The role that atmospheric water vapour plays in weather is complex and not clearly understood. 

However, US researchers say they are starting to get a handle on it after teasing out the relationship between morning soil moisture and afternoon rainfall under different atmospheric conditions.

“The prevailing wisdom… is that if you have wetter soil in the morning, you’ll have a greater occurrence of rainfall in afternoon, but it’s more complicated than that,” says Josh Welty from the University of Arizona, lead author of a paper in Geophysical Research Letters.

“On a global scale, we see evidence that you can have greater chances of afternoon rainfall over both wet and dry soil conditions, depending on atmospheric moisture.”

The project was a collaboration with researchers from the Desert Research Institute in Nevada and NASA’s Goddard Space Flight Centre. 

The team had access to five years of NASA satellite data from the Global Precipitation Measurement Mission (GPMM) and the Soil Moisture Active Passive (SMAP) satellite, as well as atmospheric moisture and movement data from the Modern-Era Retrospective analysis for Research and Applications Version 2, or MERRA-2, model, which incorporates satellite observations. 

The study focussed on days in which afternoon rainfall occurred and noted the difference between the number of rainfall days that occurred over wetter-than-average soil and the number of rainfall days that occurred over drier-than-average soil. 

The resulting analysis found that on days when wind blows in little atmospheric moisture, afternoon rainfall is more likely to occur over wetter soils or higher relative humidity. However, when wind introduces lots of atmospheric moisture, afternoon rainfall is more likely over drier soils or lower relative humidity. 

For both conditions, afternoon rainfall occurrence is more likely with warmer morning soil or air temperature.

“The results really show the complexity of the land’s influence on weather and climate,” says NASA’s Joe Santanello.

“When you add in the human factor of irrigation or land use that changes the dryness or wetness of the soils, which we currently don’t represent well in the models, we potentially have additional downstream effects on weather and climate that we haven’t foreseen.”

The next step is to assess how these relationships play out in global climate and weather forecasting models.

“Our findings are observational, but now, we want to use computer modelling to help us understand why drier or wetter soil could enhance rainfall likelihood,” says the University of Arizona’s Xubin Zeng. “We know it’s true, but we don’t quantitatively know why.”