Summer Rain Triggered By Dry Soil
September 13, 2012

Convective Precipitation Is More Likely Over Drier Soils

April Flowers for - Your Universe Online

An international team of scientists and research groups released a study which suggests a strong correlation between summer rain storms and dry soil conditions. In short, summer rains are more likely over drier soils.

The Centre for Ecology and Hydrology in the UK, the VU University Amsterdam, the Center of Meteorology CNRM in Toulouse, and the Vienna University of Technology collaborated on a detailed analysis of satellite data. Previous computer models have predicted the opposite effect, and the study calls for those models to be reconsidered.

Soil moisture can be measured with satellites using microwave radiation. Unlike visible light, microwaves can penetrate clouds. Satellites can either measure the Earth's natural microwave radiation to calculate the local soil moisture (passive measurement) or the satellite sends out microwave pulses and measures how strongly the pulse is reflected by the surface (active measurement). From this data, the soil moisture can be calculated.

The team looked at hydrological processes across six continents, examining imagery from weather satellites which track the development of storm clouds across the globe. When they matched up where new storms appeared alongside images of how wet the ground was, they were somewhat surprised.

Dr Chris Taylor from NERC Centre for Ecology & Hydrology said, "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. In those less extreme climates, with more vegetation cover, we expected the soil wetness effect would be too weak to identify."

In contrast to frontal rain systems, which move air from the ocean to the land and can cause rain over large areas, summer rainstorms are restricted to a rather small region. Summer showers frequently occur at the end of a hot day by process of "convective precipitation." Instead of moving across the land, the air moves from the hot ground upwards, forming clouds high in the air, and finally leading to localized rainstorms.

Dr Taylor added, "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 kilometers up into the atmosphere. If the soil is dry, the thermals are stronger, and our new research shows that this makes rain more likely."

"It's tempting to assume that moist soils lead to higher evaporation, which in turn stimulates more precipitation", says Wouter Dorigo of Vienna University of Technology. "This would imply that there is a positive feedback loop; moist soils lead to even more rain, whereas dry regions tend to remain dry."

Observations suggest otherwise. "We have analyzed data from different satellites measuring soil moisture and precipitation all over the globe, with a resolution of 50 to 100 kilometers. These data show that convective precipitation is more likely over drier soils", says Dorigo.

The new data contradicts established computer models, and a conclusive explanation for this has yet to be found. The researchers compared their observations with six global weather and climate models used to simulate climate change. They found that the 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.

"The air over dry soils heats up more easily. This could lead to a more intense vertical draft", Dorigo suspects. However, this cannot yet be described at a sufficient level of detail with today's computer simulations.

Dr Françoise Guichard from CNRM-GAME 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."

Results of the study are published in the journal Nature.