Friday, 7 June 2002
Dynamic properties of water and ice clouds from dual-beam airborne cloud radar data : the CARL 2000 and CARL 2001 validation campaigns
The knowledge of the cloud properties has been recently identified as a mandatory step to reach if the operational weather and climate change forecasts are to be improved. In the framework of the future space missions devoted to the monitoring of the microphysical, radiative, and dynamic properties of clouds at global scale using cloud radar and lidar combination (CLOUDSAT/CALIPSO (NASA/CNES) and EarthCARE (ESA)), there is a need for ground-based and airborne validation of the radar/lidar measurements and products from these space missions. The French RALI project addresses these issues through the development of an airborne combination of a dual-beam Doppler cloud radar (RASTA) and a dual wavelength backscatter lidar (LEANDRE 1). The synergy between the two instruments is such that in moderately thick clouds (optical depth less than around 5) the liquid/ice water content and effective radius of droplets/crystals can be retrieved from these two measurements. In this paper we focus on the retrieval of the dynamics from the dual-beam cloud radar only. We propose a method that combines the reflectivity and Doppler measurements from two non-colinear cloud radar beams to access the dynamic properties of clouds. More precisely, in this method a statistical relationship between radar reflectivity and terminal fall velocity of hydrometeors is first retrieved by assuming that for a long sampling time span (several passes through the same cloud, for instance) the mean vertical air motions vanish with respect to the mean terminal fall velocity. This underlying hypothesis is being presently evaluated using in-situ data and in-situ microphysical VT-Z relationships in liquid phase. Then, this statistical relationship is used to remove the terminal fall speed contribution from the Doppler velocities, and the along-track horizontal and vertical wind components are retrieved in a second step using a dual-Doppler wind synthesis. The accuracy of the method is presently evaluated using airborne Doppler radar observations of water and ice clouds obtained during the European CARL2000 and CARL2001 campaigns.
Supplementary URL: http://www.cetp.ipsl.fr/~protat/abstract_cphy2002.pdf