Characterizing the Linkages Between landform and Precipitation Regime in the Sierra Madre Meridional and in the Andes

Mountains play an important role in the hydrologic cycle in many parts of the world. About 25% of the world's population lives in mountainous terrain, and 60% of people rely on freshwater from mountainous regions for drinking water and other purposes. This is especially the case in the western US, in Central America and along the Andes. Whereas quantitative estimation of precipitation in mountainous regions is of critical importance, sparse raingauge networks and the operational difficulties of ground-based radar in the vicinity of high terrain, leave us without substantive observations to work with. By contrast, satellites provide a unique opportunity to look at large regions simultaneously and at high resolution. Although terrain complexity can also cause substantial uncertainty in the interpretation of remotely-sensed data, there is great value in the small-scale structure captured by high spatial resolution sensors. A comprehensive study including surface measurements, observations from the NASA TRMM satellite, and coupled land-atmosphere modeling to characterize the diurnal cycle of precipitation over the Sierra Madre Meridional (east of Mexico City) and over the Andes is currently under way. The objective of this work is to investigate the role of landform as the organizing principle of convective activity in mountainous regions and to determine whether this spatial organization can be linked to the diurnal cycle of rainfall. For this purpose, TRMM data were analyzed over the Sierra Madre and Andes Mountains using an algorithm developed by Nesbitt et al. (2000) to determine the location of precipitation features (PF's) over a time period extending from 1998 to 2004. The algorithm uses two types of data provided by the TRMM satellite: the near-surface precipitation radar (PR) and the TRMM Microwave Imager (TMI) polarization-corrected temperatures (PCT's) at 85.5 GHz. A PF is defined as an area of 75 km2 or greater in which reflectivities are greater than 20 dBZ and PCT's are less than 250 K. Diurnal, seasonal, and interannual variabilities in the number of PF's will be presented. Analyses over Mexico are performed during the summer season extending from May through September(i.e. the monsoon season), and those over South America are performed during their summer season from November to March. The results are compared to previous results over the Himalayas, and linkages between cloudiness, large-scale circulations, orography, and rainfall are proposed.