The direct assimilation of ATOVS radiances in a 3D-var assimilation system

The introduction of variational assimilation systems at many numerical weather prediction centres around the world has allowed the direct assimilation radiances as opposed to assimilation of retrieved profiles. However even when radiances are assimilated there are many options as to what degree of radiance pre-processing takes place. Various issues relating to direct radiance assimilation will be discussed, including the accuracy of the radiative transfer model, quality control and preprocessing. The launch in 1998 of the first Advanced TIROS Operational Vertical Sounder instrument has provided more information on temperature and humidity in cloudy regions. It will be described how the new temperature information has been used operationally and how this led to a considerable improvement in numerical weather forecast accuracy. Using radiances from ATOVS on NOAA-15 and TOVS on NOAA-14 forecast accuracy for 500 hPa height is improved by well in excess of one day in the southern hemisphere and by 6-12 hours in the tropics and northern hemisphere compared to a system where the radiances are not assimilated.