Summer convection over and around the subtropical Andes

Abstract

This study presents a 15-year climatology (1998–2013) of convective precipitation systems in the subtropical Andes (36° S–20° S) during the austral summer (December–February). Using the Precipitation Radar data retrieved by the Tropical Rainfall Measuring Mission satellite, we analysed the frequency, structure, and diurnal cycle of these precipitation systems. Our findings reveal that summer precipitation is predominantly convective, concentrated in large-area systems that frequently occur in the eastern subtropical Andean slopes and their adjacent plains during evening and night hours. These precipitation systems often develop deep intense convective cores (>12 km), activity of large lightning flashes, and high precipitation rates. Distinct characteristics were observed in different cross- and along-barrier regions. Over the northeastern slopes and their adjacent plains (north of 28° S), the precipitation systems usually exhibited a clear bright-band signature, indicative of a tropical convection regime, in a humid, high-instability, low-shear environment that develops large-area stratiform precipitation in the mature or decay stage. Conversely, in the southern subregion the precipitation systems over the eastern slopes and adjacent plains showed deeper radar echoes but smaller areas with less pronounced bright-band signature, suggesting a more continental nature of convection in a dry, moderate to strong wind shear environment, and with a reduced stratiform precipitation stage. Within the Andes, convective systems were quite smaller and shallower than the ones observed on the eastern side, typically developing in the midday and afternoon. The western subtropical Andean slopes experienced very scarce, shallow, and weak convection compared to the interior and eastern flank. Extratropical cyclones associated with frontal stratiform precipitation were found to even occur in summer over the western slopes south of 28° S. These findings advance our understanding of the influence of the Andes on South American rainfall and climate, highlighting the complex interplay between the atmosphere and the topography of a formidable long and high barrier.