Variational Assimilation of SSM/I Total Precipitable Water Retrievals in the CMC Analysis System

Abstract

The impact of assimilating Special Sensor Microwave/Imager (SSM/I) total precipitable water (TPW) on the Canadian Meteorological Centre (CMC) operational analyses and forecasts is evaluated. Assimilation cycles were performed for the months of July 1996 and December 1996. The agreement between the SSM/I TPW climatology and the analyzed TPW for the control case (for which only conventional observations were assimilated) was quite good (root-mean-square difference of 2.8 kg m?2), which showed that the humidity analysis for the control case was already good. As a result of assimilating SSM/I TPW and depending on the month studied, collocations with radiosondes over the oceans showed that both the analyses and the 6-h forecasts of humidity were improved in the Tropics and to a lesser degree in the Southern Hemisphere extratropics. The geopotential anomaly correlations that were computed only for the July 1996 case showed an increase of 1%?2% starting with the day 3 forecast in the Tropics and the day 4 forecast in the Southern Hemisphere extratropics. Comparison with precipitation climatological observations indicated that the CMC spectral finite element (SEF) global forecast model (which has a considerable precipitation spinup) has a hydrological cycle that is too active. The precipitation that occurs in the intertropical convergence zone (ITCZ) and South Pacific convergence zone covers an area that is too wide and the size of the areas where large-scale subsidence occurs are greatly underestimated. The net impact of assimilating SSM/I TPW was to slightly reduce the globally averaged precipitation rate and thus bring the 6-h forecasts closer to the observations. The precipitation rates were mainly decreased in the Southern Hemisphere (where the SEF model is shown to have a wet bias) and increased where the maximum ITCZ precipitation occurs. The precipitation in the large-scale subsidence zones was also reduced in agreement with the observations. The residence time of the impact of the SSM/I TPW was fairly short: about 24 h in the midlatitudes and 48 h in the Tropics. Although the humidity analysis was univariate, assimilating SSM/I TPW accelerated the Hadley cell and increased the meridional transport of humidity in the Tropics.