A texture-polarization method for estimating convective-stratiform precipitation area coverage from passive microwave radiometer data

Observational and modeling studies have revealed the relationships between convective-stratiform rain proportion and the vertical distributions of ver tical motion, latent heating, and moistening in mesoscale convective system s. Therefore, remote sensing techniques that can be used to quantify the ar ea coverage of convective or stratiform rainfall could provide useful infor mation regarding the dynamic and thermodynamic processes in these systems. In the current study, two methods for deducing the area coverage of convect ive precipitation from satellite passive microwave radiometer measurements are combined to yield an improved method. If sufficient microwave scatterin g by ice-phase precipitation is detected, the method relies mainly on the d egree of polarization in oblique-view, 85.5-GHz radiances to estimate the f raction of the radiometer footprint covered by convection. In situations wh ere ice scattering is minimal, the method draws mostly on texture informati on in radiometer imagery at lower microwave frequencies to estimate the con vective area fraction. Based upon observations of 10 organized convective systems over ocean and n ine systems over land, instantaneous, 0.5 degrees -resolution estimates of convective area fraction from the Tropical Rainfall Measuring Mission (TRMM ) Microwave Imager (TMI) are compared with nearly coincident estimates from the TRMM precipitation radar (PR). TMI convective area fraction estimates are low-biased relative to PR estimates, with TMI-PR correlation coefficien ts of 0.78 and 0.84 over ocean and land surfaces, respectively. TMI monthly average convective area percentages in the Tropics and subtropics from Feb ruary 1998 are greatest along the intertropical convergence zone and in the continental regions of the Southern (summer) Hemisphere. Although convecti ve area percentages from the TMI are systematically lower than those derive d from the PR, the monthly patterns of convective coverage are similar. Sys tematic differences in TMI and PR convective area percentages do not show a ny clear correlation or anticorrelation with differences in retrieved rain depths, and so discrepancies between TRMM version-5 TMI- and PR-retrieved r ain depths are likely due to other sensor or algorithmic differences.