Warm surface drives stronger thermal updrafts
By Summit Voice
SUMMIT COUNTY — A new international study contradicts previous findings about the relationship between dry ground and rainfall, showing that afternoon thunderstorms are more likely to develop above parched areas, where surface heat drives the development of stronger vertical thermal currents.
Several other recent research projects have suggested that dry ground intensifies local and regional droughts, but after matching imagery from weather satellites which track the development of storm clouds across the globe with images of how wet the ground was, the scientists were somewhat surprised.
“We had been looking at storms in Africa and knew that rain clouds there tended to brew up in places where it hadn’t rained in the previous few days. We were surprised to see a similar pattern occurring in other regions of the world such as the US and continental Europe,” said Dr, Chris Taylor from NERC Centre for Ecology & Hydrology. “In those less extreme climates, with more vegetation cover, we expected the soil wetness effect would be too weak to identify.”
The results have important implications for the future development of global weather and climate models which may currently be simulating an excessive number of droughts. The results suggest that existing models do the wrong thing, triggering rain over wetter soils.
The implication is that existing climate models are more likely to go into a vicious circle whereby dry soils decrease rainfall, leading to even drier soil conditions. The paper concludes that fixing this problem is a priority for scientists developing the climate models.
“Both heat and moisture are critical ingredients for rain clouds to build up during the afternoon. On sunny days the land heats the air, creating thermals which reach several kilometres up into the atmosphere. If the soil is dry, the thermals are stronger, and our new research shows that this makes rain more likely,” Taylor said.
“We need to improve climate models so that we get a better idea of what global climate change will mean on smaller regional scales over land,” said co-author, Dr. Françoise Guichard, of Meteo-France.