A case study of a Chaco low-level jet event

This paper concentrates on the analysis of the life cycle of the low-level jet (LLJ) during a summer Chaco jet event. This is accomplished through the use of the Eta/Centro de Previsao del Tempo e Estudos Climaticos (CPTEC) regional model, in order to obtain high temporal and spatial detail of the main processes taking place. Both the low-level circulation and the geopotential height evolution at different latitudes are analyzed to provide a more detailed description of the effects of topography and differential warming on the evolution of this current. This study shows that the life cycle of the particular event analyzed is not the same at the different latitudes swept by this well-organized northerly current, expanding from 158 to 32degreesS during two consecutive days. A common feature to all the examined latitudes is the presence of a diurnal cycle linked to local effects, which is more evident during the first day and a half of the simulation. This cycle was identified not only by a nocturnal maximum of the wind, but also by the oscillations of the geostrophic wind close to the surface in response to differential warming over sloping terrain. However, during the second day, the diurnal oscillation is superseded by synoptic-scale forcing. The meridional growth of this northerly current reacts basically to a deepening of the northwestern Argentinean low, consequently becoming a geostrophic response to a synoptic perturbance. However, during the final stages of this event, a northerly wind area located over the southern tip of the current, which notably increases the northerlies' penetration toward higher latitudes, develops. This last extension is mainly due to a component of ageostrophic origin. Evidence is provided in support of the hypothesis that this secondary development is a feedback between the LLJ and the precipitation at the exit region.