Lake Wheeler

Thunderstorms More Frequent In The Sandhills Region!

Recent research at the State Climate Office of North Carolina suggests that thunderstorms occur more frequently in the Sandhills Region of North and South Carolina.

The Carolina and Georgia Sandhills region of the Southeast is an elongated area of sandy soil that is located adjacent to regions of varying soil type. The Piedmont area to the northeast has soils that are mainly loam and clay-loam, while a variety of soil types exists in the Coastal Plain to the southeast. It is the differences in these soil types that create differential heating of the surface. Areas with sandy soil will more easily release their moisture to the area, allowing for a higher sensible heat flux than loam or clay-loam soils. This allows more energy to be used for heating the surface rather than evaporating moisture in the soil. Also, the heat capacity for sand is much less than that of clay or loam, so that given the same amount of energy, sandy soil would increase in temperature more than the loam or clay soils.

These differential surface heating patterns create mesoscale circulations causing clouds to form. Eventually, if conditions are right, thunderstorms will also form along the boundaries of these areas. By examining climatological precipitation in this and surrounding areas, the effects of the Sandhills region can be investigated. A period of 40 years was used to average the precipitation for 120 stations located within 50 miles of the Sandhills. Because surface heating is greatest during the summer months, these months are of most interest. Rainfall during the summer in Carolinas and Georgia is mainly the result of thunderstorm activity, so any increase in activity would be more clearly seen during this season.



July Averaged Monthly Precipitation around the Sandhills Region over the 40 years (1960-1999). The Sandhills are outlined in red.


Soil surface texture in the Carolinas and Georgia.



Official Associated Press Release

July 5, 2001

Scientists Study Why More Storms Form in the Sandhills in Summer

FOR IMMEDIATE RELEASE

When it rains it pours * especially, it seems, in the Sandhills in summer. For years, weather watchers have noticed that in the hot, muggy days of June, July and August, storms often flare up or intensify over the Sandhills region of North and South Carolina, dropping more rain there than on surrounding areas.

Scientists and students at North Carolina State University and the North Carolina State Climate Office are investigating this anomaly. They've analyzed 40 years of rainfall and climate data from the region, and are creating computer models to help explain what fuels the storms.

"The ultimate goal is not only to better understand why this phenomenon occurs, but to help meteorologists develop better ways to forecast it," said State Climatologist Dr. Sethu Raman, a professor of marine, earth and atmospheric sciences at NC State, who is leading the study.

Raman and his students have found that the Sandhills receive, on average, 1.12 more inches of rain from June through August than adjacent regions. The greatest disparity occurs in July, along a line running southwest from Lillington, N.C., to the South Carolina border. Over the past 40 years, weather stations in this corridor have received, on average, nearly one-and-a-half extra inches of rain in July alone. That may not sound like much compared to the rainfall a tropical system can produce, Raman says, but it's still significant in terms of water conservation measures and for the potential of flash flooding.

The Sandhills is a thin strip of rolling, sandy terrain that cuts across Georgia, South Carolina and southern North Carolina, dividing the Piedmont's clay soil from the sandy loam of the coastal plain. With its mild winters and fast-draining soil, the region is home to many golf courses, horse farms, retirement communities and resorts. Urbanization of the once sparsely developed landscape has occurred at a rapid pace, especially near towns like Southern Pines, N.C., Columbia, S.C., and Augusta, Ga.

These same conditions * fast-draining soil and urbanization * also play a role in triggering the summer storms, Raman and his students say. When an area with drier soil and sparse vegetation, such as the Sandhills, is located next to an area with moister soil and dense vegetation, such as the Piedmont or coastal plain, differences in surface heating occur, says Robb Ellis, an NC State junior majoring in meteorology. The drier soil heats more quickly and gives off more of the heat, warming the air above it.

This hot air rises, while the cooler air over nearby regions doesn't, causing a small area of circulation to develop. If there's a southerly wind carrying moisture from the Gulf of Mexico, as is often the case in the Southeast in summer, rain clouds can form or intensify in this circulation. Having moist, densely vegetated regions on both sides of the Sandhills * together with the presence of the sea breeze, which reaches as far inland as the Sandhills in the Carolinas * makes the process all the more potent by creating circulation on both sides, says Ellis, who first became interested in the phenomenon while growing up in Columbia, S.C., where he worked at the South Carolina State Climatological Office.

Urbanization also plays a role, Raman says. As the Sandhills' native vegetation is cleared to make way for new development, its soil can dry out even more quickly, increasing the heat differential between it and the moister soils in adjacent regions.

Data collected in the study have been used by another of Raman's students, Aaron Sims, a graduate student in meteorology from Raleigh, to create mesoscale meteorological models. These computer-generated models give the team a better idea of how all the various factors * soil moisture, surface heating, urbanization and the like * interact to create the rainfall anomaly.

"The data give us snapshots of different aspects of the phenomenon; modeling gives us a total picture. It shows us how the whole phenomenon evolves," Sims said. The models he is creating now may lead to better understand and help meteorologists accurately forecast Sandhills precipitation.

The State Climate Office is located on NC State's Centennial Campus and is a public service center of the university's College of Physical and Mathematical Sciences, with support from the College of Agriculture and Life Sciences. More information about the State Climate Office and its research and public service programs is available online at http://www.nc-climate.ncsu.edu.


NC State News Services phone: 919-515-3470 fax: 919-515-2556