Wet and dry spells in the Rio Santa Basin, Peruvian Andes : a WRF modeling case study

During the rainy season, the Rio Santa basin (Peruvian Andes) experiences wet and dry spells. These are periods with exceptional precipitation, either above or below the average, lasting for several days. Many studies observed a link between the upper-tropospheric zonal wind and the occurence of wet or dry spells. The goal of this thesis was to identify reasons for this interaction. Therefore a wet and a dry event were investigated by performing high-resolution simulations with the Weather Research and Forecasting (WRF) model. ERA-Interim reanalysis data was used to drive three nested domains with a horizontal grid size of 25 km, 5 km and 1 km. The WRF model reproduces the wet, as well as the dry spell in the Rio Santa basin. The simulated rainfall totals match rain gauge measurements, as well as estimations from the Global Precipitation Measurement (GPM) satellite reasonably well within the watershed. ^However, along the eastern part of the Andes, the model tends to overestimate precipitation. In contrast, no rainfall is predicted over the eastern lowlands, although it was observed. The model results provide insights to better understand the influence of the upper-level flow on the precipitation pattern. This flow is dominated by easterlies during the wet event, in which isolated convection develops over the Andes. Additionally, mature convection cells are triggered, preferably along the eastern foothills of the Andes. The upper-level easterlies allow these deep and organised convection cells to travel across the Andes. Even if precipitation approaching from the east only marginally reaches the Rio Santa basin, rain and thus moisture is delivered into the Andes. This facilitates local moist convection on subsequent days. In contrast to the wet event, the dry spell is characterized by a westerly flow aloft, while at the Andes crest level easterlies are present. ^Convection cells do not overcome the wind reversal and die fast. Scattered deep convection along the eastern foothills of the Andes is also very short-lived. Moreover, in the Rio Santa valley, the mentioned easterlies trigger foehn-like processes. The descending motion as well as mixing due to breaking gravity waves dries the air and inhibits convection. These results are mostly in accordance with the widely aknowledged theory proposed by Garreaud (1999). However, the unexpected easterlies at the Andes crest level during the dry spell do not fit in this framework and call for a comprehensive climatological investigation.by Maria SillerUniversity of Innsbruck, Masterarbeit, 2017(VLID)226408