LWP is typically retrieved by either solar reflectance or thermal microwave emission measurements. But both methods have their limitations. On the one hand for the interpretation of reflectance measurements, a certain cloud profile has to be assumed and microwave satellites on the other hand are limited by their coarse footprint of about 40 km. Here we use an airborne microwave radiometer to retrieve the LWP of mostly non-precipitating clouds. An airborne microwave radiometer provides the advantage of having a kilometer footprint (depending of flight altitude). To enhance the assessment of thin clouds, a cloud mask derived from a lidar backscatter channel is used to correct the clear sky bias of the radiometer retrieval, which is due to variation in the background signal. When clouds start to precipitate microwave measurements at higher frequency are more and more influenced by scattering. Scattering makes the determination of LWP more difficult. Therefore, for heavy rain producing clouds (> 1 kg/m²), we exploit the attenuation signal of a 35 GHz radar. The accuracy gain of this active and passive microwave radiometry technique is assessed with radiative transfer simulations.
We present a full-scale airborne analysis from light to strongly precipitating tropical clouds. This presentation demonstrates the ability of an airborne microwave radiometer in synergy with a water vapor and backscatter lidar and a cloud radar to characterize trade-wind cumuli on a kilometer scale with high accuracy. This study combines case studies as well as statistical analysis and theoretical simulations.