The sensitivity of convection initiation to surface heterogeneities during VORTEX-SE

Due to the difficulty in forecasting tornadoes in southeast US, the Verification of Rotation in Tornadoes in the Southeast Experiment (VORTEX-SE) was conducted in the spring of 2016 to better understand tornado formation in the conditions unique to this region around northern Alabama.  Observations collected during VORTEX-SE were used along with high-resolution simulations to determine the sensitivity of convection initiation to surface heterogeneities.  The simulations are initialized with heterogeneities in surface fluxes, local airflow regimes, and varying surface roughness lengths typical of those found in the southeast US. The heterogeneities are prescribed with varying spatial patterns and magnitudes of gradients.  We show results from both idealized simulations and several cases from the 2016 field project. 

In general, we found that the formation of CI events tends to occur over regions of high sensible and low latent heat flux associated with low-level convergence. Also, under certain conditions, more coherent nonclassical mesoscale circulations (NCMCs) develop and force CI.  The simulations show that both the strength and the extent NCMCs are sensitive to the magnitude and spatial structure of the surface heterogeneities.  The simulations also reveal that the timing and areal coverage of CI is sensitive to the spatial structure and magnitude of the gradients of the surface heterogeneity.  Additionally, we found that the size of individual convective cores is also sensitive to these parameters.  We show that heterogeneities in the surface 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).