The use of passive 118‐GHz O2 observations of rain cells for precipitation parameter estimation is demonstrated by a nonlinear statistical cell top altitude retrieval operator. The retrieval operator consists of a Karhunen‐Loève (KL) transformation followed by a rank reduction, a nonlinear operator, and a linear minimum mean square error estimator. Statistics were derived from an ensemble of 118‐GHz convective rain cell core observations over land, along with coincident estimates of the cell top altitude obtained by optical stereoscopy. The observations were made using the millimeter‐wave temperature sounder (MTS) scanning spectrometer aboard the NASA ER‐2 high‐altitude research aircraft during the Genesis of Atlantic Lows Experiment (GALE) and the Cooperative Huntsville Meteorological Experiment (COHMEX), 1986. The retrieval operator applied to MTS data yielded an rms discrepancy of 2.0 km for a combined cumulus, mature, and dissipating cell observation set and 1.6 km for the cumulus‐only set. Comparison of the 118‐GHz cell top altitude retrieval with coincident radar reflectivity data for a case of mature convection indicates that the 118‐GHz altitude retrieval tracks the 5‐ to 10‐dBZ reflectivity level over weak precipitation and the 20‐dBZ reflectivity level over strongly convective cores. A slight improvement of the 118‐GHz altitude retrieval discrepancy to 1.5 km rms is facilitated for the cumulus‐only set by including information on the atmospheric temperature profile and cell size from the 118‐GHz observations. Because of the ability of downward viewing microwave channels to probe beneath cloud tops and the sensitivity of these channels to precipitation, the 118‐GHz cell top altitude retrievals provide a more meaningful measure of the precipitation altitude than do optical or infrared retrievals. By using the KL decomposition followed by a nonlinear transformation, it is shown that 2 to 3 observable degrees of freedom exist in the 118‐GHz rain cell core perturbation spectra, implying that at least one other meteorological parameter, independent of cell top altitude, can be retrieved from these data.
