Wintertime precipitation episodes in central Chile: Associated meteorological conditions and orographic influences

Abstract

Central Chile (32 degrees-35 degrees S) is a mountainous and densely populated strip of land between the South American Pacific coast and the main divide of the Andes, 5000 m in height. In this study, wintertime precipitation episodes in central Chile are characterized using precipitation gauge, river discharge, radiosonde, and Special Sensor Microwave Imager (SSM/I) passive microwave radiometer observations over a 10-yr period (1993-2002). Precipitation episodes that typically occur as cold frontal rainstorms move over the region from west to east, within which the cross-mountain flow is blocked at lower levels. The influence of the Andes on the climatological precipitation pattern extends several hundred kilometers upstream of the coast. Over the mainland, the wintertime precipitation is most strongly related to the height of the mean topography surrounding the rain gauge sites, rather than the actual altitudes of the instruments, although higher-elevation locations are not well sampled by available rainfall observations. Between the coast and foothills of the Andes, the precipitation pattern is relatively uniform despite the complex coastal topography. On the western face of the Andes climatological enhancement factors of between 1 and 3 are inferred. Regression analysis against radiosonde data at a coastal site reveals that the precipitation is strongly related to the zonal (cross mountain) moisture flux. The strongest relationship is found when the moisture flux is multiplied by the relative humidity. This variable explains 50% of the variance in daily area average precipitation in central Chile and up to 60% of the variance in the daily precipitation recorded at individual stations. The factors contributing to events of heavy precipitation enhancement in the Andes were examined. Events of heavy, but isolated, precipitation in the Andes tend to occur in the warmer, prefrontal sector of approaching storms and are associated with unusually high moisture fluxes near to and above the crest of the mountain range. Strongly frontal episodes, characterized by widespread rainfall throughout central Chile, lead to variable, but on average rather weak, enhancement in the Andes.