the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Nov 2022
Global evaluation of Doppler velocity errors of EarthCARE Cloud Profiling Radar using global storm-resolving simulation
Abstract. The Cloud Profiling Radar (CPR) on the Earth Clouds, Aerosol, and Radiation Explorer (EarthCARE) satellite is the first satellite-borne Doppler radar (EC-CPR). In our previous study, we examined the effects of horizontal (along-track) integration and simple unfolding methods on the reduction of Doppler errors in the EC-CPR observations, and those effects were evaluated using two limited scenes in limited latitude and low pulse repetition frequency (PRF) settings. In this study, the amount of data used was significantly increased, and the area of the data used was extended globally. Not only low PRF but also high PRF settings were examined. We calculated the EC-CPR-observed Doppler velocity from pulse-pair covariances using the radar reflectivity factor and Doppler velocity obtained from a satellite data simulator and a global storm-resolving simulation. The global data were divided into five latitudinal zones, and mean Doppler errors for 5 dBZe after 10 km integration were calculated. In the case of low PRF setting, the error without unfolding correction for the tropics reached a maximum of 2.2 m s-1 and then decreased toward the poles (0.43 m s-1). The error with unfolding correction for the tropics became much smaller at 0.63 m s-1. In the case of high PRF setting, the error without unfolding correction for the tropics reached a maximum of 0.78 m s-1 and then decreased toward the poles (0.19 m s-1). The error with unfolding correction for the tropics was 0.29 m s-1, less than half the value without the correction. The results of the analyses of the simulated data indicated that the zonal mean frequency of precipitation echoes was highest in the tropics and decreased toward the poles. Considering a limitation of the unfolding correction for discrimination between large upward velocity and large precipitation falling velocity, the latitudinal variation of the Doppler error can be explained by the precipitation echo distribution.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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RC1: 'Comment on egusphere-2022-1255', Anonymous Referee #1, 05 Feb 2023
This paper is concerned with the accuracy of the vertical velocity observations that should be achieved by the nadir pointing 94GHz Doppler radar on the EarthCARE satellite when it is launched. It is an extension of the estimates in an earlier paper (HH, 2022) that used the same NICAM global model with 3.5km horizontal resolution to forward model the values of radar reflectivity and vertical velocity over 2 orbits with a prf close to 6100Hz so the folding velocity is +/- 4.8m/s so that precipitation with a terminal velocity of 6m/s would be folded and appear as an upward velocity of - 3.6m/s (the convention is that downward velocity is positive). Such upward velocities were deemed unlikely and so any upward velocity above 3m/s was unfolded by subtracting twice the folding velocity. When velocities were unfolded then the average standard deviation of the retrieved vertical velocity was reduced.
The advance in this paper is to extend the analysis firstly to 16 orbits (rather than 2 orbits in the HH paper) so that the degree of folding as a function of latitude can be found, and secondly to extend the analysis to 7500Hz so the folding velocity is 6m/s rather than 4.8m/s for 6100Hz. The advantage of the 7500Hz frequency is a reduction in the phase noise due to the reshuffling of the target caused by a Doppler width of the target assumed to be 4m/s due to satellite motion and a finite beamwidth. This leads to a lowering of the correlation between two sequential pulses (Equn 2 in this paper, equn 3 in HH).
The conclusion of this new paper (abstract lines 21-24) is:
“…the mean frequency of the precipitation echoes was highest in the tropics and decreased toward the poles. Considering a limitation of the unfolding correction for discrimination between large upward velocity and large precipitation falling velocity, the latitudinal variation of the Doppler error can be explained by the precipitation echo distribution.”
MAJOR COMMENTS
- TERMNAL VELOCITY AT 94GHz.
The conclusion of the new paper in the abstract lines 21-24 that heavier precipitation occurs in the tropics is to be expected, but it would be useful to know the values of reflectivity and the type of particles that are needed to produce the high terminal velocities above 6 m/s at 94GHz, 7500Hz and consequent folding. In most rainfall Mie scattering of the larger drops at 94GHz leads to terminal velocities much below 6 m/s.
