The assimilation of SSM/I brightness temperatures in clear skies at the Meteorological Service of Canada

David Anselmo and Godelieve Deblonde

Meteorological Service of Canada, Dorval, Quebec, Canada

Abstract

In recent years, the assimilation of satellite data has become a vital component of the global and regional assimilation systems at the Canadian Meteorological Centre (CMC). Moreover, the direct assimilation of satellite radiance measurements from AMSU-A, AMSU-B, and the GOES water vapor channel has resulted in notable improvements in the short and medium range CMC forecasts. This has been demonstrated in Observation System Experiments conducted by CMC.

In preparation for the assimilation of Special Sensor Microwave Imager (SSM/I) data in the operational 4D-Var global analysis system at CMC, two 3D-Var experiments are conducted. In the first experiment, clear-sky, open-ocean brightness temperatures from the 7 SSM/I microwave channels are added to the global analysis system. In the second experiment, stricter filtering of AMSU data is applied together with the addition of the SSM/I data. More specifically, AMSU-A CH3 (50.3 GHz) is removed due to its non-negligible sensitivity to clouds, and more aggressive filtering of AMSU-B CH2 (150.0 GHz), CH3 (183.3±1 H GHz), CH4 (183.3±3 H GHz), and CH5 (183.3±7 H GHz) is invoked using CH2 to identify cloudy pixels. In the current quality control procedures for AMSU-B, an effective precipitation screen is present, however, there is no method of detecting and removing cloudy observations. The two experiments are each executed over a 5 week period covering July of 2003 and January of 2004.

An evaluation of the results shows improvements in the analysed integrated water vapour, surface wind speed, and daily precipitation rate fields when compared against independent observations. Furthermore, gains are found in the forecasts when validated against radiosonde data and against their own analyses. Short-range QPF scores also show positive impact. Overall, the second experiment shows better results than the first. In particular, the additional filtering of AMSU-B CH2-5 is identified as an important modification to the current operational configuration.