PROPOSAL FOR A SPACEBORNE DUAL-BEAM RAIN RADAR WITH DOPPLER CAPABILITY

The potential characteristics and performances of a spaceborne dual-be am radar (or stereo radar) operating at 24 GHz, and devoted primarily to the retrieval of rain-rate structure by using the stereo-radar anal ysis, were presented in a previous study. This constitutes the startin g point of the present paper, which analyzes the feasibility and scien tific interest of adding a Doppler capability to the instrument. The c onstraints imposed by the Doppler mode are elaborated and discussed. A ccordingly, the basic design and the expected performances of a low-al titude (almost-equal-to 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 measurem ent of rain at the global scale. The scientific interest for Doppler d ata from space is investigated. Two components of the air velocity can be determined from the dual-beam spaceborne Doppler radar: the along- track component of the horizontal air velocity and a component directe d between 0-degrees and 20-degrees off the vertical. Both components c ould be estimated with an accuracy of approximately 1.2 m s-1 within e ach resolution cell in standard conditions. Two ways to exploit these data are proposed: monitoring the mesoscale wind field within stratifo rm precipitation areas, or estimating the horizontal transport of vert ical momentum associated with deep convection at the climatological sc ale.