Water vapor and soil moisture can predict afternoon rainfall

The effect that water vapor has on the weather is complex and not entirely understood. In a new study from the University of Arizona, researchers have found that atmospheric water vapor plays a central role in the interactions between soil moisture and afternoon rainfall. 

“The prevailing wisdom on the relationship between soil moisture and rainfall 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,” said study lead author Josh Welty. “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.”

In a collaboration with experts at the Desert Research Institute in Nevada and NASA’s Goddard Space Flight Center, the researchers analyzed satellite-based observations of soil moisture and afternoon rainfall in the northern hemisphere. 

The NASA satellite data examined for the study was obtained over the last five years for the Global Precipitation Measurement mission.

The study revealed that on days when the wind blows in little atmospheric moisture, afternoon rainfall is more likely to occur over wetter soils or higher relative humidity. 

By contrast, on days when wind carries in an abundance of atmospheric moisture, afternoon rainfall is more likely over drier soils or lower relative humidity. 

The researchers also determined that for both conditions, rainfall is more likely to occur in the afternoon on days with warmer air or soil temperatures in the morning.

The research builds on a previous study that identified the link between soil moisture and afternoon rainfall in the Southern Great Plains of Oklahoma. 

The new findings show that the relationship between soil moisture, afternoon rainfall, and atmospheric moisture in Oklahoma does not apply to the entire northern hemisphere.

“Over the Southern Great Plains, we found that when the wind brings less moisture, dry soils are associated with increases in rainfall amount; and when the wind brings greater moisture, wet soils are associated with increases in rainfall amount. In the current study, we find that, actually, in many regions, the opposite is true for the likelihood of afternoon rainfall,” said Welty.

Study co-author Xubin Zeng said that understanding the role of water vapor in weather is important because its effects are felt everywhere.

“For example, for the Southern Great Plains there are many tornado activities because there is water vapor moving in from the Gulf of Mexico. Also, on the California coast you talk about severe flooding from atmospheric rivers, which are corridors of concentrated water vapor that can quickly precipitate once hitting a mountain range, causing mass flooding,” said Zeng.

“Water vapor brought in by the winds is an important source to understand. In the past, people didn’t pay enough attention to it in studying how land conditions affect rainfall, potentially making their results misleading. Once we consider the wind’s movement of water vapor, the results become more robust.”

Recognizing the interactions between water vapor, soil moisture, and afternoon rainfall has become even more important now as global warming will shift their patterns. Such changes will not only have effects on weather and natural disasters, but also on agriculture, explained Zeng.

“The results really show the complexity of the land’s influence on weather and climate,” said study co-author and NASA scientist 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 study is published in the journal Geophysical Research Letters.

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By Chrissy Sexton, Earth.com Staff Writer