An informative NASA press release by Rob Gutro appeared on July 25, 2006 entitled “There’s a change in rain around desert cities”.
A summary schematic image that is presented in the press release reads,
“This image shows warm air rising from an urban area, and subsequent cloud formation. Cities tend to be one to 10 degrees Fahrenheit (.56 to 5.6 Celsius) warmer than surrounding suburbs and rural areas and the added heat can destabilize the atmosphere and change the way air circulates around cities. Added heat creates wind circulations and rising air that can produce or enhance existing clouds. Under the right conditions, these clouds can evolve into rain-producers or storms. It is suspected that converging air due to city surfaces of varying heights, like buildings, also promotes rising air needed to produce clouds and rainfall. Winds can carry these clouds to the east of the cities.”
Other excerpts from the press release are,
“Urban areas with high concentrations of buildings, roads and other artificial surface soak up heat, lead to warmer surrounding temperatures, and create “urban heat-islands.” This increased heat may promote rising air and alter the weather around cities. Human activities such as land use, additional aerosols and irrigation in these arid urban environments also affect the entire water cycle as well.”
“A study by J. Marshall Shepherd, a climatologist at the University of Georgia, Athens, used a unique 108-year-old data record and data from NASA’s Tropical Rainfall Measuring Mission (TRMM) satellite, to examine arid cities’ rainfall patterns. Shepherd found a 12-14 percent increase in rainfall in the northeast suburbs of Phoenix from the pre-urban (1895-1949) to post-urban (1950-2003) periods. This increase in rainfall is likely related to changes in the city and the lands within the city, such as more roadways and buildings in place of open natural area. The increase may also be related to changes in irrigation. However, the role of irrigation in changing the weather of cities in arid areas requires more study, Shepherd said.”
“We think that human activities, such as changing the landscape, can affect the flow of the winds associated with the U.S. southwest’s monsoon and rising air and building storms on the east side of mountains,” said Shepherd. The weather in Phoenix, in fact, is affected by both, and that can change where the rains fall.”
“Shepherd used satellite images from the Landsat satellite and the Advanced Spaceborne Thermal Emission and Reflection Radiometer instrument aboard NASA’s Terra satellite to determine expansion characteristics. He used the TRMM satellite’s rainfall data to pinpoint precipitation areas. This study shows the importance of satellite data in regions like the Middle East, where traditional measurements are sparse or inaccessible. ‘Many of the fastest-growing urban areas are in arid regimes,’ said Marshall Shepherd, author of the report just published in the online edition of the Journal of Arid Environments. ‘Because their total rainfall is low, these areas have been largely ignored in studies on how human activities affect the water cycle. But these cities are particularly sensitive to such changes, since the water supply is so critical.'”
“‘The results showed us just how sensitive the water cycle can be to human-induced changes, even under arid or drought conditions’ Shepherd said. These findings have real implications for water resource management, agricultural efficiency and urban planning.”
The title of the December 2006 Journal of Arid Environments article is “Evidence of urban-induced precipitation variability in arid climate regimes” . The abstract reads,
“The study employs a 108-year precipitation historical data record, global climate observing network observations and satellite data to identify possible anomalies in rainfall in and around two major arid urban areas, Phoenix, Arizona and Riyadh, Saudi Arabia. The analysis reveals that during the monsoon season, locations in northeastern suburbs and exurbs of the Phoenix metropolitan area have experienced statistically significant increases in mean precipitation of 12–14% from a pre-urban (1895–1949) to post-urban (1950–2003) period. Further analysis of satellite-based rainfall rates suggests the existence of the anomaly region (AR) over a 7-year period. The anomaly cannot simply be attributed to maximum topographic relief and is hypothesized to be related to urban-topographic interactions and possibly irrigation moisture. Temperature records suggest that Riyadh has experienced an adjustment in mean temperature in response to the growth of urban surfaces (e.g. the so-called urban heat island effect). While ground-based precipitation records also indicate an upward trend in mean and total precipitation in and around Riyadh in the last 10–15 years, it is difficult to attribute the increase to urbanization because other less urbanized stations in Saudi Arabia also show a similar increase. Recent satellite-based precipitation estimates indicate an AR 50–100 km north of Riyadh, but this study is not robust enough to conclusively link it to urbanization although certain climate-regime attributes suggests that it might be.”
This study provides additional evidence as to why a focus on local and regional human climate forcings needs to be elevated in importance in the climate science community.