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 nadir-looking CPR operates in three observation modes – 16, 18, 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 second-trip echo from mirror images and multi-scattering tails. This study quantitatively evaluates the applicability of these three modes using actual CPR observations, focusing on these three aspects. The standard deviation (SD) of Doppler velocity, used as an indicator of measurement noise, indicated that the 16 and 18 km modes provide more accurate Doppler measurements than the 20 km mode, with comparable SD 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 or 20 km modes are suitable for observation in these regions. The risk of overlap between genuine cloud echoes and second-trip echoes at high altitudes was highest in the 16 and 18 km modes but was largely confined to low-latitude regions (approximately 0–40°). Accordingly, without considering second-trip echo-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 second-trip echo contamination. These results demonstrate, for the first time using actual EarthCARE observations, the 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 second-trip echo contamination to determine the optimal implementation of each mode as a function of latitude.