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

We report further developments of a proposed 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 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 studies by weather forecasting centers demonstrate that winds are the second most important class of observations in reducing forecast errors after IR and microwave humidity and temperature sounders. This suggests that wind observations from this satellite should improve forecast skill further leading to better predictions of timing and location of windstorms so that mitigation activities can be better focused.

The 94GHz (3mm) frequency is needed to achieve a 1km vertical resolution as the 500m pulse approaches the earth’s surface at angles around 41°. The 2.9 by 1.8m elliptical antenna rotates once every seven seconds sweeping out a circular ground track that advances 50km for each revolution. The radar transmitter will be based on the one that has operated on Cloudsat since its launch in 2006. The Doppler velocity accurate to better than 2m/s will be derived using the returns from cloud particles from a pair of pulses separated by 20μsec (3km slant path) polarized in the horizontal (H) and vertical (V) so that the folding velocity is 40m/s and high wind speeds can be measured. The H and V pulses must propagate, scatter and be received independently. 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 and obliterates the return from the following pulse. ESA funded aircraft observations of 94GHz surface returns at high incidence angles indicate that the blind zone over the ocean will have a depth of about 1km. Ground based observations show that cross talk when the difference in the return power of two targets at ranges of 1 and 4km is comparable with the depolarisation ratio of the lower target, leads to ghost echoes and an increased random error of the velocity estimate; however these studies also confirm that such ghost echoes will be very rare from the satellite because all targets will be at a similar range.