Extending the use of ATOVS radiances over the land

Retrievals from the Advanced TIROS Operational Vertical Sounder have been assimilated operationally at the UK Meteorological Office since March 1999 through a one dimensional variational approach. However the information was almost exclusively used over the sea, with only information above 400 hPa being assimilated over land. This is because of uncertainty in the surface emissivity, difficulty in correcting for radiance biases over land and d errors in detecting cloud contamination. By carefully chosing a region where these problems are less severe and the model background is least accurate (i.e. there is more effective information in the radiances) the potential impact of radiance information over the land on numerical weather forecast accuracy can be measured. An experiment will be described where the information was assimilated at all heights in a region 30E-130E 50N-70N (corresponding approximately to Siberia). Locally and at short range the impact was a 10% fall in root mean square error for mean sea level pressure and 500 hPa height. This benefit propogates eastwards across the Pacific Ocean to impact on America at around day three of the forecast and Europe and days four to five. By day 5 root mean square errors fall by around 1-2% over the whole northern hemisphere. It is now planned to experimentally begin assimilation over other land regions.