On validating EarthCARE CPR precipitation products with different instruments at ground

Satellite-based measurements are necessary to provide reliable measurements of clouds and  precipitation on global scale. Starting from the NASA/JAXA TRMM and NASA CloudSat missions, and consolidated by the NASA/JAXA GPM (Global Precipitation Measurement) mission, radars on satellite are playing a growing important role allowing to cover remote and oceanic areas and to reveal the vertical structure of clouds and precipitation systems. The ESA/JAXA EarthCARE (Clouds, Aerosol and Radiation Explorer) satellite, in orbit since 28 May 2024, has on board a 94 GHz Cloud Profiling Radar (CPR) provided by JAXA and NICT, with Doppler capability that provides information on vertical cloud profiles and precipitation properties through complex algorithms that require physical assumptions. It is there important to validate both satellite quantitative products with independent measurements and the physical assumptions underlying the retrieval algorithms.  Missions addressing clouds and precipitation have relied on field campaigns, using suborbital flights seeking for coincidence in locations of aircraft and satellite measurements, networks of instruments for long-term statistical validation, or supersites with multiple instruments capable of collecting high quality measurements correlative to satellite measurement.

Unlike other atmospheric parameters, clouds and even more, precipitation are significantly affected by spatial variability, even within a few kilometers and have an intrinsic intermittent nature. This fact poses specific challenges in obtaining an adequate quantity of significant correlative measurements during satellite overpasses from fixed installation. Satellite radar observations are validated with ground-based measurement devices, including raingauges and disdrometers (although the satellite measurement unaffected by ground clutter are several hundred meters above) or radars. Ground-based profiling radars operating at vertical incidence have adequate vertical resolution for matching satellite radar measurements but, depending on the wavelengths adopted on satellite and at ground, could differ in sensitivity and wavelengths. The spatial coincidence of individual satellite and ground-based profiles is unlikely. The GPM-GV program includes scanning weather radars, including operational ones, to match observations, considering the different sampling volume. They have a wider vertical resolution compared to radar profilers for most distances and lack sensitivity for clouds parameters. This study  part of the project “Contribution to EarthCARE products VALidation during the commissioning phase from atmospheric observatories in Central MEDiterranean in Italy (EC-VALMED.it)“ funded by ASI consider available data collected from satellite along with datasets available in the two validation sites of Rome and Lampedusa. The evaluation of the influence of spatial variability of observed precipitation phenomena at small scale is the aim of this study, crucial to understand the representativeness of the two validation sites and to define the validation strategy to be followed to validate geophysical products of EarthCARE CPR. An experimental approach based on operational weather radar and satellite radar profiles, aims at pointing out the effect of non-coincident measurements, along effects of difference of wavelength between satellite and ground sensors, and the effect of blind zone close to the surface. To this purpose, L2 CPR data will be considered, together with measurements collected from the instruments at ground available in the validation sites (disdrometer, radar profilers) and quasi-coincident data from operational scanning radars.