Differential Absorption Radar for Water Vapor Sounding

We introduce the concept and development of a Differential Absorption Radar (DAR) for water vapor sounding of the atmosphere. We will present the status of year one of our development of DAR instrumentation. The remote sensing method uses dual frequency radar at a pair of frequencies on and off of the 183 GHz water vapor absorption line. We show that the difference in received reflectivity between these frequencies can be used to infer profiles of water vapor within clouds. Water vapor DAR would provide a useful compliment to existing water vapor sensors which are have difficulty sensing within clouds and near the Earth's surface, areas in which DAR is feasible. Observations from DAR would shed light on outstanding science problems such as identifying the controls on supersaturation in ice clouds and understanding the relationship between water vapor and clouds in Earth's boundary layer. Our theoretical studies suggest that the the water vapor concentration can be retrieved to within 1-3 gm^-3 and the column integrated water vapor can be retrieved to within 1 kgm^-2. These precisions rival those of existing water vapor remote sensing instruments.