Influence of Moscow megacity on severe atmospheric convective events: results of high-resolution modeling for multiple summers

During the warm season in the Moscow region, severe convective events like squalls, heavy showers, large hail, and tornadoes are regularly observed. Moscow megacity may intensify such phenomena due to various urban effects including modulation of heat and moisture fluxes. To quantitatively estimate this effect, we used datasets from two long-term numerical experiments based on the high-resolution COSMO-CLM mesoscale model (grid size ~1 km, output step 1 hour) for the summer seasons of 2007–2016. Experiments differ by switching on/off the urban canopy layer parameterization of TERRA_URB (labeled as URB and noURB, respectively).

We analyzed the maximum surface wind speed, the vertical component of wind speed at different levels, the hourly intensity and daily precipitation amount, and high quantiles for wind and precipitation. To analyze the possibility of mesocyclones formation, we used the Updraft Helicity (UH25) parameter.

We revealed statistically significant differences between the two experiments for the distributions of intense precipitation, vertical wind speed, and the UH25 parameter. The URB experiment showed an increase of the highest quantiles, standard deviations and partially mean values directly above Moscow and to the northeast of the city, in the leeward region compared to the noURB experiment. The compound events were analyzed as well (when both extreme precipitation and extreme surface wind speeds were noted). We found a statistically significant growth of compound events frequency for URB sample and inside the 18-km radius around the center of Moscow indicating the influence of the city.