Convection initiation under weak synoptic-scale forcing in Shanghai: favorable conditions and lifting mechanisms

Convection initiation (CI) prediction under weak synoptic-scale forcing remains a challenge. To clarify the favorable conditions for CI and identify their connection with lifting mechanisms, twelve years (2011–2022) of summer CI events over Shanghai under weak forcing are investigated by contrasting the pre-convective environments of days with and without deep convection using multi-source observations and ERA5 reanalysis. Relative operating characteristic analysis and physically constrained logistic regression model construction shows that CI is favored when the lowest 1-km temperature (lowT) ≥ 27.5 °C, the lowest 1-km wind speed (lowWS) ≤ 5.2 m s −1 , and the 900–600-hPa relative humidity (midRH) ≥ 51.1%. A logistic regression model constructed from above three parameters achieves an AUC of 0.85, a critical success index of 0.61, and a false alarm rate as low as 0.27, outperforming any single commonly used convective index. Three types of lifting mechanisms account for CI over Shanghai: (i) sea-breeze circulation alone, (ii) horizontal convective rolls (HCRs) alone, and (iii) the coupling of HCRs and sea-breeze front (SBF). Far from being a rarity over Shanghai, deep convection trigged by HCRs alone has long been underestimated. The lowWS governs the formation of the lifting mechanisms, midRH determines whether shallow convection can transition into deep convection, and lowT affects the frequency of long-lived convection. A schematic diagram of four SBFs and HCRs configurations according to distinct boundary-layer wind direction is proposed to indicate the high-frequency CI locations and storm tracks, offering forecasters a quick-look tool for Shanghai metropolis.