Using ALARO and AROME numerical weather prediction models for the derecho case on 11 August 2017

Abstract

On average, a derecho occurs once a year in Poland while bow echoes happen several times per year. On 11 August 2017, severe meteorological phenomena were observed in Poland, including extremely strong wind gusts. We focused especially on the convective windstorm of a derecho type which occurred on that date in northern and north-western Poland. A rapidly moving mesoscale convective system (MCS) resulted in a bow echo, a mesoscale convective vortex (MCV), and finally fulfilled the criteria for a derecho. To establish whether our operational models in the Institute of Meteorology and Water Management, National Research Institute (IMGW-PIB) could reproduce a derecho of such intensity as that of 11 August 2017, the results from two mesoscale numerical weather prediction models were analyzed. The Application of Research to Operation at Mesoscale (AROME) and the ALADIN & AROME (ALARO) models were applied in the non-hydrostatic regime. We also examine how models differ with respect to mesoscale convective system drivers (such as vertical wind shear and convective available potential energy) and representation of deep convection (e.g., vertical velocities, cold pool generation). Forecasts are compared with observations of wind gusts and radar data. Severe weather phenomena, such as rear inflow jet and cold pool, were predicted by both models, visible on the maps of the wind velocity at 850 and 925 hPa pressure levels and on the map of air temperature at 2 m above the ground level, respectively. Relative vorticity maps of the middle and lower troposphere were analyzed for understanding the evolution of MCV.