Assimilation of SSM/I radiances in the NCEP global data assimilation system

Kozo Okamoto

Japanese Meteorokogical Agency
Visiting NCEP/EMC


Although NCEP uses SSM/I retrieval precipitation rates and ocean surface wind speeds in the current operational global data assimilation system, no moisture information in rain-free areas from SSM/I measurements is used. Assimilating SSM/I clear radiances is an approach to make more use of SSM/I information and a first step to use measurements, including cloud/rain-affected radiances in the future, from various microwave imager instruments. SSM/I radiance assimilation is achieved by extending the current AMSU-A radiance assimilation and developing quality control (QC) and, if necessary, bias correction (BC) procedures specific to SSM/I radiances. Because the radiative transfer model used does not include cloud/rain effects, the QC and BC procedures exclude observations with significant cloud/rain signal. Proper identification of these observations is crucial for the performance of the assimilation.

It is found that, from cycle experiments of July through August 2004, the SSM/I radiances have the impacts of adding moisture in the northern hemisphere and tropics and reducing moisture in mid- to high- latitudes of the southern hemisphere. Although the resulting moisture may be excessive in the tropics, these impacts lead to the improvement of the dynamics in the analysis and forecast. In addition, the moistening is effective in reducing the spin-up of the precipitation forecast.