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Characteristics of precipitation systems in and around Bangladesh are analysed using 6 years of radar data from the Bangladesh Meteorological Department (BMD). Precipitation systems are classified as arc, line, or scattered types according to their shape; then as small, medium, or large according to their length; and as stationary, slow, or fast according to their propagation speed. Arc-, line-, and scattered-type systems are found 230 (29%), 117 (15%), and 442 (56%) times, respectively, from April to September during the analysis period (2000)(2001)(2002)(2003)(2004)(2005). These arc-, line-, and scattered-type systems have average speeds of 11.0, 7.1, and 5.8 m/s, horizontal lengths of 185, 184, and 268 km, and approximate lifetimes of 4.3, 4.0, and 4.8 h, respectively. Scattered-type systems dominate in the monsoon period (June, July, August, and September), while arc-type systems dominate in the pre-monsoon period (April and May). Line-type systems occur with nearly equal frequency in both periods. The monsoon systems are large and stationary or slow moving. In contrast, pre-monsoon systems are small and fast moving. Pre-monsoon systems propagate southeastward whereas monsoon systems propagate to the northeast, northwest, or southeast. A large number of systems occur in the southern, eastern, and northern parts of Bangladesh. Of the 442 scattered-type systems, 244 are scattered-type systems having wide areal coverage (SWAC), which extends out of radar range, and speeds too small to calculate. About 97% of SWACs develop during the monsoon period and contribute greatly to monsoon rainfall in this region.

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This paper describes a transition of rainfall characteristics related to the moistening of the land surface over the central Tibetan Plateau. This transition was observed three weeks after the onset of the summer rainy season of 1998. The objective is to clarify the potential of the plateau surface to modify the characteristics of monsoon rainfall. Summer rain events were first separated according to large-scale conditions into three types: one with a near-surface heat low and a Tibetan upper high, one with a near-surface low associated with a midlatitude trough, and one without a near-surface low. The first type was studied in further detail because of its intraseasonal variability in the rainfall amount (from 2.8 mm day−1 in June to 5.7 mm day−1 in August). The smaller amounts of the diurnal rain in June than July are related to the evaporation of precipitation within a drier and deeper subcloud layer. The moistening of this layer was related to the increase in the soil moisture and activation of vegetation. These results suggest a significant impact of the plateau surface upon the modification of the rainfall characteristics. This impact is the largest under the condition with a near-surface heat low forming due to strong solar heating.