A census of precipitation features in the tropics using TRMM: Radar, ice scattering, and lightning observations

An algorithm has been developed to identify precipitation features (greater than or equal to 75 km(2) in size) in two land and two ocean regions durin g August, September, and October 1998. It uses data from two instruments on the Tropical Rainfall Measuring Mission (TRMM) satellite: near-surface pre cipitation radar (PR) reflectivities, and TRMM Microwave Imager (TMI) 85.5- GHz polarization corrected temperatures (PCTs). These features were classif ied by size and intensity criteria to identify mesoscale convective systems (MCSs), precipitation with PCTs below 250 K, and other features without PC Ts below 250 K. By using this technique, several hypotheses about the conve ctive intensity and rainfall distributions of tropical precipitation system s can be evaluated. It was shown that features over land were much more int ense than similar oceanic features as measured by their minimum PCTs, maxim um heights of the 30-dBZ contour, and 6-km reflectivities. The diurnal cycl e of precipitation features showed a strong afternoon maximum over land and a rather flat distribution over the ocean, quite similar to those found by others using infrared satellite techniques. Precipitation features with MC Ss over the ocean contained significantly more rain outside the 250-K PCT i sotherm than land systems, and in general, a significant portion (10%-15%) of rainfall in the Tropics falls in systems containing no PCTs less than 25 0 K.Volumetric rainfall and lightning characteristics las observed by the L ightning Imaging Sensor aboard TRMM) from the systems were classified by fe ature intensity; similar rain amounts but highly differing lightning flash rates were found among the regions. Oceanic storms have a bimodal contribut ion of rainfall from two types of systems: Very weak systems with little ic e scattering and moderately strong systems that do not produce high lightni ng hash rates. Continental systems that produce the bulk of the rainfall la s sampled) are likely to have higher lightning flash rates, which are shown to be linked to stronger radar and ice-scattering intensities.