A multi-city analysis of the UHI-influence on warm season rainfall - What is the role of the nocturnal instability?

This study evaluates the influence of the Urban Heat Island (UHI) on warm season precipitation for several US cities by using contemporary high-resolution observational datasets. Spatial (local and downwind) rainfall anomalies associated with propagating and non-propagating storms are analyzed using statistical methods. A diurnal trend analysis suggests that the UHI has a dominant influence on precipitation for inland cities (Minneapolis and Washington D.C.), especially during afternoon and nocturnal hours. Propagating storms appear to converge over the urban region, but the rainfall intensification occurs downwind of the city. The nocturnal precipitation anomaly, which has received less attention in the past, is significant and comparable to the daytime anomaly. For coastal cities (New York City, Providence and Cleveland), a local increase in the frequency and intensity of extreme positive rainfall anomalies may occur during daytime due to the UHI-sea breeze interaction. For non-propagating storms, nocturnal rainfall enhancement is evident downwind of an inland city (Dallas), whereas a local daytime increase in the frequency of heavy rainfall events is observed for a coastal city (Houston). Nocturnal and downwind UHI-impacts on rainfall are less obvious for coastal cities but appear rather significant for inland urban areas.

Model sensitivity experiments are carried out over Minneapolis to inspect the role of the urban land cover in modifying rainfall patterns associated with nocturnal propagating storms. It is found that the urban region behaves like a “hot spot” or favored location for propagating storm cells due to increased surface convergence, lower wind speeds, and a cyclonic turning of surface winds as the storm approaches. These conditions appear to create a low-pressure anomaly over the urban center which in turn may influence the storm cell position. Although it is likely that the nocturnal UHI contributes to increased surface convergence in the urban environment, the wind anomaly may be caused due to other factors.