Assimilation and Modeling of the Atmospheric Hydrological Cycle in the ECMWF Forecasting System

Abstract

Several new types of satellite instrument will provide improved measurements of Earth's hydrological cycle and the humidity of the atmosphere. In an effort to make the best possible use of these data, the modeling and assimilation of humidity, clouds, and precipitation are currently the subjects of a comprehensive research program at the European Centre for Medium-Range Weather Forecasts (ECMWF). Impacts on weather prediction and climate reanalysis can be expected. The preparations for cloud and rain assimilation within ECMWF's four-dimensional variational data assimilation system include the development of linearized moist physics, the development of fast radiative transfer codes for cloudy and precipitating conditions, and a reformulation of the humidity analysis scheme. Results of model validations against in situ moisture data are presented, indicating generally good agreement—often to within the absolute calibration accuracy of the measurements. Evidence is also presented of shortcomings in ECMWF's humidity analysis, from the operational data assimilation and forecasting system in 2002, and from the recently completed ERA-40 reanalysis project. Examples are shown of biases in the data and in the model that lead to biased humidity analyses. Although these biases are relatively small, they contribute to an overprediction of tropical precipitation and to an overly intense Hadley circulation at the start of the forecast, with rapid adjustments taking place during the first 6–12 h. It is shown that with an improved humidity analysis this long-standing “spindown” problem can be reduced.