Physical mechanisms forcing the warm season diurnal cycle of precipitation in southeastern South America

The physical mechanisms associated with precipitation in southeastern South America during spring are investigated using short term integrations with the regional mesoscale Eta model. An evaluation of the model's performance using in-situ measurements of precipitation and satellite estimates, reveals that the model performed satisfactorily in the subtropics and extratropics.

An important component of the circulation reproduced in the model forecasts is the Low-level Jet (LLJ) east of the Andes that feeds moisture from the Amazon basin to higher latitudes. The LLJ appears to be topographically bounded and diurnally modulated, and its maximum intensity occurs at about 03 UTC (near local midnight). The model forecasts also show that the diurnal cycle of the LLJ favors increased nighttime moisture flux convergence in southeastern South America. This convergence occurs mostly in the lower troposphere, and as a result there is widespread nighttime ascending motion and increased precipitation. Hence, the diurnal cycle of precipitation can be related to a dynamical forcing associated with the nighttime increase of the LLJ. The results are consistent with previous observational studies that also show a nighttime maximum of precipitation over the region.

A second regime of precipitation is noticed toward the east, where values are largest during the day. In this case, no widespread moisture flux convergence and ascent are found. Instead, the forcing seems the result of convective instability triggered by a more localized sea-land circulation enhanced by the topographic effects of the low mountains in southern Brazil.