Land use and storms

Do you ever hear something you just can’t stop thinking about? About ten years ago, I heard a talk by Roger Pielke, Sr., where he compared the weather over northern Texas for two days, 100 years apart. The weather – high and low pressure areas, temperatures, and humidity – started out the exactly the same on two days in the world created by his version of a computer weather forecast model. But the land was different. It’s like someone took the weather from a 1991 TV weather map and put the highs, lows, and fronts on the United States of 1881, when there were only 44 states, and the land was prairie. And there was no television.

He picked a day when he knew there had been a strong thunderstorm – it had been observed and very carefully documented in 1991 by scientists during a field program. Since the storm already happened, he knew where the storm would form. And it’s a common place for storms to form – along a line where warm, moist air from the Gulf of Mexico meets dry hot air from the southwest United States. This line is called the “dry line.” It commonly forms in northern Texas. Storm forecasters watch the dry line closely to see whether new storms are forming.

Why the dry line? When air from two places flows together, something has to happen. The air can go sideways and “squirt” out of the way, or it can go up, or it can move sideways and up. This isn’t enough detail to know whether a storm will form, though. For a storm to form, the air has to go up over a small area, somehow. And it has to be moist enough to make a cloud.

That’s just the first step—the rising air cools. When the air gets cool enough, water starts condensing around tiny dust particles, and you get a cloud. If the air continues to rise – which it will if it is lighter (warmer) than the air at the same level outside the cloud – a very big cloud can form. And rain. Add to the mix a little wind change with height – we want the cloud to tilt over so the rain won’t fall into the updraft and kill the cloud. There is a very nice animation of a cloud forming on the GLOBE online Cloud Module.

We use computer models that describe how air behaves to predict where and how much air will rise. The model used by Pielke and his colleagues also describes how the soil and plants heat and moisten the air.

In the first model run, he put prairie into the model. In the second run, he put modern land use – including irrigation. After very carefully making sure that the beginning conditions are similar, they ran the model twice, once with the “old” land use and once with the “new.” The results were surprisingly different.

The “modern” computer run formed the strong thunderstorm that was observed. But the 1891 computer run formed much smaller clouds. Why?

The “modern” surface types heated at different rates. This leads to patches of warm air that start rising; and cool patches of air that move in to take their place. A “circulation” is set up with the updrafts over the warm region. This circulation helps get an updraft – and hence – a cloud, started in one place. This means the moist air near the surface can go up in one place, instead of many, concentrating the “energy” into one big storm; in this case, right on the dryline, where the air was already flowing together.

Next time – are there more storms than there used to be?

Thanks to Roger, Pielke, Sr. for supplying with journal articles on this topic.