In the past 30 years, several observational and climatological studies have suggested that major urban areas may have an appreciable effect on meso-gamma scale weather features and patterns under certain synoptic regimes. As a result, the signature of the "urban heat island effect" is resolvable in the rainfall patterns of the metropolitan area. However, a recent U.S. Weather Research Program panel concluded that more research is needed in this area. In response to this recommendation, NASA and other agencies initiated programs such as the Atlanta Land-use Analysis: Temperature and Air Quality Project (ATLANTA). Such programs aim to identify and understand how urban heat islands impact the environment. Rainfall modification is a small subset of the environmental changes associated with urban heat islands.

Here, a novel approach is introduced to correlating urbanization and rainfall modification. This study represents one of, if not, the first attempt to identify, quantify, and explain rainfall modification by urban areas using satellite-based rainfall measurements. Previous investigations used rain gauge networks and ground-based radar. Three years of mean monthly rainfall rates derived from the Tropical Rainfall Measuring Mission’s (TRMM) Precipitation Radar are employed. Analysis of data at half-degree latitude resolution (~55 km) enables identification of rainfall patterns in a 50-100 km radius around the major metropolitan areas of Atlanta, San Antonio, Waco, and Dallas/Ft. Worth during the warm season. Preliminary results reveal an average increase of 7.4% in monthly rainfall rates (relative to a mean upwind CONTROL area) over the metropolis but an average increase of 32% in monthly rainfall rates within 55 kilometers downwind of the metropolis. Some portions of the downwind area exhibit increases as high as 62%. This result is quite consistent with ground instrument-based studies of St. Louis and Chicago almost two decades ago. Furthermore, results in the Atlanta area are consistent with recent observational finding reported during ATLANTA following the1996 Summer Olympics.

The most prevalent hypotheses concerning urban-induced modification of rainfall include: (1) atmospheric destabilization through existence or enhancement of the mesoscale circulation associated with the urban heat island, (2) increased convergence or turbulence due to increased surface roughness, (3) increased low-level moisture content due to urban emissions, or (4) modification of microphysical processes due to increased condensation nuclei sources. Future components of this study seek to investigate, using a mesoscale model, the relative influences of these hypothesized factors on rainfall modification. Such research has implications for urban planning, water resource management, and understanding human impact on the environment.