Numerical simulation of a violent supercell tornado over Vienna airport initialized and initiated with a cloud model

Abstract

Abstract Supercell storm that occurred at the Vienna International Airport - “Schwechat” on July 10th, 2017, produced large hail and tornado with the strength of F2 on the Fujita scale, which corresponds to wind speeds between 118\xa0km/h. and 180\xa0km/h. (~33–50\xa0m/s). Doppler Weather radar of the Austro Control Association has successfully captured the evolution of a supercell storm and the characteristic patterns representative for the appearance of tornado. The present study examines the main triggering factors responsible for initiation of supercell storm and the main ingredients for formation of tornado. The results from a number of sensitivity tests using the advanced research version of the Weather Research and Forecasting WRF-ARW model, indicate the existence of very unstable atmospheric conditions (e.g. large surface CAPE, enhanced wind shear and veering, high storm helicity index, differential heating induced by the strong local forcing environment) favorable for initiation and development of a tornadic supercell. A three-dimensional (3-D) cloud model simulation at a 500\xa0m horizontal grid length quite well captured some important characteristics of a supercell storm dynamics (e.g., near surface convergence, strong rotating updrafts, vertical vorticity evolution and formation of mesocyclone) and cloud microphysics (evolution and transformation of a cloud hydrometeor fields, formation and fallout of precipitation). The radar reflectivity patterns identify a two downdraft cores, a hook-shaped echo at the rear side, and bounded weak echo region at a cloud mature phase. A tornado-like signature represents a quite useful ingredient for initializing and initiation of a tornado evolved from a supercell cloud at a small sub-domain (D2). The model runs at 50\xa0m and 10\xa0m horizontal and vertical grid spacing within a sub-domain (D3), respectively show evident advantage in simulation of a tornado-scale vortex, initiation, evolution and dissipation, relative to a coarser model runs. The sensitivity experiments using this novel technique revealed that such small-scale local hazards as tornadoes could be reproduced quite successfully with a close resemblance to observations. The initiation of a vortex tornado that evolves from a simulated supercell storm under open lateral boundary conditions allows for the selection of a smaller area and cloud simulation with a very fine resolution, capable of capturing smaller associated tornado vortices. The proposed method shows certain advantages over other models in terms of computing time savings and can serve as a reliable tool in various applications of short-term forecasting and local hazard warning.