Soundings

Humidity retrieval from SSM/T-2 measurements

An algorithm has been developed to retrieve upper tropospheric water vapor (UTW) in the 500-200 hPa layer from SSM/T-2 measurements of radiation around 183 GHz. The algorithm is based on physical relaxation procedures utilizing statistical covariance information to provide an initial guess profile and to constrain the updating step in relaxation process. A method is used to remove SSM/T-2 brightness temperature bias compared with the collocated simulated brightness temperature. Correction functions are proposed for three channels of F14 SSM/T-2 around 183 GHz. The algorithm is validated against radiosonde observations and collocated SSM/T-2 brightness temperatures. Under the clear-sky and weak cloudy conditions, the retrieved UTWs showed an rms error of 0.68 kgm-2 and a near zero bias. The retrieval provides an independent source of satellite data of water vapor in the upper troposphere, augmenting the upper tropospheric humidity information from thermal infrared measurements around 6.3 um water vapor absorption band.

Click for larger version

Figure 1: Monthly mean upper tropospheric water vapor (kg m-2) for January 1998, retrieved from (a) SSM/T-2, and (b) TOVS measurements. Missing areas are represented by blanks.