Structure of Florida thunderstorms using high-altitude aircraft radiometer and radar observations

Abstract

This paper presents an analysis of a unique radar and radiometer dataset from the National Aeronautics and Space Administration (NASA) ER-2 high-altitude aircraft overflying Florida thunderstorms on 5 October 1993 during the Convection and Moisture Experiment (CAMEX). The observations represent the first ER-2 Doppler radar (EDOP) measurements and perhaps the most comprehensive multispectral precipitation measurements collected from a single aircraft. The objectives of this paper are to 1) examine the relation of the vertical radar reflectivity structure to the radiometric responses over a wide range of remote sensing frequencies, 2) examine the limitations of rain estimation schemes over land and ocean backgrounds based on the observed vertical reflectivity structures and brightness temperatures, and 3) assess the usefulness of scattering-based microwave frequencies (86 GHz and above) to provide information on vertical structure in the ice region. Analysis focused on two types of convection: a small group of thunderstorms over the Florida Straits and sea-breeze-initiated convection along the Florida Atlantic coast. Various radiometric datasets are synthesized including visible, infrared (IR), and microwave (10-220 GHz). The rain cores observed over an ocean background by EDOP, compared quite well with elevated brightness temperatures from the Advanced Microwave Precipitation Radiometer (AMPR) 10.7-GHz channel. However, at higher microwave frequencies, which are ice-scattering based, storm evolution and vertical wind shear were found to be important in interpretation of the radiometric observations. As found in previous studies, the ice-scattering region was displaced significantly downshear of the convective and surface rainfall regions due to upper-level wind advection. The ice region above the rain layer was more opaque in the IR, although the 150-and 220-GHz brightness temperatures Tb approached the IR measurements and both corresponded well with the radar-detected ice regions. It was found that ice layer reflectivities and thicknesses were approximately 15 dBZ and a few kilometers, respectively, for detectable ice scattering to be present at these higher microwave frequencies. The EDOP-derived rainfall rates and the simultaneous microwave Tb's were compared with single-frequency forward radiative transfer calculations using a family of vertical cloud and precipitation water profiles derived from a three-dimensional cloud model. Over water backgrounds, the lower-frequency emission-based theoretical curves agreed in a rough sense with the observed radar rainfall rate-Tb data points, in view of the uncertainties in the measurements and the scatter of the cloud model profiles. The characteristics of the ice regions of the thunderstorms were examined using brightness temperature differences ΔTb such as Tb (37 GHz) - Tb (220 GHz). The ΔTb's (150-220, 89-220, and 37-86 GHz) suggested a possible classification of the clouds and precipitation according to convective cores, elevated ice layers, and rain without significant ice above the melting layer. Although some qualitative classification of the ice is possible, the quantitative connection with ice path was difficult to obtain from the present observations.