Variational analysis of total water using AMSU

Abstract

Vertically integrated cloud liquid water has been successfully analysed from both the Special Sensor Microwave Imager (SSM/I) and the Advanced Microwave Sounding Unit (AMSU). However microwave radiometers provide very little information on the vertical cloud liquid water profile. When considering the assimilation of microwave sounding channels, such as those on the AMSU or the Special Sensor Microwave Imager Sounder (SSMIS) the sensitivity of the measurements to cloud liquid water is dependent on the altitude of the cloud. Cloud well below the peak of the weighting function will tend to increase the measured brightness temperatures whereas cloud at or above the weighting function peak will tend to lower the measured brightness temperatures. Therefore if temperature and humidity sounding data are to be assimilated where cloud is present it will be necessary to analyse simultaneously the temperature, humidity and cloud profiles. One approach is to use a one dimensional variational analysis (1D-var) with a single moisture control variable, total water, where total water is the sum of water vapour, cloud liquid water, cloud ice water, rain and snow. For simplicity in this study rain and snow are neglected and ice cloud is assumed to be transparent. The analysis of cloud from the total water approach is compared with a variational analysis with separate cloud and water vapour control variables. The suitability of a total water control variable for variational analysis of cloudy AMSU, SSM/I or SSMIS data is then discussed.