A Simple Boundary Layer Parameterization of Heterogeneous Land Surfaces Applied to Convective Updraft Prediction

A simple surface parameterization scheme with high spatial resolution is tested as a prediction tool for tornado-producing updrafts during severe thunderstorms. The effects of land surface heterogeneity, long a challenge for large-scale models based on Monin-Obukhov similarity theory (MOST), are quantified using a downscaling parameterization to estimate variations in localized surface fluxes. The land surface is defined at 30 m resolution by remotely-sensed data from the Shuttle Radar Topography Mission and National Land Cover Database, including layers of surface elevation, land cover, tree canopy fraction, and soil imperviousness. An integrated wind model based on the European Wind Atlas application of Jackson-Hunt theory to complex orography is employed to resolve spatially-varying momentum flux profiles. Temperature and moisture flux are then evaluated locally using Deardorff soil and vegetation parameterizations. As MOST breaks down to yield local-free-convection (LFC) similarity in the limit of free convection, the boundary layer stratification is captured just before convection to identify likely updraft locations based on point-specific maxima of surface-based convective available potential energy (SBCAPE).
An example case, the tornado of August 6^th, 2017 in Tulsa, Oklahoma, is explored: the boundary layer simulation highlights a topographical heat island likely responsible for introducing an inflow singularity within the nocturnal convective line, evidenced by radar reflectivity and base velocity imagery. More broadly, GIS analysis of significant (EF2 or greater) Oklahoma tornado tracks since 2011 indicate that elevated SBCAPE is commonly found 5-10 minutes upwind from locations of tornado touchdown or intensification. Furthermore, many ‘negative’ cases show a hook echo on radar reflectivity imagery downwind of simulated updraft sources. This study suggests a preliminary observational link between localized SBCAPE maxima and updraft cyclogenesis downwind.