- MULTIPLE SCATTERING.
The authors appear to have neglected the effect of multiple scattering which leads to very noisy phase returns due to the differing path lengths of the multiply scattered photons. This effect becomes important for rain rates above 5 mm/hr and will drastically degrade the quality of the Doppler, see Matrosov et al. 2008. https://doi.org/10.1175/2008JTECHA1095.1
- MODEL RESOLUTION.
The model has a resolution of 3.5km so the size of the features that can be represented is probably greater than 10km. This means that the forward modeled values of reflectivity for each km in the horizontal will not be independent but will be smoothed, and secondly the full range of updrafts and downdrafts will not be resolved. In the current analysis based on the NICAM model it is assumed that any updraft above 3m/s is deemed to be unlikely, but in reality updrafts much higher than this do occur on the km or sub km scale.
Less important issues with the presentation. The paper relies too much on quoting results from HH. It would read better is some explanations were provided.
- a) Rather than quoting equation (2) and referring to the HH paper for explanations, the introduction should have a short paragraph explaining that the Doppler is retrieved by estimating the phase change of the returned signal from a target from successive transmitted pulses.
- b) Include a couple of sentences explaining that as a spacecraft is a moving platform with a finite beam-width the targets have a high Doppler width (4m/s) and so the reshuffling of the targets in the time between two transmitted pulses leads to a rapid lowering of the correlation of the phases and higher Doppler errors. .
- c) The terminology “high mode PRF” (e.g. the caption to figure 3) can be confusing. Does “high” refer to the PRF or the maximum altitude? Better to say high prf /lower maximum altitude.
Citation: https://doi.org/10.5194/egusphere-2022-1255-RC1 -
AC1: 'Reply on RC1', Yuichiro Hagihara, 16 Mar 2023
Dear referee,
We really appreciate the reviewer’s efforts. Your comments definitely help us to improve our paper.
The following is a point-by-point response to the specific comments:
MAJOR COMMENTS
1. TERMNAL VELOCITY AT 94GHz.
The conclusion of the new paper in the abstract lines 21-24 that heavier precipitation occurs in the tropics is to be expected, but it would be useful to know the values of reflectivity and the type of particles that are needed to produce the high terminal velocities above 6 m/s at 94GHz, 7500Hz and consequent folding. In most rainfall Mie scattering of the larger drops at 94GHz leads to terminal velocities much below 6 m/s.RESPONSE:
In H22, Fig. 9(a) shows a 2D-histogram of Vjsim without the random error as a function of the Ze for the precipitation case. Large fall velocities exceeding 6 m/s are not seen as you pointed out. As shown in Fig. 9(b-d) in H22, considering the random error due to the Doppler broadening, the velocity folding occurs.
2. MULTIPLE SCATTERING.
The authors appear to have neglected the effect of multiple scattering which leads to very noisy phase returns due to the differing path lengths of the multiply scattered photons. This effect becomes important for rain rates above 5 mm/hr and will drastically degrade the quality of the Doppler, see Matrosov et al. 2008. https://doi.org/10.1175/2008JTECHA1095.1RESPONSE:
We thank for your important remarks. However, the Doppler effect of multiple scattering was not considered in this study because of its complication, and the issue will be the subject of future research.
3. MODEL RESOLUTION.
The model has a resolution of 3.5km so the size of the features that can be represented is probably greater than 10km. This means that the forward modeled values of reflectivity for each km in the horizontal will not be independent but will be smoothed, and secondly the full range of updrafts and downdrafts will not be resolved. In the current analysis based on the NICAM model it is assumed that any updraft above 3m/s is deemed to be unlikely, but in reality updrafts much higher than this do occur on the km or sub km scale.RESPONSE:
There may be fast updrafts on the km or sub km scale. However, such events is rare globally and would be negligible in statistics such as latitudinal zonal means. We are focusing on global statistical results and therefore we use the NICAM. When higher horizontal resolution NICAM data becomes available, we would like to study similar evaluation with it.
