Monitoring of observation and analysis quality by a data assimilation system.

Abstract

Operational data assimilation systems use short-range forecasts to provide the background, or first-guess, field for the analysis. We make a detailed study of the apparent or perceived error of these forecasts when they are verified against radiosondes. On the assumption that the observational error of the radiosondes is horizontally uncorrelated, the perceived forecast error can be partitioned into prediction error, which is horizontally correlated, and observation error, which is not. The calculations show that in areas where there is adequate radiosonde coverage, the 6-hour prediction error is comparable with the observation error. In the Northern Hemisphere midlatitudes, the forecasts account for most of the evolution of the atmospheric state from one analysis to the next. The situation is rather different in the Southern Hemisphere. -from Authors