Numerical Studies of Precipitation in Tropical Mountainous Terrain

The aim of this study is to examine convective activity and precipitation in tropical mountainous regions during the Northern American Monsoon season (June, July, August, and September). The focus regions are the Sierra Madre del Norte and del Sur, Mexico where there are significant orographic influences on precipitation, which include orographic blocking, gap wind, barrier jet and rain shadow effects, even where the average mountain height is below 1500 m. The working hypothesis is that orographically-induced local circulation as well as thermal convection in mountainous regions play an important role in the regional hydrological cycle and water budget over Mexico. Two numerical studies during the 2003 monsoon season were conducted using the PSU/NCAR MM5 model. The first simulation is focused on the Sonora river basin in northern Mexico. The second experiment is focused on the vicinity of Mexico City. The grid resolutions of both cases are 27 km, 9 km, and 3km. The initial and boundary conditions are derived from the NCEP ETA model data at 40 km resolution. The 3 km simulation results are used in turn to drive a cloud resolving model (CRM) at 1 km horizontal grid resolution coupled to a hydrological model. The goal is to characterize the diurnal cycle of precipitation in mountainous terrain and to develop an understanding of the physical processes that determine the monsoon hydrometeorology in the Sierra Madre. In addition, the model simulation data provide an opportunity to examine TRMM precipitation products and rain gauges data in mountainous regions. Results from this study will be compared with previous findings in the region of the Himalayas as well as Indian monsoon.