Impact of wind bogus and cloud- and rain-affected SSM/I data on tropical cyclone analyses and forecasts

In the context of its responsibilities as a Regional Specialised Meteorological Centre (RSMC) for the Southwest Indian Ocean, Meteo-France operates a tropical cyclone warning centre which sends advisories to all the concerned countries in the area. In assistance to the forecasters and as part of the operational suite for the short-range forecasts, a limited-area model (ALADIN Reunion) is run with four daily analyses. Assimilated observations include conventional observations - radiosondes, buoys, surface stations, aircraft reports, upper-wind reports - and satellite observations. The latter include QuikSCAT surface winds, atmospheric motion vectors from geostationary satellites, and radiances from polar-orbiting satellites. Assimilation of satellite radiance data is done in clear-sky conditions, and thus cannot be used in the vicinity of tropical cyclones. Two new sources of pseudo-observations are investigated that can bring new information content to those regions: Total Column Water Vapour (TCWV) pseudo-observations deduced from cloudy/rainy SSM/I data, as well as a 3D wind bogus. The TCWV algorithm is obtained from SSM/I brightness temperatures through a simple statistical regression from the ID-Var analyses of the European Centre for Medium-Range Weather Forecasts, which are derived from complex inversion methods using moist physics and radiative transfer models. The 31) wind bogus is derived from structural information contained in the tropical cyclone advisories issued by the RSMC and contains a low-level vortex composed of two concentric rings of eight winds each, at each of the surface, 850, 700 and 500hPa levels. Forecast scores and fit of the model to the observations are improved and indicate a positive impact of those new datasets. Structural validation is investigated through the comparison of model and TMI 'observed' rain rates: it is found that assimilating and cycling cloudy/rainy TCWV helps achieve more realistic cyclonic features. Copyright (C) 2008 Royal Meteorological Society