Active and passive microwave remote sensing of precipitating storms during CaPE. Part I: Advanced microwave precipitation radiometer and polarimetric radar measurements and models

The Advanced Microwave Precipitation Radiometer (AMPR), an across-track scanning, four-channel (10.7, 19.35, 37.1, 85.5 GHz) total-power radiometer system, was instrumented aboard a NASA ER-2 aircraft during the 1991 CaPE (Convection and Precipitation/Electrification) project in central Florida. At a 20 km flight altitude, the AMPR provides fine-scale microwave imagery of Earth surfaces and its atmosphere, and is well-suited for diverse hydrological applications. During overflights of precipitation, coincident ground-based radar measurements were taken with the NCAR CP-2 dual-frequency, dual-polarization radar system. After remapping the radar data into a format compatible with the AMPR scanning geometry, the radar-derived profiles of rain, melting, and frozen hydrometeors are compared against the AMPR equivalent blackbody brightness temperature (T_B) imagery. Microwave radiative transfer modeling procedures incorporating the radar-derived hydrometeor profiles were used to simulated the multifrequency AMPR imagery over both land and ocean background ER-2 flights. Within storm cores over land, columnar ice water paths up to 20 kgm^-2 gradually depressed the 85 GHz T_B as low as 100 K. The presence of tall vertical reflectivity columns encompassing > 20 kgm^-2 columnar ice water path often produced 37 GHz T_B<85 GHz T_B directly over the core. Over ocean, the 10 GHz channel provided the clearest correlation with the rainfall amounts, whereas the 19 GHz channel saturated near 260 K past 10-15 mm hr^-1 rain rate as determined by radar. Scattering by ice and melting ice at 37 GHz produced T_B ambiguities over both raining and clear-ocean regions. Sensitivity to the columnar mixed phase region via the intermediate frequencies (19 and 37 GHz) is demonstrated and explained with the radar-derived T_B modeling. By superimposing vertical profiles of cloud liquid water (which this radar cannot measure) upon the radarinferred hydrometeor structure, additional information on the location of the peak cloud water and its amount relative to the vertical ice structure can be noted, along with a possible inference of the dominant ice particle size within the upper storm core. These results suggest that as the resolution of passive radiometric measurements approaches dimensions where the antenna beams become increasingly filled by the cloud, precipitation retrieval via multifrequency T_B input is well-suited to a vertical profiling-type algorithm. This is further examined in Part II of this manuscript, where the radarderived vertical hydrometeor profiles are used to test the applicability of a multispectral cloud model-based approach to passive microwave precipitation retrieval from space.