| 25 Mar 2026
Observation modes of EarthCARE/CPR with different Doppler measurement accuracy: Evaluation of their applicability
Abstract. An accurate characterization of cloud vertical motion is essential for understanding cloud microphysical and dynamical processes. The Cloud Profiling Radar (CPR) onboard the Earth Cloud Aerosol and Radiation Explorer (EarthCARE) satellite, launched in May 2024, enables the first global measurements of Doppler velocity from space. The CPR operates in three observation modes—16-km, 18-km, and 20-km modes—each characterized by a distinct pulse repetition frequency (PRF), which determines the Doppler velocity data quality, the maximum observable altitude, and the likelihood of spurious high-altitude echoes known as mirror images. This study quantitatively evaluates the applicability of these three modes using actual CPR observations, focusing on these three aspects. The standard deviation (STD) of Doppler velocity, used as an indicator of measurement noise, indicated that the 16-km and 18-km modes provide more accurate Doppler measurements than the 20-km mode, with comparable STD values between the former two. Clouds above 16 km were primarily observed between 0° and 40° latitude, while clouds exceeding 18 km were rare, suggesting that the 18-km or 20-km modes are suitable for observation in these regions. The risk of overlap between genuine cloud echoes and mirror images at high altitudes was highest in the 16-km and 18-km modes but was largely confined to low-latitude regions (approximately 0°–40°). Accordingly, without considering mirror image-related risks, the 16-km mode is preferable at latitudes above 40°, where high clouds are infrequent and Doppler measurement accuracy is highest. In contrast, the 18-km mode provides an optimal balance between Doppler accuracy and vertical coverage at lower latitudes. It should be noted, however, that high-PRF modes inherently increase the likelihood of mirror image contamination. These results demonstrate, for the first time using actual EarthCARE observations, trade-offs among Doppler measurement accuracy, observation height, and spurious echo contamination across CPR operational modes. Future work should involve continuous assessments of the balance between Doppler accuracy and mirror image contamination to determine the optimal implementation of each mode as a function of latitude.
Competing interests: At least one of the (co-)authors serves as editor for the special issue to which this paper belongs.
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The paper presents a thorough characterization of the observations mode of the EarthCARE CPR.
In particular it investigates the impact of the different modes on two aspect: 1) Doppler velocity errors , 2) occurrence of second-trip echoes. The paper is very important because it provides a description of the pros and cons associated to the different modes that have been, are and will be used at different latitudes and in different mission periods.
I have some major comments that I would like the authors address to make the paper more clear and coincise.
1) The authors mention several times that they compute the standard deviation of Doppler velocity in several part of the paper. But this is left vague. The authors need to say across what ensemble such standard deviation is computed. This is very relevant in relation to the impact that what the authors refer to as "natural variability" std means. For instance if I understand correctly in Fig.4 the second row represent the std computed for each pixel in lat-height including all data with reflectivity between -20 dBZ and 0 dBZ (for 1 km Doppler velocities?). In that case the impact of the random component will be marginal (I expect) because in such a wide range of reflectivities there will be a large natural variability. In fact the authors do not present any quantitative intercomparison between the standar deviations of the different modes derived from Fig.4 (am I correct?). In fact I do not see the reason of plotting Fig.4 apart for documenting the behaviour of the mean Doppler velocity as a function of latitude and height (which by the way appears quite puzzling to me, why there are constant updrafts in the upper levels?). Also statement like in the conclusions (Line 395-397) are not very general since they are applicable to a given ensemble with the relative impact of the natural variability being very different from ensemble to ensemble.
2) Fig3 and Fig.5: Instead of plotting Fig.3 like it is it would be much better to merge Fig.3 in the top panel of Fig.5 (accounting for the integration length of course). For Fig.5 it would be good again to specify across which domani the std is taken. The intercomparison between the two curves will indeed explain what is the natural variability component (which shoul be kind of independent from the PRF but should depend on he integration length). It would be good to see some attempt to separate the impact of the random component (to verify whether it follows the expected behaviour).
3) I am not so sure Fig.6 adds much on what already shown in Fig.5.
4) terminology: instead of using "mirror" frequency and similar expression I would use "second-trip echoes" frequency because the origin can actually be different (not necessarily mirror images)
5) "Cloud overlap risk" in Sect 3.5 : I would highlgiht caveats to the proposed definition. Indeed using a product of probabilities is correct if the two events are independent (which I would not say is the case).
Minor comments:
1) Fig 9, last row. Cloud and mirror overlap = is this the same as cloud overlap risk? Then use the same name
2) Eq.3: why are you introducing C (C=1 in Table 1) I would delete C from the Eqaution and the TAble.
3) Table 1: sigma_sm=: in the third colum there is an expression that is correct for sigma_sm not the square of it. Also not clear to me what values of V_s is used
4) Line 188: this is due to pulse strecthing as documented in Xu et al., 2025 https://doi.org/10.5194/egusphere-2025-5421