An airborne radar and lidar combination to document microphysical and radiative parameters of clouds

Clouds are an important component of the Earth’climate system. However their parametric representation in large scale circulation model is recognized as insufficient.

Earth CARE (Earth Cloud Aerosol Radiation Explorer) is an ESA mission aiming to adress this question. It plans to combine on the same spaceborne platform a cloud radar and a lidar to retrieve the microphysical and radiation properties of clouds.

RALI (RAdar-LIdar) developed at IPSL (France), which combines the 95 GHz cloud radar of the CETP and the 0.5 µm wavelength backscattering lidar of the Service d'Aéronomie, is an interesting airborne demonstrator for this mission.

The first test of RALI was succesfully accomplished during the last CARL 2000 field project (in november 2000, in Brest, France), where both instruments were mounted on board the french ARAT aircraft. The Meteo-France MERLIN aircraft, instrumented with mycrophysical probes of the GKSS (Germany), was flying below the ARAT at the same time.

In order to derive the radiative and microphysical properties of clouds, a synergetic algorithm has been developed. It combines the backscatter coefficient, b, from the lidar and the apparent reflectivity, Z_e, from the radar to infer properties of the particle size distribution. The principle of this algorithm is to apply in parallel the Hitschfeld-Bordan algorithm to the radar and the Klett algorithm to the lidar. Taken separately, these two algorithms are unstable, but by considering a mutual constraint, it is shown that a stable solution can be established.

This solution formulates the retrieval of attenuation parameters of lidar and radar, which allow us, by combining retrieved reflectivity of the radar and backscattering coefficient of the lidar, to access microphysical and radiative parameters of clouds.

The results of the synergetic algorithm applied on real data are validated by microphysical in-situ measurements.