Spatiotemporal Variation of the Vertical Gradient of Rainfall Rate Observed by the TRMM Precipitation Radar

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Abstract
Seasonal and spatial variation of the vertical gradient of rainfall rate was investigated using global precipitation data observed by the Precipitation Radar (PR) on the Tropical Rainfall Measuring Mission (TRMM) satellite. The vertical gradient was rendered by features of downward decreasing (DD) or downward increasing (DI) rainfall rate in the lower part of the profile. The DD profiles dominated tropical interior landmasses such as Africa and the Brazilian Plateau in summer. The DI profiles were observed over land in winter and over ocean except for regions with very little rainfall. In addition, DI profiles appeared during the height of the wet season even over the tropical landmasses, such as the mature monsoon period over inland India and over the Amazon River basin. Individual precipitation systems were also investigated in terms of their areally averaged DD and DI characteristics mainly over India. Deep (shallow) profiles tended to be DD (DI) for all seasons except the premonsoon season. As the rain area increased, the vertical gradient of rainfall rate decreased (DD tendency). Embedded in the dominant DD signature for deep storms, deep but significant DI profiles were observed in every month. They characterized the precipitation in the premonsoon season. More than half of the mesoscale/ synoptic-scale systems (rain areas >104 km2) having the significant DD or DI regions had both of them as part of their slant cores. The vertical gradients for these systems had a similar trend for both their stratiform and convective parts. During the mature period of the southwest monsoon, the number of small systems that were DI and widespread systems with moderate vertical gradient increased.