Statistical Retrieval of Precipitation Cell- Top Altitude Using Passive 118-GHz Observations

The embedding of cell-top altitude information in 118-GHz observations occurs via two mechanisrrm. First, a statistical dependence exists between the cell-top altitude and the brightness temperature of the cell top. In the case of cells with tops below the freezing level, higher cell-top altitudes are associated with increased precipitation, and hence increased absorption, which produces decreases in brightness for land backgrounds. In the case of cells extending above the freezing level increasing quantities of ice occur in the cell top. The presence of ice causes strong scattering of the cold cosmic baCkground radiation, producing large negative perturbations in brightness temperature. Second, the altitude distribution of atmospheric water in the troposphere and lower stratosphere can be probed by virtue of the successively higher peaking altitudes of the 118-GHz clear-air temperature weighting functions [51. The 118-GHz clear-air weighting functions peak at altitudes ranging from the surface, for transparent frequencies located -2.5 GHz from the 118.750-GHz line center, to -35 km (well above most clouds) for frequencies at the line center. Abetract. A non-linear statistical retrieval operator for precipitation cell-top altitude uain~ hi~h spatial-resolution passive 118-GHz O2 brightne88 spectra is demonstrated. The retrieval operator conlists of a Karhunen-Loeve (KL) transformation followed by a rank reduction, a linearization, and a linear minimum mean-square-error estimator. Information from the 118-GHz data on the ambient at. m~pheric temperature profile and the precipitation cell size is also incorporated into the linear stage of the retrieval operator. The RMS retrieval error is 1.5 km for cumulus-stage cells with tops ranging from 1.5 to 16 km. The sensitivity of nadiral 118-GHz spectra to the cell-top altitude is predominantly due to the scattering and absorption of radiation originating from low , warm atmospheric level. by colder liquid and frozen precipitation. This effect causes cold perturbations in the brightness spectrum, which typically become 8ironger with increasing cell-top altitude. The different peakingaltitudes of the clear-air 118-GHz weighting functions provide additional "altitude-slicing" sensitivity.