Springtime Precipitation and Water Vapor Flux over Southeastern South America

Abstract

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 as well as satellite estimates reveals that the model performed satisfactorily in the subtropics and extratropics. Deficiencies in tropical Brazil are partly related to the model?s convective adjustment scheme and possibly to surface parameterizations as well. The model forecasts reproduce all observed centers of precipitation south of about 20°S, although in some cases the magnitude is somewhat smaller. Of particular relevance for this study is the finding that spatial correlations between the model forecast and observed precipitation over Cuenca del Plata are almost as high as those obtained for the Mississippi River basin using forecasts of the National Centers for Environmental Prediction operational Eta Model. Cuenca del Plata is a basin in southeastern South America that is the water resource for a largely populated area and is well known for its agricultural production and other factors that sustain the region?s economies. An important component of the circulation reproduced in the simulations is the low-level jet east of the Andes that feeds moisture from the Amazon basin to higher latitudes. It has a diurnal cycle with a nighttime maximum that favors increased moisture flux convergence in southeastern South America. This convergence, in turn, is associated with generalized nighttime ascent and precipitation. The results are consistent with previous observational studies that show a nighttime maximum of precipitation over the region. A second regime of precipitation is found toward the eastern coast, where maximum daytime precipitation appears to be associated with a convectively unstable atmosphere, with convection being triggered by a sea?land breeze enhanced by the topography of southern Brazil. These diurnal regimes of precipitation have a significant impact in the atmospheric water cycle in Cuenca del Plata. The basin-averaged vertically integrated moisture flux convergence is about 4 mm day?1 and almost doubles the spring values for the Mississippi River basin. The large values may be related to the particular conditions of the period under analysis and the stronger low-level jet. The results reported here provide a preliminary description of the basin-averaged moisture flux convergence and its diurnal variability, but basin-averaged precipitation is still the component that needs to be improved. It is assumed that a blend of observations and high-resolution satellite estimates will be needed to complete the description of the atmospheric water cycle.