Impact of Surface Drag on Surface Kinematic and Thermodynamic Boundaries in Simulated Supercell Thunderstorms

Long-standing conceptual models of supercell thunderstorms have typically featured two main surface boundaries with both kinematic and thermodynamic gradients: the so-called rear-flank and forward-flank gust fronts (RFGF and FFGF). However, recent studies from both observational and modeling perspectives have found more complicated surface kinematic and thermodynamic structure--particularly in the forward flank region--than accounted for by this model. These manifest as multiple dynamic convergence boundaries that may or may not be accompanied by substantial thermodynamic gradients. Understanding the nature of these surface boundaries is germane to outstanding questions about tornadogenesis, as they may be sources of vertical and/or horizontal vorticity for the developing tornado. Concurrently with the recognition of this additional complexity in supercell surface boundary structure, other recent simulation studies have suggested that horizontal vorticity generated by surface drag may be a more important source of vorticity for tornadoes than previously thought.
In both cases, the role of surface drag has yet to be fully clarified, especially since until recently idealized simulations of supercells have historically mostly neglected it. In this study, we turn our attention primarily to the role of surface drag on the development and characteristics of surface boundaries in high-resolution simulations of supercell thunderstorms. Preliminary results indicate that inclusion of surface drag substantially alters the surface kinematic and thermodynamic gradients and boundaries in simulations of a strongly tornadic supercell, particularly in the forward flank region. In general, forward flank boundaries are less well-defined and surface winds are also greatly reduced throughout both the rear-flank and forward-flank regions, except very near the simulated tornado-like vortex. Overall results are similar to those of some recent real-data-driven “full physics” supercell tornado simulation studies which include surface drag. We discuss and analyze these differences and implications for modeling of supercells and attendant tornadoes. Additionally, we compare and contrast methods for maintaining the large scale environmental wind profile when surface drag is included in idealized simulations.