Urban effects on precipitation: A synthesis of the literature

The published literature on urban effects on precipitation is filled with apparently conflicting results, which include case studies and climatological studies that show spatial distributions in which urban precipitation values are decreased over the city, increased over the city, increased over the downwind urban edge, and/or increased further downwind of the city. In most of these studies, investigators could only offer intuitive suggestions as to the possible cause(s) of these observed effects, e.g., aerosols, urban heat island (UHI), roughness, mixing, and/or convergence.

When these studies provide information on flow direction and storm type, the vast majority of them show results that fall into three groups, as follows:

• In otherwise calm conditions, an UHI centered over the city produces convergence and an area of isolated convective precipitation over the city.
• With a strong prevailing flow, the UHI is weak, and the regional flow is divided as air flows around the city due to its building barrier effect. Precipitating areas approaching the city thus divide, with precipitation maxima found on the lateral edges of the city and downwind of the city, where the regional flow re-converges.
• An intermediate case exists when a moderate prevailing flow advects the UHI, convergence area, and precipitation maxima to the downwind urban edge.

Results of the current study show a grouping of most of the previous studies in the literature indto one or more of the above three groups. For example, the current authors have carried out a summer climatological study in NYC and a series of six summer case studies in Atlanta that both show examples of the first and third groups listed above.