Atmospheric precipitation patters under the influence of St. Petersburg

The metropolis influences precipitation in different directions. As a rule, a weakening of low and medium intensity precipitation is observed over the city, and an increase in the most intense precipitation. The main mechanisms of the city's influence on precipitation include: the urban heat island effect (increased ascending currents leading to increased cloud formation and precipitation), specific urban landscape, and high concentration of aerosols. In addition, the metropolis of St. Petersburg is especially influenced by the proximity of Lake Ladoga and the Gulf of Finland. If the air humidity is insufficient for cloud formation, powerful convective currents formed over the central part of the city are an obstacle to advective air currents entering the windward part of the city. Incoming air masses experience additional forced lift, resulting in cloudiness and precipitation. High relative humidity of the atmospheric air contributes to increased convective instability and pollution of air masses over the city, which favors cloud formation. In the process of transformation of clouds from cumulus to powerful cumulus and cumulonimbus, they are displaced under the influence of the prevailing transfer of air masses. Precipitation falls mainly in the leeward areas of the city and beyond.
Recently, the problem of forecasting cases of precipitation over the city has become acute, when the precipitation zone "breaks" over the city and when, on the contrary, "sharply intensifies". For example, one of such "special" cases was observed on July 31, 2024. Quite strong ascending flows were observed over the city, but the cloudiness was destroyed, not intensified. Among the interesting cases for modeling the ongoing processes, the following dates were also selected: January 5, 2023, June 22, 2023, etc. (where either the effect of weakening or the effect of intensifying precipitation over the city is clearly observed).
Numerical experiments were carried out based on the WRF-ARW model. As a result of the work carried out, an archive of cases of atypical precipitation was created, a synoptic analysis of the situations was carried out, numerical experiments were conducted to model the selected cases, and maps of radar characteristics were prepared to understand the trigger mechanisms of precipitation behavior over the city and in the immediate suburbs.