Measurement and Analysis of Orographic Precipitation in the Himalayas - First Results from the TRMM Hydrometeorological Network in Central Nepal

In the spring of 1999, a network of sixteen meteorological stations was installed at elevations ranging from 500 to 4,500m from the leading edge of the Himalayas to the accessible headwaters of the Marsyandi river basin in Central Nepal. Two types of stations were installed: 1) high-stand stations, which were placed at elevations above 1,000m; and 2) low-stand stations at lower elevations. The high-stand stations consist of 10-m towers equipped with temperature and relative humidity probes, wind speed and wind direction sensors, a sonic ranger to make snow-depth measurements above the expected thick snow cover, and a solar panel. Tipping bucket raingauges are on separate stands away from the towers. The snow instrumentation is augmented with passive gamma-ray loggers at high elevations. The low-stand stations consist of 1-m towers on wich temperature and relative humidity probes, and a tipping bucket raingauge were installed. Both station types have autonomous data-logging systems, and high altitude stations are backed up by solar panels. Streamflow is monitored at the outlet of several catchments within the Marsyandi. In this paper, we present results from the analysis of the data collected during the 1999 monsoon season. This includes the characterization of the spatial and temporal effects associated with the orographic enhancement of rainfall, and storm statistics such as inter-storm arrival times, rain and snowfall intensity, and duration. The effectiveness of the monitoring network is assessed by using streamflow data as an areal integrated measure of distributed precipitation. These data are subsequently compared with TRMM precipitation products. Finally, we report on the preliminary conclusions from ongoing research to identify spatial relationships between ground observations and TRMM precipitation estimates as a function of regional-scale atmospheric conditions, storm path and terrain features.