Regional, seasonal, and diurnal variations of convection in the Himalayan region

This study uses eight years of Tropical Rainfall Measurement Mission (TRMM) Precipitation Radar (PR) data to document the regional, seasonal, and diurnal distribution of precipitating systems in the Indian subcontinent, a region where the strength and structure of convection is strongly dominated by the Himalayan Mountain Range. Following Houze et al. (2007), we classify precipitating systems according to their convective/stratiform nature and dimensions as: deep convective systems (40 dBZ echo > 10 km in height), wide convective systems (40 dBZ echo area > 1,000 km²), and broad stratiform systems (area > 50,000 km²).

During the monsoon season (June to September), deep convective systems occur almost exclusively over land, especially near the western terrain indentation between the Himalayan, Karakoram, and Hindu Kush mountains. The intense, localized convection arises as moist monsoonal flow from the Arabian Sea is lifted over the western indentation foothills and is able to break through a capping inversion formed by dry air flowing off the Afghan mountains. This mechanism is also responsible for the formation of wide convective systems in the western terrain indentation; however, wide convective systems are more evenly distributed over the whole Indian subcontinent and surrounding oceans. Wide convective systems also occur frequent over the Bay of Bengal, apparently in connection with Bay of Bengal depressions. Broad stratiform systems are primarily found over the oceans. They occur preferentially on the eastern parts of the Bay of Bengal and Arabian Sea, upstream of the coast, suggesting that during these events the monsoon flow is blocked by the coastal mountains (Grossman and Durran 1984). The location of broad stratiform systems in the eastern parts of the Bay of Bengal and Arabian Sea coincides with the climatological maxima of monsoon precipitation (e.g., Xie et al. 2006). Broad stratiform systems also occur near the eastern indentation of the terrain, often in connection with Bay of Bengal depressions.

Each type of system has a characteristic diurnal cycle. Deep convective systems occur almost exclusively in the afternoon and evening hours (13-23 Mean Solar Time [MST]), as a result of solar heating. Over land, wide convective systems have a bimodal distribution, with afternoon/evening (16-20 MST) and late night/morning (02-06 MST) maxima. The late night/morning wide convective systems are primarily located along and slightly upstream of the Himalayan foothills, suggesting that they form as moist monsoonal flow converges with down-valley flow exiting the Himalayan valleys. Maritime wide convective systems and broad stratiform systems show a lower-amplitude diurnal cycle, with an evening (18-01 MST) minimum, as is frequently found in oceanic cloud cover and convection.

            During the pre-monsoon season, represented by the month of May, the maximum occurrence of deep and wide convective systems is near the Indian east coast. These systems occur along the Indian dry-line, which forms in this season as a shallow layer of moist maritime air is capped by dry continental flow from the Afghan mountains and the Indian Thar Desert (Weston 1972). Very few broad stratiform systems are observed during the pre-monsoon season.