Processes Controlling the Location of Convection Initiation over Complex Terrain in a Sheared, Conditionally Unstable Environment

Previous studies using satellite and radar data show that some of the most intense convective storms initiate over and near the Sierras de Córdoba (SDC) mountain range in Argentina. Analysis of a multi-year catalog of radar-observed convection initiation (CI) cases over the north-south oriented SDC range reveals the presence of a daytime hotspot to the southeast of the main ridgeline. Significant knowledge gaps remain as to how regional topography modifies the mesoscale flow and which mesoscale processes contribute to CI over this hotspot. This study aims to explore the role of thermally forced daytime upslope flows, the interaction of shear and the northerly South American Low-level Jet (SALLJ) to low-level convergence and eventually daytime CI.

Numerical simulations of CI over the SDC mountains were conducted using Cloud Model 1 at horizontal grid spacings of 1 km. The environmental thermodynamic and wind profiles were derived from IOP4 of the RELAMPAGO field campaign, which collected radiosonde and other data useful for model initialization and evaluation. This case was chosen because the pre-convective environment wind profile resembles the composite wind profile of the daytime CI cases from prior studies. The simulations began at 1130 UTC (0830 LT) and ended at 2130 UTC (1830 LT). Analysis of the 10 h simulations shows that a convergence line develops to the east of the main ridgeline. Data from GOES-16 satellite shows the presence of a convergence line similar in location and shape to the one seen in the model output. The maximum reduction in CIN takes place over the southern portion of this line i.e., to the southeast of the crest, where pooling of high equivalent potential temperature takes place. Observations taken from various instruments deployed during the campaign confirm this spatial distribution of surface equivalent potential temperature around the mountains. In order to isolate the role of daytime upslope flows in the development of this convergence line, two additional simulations were conducted: one without background winds and the other one without diurnal heating. Output from these additional simulations indicate: first, it is likely that higher terrain in the southern part of the SDC enhances convergence in this area; second, upslope flows help in maintaining the strength and position of the convergence line, which can also be seen in the simulation without diurnal heating.