We will add this explanation in the revised manuscript.
1. a) Rather than quoting equation (2) and referring to the HH paper for explanations, the introduction should have a short paragraph explaining that the Doppler is retrieved by estimating the phase change of the returned signal from a target from successive transmitted pulses.RESPONSE:
We agreed with your suggestion. We will add the following sentence in the revised manuscript.
"The EC-CPR measures Doppler velocities using the pulse-pair method. It measures phase shift of echoes from two successive transmitted pulses."
2. b) Include a couple of sentences explaining that as a spacecraft is a moving platform with a finite beam-width the targets have a high Doppler width (4m/s) and so the reshuffling of the targets in the time between two transmitted pulses leads to a rapid lowering of the correlation of the phases and higher Doppler errors.RESPONSE:
We agreed with your comment. We will add the following sentence in the revised manuscript.
"Since the EC-CPR is a finite beamwidth on fast moving spaceborne platform, targets have a broad Doppler width, which causes a worsening of the correlation of the phase. Then, large Doppler errors are introduced."
3. c) The terminology “high mode PRF” (e.g. the caption to figure 3) can be confusing. Does “high” refer to the PRF or the maximum altitude? Better to say high prf /lower maximum altitude.RESPONSE:
We agreed with your suggestion. It is indeed confusing, so we will change them in the revised manuscript as follows:
"high mode PRF" to "PRF of the high mode (lower PRF)" and
"low mode PRF" to "PRF of the low mode (higher PRF)".
Thank you very much.Citation: https://doi.org/10.5194/egusphere-2022-1255-AC1
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RC2: 'Comment on egusphere-2022-1255', Anonymous Referee #2, 06 Feb 2023
The paper assesses the vertical velocity error in the EarthCare CPR observations at different integration path scales for two different PRF patterns. It builds upon the previous study of the same authors, but for a smaller sample size. The analysis is based on the radar simulations applied to the NICAM precipitation resolving model. The model output is first interpolated at the CPR observation grid, and then the radar moments of the interest are obtained.
The paper shows reduced errors for higher PRF sampling thanks to a reduction in the phase noise and increased folding velocity threshold. Moreover, a simple unfolding technique reduces additionally the reported errors that affect the most the Doppler velocity measurements in the tropics.
Major comments:
1. The authors assume a constant Doppler spectrum width of 4 m/s. It would be beneficial for readers to explain why this value is a reasonable estimate given the spacecraft speed and the beam-width of the radar. Moreover, it should be also mentioned that the Doppler spectrum width depends on the observed hydrometeors distribution, i.e., for heavy rain the with will be additionally increased.
2. Although, a long integration path (10 km) seams to be a tempting approach to reduce uncertainty in the Doppler measurements, the authors do not assess the effect of a long scale signal decorrelation and non-uniform beam filling effects in such a large sampling volumes. Moreover, the horizontal resolution of 10 km prevents studies on the small scale features like localized convection, the characterization of which is one of the objectives of space-borne Doppler radar missions.
3. Does the radar simulator accounts for multiple scattering? This issue has been demonstrated to have a destructive effect on the quality of the Doppler measurements.
Minor comments:
1. The terminology "high-mode PRF" can be misleading. It would be better to use "low PRF mode" or "high tropopause mode".
2. It feels like some of the discussions can be reduced in length, e.g., when the high PRF mode is compared with the low PRF mode.
Some minor comments are also included in the attached file.
-
AC2: 'Reply on RC2', Yuichiro Hagihara, 16 Mar 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1255/egusphere-2022-1255-AC2-supplement.pdf
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AC2: 'Reply on RC2', Yuichiro Hagihara, 16 Mar 2023
-
EC1: 'Comment on egusphere-2022-1255', Robin Hogan, 20 Mar 2023
Thank you for submitting your replies to the reviewers' comments. In preparing the revised version of your manuscript, please note the following:
1. Regarding Reviwer 1's comment 1: your reply provides an important piece of explanatory information - please ensure that the revised paper includes a sentence or two explaining this to the reader, including a reference to the relevant figure in H22.
