Yunji Zhang1, Fuqing Zhang2, David J. Stensrud2, Zhiyong Meng1
1 Laboratory for Climate and Ocean–Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China
2 Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania
Corresponding author: zymeng@pku.edu.cn(Web Page)
ABSTRACT
The practical predictability of severe convective thunderstorms during the 20 May 2013 severe weather event that produced the catastrophic enhanced Fujita scale 5 (EF-5) tornado in Moore, Oklahoma, was explored using ensembles of convective-permitting model simulations. The sensitivity of initiation and the subsequent organization and intensity of the thunderstorms to small yet realistic uncertainties in boundary layer and topographical influence within a few hours preceding the thunderstorm event was examined. It was found that small shifts in either simulation time or terrain configuration led to considerable differences in the atmospheric conditions within the boundary layer. Small shifts in simulation time led to changes in low-level moisture and instability, primarily through the vertical distribution of moisture within the boundary layer due to vertical mixing during the diurnal cycle as well as advection by low-level jets, thereby influencing convection initiation. Small shifts in terrain led to changes in the wind field, low-level vertical wind shear, and storm-relative environmental helicity, altering locally enhanced convergence that may trigger convection. After initiation, an upscale growth of errors resulting from deep moist convection led to large forecast uncertainties in the timing, intensity, structure, and organization of the developing mesoscale convective system and its embedded supercells.
Citation: Zhang Y., F. Zhang, D. Stensrud, and Z. Meng, 2015: Predictability of the Tornadic Thunderstorm Event in Oklahoma on 20 May 2013: Sensitivity of Convection Initiation and Organization to Small Changes in Synoptic Timing and Topographical Forcing, Monthly Weather Review, 143, 2973-2997. (PDF)
