WIVERN: A Mission to Observe Global in-cloud Winds, Clouds and Precipitation as part of the ESA Earth Explorer Programme EE11

The WIVERN (WInd VElocity Radar Nephoscope) mission, now in Phase 0 studies of the ESA Earth Explorer program, will have a single instrument, a conically scanning 94GHz Doppler radar that will provide profiles of winds in cloudy areas sampling an 800km swath at an incidence angle of 41.6. The Doppler lidar on the ESA Aeolus mission launched in 2018 with a 1km swath provided the first direct observations of winds in clear air and thin clouds and demonstrated the impact of direct wind observations in reducing NWP forecast errors. Assimilation of the Aeolus winds into the ECMWF global forecast model led to a 3% reduction in forecast errors, the largest impact of any single instrument. Accordingly, ESA has decided to launch two Aeolus follow-on missions in the 2030 decade. The WIVERN Doppler radar would complement these Aeolus winds by delivering in-cloud winds with an average revisit time of just over a day for each 20 by 20km area of the Earth’s surface up to +/- 80 latitude. Studies using the Ensemble Data Assimilation (EDA) technique predict this should lead to a very significant further improvement in forecast model performance. The WIVERN mission will also strengthen the cloud and precipitation observing capability of the Global Observing system by supplying a continuation of the 94GHz reflectivity profiles of clouds and precipitation that started in 2006 with CloudSat and will continue to 2030 with EarthCARE.

WIVERN’s W-band (94GHz) radar will have a 3m antenna rotating once every 5 seconds, a footprint of about 1km at the surface and a vertical resolution of 600m. A key element is the use of closely spaced pulse pairs one of which is H polarised the other V polarised and a separation of 20us, so that the target does not have time to reshuffle, and the folding velocity is 40m/s and the high velocities associated with wind-storms can be retrieved. In this paper we will discuss the scientific objectives of the mission and will outline some of the technical challenges of the measuring technique. In particular we will discuss how to correct for wind biases introduced by the satellite motion and wind shear across the beam, how to account for cross-talk between the H and V returns due to depolarisation by meteorological targets, how to calibrate the instrument and how to identify mis-pointing of the antenna that could affect Doppler accuracy. We will also present examples of Level 1 products via end-to-end simulations applied to high resolution cloud resolving models and expected performances of the instrument in terms of cloud/precipitation and wind coverage.