yielded random errors in surface pressure measurement. On the other hand, uncertain atmospheric temperature and water vapor mixture profiles resulted in systematic error of surface pressure. Consequently, controlling the error of surface pressure within 0.1 % proved challenging. Under a strict implementation of the error budget, the time resolution is 6.25 s and along-orbit distance resolution is 44 km, and the results showed that 765.6735/765.4637 nm is suitable as the working wavelength pair. Further, error could be expected to within 0.2–0.3 % for the cumulative average of 625 ocean surface laser pulse echoes, cumulative average of more than 144 pulse echoes on land, and observation from the 400 km orbit.
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Status: closed
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RC1: 'Comment on amt-2022-69', Anonymous Referee #1, 13 May 2022
It is my believe, that while this work represents a nontrivial amount of effort, it seems to be primarily a reexamination of existing methods for know oxygen A band wavelengths. It follows a very traditional error analysis and arrives at similar over all conclusions. Changes in topography over land and surface reflectance over the ocean are primary drivers in LIDAR measurements of surface pressure. While this is true, what is also most likely true is that the surface pressure over the spatial scales required to obtain the measurement also also plays a predominate role in the error analysis, and the longer one averages samples the more dominate this term becomes. These changes are at the crux of the problem. While this work address some of the requirement needs it does not provide a constraint on others, and seems to set a scale length to meet the design, instead of developing a design that meets the requirements. Currently, production NWP model cell sizes, on global scales, are consistently on the order of 15km, This work seems to have backed into a 44km measurement size purely based the need to beat down the measure noise, and not based on model or observational needs. This indirectly assuming that pressure is in some way shape or form stable/static over the defined extent. The examples provided as rational for such measurements, clearly have very dynamic behavior on these scales.
A more pertinent question, might be what is the appropriate path length for these type of measurements, and how many samples over land, for a typical/prescribed LEO orbit, fall in the category of having less than a 2 meter change in height over any 44km or other path length. how does one introduce this constraint into an error analysis of this type, how many sample might be expected and where?
While this work seems adopt a rigorous approach to compute atmospheric absorptions values, these could most likely be achieved using some high fidelity community RT model that may better address the interplay between/contamination of other species e.g. H20.
Finally, I find the summary/conclusions lacking.
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AC1: 'Reply on RC1', Yu Dong, 06 Oct 2022
（1）With regard to the "global production NWP model unit size, our work seems to have regressed to the measurement size of 44km", not only to meet the needs of eliminating measurement noise, but also based on the empirical data of laser remote sensing instruments that have been launched internationally, which is limited to the emission capacity of spaceborne lasers (repetition frequency multiplied by pulse energy), The satellite borne wind laser optical radar ALADIN of the ADM-AEOLUS mission launched by the European Space Agency in 2018, the laser repetition frequency is 50Hz, the average time of laser pulses is 7s, and the gap period is 21s. The original design orbital height is 408km, and the along orbit laser pulse accumulation horizontal distance unit is 50km (plus the upper span 200km horizontal distance resolution). The actual orbital height after liftoff is 320km, the laser pulse energy is 72mJ, and the along orbit accumulation distance unit is~88km, It is more than 50km of the original design. Just referring to the actual capability of ALADIN, we propose a 6.25s pulse accumulation time, and the corresponding horizontal distance measurement unit is 44km.
（2) "For typical/specified LEO orbits, how many samples on land belong to the category where the height change is less than 2 meters in any 44 km or other path length. How to introduce this constraint in this type of error analysis, how many samples are needed and where?"
In order to make the height change more rigorous, the manuscript will make major changes to this, and the title "Performance evaluation of an integrated path different absorption LIDAR model for surface pressure from low Earth orbit" will be changed to "Performance evaluation of an integrated path different absorption LIDAR model for local surface pressure from low Earth orbit", The text will delete the content about "land". The text will not discuss the situation of land surface temporarily, which has made the discussion more simple. The atmospheric pressure of sea surface is very important for weather forecast.
（3) The relationship between differential optical thickness and atmospheric pressure in the past literature is reviewed.
-
AC1: 'Reply on RC1', Yu Dong, 06 Oct 2022
Status: closed
-
RC1: 'Comment on amt-2022-69', Anonymous Referee #1, 13 May 2022
It is my believe, that while this work represents a nontrivial amount of effort, it seems to be primarily a reexamination of existing methods for know oxygen A band wavelengths. It follows a very traditional error analysis and arrives at similar over all conclusions. Changes in topography over land and surface reflectance over the ocean are primary drivers in LIDAR measurements of surface pressure. While this is true, what is also most likely true is that the surface pressure over the spatial scales required to obtain the measurement also also plays a predominate role in the error analysis, and the longer one averages samples the more dominate this term becomes. These changes are at the crux of the problem. While this work address some of the requirement needs it does not provide a constraint on others, and seems to set a scale length to meet the design, instead of developing a design that meets the requirements. Currently, production NWP model cell sizes, on global scales, are consistently on the order of 15km, This work seems to have backed into a 44km measurement size purely based the need to beat down the measure noise, and not based on model or observational needs. This indirectly assuming that pressure is in some way shape or form stable/static over the defined extent. The examples provided as rational for such measurements, clearly have very dynamic behavior on these scales.
A more pertinent question, might be what is the appropriate path length for these type of measurements, and how many samples over land, for a typical/prescribed LEO orbit, fall in the category of having less than a 2 meter change in height over any 44km or other path length. how does one introduce this constraint into an error analysis of this type, how many sample might be expected and where?
While this work seems adopt a rigorous approach to compute atmospheric absorptions values, these could most likely be achieved using some high fidelity community RT model that may better address the interplay between/contamination of other species e.g. H20.
Finally, I find the summary/conclusions lacking.
-
AC1: 'Reply on RC1', Yu Dong, 06 Oct 2022
（1）With regard to the "global production NWP model unit size, our work seems to have regressed to the measurement size of 44km", not only to meet the needs of eliminating measurement noise, but also based on the empirical data of laser remote sensing instruments that have been launched internationally, which is limited to the emission capacity of spaceborne lasers (repetition frequency multiplied by pulse energy), The satellite borne wind laser optical radar ALADIN of the ADM-AEOLUS mission launched by the European Space Agency in 2018, the laser repetition frequency is 50Hz, the average time of laser pulses is 7s, and the gap period is 21s. The original design orbital height is 408km, and the along orbit laser pulse accumulation horizontal distance unit is 50km (plus the upper span 200km horizontal distance resolution). The actual orbital height after liftoff is 320km, the laser pulse energy is 72mJ, and the along orbit accumulation distance unit is~88km, It is more than 50km of the original design. Just referring to the actual capability of ALADIN, we propose a 6.25s pulse accumulation time, and the corresponding horizontal distance measurement unit is 44km.
（2) "For typical/specified LEO orbits, how many samples on land belong to the category where the height change is less than 2 meters in any 44 km or other path length. How to introduce this constraint in this type of error analysis, how many samples are needed and where?"
In order to make the height change more rigorous, the manuscript will make major changes to this, and the title "Performance evaluation of an integrated path different absorption LIDAR model for surface pressure from low Earth orbit" will be changed to "Performance evaluation of an integrated path different absorption LIDAR model for local surface pressure from low Earth orbit", The text will delete the content about "land". The text will not discuss the situation of land surface temporarily, which has made the discussion more simple. The atmospheric pressure of sea surface is very important for weather forecast.
（3) The relationship between differential optical thickness and atmospheric pressure in the past literature is reviewed.
-
AC1: 'Reply on RC1', Yu Dong, 06 Oct 2022
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