Orographic rainfall hot spots in the Andes-Amazon transition according to the TRMM precipitation radar and in situ data

摘要

The Andes-Amazon transition, along the eastern Peruvian Andes, features "hot spots" with strong precipitation. Using 15 years of Tropical Rainfall Measuring Mission PR data we established a robust relation between terrain elevation and mean surface precipitation, with the latter peaking around 1000 m above sea level (asl), coinciding with the moisture flux peak of the South American Low Level Jet (SALLJ). There is strong diurnal variability, with afternoon (13-18 LT) convection in the Amazon plains, while on the eastern slopes (1000-2000 m asl), after the forcing associated with the thermal heating of the Andes subsides, convection grows during the night and surface precipitation peaks around 01-06 LT and organizes into mesoscale convective systems (MCSs). These then displace downslope to an terrain elevation of 700 m asl with stratiform regions spreading upslope and downslope and then decay during the remainder of the morning. The large MCSs contribute with at least 50% of daily rainfall (60% of the 01-06 LT rainfall). On synoptic scales, the large MCSs are more common in stronger SALLJ conditions, although subtropical cold surges are responsible for 16% of the cases.