Rainfall in the nation’s capital region is irregular in time and space. In the past winter, this region experienced a mild drought; however, it has received an abnormally high amount of rainfall since March. The excessive rainfall not only has alleviated the previous drought, replenishing reservoirs and raising the water table, but it has also created other problems, such as, flash floods, increasing insect population, canceled outdoor activities, etc.
Figure 1 is an accumulated rainfall map for the region. Over 400 mm rainfall has been observed in the DC area during the period between March 1 and June 17 (note the red area adjacent to the "half-diamond" outline of the city border next to the Potomac River). The figure also shows that not all areas received the same amount of rainfall. In the surrounding areas, such as, in northwest Maryland, less than half of the rainfall was received. The intensification of rainfall near the city core could be due to increased heat from the city, due to more buildings and pavement, or it could be due to a cloud "seeding" effect caused by pollution aerosols. Published studies have shown that both factors can affect storm formation near large cities. On the other hand, this pattern could just be due to consistent meteorological conditions in the region during spring 2009.
Figure 2 is an animation showing an explosive rainstorm development in the DC area on June 2, 2009.
The rainfall and the IR data were provided by the NASA Tropical Rainfall Measuring Mission (TRMM). Figure 1 was generated by the TRMM Online Visualization and Analysis System (TOVAS: http://disc2.nascom.nasa.gov/Giovanni/tovas/). The animation was generated by the Hurricane Data Analysis Tool (http://disc.sci.gsfc.nasa.gov/hurricane/trmm_quikscat_analysis.shtml).