Orographic Rainfall Hotspots in the Andes-Amazon Transition According to the TRMM Precipitation Radar and in Situ Data

The Andes-Amazon transition, along the eastern Peruvian Andes, features “hotspots” with strong precipitation. 15 years of TRMM PR data allowed establishing a robust relation between mean precipitation and terrain elevation, with a peak around 1000 masl, that coincides with the moisture flux profile of the South American Low Level Jet (SALLJ). However, there is strong diurnal variability. In the afternoon (13-18 LT), convection develops as is common in the Amazon plains. In the evening through the next morning, as the forcing associated with the thermal heating of the Andes subsides, convection grows, with surface precipitation peaking at 1000-2000 masl (01-06 LT), and organizes into mesoscale convective systems (MCSs) that displace downslope, with surface rainfall peaking at 700 masl and stratiform regions spreading upslope and downslope, decaying during the morning (07-12 LT). The large MCSs contribute with at least 50% of daily rainfall (40 % of daily rainfall between 01-06 LT). On synoptic scales, the large MCSs are more common in stronger SALLJ conditions, although subtropical cold surges are responsible for 16% of the cases.

The spreading of MCS over the central Andes of Peru above 3000 masl at early morning was observed by a vertical profiler Ka-band radar, called Mira 35-c, recently installed in the laboratory of atmospheric microphysics and radiation (LAMAR) located in the Mantaro Valley. The Mira 35-c captures the stratiform structure of early morning precipitation in agreement with TRMM observations.