Mesoscale Influences of Land Use, Topography, Antecedent Rainfall, and Atmospheric Conditions on Summertime Convective Storm Initiation under Weak Synoptic-Scale Forcing

Abstract

Abstract Throughout the summer months in the Southeast United States (SEUS), the initiation of isolated convection (CI) can occur abundantly during the daytime with weak synoptic support (e.g., weak wind shear). Centered around this premise, a dual-summer, limited-area case study of CI events concerning both geographical and meteorological features was conducted. The goal of this study was to help explain SEUS summertime CI in weak synoptic environments, which can enhance CI predictability. Results show that spatial CI nonrandomness event patterns arise, with greater CI event density appearing over high elevation by midday. Later in the day, overall CI event counts subside with other mechanisms/factors emerging (e.g., urban heat island). Antecedent rainfall, instability, and moisture features are also higher on average where CI occurred. In a random forest feature importance analysis, elevation was the most important variable in dictating CI events in the early to midafternoon while antecedent rainfall and wind direction consistently rank highest in permutation importance. The results cumulatively allude to, albeit in a muted, nonsignificant statistical signal, and a degree of spatial clustering of CI event occurrences cross the study domain as a function of daytime heating and contributions of features to enhancing CI probabilities (e.g., differential heating and mesoscale thermal circulations). Significance Statement Widespread isolated thunderstorms in the Southeast United States summer season with weak synoptic support have been commonly observed. With forecasting these remaining a challenge, a dual-summer intercomparison of geographical/meteorological features with convective initiation events was conducted. Radar data with a minimum threshold for convective initiation detection (35 dBZ) were used. Spatial nonrandomness was discovered with greater event density appearing over higher elevation by midday. Features such as prior rainfall and atmospheric instability/moisture were higher on average where initiation occurred. In a feature importance analysis, elevation ranked higher in the early to midafternoon hours while antecedent rainfall and wind direction ranked highest overall in permutation importance. These results allude to the contribution of localized phenomena to the nonrandomness (e.g., mesoscale circulations).