WIVERN: An EE9 project to observe Global Winds

University of Reading with STFC RAL and University of Leicester

Project Funded in the CEOI-ST EO9 Call for Mission and Technology Studies in preparation for future ESA Earth Explorer Missions

Fast Track Project

PROPOSAL: Building on recent ESA and CEOI studies, we propose a spaceborne Dopplerised 94GHz radar pointing off-nadir across track to provide global measurements of winds, rainfall and ice water content using the radar returns from cloud and precipitation particles. The cloud observations will have a similar swath to the clear-air ADM-Aeolus wind mission but with improved along track and vertical resolutions.

WHY? Windstorms in Europe caused £18.5B of damage in Europe in 1999, £1.5B in 2009 and £1.3B in 2010. High losses also result from flooding. The loss of life in tropical cyclones is decreasing due to improved forecasts and better warnings: compare the 138,000 deaths in Myanmar due to cyclone Nargis in 2008 with the 43 deaths from the more powerful Indian super-cyclone, Phailin, in October 2013. Better observations would lead to more accurate forecasts with improved timing and location so that mitigation activities can be better focussed. The in-cloud winds from this project would complement the clear air winds from the ESA explorer ADM/AEOLUS when it is launched in 2017.

HOW? The 94GHz (3mm) radar will use the same transmitter that has operated flawlessly on CloudSat since its 2006 launch, but with a 2.9 by 1.8m elliptical antenna will have a very narrow beam and so even with a slant path will have a vertical resolution and blind-zone of less than 1km. The use of longer chirped pulse to increase mean transmitted power; pulse compression will lead to much improved resolution along the beam than for the 500m long CloudSat/EarthCARE transmitted pulse. The Doppler shift and hence winds will be derived using the returns from twin pulses, alternately polarised in the horizontal and vertical. Rain rates will be estimated from the gradient of reflectivity (Z) and ice water content from Z.

WHAT NEEDS TO BE DONE? a) Use Chilbolton data to evaluate the rainrate algorithm and CloudSat data to quantify the extent of H-V cross talk b) Further development of an end-to-end simulation tool predicting the radar signals and accuracy of wind retrievals from cloud scenes in weather forecast models and reflectivity gradients from CloudSat; the use of this tool to predict how the retrieved winds and rainfall estimates are affected by the choice of chirp configuration and how the different time separations of the H and V influence the blind zones and vertical resolution; consideration of the implications of these choices on the mission cost. c) Consult NWP users on the effect of these trade-offs on their requirements. d) Ground based observations to evaluate the chirp range sidelobes and how they affect the accuracy of the retrieved winds close to the earth’s surface. e) Preparation of documents for the EE9 proposal.