TRMM precipitation radar reflectivity profiles as compared with high-resolution airborne and ground-based radar measurements

摘要

Orbital Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) products are evaluated by simultaneous comparisons with high-resolution data from the high-altitude ER-2 Doppler radar (EDOP) and ground-based radars. The purpose is not to calibrate any radar or to validate surface rainfall estimates, but rather to evaluate the vertical reflectivity structure, which is important in TRMM rain-type classification and estimation of latent heating profiles. The radars used in this study have considerably different viewing geometries and resolutions, demanding nontrivial mapping procedures in common earth-relative coordinates. Mapped vertical cross sections and mean profiles of reflectivity from the PR, EDOP, and ground-based radars are compared for six cases. These cases cover a stratiform frontal rainband, convective cells of various sizes and stages, and a hurricane. For precipitating systems larger than the PR footprint size, PR reflectivity profiles compare very well with high-resolution measurements thresholded to the PR minimum reflectivity, and derived variables such as bright- band height and rain types are accurate, even at off-nadir PR scan angles. Convective rainfall is marked by high-horizontal reflectivity gradients; therefore its reflectivity distribution is spread out because of the PR antenna illumination pattern and by nonuniform beamfilling effects. In these cases, rain-type classification may err and be biased toward the stratiform type, and the average reflectivity tends to be underestimated. The limited sensitivity of the PR implies that large portions of the upper regions of precipitation systems remain undetected. This implication applies to all cases, but the discrepancy is larger for smaller cells for which limited sensitivity is compounded by incomplete beamfilling. These findings have important implications for gridded TRMM products such as monthly mean rainfall.