A Proposed Satellite to Provide Global Winds, Ice Water Content and Rainfall

We propose a conically scanning space-borne broad-swath Dopplerised 94GHz to provide global measurements of winds, rainfall and cloud ice water content using the radar returns from cloud and precipitation particles. The observations will have 50km horizontal and 1km vertical resolution with several visits every day at European latitudes. Windstorms are the single most damaging meteorological phenomenon in Europe with high losses also resulting from flooding. The loss of life in tropical cyclones and hurricanes is decreasing due to improved forecasts and warnings. Assimilation of the wind observations from this satellite into forecast models should improve their skill further leading to better predictions of timing and location so that mitigation activities can be better focussed.

The 94GHz (3mm) frequency is needed to achieve a 1km vertical resolution as the 500m pulse approaches the earth's surface at angles around 45deg. The 2.9 by 1.8m elliptical antenna rotates once every seven seconds sweeping out a broad swath on the ground. Two configurations are being considered with swaths of 800 and 1800km; the broader swath having more frequent visits but with lower sensitivity detecting fewer clouds at a reduced spatial resolution. Studies of this trade off with NWP forecast models are in progress. The satellite will have the same 94GHz transmitter that has operated flawlessly on Cloudsat since its launch in 2006. The Doppler velocity will be derived using the returns from cloud particles from a pair of two closely spaced pulses polarized in the horizontal (H) and vertical (V) so that the folding velocity is larger than the highest wind speeds. The H and V pulses must be propagate, scatter and be received independently; ground based observations are being analyzed to quantify how often depolarizing cloud targets and multiple scattering lead to cross talk between the H and V returns, and how such occasions can be identified and any errors in the inferred velocity corrected. A short separation of the pulses is also needed to minimize the blind zone close to the surface where observations will not be possible; when the first pulse impacts on the depolarizing surface it will obliterate the return from the following pulse. ESA funded aircraft observations over the next twelve months of ocean and land surface returns at high incidence angles will help to quantify the depth of this blind layer.