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Abstract
Passive microwave observations with two or more frequency channels in the vicinity of the 22 GHz water vapour line have been used for decades to retrieve liquid water path (LWP) and integrated water vapour (IWV). In addition, such observations are used in sensor synergy approaches to constrain radar measurements and to complete the picture of the atmosphere as obtained by a multi-sensor approach.
LWP retrievals usually make some a-priori assumptions about the underlying cloud micro-physical properties when retrieving LWP from microwave brightness temperatures. The assumption of small cloud droplets (Rayleigh scattering behaviour) is most common, but leads to substantial errors when larger drops (precipitation) are present.
In the past, the authors proposed a method to detect the presence of larger (Mie scattering) drops in clouds by using dual polarized observations, exploiting the non-spherical shape of falling rain drops which leads to a substantial degree of linear polarization in the emitted radiation. This method increases LWP retrieval accuracy in the presence of cloud/rain mixtures and thus extends the range of weather conditions in which LWP estimation is still useful towards fully developed rainfall. We obtain a partitioning of LWP to a cloud and rain fraction. Furthermore, there will be limited insight into the cloud micro-physics and drop size distribution of the observed cloud.
We will introduce the proposed polarized LWP estimation method and present the steerable multi-channel dual polarized radiometers which recently have been built at RPG. First simple statistical retrieval techniques are now established at RPG, and we will show results from measurements during raining and non-raining conditions.