Raleigh N&O

Published: Thursday, July 5, 2001 4:27 a.m. EDT

Sandy soil breeds big storms Data suggest lightning, rain drawn to Sandhills region

By BOB WILLIAMS, Staff Writer

North Carolina's Sandhills region, known for its golf resorts and big military bases, also has the distinction of breeding an unusual number of summer thunderstorms. Why? A new study says the answer is in the sand.

Researchers at N.C. State University say the sandy soils of south-central North Carolina, where there is also little vegetation, heat more quickly than do either the clay-rich Piedmont to the west or the darker, sandy loam of the coastal plain to the east.

The result is that warm and cool air are constantly pushing against one another, forming a classic breeding ground for big storms. The phenomenon, called convergence, is particularly pronounced during the summer months.

The researchers hope their work will help lead to more accurate forecasts on summertime storms, potentially saving lives and property.

North Carolina ranks third in the nation in the number of lightning-related deaths, and fourth in lightning-related injuries. Flash floods are a killer often related to summertime storms.

"Knowing that this trigger is always sitting down there in the Sandhills gives us one more tool to understanding and predicting the weather," said state Climatologist Sethu Raman, who worked on the project along with fellow researchers Robb Ellis and Aaron Sims, both students at N.C. State.

The research was prompted by the unusual number of summertime storms that seemed to emerge in the Sandhills, that narrow slice of south-central North Carolina that includes the sprawling Fort Bragg military reservation and the golf meccas of Pinehurst and Southern Pines.

"You've got this relatively narrow strip where the soil is sandy and has relatively little vegetation," said Raman. "And it sits right between two areas where the soils are much richer and the vegetation is thicker."

There are other factors as well.

If there is a wind coming out of the south from the Gulf of Mexico -- a frequent pattern in summer -- thunderheads can form and quickly intensify over the Sandhills.

Another common summer pattern -- a persistent sea breeze off the Atlantic -- often reaches as far inland as the Sandhills, making the process even more pronounced.

So how much more rain do the Sandhills get?

The three researchers found that the Sandhills receive, on average, 1.12 more inches of rain during the summer months than do adjacent regions. The greatest disparity occurs in July, along a line running southwest from Lillington to the South Carolina border.

Over the past 40 years, weather stations in that corridor have received, on average, nearly 1.5 extra inches of rain in July alone. Although that may not sound like much, Raman said it is significant in terms of water conservation measures and for the potential of flash flooding.

Ellis said he first observed the phenomenon growing up in Columbia, S.C.

"I knew it was happening, but the question was why," said Ellis, who is studying meteorology and climate. "I wanted to figure it out."

Raman said increasing development in the Sandhills in recent years has probably magnified the phenomenon.

"Basically, more development means more buildings and pavement, and less vegetation," said Raman. "You have even fewer trees or other ground cover cooling things off, so it increases the effect."

Sims, a graduate student in meteorology from Raleigh, used data gathered during the research to create large-scale computer models of the rainfall anomaly in the Sandhills.

Those models showed how soil moisture, surface heating, and urbanization interact to create the higher rainfall totals.

Oddly enough, Raman said the same basic weather principles caused the extremely heavy rains that fell on parts of the Triangle during Hurricane Fran in 1996.

"What happened during Fran is that there were some very heavy rains in the Triangle area in the days leading up to the hurricane," he said. "Those heavy rains were fairly isolated, however. That meant when Fran hit, you temporarily had these relatively warm areas next to cool areas, just like you do all the time down in the Sandhills."

The researchers say the Sandhills pattern is present all year long, but is much more evident in summer.

"In the winter time, the weather patterns are much bigger and you can usually see them coming days ahead of time," said Raman. "The summer is just the opposite -- big storms are very localized events that can pop up quickly. That makes the anomaly down in the Sandhills much easier to observe during the summer."

The researchers stress that summertime storms can happen anywhere in North Carolina, but that all of the ingredients needed to create a big thunderstorm are present most of the time in the Sandhills.

"What we are saying is that the Sandhills are more likely to spawn big storms than other parts of the state where these conditions aren't present," said Raman. "At the same time, there will be days when there are big storms in other places and there will be nothing in the Sandhills. It just depends on how things come together."

Staff writer Bob Williams can be reached at 829-4656 or bobw@newsobserver.com

-- Anonymous, July 05, 2001