Where does it rain on a hot day's afternoon? New satellite data show that soil moisture plays an important role. It influences precipitation in a way which is quite different from what models have predicted so far.
Summer rain is more likely over drier soil -- this is the conclusion scientists have drawn from a detailed analysis of satellite data. State-of-the-art computer models predict the opposite effect; these models must now be reconsidered, says the study published in the journal Nature. Several international research groups were involved in the project: The Centre for Ecology and Hydrology (Wallingford, UK), the VU University Amsterdam, the Center of Meteorology CNRM in Toulouse, and the Vienna University of Technology.
Convective Showers: Hot Air Moves Up
Frontal rain systems, moving from the ocean across the land, can lead to rain over large areas. Summer showers, which frequently occur at the end of a hot day, are often restricted to a rather small region. This kind of rain is a completely different phenomenon. Instead of moving across the land, the air moves from the hot ground upwards, forming clouds high up in the air, and finally leading to rain. This is called "convective precipitation."
Does Soil Moisture Lead to More Rain?
"It's tempting to assume that moist soils lead to higher evaporation, which in turn stimulates more precipitation," says Wouter Dorigo (Vienna University of Technology), one of the authors of the study. "This would imply that there is a positive feedback loop: moist soils lead to even more rain, whereas dry regions tend to remain dry." But 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 Wouter Dorigo.
The new data contradicts established computer models. A conclusive explanation for this effect has yet to be found. "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.
Microwaves from Space
Soil moisture can be measured with satellites using microwave radiation. Unlike visible light, microwaves can penetrate clouds. Satellites can either measure 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.
- Christopher M. Taylor, Richard A. M. de Jeu, Françoise Guichard, Phil P. Harris, Wouter A. Dorigo. Afternoon rain more likely over drier soils. Nature, 2012; DOI: 10.1038/nature11377
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