Proposal for a Spaceborne Dual-Beam Rain Radar with Doppler Capability

The potential characteristics and performances of a spaceborne dual-beam radar (or stereo radar) operating at 24 GHz, and devoted primarily to the retrieval of rain-rate structure by using the stereo-radar analysis, were presented in a previous study. This constitutes the starting point of the present paper, which analyzes the feasibility and scientific interest of adding a Doppler capability to the instrument. The constraints imposed by the Doppler mode are elaborated and discussed. Accordingly, the basic design and the expected performances of a low-altitude (≈ 500 km) spaceborne dual-beam Doppler radar are proposed. It is shown that a slight increase in system complexity is needed to perform significant additional Doppler measurements without jeopardizing the primary objective, that is, the quantitative measurement of rain at the global scale. The scientific interest for Doppler data from space is investigated. Two components of the air velocity can he determined from the dual-beam spaceborne Doppler radar: the along-track component of the horizontal air velocity and a component directed between 0° and 20° off the vertical. Both components could be estimated with an accuracy of approximately 1.2 m s−1 within each resolution cell in standard conditions. Two ways to exploit these data are proposed: monitoring the mesoscale wind field within stratiform precipitation areas, or estimating the horizontal transport of vertical momentum associated with deep convection at the climatological scale.