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.