Characterizing the Diurnal Cycle of Low-Level Circulation and Convergence Using CFSR Data in Southeastern South America

Abstract

The focus of this study is the characterization of the diurnal cycle of low-level wind and divergence field (under two different synoptic situations observed during the South American Low-Level Jet Experiment) within the South American domain encompassed between 20° and 35°S east of the Andes, using Climate Forecast System Reanalysis (CFSR). The objective is to highlight the existence of a spatial variation of these quantities and differences in the strength of their diurnal cycle between the two synoptic situations. Inertial oscillations and thermally driven circulations as well as convection-related contributions to mesoscale convergence and their implications for deep convection initiation/maintenance are addressed in each selected subregion. Prevalence of synoptic-scale forcing over the diurnally forced circulations, or vice versa, is also analyzed. Both mesoscale wind diurnal cycle and related divergence fields are sensitive to varying synoptic conditions and display regional variability. Thermal circulations related to topographical features superpose on the diurnal inertial oscillation that, while present in the whole domain, dominates the central plain subregions. The most evident diurnal cycle in the divergence field is restricted to sloped areas just to the east of the Andes and the Sierras de Córdoba where the mesoscale component of the divergence field is responsible for modulating the total divergence. CFSR provides a broad perspective of low-level circulation over southeastern South America (SESA) during the specific 15-day period. Results from this study might stimulate future research on a relationship between low-level circulation and the initiation of convection in SESA using CFSR to perform high-resolution simulations.