Modeled Chaco low-level jets and related precipitation patterns during the 1997-1998 warm season

Chaco jet events (CJEs) are a subset of South American low-level jet events to the east of the Andes, characterized by enhanced poleward penetration and by a strong impact on precipitation over southeastern South America. The present study uses the Eta model short range weather forecasts produced operationally in the Brazilian Center for Weather Forecasts and Climate Studies (Centro de Previsao de Tempo e Estudos Climaticos, CPTEC) to characterize the CJEs and the related precipitation during the 1997-1998 warm season. An enhanced diurnal cycle in precipitation with respect to that found during the warm season mean can be recognized during CJEs in Eta/CPTEC model output, with preference for a nocturnal maximum over southern Brazil, Uruguay, and the central part of northern Argentina, and a daytime maximum near high topography (northwestern Argentina, the Brazilian Planalto). The analysis of thermodynamic and dynamic forcing appearing during CJEs, helps to explain the modeled precipitation cycle: the nocturnal maximum is mostly explained by enhanced low-level convergence at night, while the diurnal one is mainly a response to radiative warming. Boundary-layer convergence, and convective instability, present within the CJEs environment, work together to provide both dynamic forcing and potential for convection. The simulated precipitation cycle is complemented with surface observations of "current weather" that corroborate the main oscillations found in simulated precipitation.