2. Regarding multiple scattering raised by both reviewers, please include a short discussion of this in the revised paper (e.g. in the conclusions). Reviewer 1 suggested Matrosov et al. (2008) as a possible reference: this is certainly valid to show the effect of multiple scattering on Z but does not mention Doppler. For that you could cite Battaglia and Tanelli (2010: 10.1109/TGRS.2010.2052818).
3. Regarding Reviewer 2's comment about line 74, please ensure that the revised paper includes this explanation of the meaning of the threshold, and the justification for the use of a value of 20.
Citation: https://doi.org/10.5194/egusphere-2022-1255-EC1
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2022-1255', Anonymous Referee #1, 05 Feb 2023
This paper is concerned with the accuracy of the vertical velocity observations that should be achieved by the nadir pointing 94GHz Doppler radar on the EarthCARE satellite when it is launched. It is an extension of the estimates in an earlier paper (HH, 2022) that used the same NICAM global model with 3.5km horizontal resolution to forward model the values of radar reflectivity and vertical velocity over 2 orbits with a prf close to 6100Hz so the folding velocity is +/- 4.8m/s so that precipitation with a terminal velocity of 6m/s would be folded and appear as an upward velocity of - 3.6m/s (the convention is that downward velocity is positive). Such upward velocities were deemed unlikely and so any upward velocity above 3m/s was unfolded by subtracting twice the folding velocity. When velocities were unfolded then the average standard deviation of the retrieved vertical velocity was reduced.
The advance in this paper is to extend the analysis firstly to 16 orbits (rather than 2 orbits in the HH paper) so that the degree of folding as a function of latitude can be found, and secondly to extend the analysis to 7500Hz so the folding velocity is 6m/s rather than 4.8m/s for 6100Hz. The advantage of the 7500Hz frequency is a reduction in the phase noise due to the reshuffling of the target caused by a Doppler width of the target assumed to be 4m/s due to satellite motion and a finite beamwidth. This leads to a lowering of the correlation between two sequential pulses (Equn 2 in this paper, equn 3 in HH).
The conclusion of this new paper (abstract lines 21-24) is:
“…the mean frequency of the precipitation echoes was highest in the tropics and decreased toward the poles. Considering a limitation of the unfolding correction for discrimination between large upward velocity and large precipitation falling velocity, the latitudinal variation of the Doppler error can be explained by the precipitation echo distribution.”
MAJOR COMMENTS
- TERMNAL VELOCITY AT 94GHz.
The conclusion of the new paper in the abstract lines 21-24 that heavier precipitation occurs in the tropics is to be expected, but it would be useful to know the values of reflectivity and the type of particles that are needed to produce the high terminal velocities above 6 m/s at 94GHz, 7500Hz and consequent folding. In most rainfall Mie scattering of the larger drops at 94GHz leads to terminal velocities much below 6 m/s.
- MULTIPLE SCATTERING.
The authors appear to have neglected the effect of multiple scattering which leads to very noisy phase returns due to the differing path lengths of the multiply scattered photons. This effect becomes important for rain rates above 5 mm/hr and will drastically degrade the quality of the Doppler, see Matrosov et al. 2008. https://doi.org/10.1175/2008JTECHA1095.1
- MODEL RESOLUTION.
The model has a resolution of 3.5km so the size of the features that can be represented is probably greater than 10km. This means that the forward modeled values of reflectivity for each km in the horizontal will not be independent but will be smoothed, and secondly the full range of updrafts and downdrafts will not be resolved. In the current analysis based on the NICAM model it is assumed that any updraft above 3m/s is deemed to be unlikely, but in reality updrafts much higher than this do occur on the km or sub km scale.
