Divergent Urban Signatures in Rainfall Anomalies Explained by Pre‐Storm Environment Contrast

Diverse urban‐induced rainfall anomalies across different cities highlight the need for additional insights into land‐atmosphere interactions over complex urban environments. Based on empirical analyses of 144 warm‐season storms and high‐resolution numerical simulations over Nanjing, China, we show divergent patterns of urban‐induced rainfall anomalies for storms with contrasting synoptic conditions, despite of rainfall enhancement over downtown from a climatological perspective. We propose two simple gage‐based metrics to characterize both the thermal and turbulent conditions in pre‐storm environment, and classify storms into different groups. Our results show that elevated rainfall magnitudes and heavy rainfall frequency are equally expected in either downtown or suburb regions (upwind or downwind). This is mainly dictated by the relative dominance of urban‐induced thermal perturbations and mechanical turbulence (i.e., related to surface roughness) under different synoptic conditions. We develop four paradigms of urban rainfall modification, and thus provide a predictive understanding of rainfall anomalies in urban environments. Plain Language Summary The diverse characteristics of urban rainfall anomalies from previous studies highlight intricate processes about the land‐atmosphere interactions and urban rainfall modification. We conduct empirical analyses of in situ rainfall observations and numerical analyses of high‐resolution Weather Research and Forecasting model simulations during the warm season months for 2014–2017 over and around Nanjing, China. Although the urban‐induced temperature anomalies dictate enhanced rainfall over downtown from the climatological perspective, positive rainfall anomalies for storms with different pre‐storm environments are observed over suburbs, including different positions over upwind and downwind regions. We categorize 144 storms into different groups based on two independent indices characterizing urban‐induced thermal perturbations and mechanical turbulence prior to each storm. Our results emphasize that the relative dominance of thermal and turbulent conditions in pre‐storm environment plays a critical role in determining the divergent urban rainfall anomalies. An in‐depth understanding of urban rainfall modification is provided to contribute to hydrometeorological engineering design and fine‐scale flood forecasting. Key Points We show divergent urban‐induced rainfall anomalies with contrasting thermal and turbul