The Role of Surface Heterogeneities on Low-Level Vorticity Production and Boundary Layer Characteristics 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 AL. Observations collected during VORTEX-SE were used along with high-resolution simulations to determine the role of surface heterogeneities in the production/enhancement of low-level vorticity and in boundary layer characteristics such as vertical profiles of vertical velocity skewness and kurtosis. 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. Several cases from the 2016 field project are examined. Vertical profiles of skewness and kurtosis computed from the observations compare well with those computed from the simulations. In general, low-level horizontal vorticity is produced by gradients in surface heat flux and surface roughness lengths, with vorticity vectors aligned along the mean flow. This has implications for tornado formation as thunderstorms may ingest this vorticity and acquire rotation.