Less important issues with the presentation. The paper relies too much on quoting results from HH. It would read better is some explanations were provided.
- a) Rather than quoting equation (2) and referring to the HH paper for explanations, the introduction should have a short paragraph explaining that the Doppler is retrieved by estimating the phase change of the returned signal from a target from successive transmitted pulses.
- b) Include a couple of sentences explaining that as a spacecraft is a moving platform with a finite beam-width the targets have a high Doppler width (4m/s) and so the reshuffling of the targets in the time between two transmitted pulses leads to a rapid lowering of the correlation of the phases and higher Doppler errors. .
- c) The terminology “high mode PRF” (e.g. the caption to figure 3) can be confusing. Does “high” refer to the PRF or the maximum altitude? Better to say high prf /lower maximum altitude.
Citation: https://doi.org/10.5194/egusphere-2022-1255-RC1 -
AC1: 'Reply on RC1', Yuichiro Hagihara, 16 Mar 2023
Dear referee,
We really appreciate the reviewer’s efforts. Your comments definitely help us to improve our paper.
The following is a point-by-point response to the specific comments:
MAJOR COMMENTS
1. TERMNAL VELOCITY AT 94GHz.
The conclusion of the new paper in the abstract lines 21-24 that heavier precipitation occurs in the tropics is to be expected, but it would be useful to know the values of reflectivity and the type of particles that are needed to produce the high terminal velocities above 6 m/s at 94GHz, 7500Hz and consequent folding. In most rainfall Mie scattering of the larger drops at 94GHz leads to terminal velocities much below 6 m/s.RESPONSE:
In H22, Fig. 9(a) shows a 2D-histogram of Vjsim without the random error as a function of the Ze for the precipitation case. Large fall velocities exceeding 6 m/s are not seen as you pointed out. As shown in Fig. 9(b-d) in H22, considering the random error due to the Doppler broadening, the velocity folding occurs.
2. MULTIPLE SCATTERING.
The authors appear to have neglected the effect of multiple scattering which leads to very noisy phase returns due to the differing path lengths of the multiply scattered photons. This effect becomes important for rain rates above 5 mm/hr and will drastically degrade the quality of the Doppler, see Matrosov et al. 2008. https://doi.org/10.1175/2008JTECHA1095.1RESPONSE:
We thank for your important remarks. However, the Doppler effect of multiple scattering was not considered in this study because of its complication, and the issue will be the subject of future research.
3. MODEL RESOLUTION.
The model has a resolution of 3.5km so the size of the features that can be represented is probably greater than 10km. This means that the forward modeled values of reflectivity for each km in the horizontal will not be independent but will be smoothed, and secondly the full range of updrafts and downdrafts will not be resolved. In the current analysis based on the NICAM model it is assumed that any updraft above 3m/s is deemed to be unlikely, but in reality updrafts much higher than this do occur on the km or sub km scale.RESPONSE:
There may be fast updrafts on the km or sub km scale. However, such events is rare globally and would be negligible in statistics such as latitudinal zonal means. We are focusing on global statistical results and therefore we use the NICAM. When higher horizontal resolution NICAM data becomes available, we would like to study similar evaluation with it.
We will add this explanation in the revised manuscript.
1. a) Rather than quoting equation (2) and referring to the HH paper for explanations, the introduction should have a short paragraph explaining that the Doppler is retrieved by estimating the phase change of the returned signal from a target from successive transmitted pulses.RESPONSE:
We agreed with your suggestion. We will add the following sentence in the revised manuscript.
"The EC-CPR measures Doppler velocities using the pulse-pair method. It measures phase shift of echoes from two successive transmitted pulses."
2. b) Include a couple of sentences explaining that as a spacecraft is a moving platform with a finite beam-width the targets have a high Doppler width (4m/s) and so the reshuffling of the targets in the time between two transmitted pulses leads to a rapid lowering of the correlation of the phases and higher Doppler errors.RESPONSE:
We agreed with your comment. We will add the following sentence in the revised manuscript.
