EarthCARE Doppler Velocity validation using ACTRIS ground-based cloud radar network

Abstract. The Earth Cloud, Aerosol, and Radiation Explorer (EarthCARE), launched in May 2024 by ESA and JAXA, carries the first spaceborne Doppler cloud profiling radar (CPR) capable of providing global observations of hydrometeor vertical motion. The retrieval of Doppler velocities from space is technically challenging due to satellite motion–induced spectral broadening, non-uniform beam filling, low signal-to-noise ratios, and antenna mispointing. In this study, we evaluate EarthCARE CPR Level-1b (L1b) and Level-2a (L2a) uncorrected and best estimate Doppler velocity products during the first 18 months of the mission (June 2024–November 2025) using ground-based cloud radar observations from the European Aerosol, Cloud, and Trace Gase Research Infrastructure (ACTRIS) network. Ground-based measurements are forward-simulated with an orbital radar simulator to ensure geometric and sampling consistency with the CPR, enabling a harmonised, profile-based comparison across multiple sites in Europe and the polar regions.

After applying a harmonised preprocessing and filtering procedure to ensure dataset consistency, systematic Doppler velocity biases are quantified for individual overpasses and statistically across sites. The uncorrected L1b and L2a Doppler products exhibit biases that correlate with the ESA antenna mispointing estimate, confirming the dominant contribution of this effect. In contrast, the L2a best-estimate Doppler velocity product effectively mitigates these biases. For sites with sufficient overpass sampling for robust statistics, including Ny-Ålesund, Lindenberg, Jülich, and Neumayer Station, the median residual bias of the best-estimate product ranges from −0.05 ms^-1 to 0.01 ms^-1, well within the mission requirement of 0.1 ms^-1. Sensitivity analyses indicate only a weak dependence of the retrieved bias on spatial collocation distance (≤ 100 km) and temporal averaging window (±30 min).

These results demonstrate the robustness of the EarthCARE CPR Doppler velocity retrieval following application of the L2a correction scheme and confirm the suitability of the ACTRIS cloud radar network for long-term validation. The validated performance of the L2a best-estimate product provides confidence in the use of EarthCARE data for global investigations of cloud dynamics and hydrometeor vertical velocity climatologies.