Three-Dimensional Features of Summer Monsoon Precipitation Seen from TRMM/PR and Latent Heat Release over South Asia

This study using the Precipitation Radar (PR) data acquired by the Tropical Rainfall Measuring Mission (TRMM) satellite clarifies the three-dimensional rainfall pattern over south Asia. The southern foot of the Tibetan Plateau (Himalayas), Assam-Bengal and west of Western Ghats are well known for heavy-rainfall because of orographic rainfall in the summer monsoon season. The latent heat release over these regions is a major energy source for monsoon circulation. Therefore, quantitative features of the vertical profile of precipitation and latent heat release are very important for understanding monsoon and general circulation in the summer monsoon season. Here, for comparison, latent heat release is computed with the ECMWF operational with 0.5 grid resolution data set for the same period.

Although summer precipitation over the foot of the Himalayas and west of Western Ghats is almost the same, the rain top height of the former is much higher than that of the latter. Since the horizontal resolution of PR data is 4.3 km and vertical resolution is 250m, precipitation over the steep slope in the foothills of the Himalayas (500-5,500m height) related to mountain topography is clearly observed.

An active-break cycle is known to dominate the Indian monsoon region. Therefore, as a first step, composite precipitation patterns of active and break phases for the summer 1998 were produce for each height level, vertical cross sections are also made. In the break phase, strong convective precipitation with high rain top height is observed in the southern foothills of the Himalayas and Assam-Bengal region. Comparing latent heat profile composite patterns of the same period, large patterns are quite consistent with precipitation patterns. However, the strong water vapor convergence (much latent heat release) zone of the break phase is located south of the strong precipitation zone.

In the second step, diurnal variations of three-dimensional precipitation over these areas are shown. Intense diurnal variation was observed over the foot of the Himalayas. Strong convective precipitation is observed at the foot (under 1,000m height) of the mountains, especially from midnight (18UTC, 00LT) to morning (00UTC, 06LT), that exhibits the well-known band structure even above 6km. Latent heat release by ECMWF data also indicates stronger pattern for 18 and 00 UTC than those of 06 and 24 UTC of this region. The precipitation patterns and latent heat patterns were similar on macroscopic scales of 1 km in altitude and 500 km in distance.