VARIATIONAL ANALYSIS OF HUMIDITY INFORMATION FROM TOVS RADIANCES

The impact of assimilating TOVS radiance data in the European Centre f or Medium-Range Weather Forecasts humidity analysis is evaluated. It h as been found that the introduction of a one-dimensional variational a nalysis scheme (1DVAR) applied to the TOVS radiances significantly imp roves the representation of many aspects of the hydrological cycle. Th e theoretical information content of the TOVS radiance data is discuss ed and found to be consistent with significant changes observed in the mid upper-tropospheric moisture fields when the radiance data are ass imilated. In particular, a tendency of the model (without radiance ass imilation) to produce a tropical humidity structure that is far too dr y is removed, and excessively moist conditions in the southern sub-tro pics are improved. It is argued that the latter problem originates fro m the use of operational NESDIS retrieved products in the analysis. Th e humidity adjustments caused by the assimilation of TOVS radiances ar e accompanied by significant changes in the model dynamics, especially the description of the tropical Hadley circulation. One such case is described in detail, where the moistening of the tropics and drying of the sub-tropics has resulted in a stronger mean analysed meridional c irculation in the Atlantic. The analysis changes are also shown to imp rove the medium-range forecasting of humidity, together with some asso ciated benefit in the prediction of cloud and precipitation.