Springtime precipitation and water vapor flux over southeastern South America

The physical mechanisms associated with precipitation in southeastern South America Juring 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 th:lt the model performed satisfactorily in the subtropics and extr atropics. 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 s outh of about 20 degrees S, although in some cases the magnitude is somewha t smaller. OF particular relevance for this study is the finding that spati al correlations between the model forecast and observed precipitation over Cuenca del Plata are almost as high as those obtained for the Mississippi R iver basin using forecasts of the National Centers for Environmental Predic tion operational Eta Model. Cuenca del Plata is a basin in southeastern Sou th America that is the water resource for a largely populated area and is w ell known for its agricultural production and other factors that sustain th e region's economies. An important component of the circulation reproduced in the simulations is the low-level jet east of the Andes that feeds moistu re from the Amazon basin to higher latitudes. It has a diurnal cycle with a nighttime maximum that favors increased moisture Aux convergence in southe astern South America. This convergence. in turn, is associated with general ized nighttime ascent and precipitation. The results are consistent with pr evious observational studies that show a nighttime maximum of precipitation over the region. A second regime of precipitation is found toward the east ern coast, where maximum daytime precipitation appears to be associated wit h a convectively unstable atmosphere, with convection being triggered by a sea- land breeze enhanced by the topography of southern Brazil. These diurn al regimes of precipitation have a significant impact in the atmospheric wa ter cycle in Cuenca del Plata. The basin-averaged vertically integrated moisture Aux 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 Aux convergence and its diurnal variability, bur basin-averaged precipitation i s 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.