Global Assessments of Doppler Velocity Measurement Errors of EarthCARE Cloud Profiling Radar Using Global Storm-Resolving Simulation

The Earth Clouds, Aerosol and Radiation Explorer (EarthCARE) is a joint satellite mission between Japanese Aerospace Exploration Agency (JAXA) and European Space Agency (ESA). One of the features of this mission is the Cloud Profiling Radar (EC-CPR) with Doppler capability.

Vertical Doppler velocity measurement from a moving platform suffers from Doppler broadening and folding (aliasing). We simulated pulse-pair covariances using the radar reflectivity factor and Doppler velocity obtained from a satellite data simulator and a global storm-resolving simulation (NICAM). From those covariances, we calculated the Doppler velocity including Doppler broadening and folding errors. We globally evaluated accuracy of Doppler velocities with latitudinal change of cloud and precipitation scene and with latitudinal change of pulse repetition frequency (PRF) change. Using the EarthCARE CPR Level 2 algorithm, we derive horizontally integrated and unfolded Doppler velocity. Then, we expected Doppler errors will be reduced. Those data are compared with error-free original Doppler velocity and the standard deviation of the Doppler error are assessed for this study.

We separated the data into five latitudinal zones to examine latitudinal changes. In the case of low PRF setting, the error of echoes of 5 dBZe with 10 km integration without unfolding correction for the tropics reached a maximum of 2.2 m/s and then decreased toward the poles (0.43 m/s). However, if we applied unfolding correction, those value for the tropics became much smaller at 0.63 m/s. In the case of high PRF setting, the error without unfolding correction for the tropics reached a maximum of 0.78 m/s and then decreased toward the poles (0.19 m/s). Then, the unfolding correction were applied, those value for the tropics became 0.29 m/s, which was less than half the value without the correction.

Latitudinal change of the Doppler errors can be explained mainly by appearance frequency of precipitation echoes of each latitudinal zone. Basically, we apply Doppler unfolding correction in the EarthCARE CPR Level 2 algorithm. However, there is also a limitation of the unfolding correction for discrimination between large upward velocity and large precipitation falling velocity, especially in the tropic. Then, the Doppler errors are larger in the tropic and less in the poles.