WIVERN: An EE9 project to observe Global Winds

Lead Organisation: University of Reading
Project Lead: Anthony Illingworth

Partners: University of Leicester

The WIVERN mission employs a space-borne dual-polarisation Doppler conically scanning 94 GHz radar measuring in-cloud line-of-sight winds over an 800 km wide ground-track using the returns from cloud and precipitation targets, satisfying the WMO requirements for Numerical Weather Prediction models of horizontal winds with 2 ms-1 accuracy, ~50 km horizontal and 1 km vertical resolution, and daily sampling poleward of 50°. Specifically WIVERN would provide:

a) Global measurements of in-cloud line-of-sight winds with two major applications.

1. Assimilation of the large-scale (> 20 km) in-cloud wind observations into Numerical Weather Prediction (NWP) models to further improve their skill. Currently, in-cloud wind measurements are sparse and only available from instrumented aircraft taking off and landing.

2. Fluctuations in the velocities on the km scale should provide information on the characteristics of convective motions. Representation of convection and the links between dynamics and precipitation are major challenges requiring global observations. NWP and climate models use similar parameterisation schemes so better NWP models would also improve climate models.

b) Unique observations of profiles of winds within tropical cyclones and hurricanes that are currently not available operationally but are possible only from occasional penetrations by instrumented aircrafts.

c) Global rainfall, snowfall and cloud ice water content from the radar reflectivity of the hydrometeors, as has been provided by CloudSat since 2006 but with a forty fold increase in sample coverage. Assimilation of CloudSat reflectivity data shows a small positive impact in reducing forecast error so WIVERN reflectivity with its 800 km wide ground track promises a more significant reduction.

WIVERN will use the same 94 GHz transmitter tube that CloudSat has operated beyond expectation since the 2006 launch, with 3.3 μs (500 m) pulses and a similar pulse repetition frequency. WIVERN’s major advance will be to have two tubes transmitting closely spaced pulse pairs polarised horizontally (H) and vertically (V) so that, for the first time, the high Doppler velocities of winds can be measured from space. From the demonstrated sensitivity of CloudSat, the observed climatology of radar reflectivity profiles, and well-established Doppler theory, it is expected that WIVERN will, each day, provide about one million wind observations averaged over 20 km with 2 ms-1 or better accuracy. These data should have the following advantages over currently available wind measurements:

a) They are available within the deep clouds of active and developing weather systems over oceans and remote land where current observations are sparse (especially true for tropical cyclones and hurricanes).

b) The CloudSat climatology predicts that WIVERN winds should be uniform with height, thus eliminating the pronounced dip in the frequency of AMV and aircraft winds between 800 and 400 hPa.

c) The height of WIVERN radar observations is expected to be known to 800 m so that any spatial correlation of height errors should be much less severe than AMVs and less thinning should be needed.

The in-cloud observations from WIVERN will complement the predominantly clear air winds from Aeolus, with much increased coverage.

Current SRL is with confidence 3 with a road map to level 5. Schemes for monitoring the azimuthal pointing knowledge using ground targets and for identifying and correcting any wind biases due to non-uniform beam filling need further rigorous testing and validation.

Industry estimates the TRL is 3 and has prepared a road map to TRL 5. Work is needed in the next two years on, for example, optimising the feed to the rotating antenna using a quasi-optic feed or a rotating joint, and development of a bespoke angular momentum compensation system. A recent ESA study has analysed the optimum antenna configurations but further refinement is needed.