Mesoscale Practical Predictability of the 20 May 2013 Tornadic Thunderstorm Event in Oklahoma

The EF-5 tornado that hit Moore, Oklahoma during local afternoon of 20 May 2013 was one of the deadliest and costliest tornado in United States in recent years. Using ensembles of WRF with a convection-permitting horizontal grid spacing of 1 km, this study focused on the practical predictability of its parent thunderstorm and accompanying convective systems, dealing with two common sources of errors that occur frequently in current numerical models: those from delaying or advancing in synoptic conditions, and those from location discrepancies of initiated convections.

The delaying or advancing of simulated synoptic conditions was explored by using initial conditions that came from model simulations at different times. Results showed that convection initiation (CI) in the ensemble generally synchronized with their respective initial conditions. The timing of convection initiation was strongly regulated by environmental conditions and development of the PBL. Low-level moisture was considerably influenced, primarily through changes in its vertical distribution within the boundary layer due to vertical mixing during the diurnal cycle as well as advection by low level jet. As a consequence, some of the members had richer moisture, larger instability (greater CAPE), weaker inhibition (less CIN and lower LFC) as well as earlier removal of capping inversion within PBL, leading to an earlier CI. Sensitivity experiments also proved the crucial role that properties within the PBL played in determining the timing of CI in each simulation.

The location discrepancies of initiated convections were explored by moving the underlying surface of the model. Results showed that changing the topography modified not only the timing of CI but also the development and the organization of the simulated thunderstorms. Changes in terrain height altered the moisture and CAPE within the PBL, creating different environmental conditions in which convection initiated. While the terrain height changes did not systematically alter the overall flow pattern, the dynamic responses of wind fields to the smaller scale topography details, such as hill and valleys, resulted in locally enhanced low-level convergence via changes in wind speed, wind direction, or variations in vertical wind shear within the PBL, which might influence locations of CI; on the other hand, changes in the low-level storm-relative environmental helicity, which is a vertically integrated parameter, might influence the development and organization of subsequent convective thunderstorms.