Characteristics of TRMM-Observed Seasonal Precipitation in the Central Andes

The Andes extend from the west coast of Colombia (10N) to the southern tip of Chile (53S). In southern Peru and Bolivia, the Central Andes is split into separate eastern and western cordilleras, with a high plateau (≥ 3000 m), the Altiplano, between them. Because 90% of the Earth's tropical mountain glaciers are located in the Central Andes, our study focuses on this region, defining its zonal extent as 7S-21S and the meridional extent as the terrain 1000 m and greater. Although intense convection occurs during the wet season in the Altiplano, it is not included in the lists of regions with frequent or the most intense convection. The scarcity of in-situ observations with sufficient density and temporal resolution to resolve individual storms or even mesoscale-organized cloud systems and documented biases in microwave-based rainfall products in poorly gauged mountainous regions have impeded the development of an extensive literature on convection and convective systems in this region. With the tropical glaciers receding at unprecedented rates, leaving seasonal precipitation as an increasingly important input to the water balance in alpine valley ecosystems and streams, understanding the nature and characteristics of the seasonal precipitation becomes increasingly important for the rural economies in this region.

This study took advantage of the University of Utah's Precipitation Features database compiled from 14 years of TRMM observations (1998-2012) to analyze the characteristics, principally size, intensity, and rainfall contributions, of the entire spectrum of precipitation features (PF) in the study region. This analysis was supplemented by ERA-Interim reanalysis and WRF simulations of multi-day rainy events in selected areas of the study region. Previous work in analyzing PFs in regions such as South Asia and West Africa set size and/or intensity criteria to eliminate from consideration PFs that were deemed too small or weak to contribute meaningfully to regional rainfall totals. In this study, we show that small and weak PFs do contribute a non-negligible share of the seasonal precipitation, in both wet and dry seasons. We compare our results to PFs in similar regions such as the Himalayas and show that the PF population and therefore the convective environment in this region is unique, producing a much larger population of small, weak PFs and a much smaller population of large, intense PFs. We present our hypotheses why weak convection dominates the seasonal precipitation and how the population characteristics may shift due environmental changes brought on by glacier recession and warming global temperatures.