A Comparison of Observed and Modeled Brightness Temperatures for Hurricane Erin (2001)

The Advanced Microwave Precipitation Radiometer (AMPR) is a four-frequency passive microwave radiometer with high spatial resolution. On the mission to Hurricane Erin (2001) during the Fourth Convection and Moisture Experiment (CAMEX-4), the AMPR onboard the ER-2 high-altitude aircraft measured brightness temperatures (Tb) at 10.7, 19.35, 37.1 and 85.5 GHz channels. The observed Tb are compared against those calculated from a microwave Radiative Transfer Model (RTM) that uses hydrometeor fields obtained from high-resolution simulations of Erin conducted using the Fifth-Generation PSU/NCAR Mesoscale Model (MM5). The 10.7 and 19.35 GHz channels are most sensitive to the thermal emission from rain. At 10.7 GHz, the simulation overestimates the frequency of Tb greater than 235 K, but underestimates that between 150 and 235 K. This shows that the simulation overpredicts the frequency of heavy rain and underestimates that of light rain. The comparison of simulated radar reflectivity factor (Z) with that observed by the NASA ER-2 Doppler Radar (EDOP) further verifies this point. At 37.1 GHz, the simulation overestimates the frequency of Tb between 190 K and 255 K, and underestimates that between 255 K and 285 K. By artificially removing the graupel from the modeled hydrometeor fields, the frequency distribution of Tb at 37.1 GHz and at 85.5 GHz more closely resembles the AMPR observations.