Clear-sky column water vapor retrievals using the Airborne Imaging Microwave Radiometer (AIMR)

The brightness temperature measurements from the Airborne Imaging Microwave Radiometer (AIMR) microwave radiometer 37 GHz and 90 GHz channels are used to determine the column water vapor amount under clear-sky conditions during the Indian Ocean Experiment (INDOEX) campaign in 1999. The retrieval algorithm is based on the integrated relative humidity profiles measured by numerous dropsondes released from the National Center for Atmospheric Research (NCAR) C-130 aircraft that operated during the 2 month Intensive Field Phase (IFP) of INDOEX. The highest correlations of the linear relationship between the AIMR brightness temperature measurements and the dropsonde column water vapor values are for the nadir brightness temperatures of the 37 GHz and 90 GHz AIMR channels. The column water vapor, w, given with units of kg m^-2, may be expressed in terms of the 90 GHz nadir brightness temperature, T_B, via the expression, w = -209.4 + 1.025 T_B, whereas in terms of the 37 GHz AIMR nadir brightness temperature, w = -277.8 + 1.835 T_B. These expressions are valid when AIMR brightness temperature measurements about the NCAR C-130 are made at high altitudes (above 5 km) and when the atmospheric column is cloud-free below the aircraft. Other relationships between the polarized components of the AIMR brightness temperature measurements as well as various combinations (differences) of these polarized components with the column water vapor did not identify any parameterizations to the algorithm better than that seen with the mean of the raw brightness temperature components.