The Simulated Effect of Surface Flux Heterogeneity on Convection Initiation in the Southeast US

Motivated by the difficulty in predicting convection initiation (CI) over the southeast US during the warm season, this study presents a series of idealized large-eddy simulations that test the sensitivity of cumulus (CU) formation and subsequent CI to local variations in surface fluxes and airflow regimes. The simulations are initialized with heterogeneities typical of this region during this time of year as observed in northern AL in late summer 2014. The fluxes are initialized according to either a random tile or checkerboard configuration of varying horizontal scale, and the winds are initialized with varying speeds. In all cases, the total surface flux was homogeneous, with any give tile having either a high (low) sensible and low (high) latent heat flux. The initial temperature and moisture profiles are identical in all simulations. In general, CU formation and CI tend to occur over regions of high sensible and low latent heat flux associated with low-level convergence. The simulations reveal that the timing and areal coverage of CU and later CI is sensitive to the spatial structure of the surface flux heterogeneity and mean boundary layer wind. Additionally, the size of individual CU and cumulonimbi are also sensitive to these parameters. The heterogeneities in the surface fluxes influence the boundary layer structures that develop and in turn produce the microscale updrafts that force air to the lifting condensation level (LCL) and level of free convection (LFC).