Abstract

Aiming at the statistical and synoptical characterization of summer precipitation events in the andean portion of central Chile and their impact on the local river discharge, 94 summer rainfall events within the period 1970 - 1992 were defined. For that purpose daily rainfall records in 4 stations above 2000 m elevation and daily average runoff data (1980-1990) in 2 pre-andean stations of the Aconcagua and Maipo watersheds, were utilized. The characteristic weather patterns during those events were determined through pressure-time composite of temperature, relative humidity and wind anomalies at Quintero and Mendoza (radiosonde stations) and through objective analyses from the European Center for Medium Range Weather Forecasting. Considering the magnitude of the zonal wind omponent at 500 hPa over Quintero (top of the mountain range), the rainfall events were classified into two categories: those with a strong westerly flow (PW) and those associated with weak or inverted (easterly) westerly flow (PE). In the PW cases the moisture advection is associated with a deep extratropical trough off central Chile. The decay of the subsidence inversion, the strengthening of the westerly flow and the upper-troposphere divergence favor the upslope moisture edvection and the subsequent onset of convection. In the PE cases central Chile stays to the SW of a short-wave trough with its axis strongly tilted in a NW-SE direction, with weak geopotential gradients and a frequent cut-off low to the N-NE of the study area. The cold air associated with the upper air trough and the warming of the lower troposphere due to a Foehn effect reduce the static stability resulting in potentially unstable conditions. The moisture advection in this case is from the east, centered at about 700 hPa over Mendoza. The precipitation is convective, being typically distributed into 5 to 6 episodes in the DJF season. Characteristic episode durations range from 1 to 7 days with daily totals very seldom exceeding 10 mm. No significant differences were found in the rainfall ammounts between PW and PE episodes.