Retrieval of Liquid Water Path Inside Nonprecipitating Clouds Using TMI Measurements

Quantitative estimates of liquid water path (LWP) in clouds using satellite measurements are critical to understanding of cloud properties and the assessment of global climate change. In this paper, the relationship between microwave brightness temperature (TB) and LWP in the nonprecipitating clouds is studied by using satellite microwave measurements from the TRMM Microwave Imager (TMI) onboard the Tropical Rainfall Measuring Mission (TRMM), together with a radiative transfer model for microwave radiance calculations. Radiative transfer modeling shows that the sensitivity is higher at both 37.0 and 85.5 GHz horizontal polarization channels for the LWP retrievals. Also, the differences between the retrieved values responding to TBs of various channels and the theoretical values are displayed by the model. Based upon above simulations, with taking into account the factor of resolution and retrieval bias for a single channel, a nonprecipitating cloud LWP in the summer subtropical marine environment retrieval algorithm is formulated by the combination of the two TMI horizontal polarization channels, 37.0 and 85.5 GHz. Moreover, by using TMI measurements (1B11), this algorithm is applied to retrieving respectively LWPs for clear sky,nonprecipitating clouds, and typhoon precipitating clouds. In the clear sky case, the LWP changes from -1 to 1 g m-2, and its mean value is about 10-5 g m-2. It indicates that, using this combination retrieval algorithm, there are no obvious systemic deviations when the LWP is low enough.The LWP values varying from 0 to 1000 g m-2 in nonprecipitating clouds are reasonable, and its distribution pattern is very similar to the detected results in the visible channel of Visible and Infrared Scanner (VIRS) on the TRMM. In typhoon precipitating clouds, there is much more proportion of high LWP in the mature phase than the early stage. When surface rainfall rate is lower than 5 mm h-1, the LWP increases with increasing rainfall rate.