Radar, passive microwave, and lightning characteristics of precipitating systems in the tropics


The bulk radar reflectivity structures, 85- and 37-GHz brightness temperatures, and lightning charateristics of precipitating systems in tropical Africa, South America, the east Pacific, and west Pacific are documented using data from the Tropical Rainfall Measuring Mission (TRMM) satellite during August, September, and October of 1998. The particular focus is on precipitation features [defined as a contiguous area ≥75 km2 with either a near-surface reflectivity ≥20 dBZ or an 85-GHz polarization-corrected temperature (PCT) ≤ 250 K] with appreciable rainfall, which account for the bulk of the total rainfall and lightning flash density in their respective regions. Systems over the tropical continents typically have greater magnitudes of reflectivity extending to higher altitudes than tropical oceanic systems. This is consistent with the observation of stronger ice scattering signatures (lower 85- and 37-GHz PCT) in the systems over land. However, when normalized by reflectivity heights, tropical continental features consistently have higher 85-GHz PCT than tropical oceanic features. It is inferred that greater supercooled water contents aloft in the tropical continental systems contribute to this brightness temperature difference. Lightning (as detected by the Lightning Imaging Sensor) is much more likely in tropical continental features than tropical oceanic features with similar brightness temperatures or similar reflectivity heights. Vertical profiles of radar reflectivity add additional information to the nonunique lightning-brightness temperature relationships showing that features with lightning tend to have greater magnitudes of reflectivity and smaller decreases of reflectivity with height above the freezing level than systems without detected lightning. Regional comparisons of the lightning, radar, and microwave signatures of precipitating features show that, over the oceans, the west Pacific has the highest frequency of intense precipitation features (by minimum PCT or maximum reflectivity height). Over land, the intense precipitation features occur more frequently in Africa. These observations are consistent with the relative differences in lightning flash density between the land and ocean regions. The quantitative database of land and ocean features presented here provides a substantial observational framework against which cloud and radiative transfer model results can be tested.

Monthly Weather Review