Implementation of 1D+4D-Var assimilation of precipitation-affected microwave radiances at ECMWF. II: 4D-Var



This paper presents the operational implementation of a 1D+4D-Var assimilation system of rain-affected satellite observations at ECMWF performed on 28 June 2005. The first part describes the methodology and performance analysis of the 1D-Var retrieval scheme in clouds and precipitation that uses Special Sensor Microwave/Imager (SSM/I) microwave radiance observations for the estimation of total-column water vapour (TCWV). This part describes the technical implementation of the TCWV observations in 4D-Var as well as the impact analysis.

The effect of the TCWV observations implied by precipitation on the 4D-Var analyses is significant and the total information content is comparable to that of the SSM/I, High-resolution InfraRed Sounder and Advanced Microwave Sounding Unit (AMSU-B) radiances. Regions with systematic drying in the analysis persist throughout the forecast while moistening is removed by precipitation after 1–2 days. The corresponding divergence increments reflect the feedback between moisture and dynamics. Forecast evaluation using model analyses exhibits mostly positive relative-humidity forecast scores, in particular at 700 hPa and in the Tropics. Some short-term negative forecast scores are observed for geopotential near 1000 hPa and in the southern hemisphere between days 2–4. Wind scores vary greatly between regions and different forecast lengths. Tropical cyclone tracking forecasts are only slightly affected by a reduced location error spread through the rain assimilation. Comparison to dropsonde observations of wind and temperature shows improvement as does TCWV analysis validation against independent observations from Jason radiometer data. The system has been implemented operationally in June 2005 and will be further developed towards a direct 4D-Var assimilation of radiances in clouds and precipitation. Copyright © 2006 Royal Meteorological Society