STATISTICAL TEMPERATURE PROFILE RETRIEVALS IN CLEAR-AIR USING PASSIVE 118-GHZ O-2 OBSERVATIONS

Linear statistical temperature profile retrievals from nadiral passive 118-GHZ O2 spectra are demonstrated using time- and space-coincident microwave observations and radiosonde profiles. The nadiral aircraft-based brightness observations were made using the Millimeter-wave Temperature Sounder (MTS) instrument aboard the NASA ER-2 high-altitude platform during the Genesis of Atlantic Lows Experiment (GALE) and the Cooperative Huntsville Meteorological Experiment (COHMEX). Separate retrievals are demonstrated for winter and summer midlatitude conditions in clear-air over land; observations during both day and night are included. The retrieval operator is a linear-statistical minimum mean-squared-error estimator. A purely statistical retrieval operator circumvents the effects of radiative transfer model uncertainties and biases in either the radiosonde or MTS data. The retrieved profile root-mean-square errors are approximately 0.7 - 1.2 K for either the winter or summer tropospheres. These are substantially smaller than the a priori temperature profile variations, demonstrating that 118-GHz aircraft or satellite observations can provide useful information on atmospheric vertical thermal structure. However, classification of retrieval operators by prevailing meteorological conditions appears to be important for operational purposes. Correlations between MTS antenna temperatures and radiosonde kinetic temperatures are nearly consistent with calculations based on a radiative transfer model, indicating both the validity of the model and the sensitivity of the MTS channels to atmospheric temperature. The predicted and observed correlations exhibit the same general trends, although the predicted values are usually slightly greater.