Summer in the Mid-South can be quite hot as daytime temperatures reach the low to mid nineties each day. Add to that the rich flow of moisture from the Gulf of Mexico pushing dew points into the sixties and you get a heat index of 100+ degrees. Needless to say a summertime shower can bring instant yet brief relief from the sweltering heat and much needed moisture to a rapidly evaporating soil. The problems facing the forecaster in the Mid-South on a typical summer day are when will it rain, where will it rain, and how much will it rain. Though most of this subtropical environment is dominated by rural communities and agriculture there are a few larger cities and urban areas, the largest being Memphis Tennessee. Situated on the east bank of the Mississippi River at an elevation of 473 feet above sea level, the Memphis metropolitan area is home to 1.2 million people. It is the second largest city in the southeast and the seventeenth largest city in the United States. Needless to say, the urban heat island (UHI) is a contributing factor to the summertime temperatures. Studied have shown that city temperatures can range from 5 to 10 degrees higher than that the surrounding rural areas. This is known as the urban heat island (UHI) effect. But, can the UHI also impact precipitation on the mesoscale environment in and around these cities? The answer is YES, INDEED. In a recent study NASA employed the TRMM (Tropical Rainfall Measuring Mission) satellite to analyze the rainfall distribution over major cities in the U.S. Although Memphis was not one of the cities in the study, the results were essentially the same for those that were. The remote sensing data showed that the UHI increased summer rain over and downwind of major cities. The rainfall in those two primary areas exceeded rainfall in the downwind locations by 48 percent and to as much as 116 percent. As the air in the city warms two important factors occur, the warmer air over the city rises faster and a thermal gradient is created between the city and the surrounding rural areas. These two factors combine to create an area of convergence and lead to an enhanced lifting of the airmass over the city. Another interesting aspect is the higher level of pollutants contained in this rising urban airmass which in turn leads to an increase in the number of condensation nuclei in atmosphere. All of these factors combine to create an increase in clouds and under the right circumstances an increase in the amount of rainfall, but not for everyone. Because the air over the city is warmer it has the capacity to hold more moisture. In order to produce the rain an ample supply of moisture must also be available. Fortunately, in Memphis an abundance of available moisture is rarely a problem. Each summer the subtropical high sets up over the eastern Atlantic. Its center is typically in the area of the island of Bermuda, thus the name Bermuda High. As this system builds over the summer months, ridging as far west as the Rockies, it brings an ample and somewhat steady stream of moisture into the area. It also contributes to a fairly consistent south southeasterly wind flow at the surface and aloft. As convection increases and the peak of the diurnal heating cycle is reached, isolated to widely scattered thunderstorms develop. Many of these rain producing events occur in the heart of the city as a result of the convergence taking place in the areas of maximum heating and the increased upward vertical velocity. Other areas of increased activity lie along the periphery of the city or in the downwind locations. The reason for precipitation in these two areas is actually very simple. Because the air over the city is warmer it has the capacity to hold more moisture and it must be lifted higher in order to condense. As you reach the outer edges of the city that capacity decreases due to the gradual decrease in temperature. This is also the same area that is experiencing confluence due to the thermal gradient that exists between the city and the rural area which can give the moisture enough lift to break the cap that is in place and produce rain. As for the rain or thunderstorm that develops in the downwind position, this is a result of the moisture rich airmass that was once in place over the city that has now drifted into an airmass of cooler temperatures thus increasing the relative humidity of this incoming parcel of air which in turn increases its ability to produce precipitation. One factor that will limit the production of rain is a west to southwest flow. The center of the Bermuda High can be in such a position and somewhat weaker state that it contributes to this wind pattern. Drier continental air will flow into the region limiting the amount of available moisture and the ability to produce rain. Another limiting factor is a stronger Bermuda high that centers over the southeastern U.S. If it remains in place for a number of days it will still produce an ample supply of moisture but due to the close proximity of the center of the high a weaker wind at the surface. This along with the resulting subsidence will produce a stagnant airmass near the surface and a stronger cap aloft. As a result, fewer thunderstorms, if any, are produced and those that do develop tend to begin and end in the same location or drift slightly downwind. In conclusion, after researching this topic a few techniques can help with the problems listed at the introduction to this paper. The forecaster must realize that daily rainfall predictions outside of a larger synoptic influence such as a frontal passage or tropical event will be isolated to widely scattered. Areas most likely to receive precipitation will be in and around the city or in the downwind location. The most likely time for precipitation will be near the peak of the daytime maximum temperature. Monitoring temperatures, surface winds, area dew points, and water vapor imagery will help with identifying moisture rich areas thus giving the forecaster an idea of areas to target for potential rain or thunderstorm development. And as far as how much, well that's still a tough call and amounts will vary but research has shown that the heaviest areas of precipitation will occur in the city or in the downwind locations. References: Bornstein and Lin: Urban Heat Islands and Summertime Convective Thunderstorms in Atlanta: Three Case Studies. 1999 Orville, Nielsen-Gammon, Zhang, and Collins: The Houston Environmental Aerosol Thunderstorm (HEAT) Project. 2005 NASA: Here Comes Urban Heat http://science.nasa.gov/headlines/y2000/essd16mar_1m.htm O'Hara and Roberge: The Effects of the Baltimore Heat Island and Its Particulate Matter On Precipitation. Dixon and Mote: Patterns and Causes of Atlanta's Urban Heat Island-Initiated Precipitation. 2003 |