A Large Mid-Latitude City Intensifies Severe Convective Events: Evidence from Long-Term High-Resolution Simulations

Abstract

Urban areas are vulnerable to climate and weather extremes, including severe convective events. However, large cities can themselves intensify such events, although the magnitude of this intensification remains uncertain. In this study, we quantify the influence of Moscow on severe convective events by using simulations with the convection-permitting COSMO-CLM model with 1-km resolution for multiple summers for 2007-2016 and analyzing the difference between simulations with and without the urban canopy parameterization (URB and noURB, respectively). We found a significant difference between these simulations for extreme precipitation and wind characteristics. Within the city boundary, the difference is about 20% for 30 mm daily precipitation events and 70% for 50 mm events. Strong wind events are twice as frequent in the URB simulation. The urban canopy significantly increases the magnitude and variability of extreme quantiles for vertical velocities. We found more than a 50% increase in compound events with both wind and precipitation exceeding their 0.99 quantiles and in events with the 2-5 km updraft helicity exceeding critical values, which serve as a good proxy for mesocyclone formation. Our findings may help to refine the population bias and can be used to specify the predicted changes in convective hazards in mid-latitude cities.

Keywords: severe convective events, urban impact, urban precipitation, updraft helicity, compound events, convective permitting simulations