Counterintuitive, models wrong – rainfall more likely over drier soil

From the Vienna University of Technology , what appears to be a negative drought feedback mechanism has been observed.

Summer rain more likely over drier soils

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 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.

here’s a second press release:

From the Centre for Ecology & Hydrology

Parched soils trigger more storms

Afternoon storms are more likely to develop when soils are parched, according to a new study published this week in Nature which examined hydrological processes across six continents.

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 research team included scientists from the UK, Holland, Austria and France and was led by Dr Chris Taylor from the NERC Centre for Ecology & Hydrology in the UK.

The scientists examined 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.”

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.

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 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.”

Co-author Dr Françoise Guichard from CNRM-GAME (CNRS and Meteo-France) 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.”

The research team came from the NERC Centre for Ecology & Hydrology in the UK, CNRM-GAME (CNRS and Meteo-France) in France, Vrije Universiteit Amsterdam in the Netherlands, and the Vienna University of Technology in Austria.