Retrieval and characterization of cloud liquid water path using airborne passive microwave data during INDOEX

Abstract

During the 1999 intensive observation period of the Indian Ocean Experiment (INDOEX), the Airborne Imaging Microwave Radiometer (AIMR) was deployed on the National Center for Atmospheric Research (NCAR) C‐130 aircraft to measure up welling microwave radiation that can be used to retrieve cloud liquid water path (LWP). In this study, we present a LWP retrieval algorithm that is optimized for tropical atmospheric conditions, typical of conditions observed in INDOEX. Radiative transfer modeling and error analysis are conducted for the four AIMR channels, to guide selection of AIMR channels used for the LWP retrievals. Results show that the horizontal polarization channels outperform vertical polarization channels at both 37 and 90 GHz. Additionally, for LWP less than ∼300 g m−2, the best results are expected from the 90 GHz horizontal polarization channel, while the 37 GHz horizontal polarization channel performs better for higher LWPs. On the basis of these findings we formulated the LWP retrieval algorithm from the combination of the retrievals of 37 and 90 GHz horizontal polarization channels. Results of several indirect validations show that in nearly clear condition the LWP retrievals have essentially no bias and a random error of about 28 g m−2. The image of the retrieved LWP compares well with observations by a 0.64 μm visible channel, and the magnitude of the retrieved LWP for large convective cells is comparable to the estimation based on in situ measurements. It is also shown that the retrieved LWPs for convective cells are smaller than those estimated by assuming adiabatic process while the two have a similar trend in the LWP versus cloud top temperature diagram. By analyzing all available AIMR observations, it is found that the mean LWP for cloudy pixels measured during the INDOEX experiment is about 50 g m−2. A significant north‐south gradient of the mean LWP is found in INDOEX domain during this period, with the mean LWP in the region south of 5°S being twice as high as that in the region north of 50N. The LWP frequency distribution shows that clouds with larger LWPs occur more often in the southern region than in the northern region.