"Since the EC-CPR is a finite beamwidth on fast moving spaceborne platform, targets have a broad Doppler width, which causes a worsening of the correlation of the phase. Then, large Doppler errors are introduced."
3. c) The terminology “high mode PRF” (e.g. the caption to figure 3) can be confusing. Does “high” refer to the PRF or the maximum altitude? Better to say high prf /lower maximum altitude.RESPONSE:
We agreed with your suggestion. It is indeed confusing, so we will change them in the revised manuscript as follows:
"high mode PRF" to "PRF of the high mode (lower PRF)" and
"low mode PRF" to "PRF of the low mode (higher PRF)".
Thank you very much.Citation: https://doi.org/10.5194/egusphere-2022-1255-AC1
-
RC2: 'Comment on egusphere-2022-1255', Anonymous Referee #2, 06 Feb 2023
The paper assesses the vertical velocity error in the EarthCare CPR observations at different integration path scales for two different PRF patterns. It builds upon the previous study of the same authors, but for a smaller sample size. The analysis is based on the radar simulations applied to the NICAM precipitation resolving model. The model output is first interpolated at the CPR observation grid, and then the radar moments of the interest are obtained.
The paper shows reduced errors for higher PRF sampling thanks to a reduction in the phase noise and increased folding velocity threshold. Moreover, a simple unfolding technique reduces additionally the reported errors that affect the most the Doppler velocity measurements in the tropics.
Major comments:
1. The authors assume a constant Doppler spectrum width of 4 m/s. It would be beneficial for readers to explain why this value is a reasonable estimate given the spacecraft speed and the beam-width of the radar. Moreover, it should be also mentioned that the Doppler spectrum width depends on the observed hydrometeors distribution, i.e., for heavy rain the with will be additionally increased.
2. Although, a long integration path (10 km) seams to be a tempting approach to reduce uncertainty in the Doppler measurements, the authors do not assess the effect of a long scale signal decorrelation and non-uniform beam filling effects in such a large sampling volumes. Moreover, the horizontal resolution of 10 km prevents studies on the small scale features like localized convection, the characterization of which is one of the objectives of space-borne Doppler radar missions.
3. Does the radar simulator accounts for multiple scattering? This issue has been demonstrated to have a destructive effect on the quality of the Doppler measurements.
Minor comments:
1. The terminology "high-mode PRF" can be misleading. It would be better to use "low PRF mode" or "high tropopause mode".
2. It feels like some of the discussions can be reduced in length, e.g., when the high PRF mode is compared with the low PRF mode.
Some minor comments are also included in the attached file.
-
AC2: 'Reply on RC2', Yuichiro Hagihara, 16 Mar 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1255/egusphere-2022-1255-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Yuichiro Hagihara, 16 Mar 2023
-
EC1: 'Comment on egusphere-2022-1255', Robin Hogan, 20 Mar 2023
Thank you for submitting your replies to the reviewers' comments. In preparing the revised version of your manuscript, please note the following:
1. Regarding Reviwer 1's comment 1: your reply provides an important piece of explanatory information - please ensure that the revised paper includes a sentence or two explaining this to the reader, including a reference to the relevant figure in H22.
2. Regarding multiple scattering raised by both reviewers, please include a short discussion of this in the revised paper (e.g. in the conclusions). Reviewer 1 suggested Matrosov et al. (2008) as a possible reference: this is certainly valid to show the effect of multiple scattering on Z but does not mention Doppler. For that you could cite Battaglia and Tanelli (2010: 10.1109/TGRS.2010.2052818).
3. Regarding Reviewer 2's comment about line 74, please ensure that the revised paper includes this explanation of the meaning of the threshold, and the justification for the use of a value of 20.
Citation: https://doi.org/10.5194/egusphere-2022-1255-EC1
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Cited
Yuichi Ohno
Hiroaki Horie
Woosub Roh
Masaki Satoh
Takuji Kubota
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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