Five Questions About Rainfall and Drought for Ana Barros

It's complicated: Rainfall is affected by global patterns, and landforms.
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Mar 17, 2008

Ana P. Barros is a professor of environmental engineering in the Pratt School of Engineering who studies the water cycle and how land, air and water interact.

Does being east of the mountains make the drought worse in North Carolina? Would we get more rain if they weren't in the way?

Locations downslope and downwind of mountains with regard to regionally predominant storm tracks tend to receive significantly less rainfall (up to 300% ) than upslope and upwind regions. This is the so-called the “rainshadow” effect that explains the climate and vegetation differences for example across the Cascade Mountains in Washington State. This is not quite what happens in North Carolina. Droughts in North Carolina are associated with a shift in regional atmospheric circulation patterns from southerly and carrying very high moisture content to produce rainfall on average (this is called high moisture convergence) to significantly drier northerly patterns associated with anti-cyclonic circulation . These changes are linked to inter-annual and multiyear global-scale changes in atmospheric circulation that also lead to a significant decrease in the frequency of tropical storms including tropical cyclones and hurricanes originating from the Atlantic Ocean, which account for the majority of the summertime rainfall in North Carolina. Furthermore, such changes are also associated with warmer temperatures. The increase in temperature in North Carolina leads to increase in evapotranspiration from vegetation and evaporation from lakes and reservoirs, decreasing soils moisture and water levels. Prolonged absence of rainfall or low rainfall amounts coupled with increased evapotranspiration of near-surface soil moisture is the recipe to extreme drought.

I heard the La Nina effect in the South Pacific is messing up our weather in the SE United States. How can that be possible?

La Nina is a multi-year change in atmospheric circulation pattern caused by cooling of the central and eastern Equatorial Pacific Ocean which results in a severe reduction of the influx of atmospheric moisture to North Carolina (see answer above), a decrease in the number of summertime tropical storms from the Atlantic ocean, and warmer weather in North Carolina. During La Nina the storm tracks out of the Pacific Ocean move northward causing floods in the West coast, whereas high-pressure (anti-cyclonic circulation) prevails over the East coast, and the number of rain-producing weather systems crossing North Carolina decreases dramatically.

It seems like being in drought and having dry soils and dead vegetation makes it even less likely to rain because there's less evaporation. Is that the case or not?

Yes, this is true. Clouds form when moist air parcels are forced to cool to a temperature at which condensation can take place. The way moist air is forced to cool is by raising up in the atmosphere to colder levels. When soil moisture decreases such that the upper soil layers are very dry, then there is less evapotranspiration, the atmospheric boundary layer becomes very dry and warmer temperatures can be found at higher levels in the atmosphere during the daytime. This prevents the formation of low-level clouds because there is less moisture and the condensation can take place only at very high altitudes. In turn, a decrease in low level cloudiness leads to increased direct solar radiation reaching the land surface, and thus higher air temperatures throughout the lower troposphere, further decreasing the relative humidity. Another feedback is that rainfall originating from high clouds can be evaporated before it reaches the ground.

If the Earth warms up by just 2 degrees or whatever, does that mean it's going to rain less or more?

At a local place, if the temperature increases and the moisture availability remains the same, the atmosphere can hold more moisture before condensation takes place, and therefore an increase in temperature should lead to fewer clouds and less rain. However, the amount of rainfall at any one place on Earth is not directly related to the average temperature of the planet, but rather to how that average increase in temperature manifests itself regionally in terms of atmospheric moisture fluxes and temperature. Indeed, even if there was no change in climate, the fundamental cause of atmospheric circulation is the gradient in solar radiation between the Equator and the poles, that is why there are significant differences in temperature between these regions. This is further complicated by the presence and relative distributions of oceans and continents, clouds, etc. The average temperature of the Earth is a broad brush indicator of climate, but it is not enough to answer this question. Therefore, some regions may see an increase in rainfall, whereas other regions may see a decrease, and others may see no change. Climate model forecasts suggest a decrease in atmospheric moisture influx to North Carolina in the future.

Could you explain what is meant by the water table and how the effects of people drilling wells on their property may impact the overall water table for an area?

In simple terms, the water table is the surface that separates unsaturated from saturated soil layers. That means that all open spaces in between soil particles below the water table are filled with water. This water is called groundwater and the soil-water storage system is referred to as an aquifer. When people drill wells into an aquifer, the pumps force water to move out of the aquifer. Because of gravity, the water that is at shallower depths in the aquifer moves downward to occupy the open spaces that were left empty by the pumped water, and as the water moves downward the water table moves with it. Unless the water is replenished from above by infiltration of rainfall, the amount of groundwater can decrease dramatically over time, and depending on the size of the aquifer, as the water table becomes increasingly lower, the costs of drilling and pumping groundwater can be prohibitive. If there are multiple wells extracting water from an aquifer, there is a cumulative effect to lower the water table. If groundwater extraction increases during a drought to increase water supply, water table recovery can take a long time (months to years) even if it rains a lot because the natural flow rates of water in soils are very, very slow as compared to pumping rates or rainfall rates.

Next Month:

Psychologist Elizabeth Brannon studies how human adults and children understand numbers, and compares us with our primate cousins monkeys and lemurs to better understand how our numerical ability may have developed and evolved. Ever wonder how babies count things without the words for numbers? Why would a monkey choose the bunch with five bananas instead of the bunch with three -- does she know how to count? Ask your own questions for Dr. Brannon, and see them answered here next month.


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