Nav: Home

Decoding the skies: The impact of water vapor on afternoon rainfall

May 05, 2020

The role that atmospheric water vapor plays in weather is complex and not clearly understood. However, University of Arizona researchers have started to tease out the relationship between morning soil moisture and afternoon rainfall under different atmospheric conditions in a new study in the journal Geophysical Research Letters.

"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 lead author Josh Welty, a UArizona doctoral student in the Department of Hydrology and Atmospheric Sciences. "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 team, which also included researchers from the Desert Research Institute in Nevada and NASA's Goddard Space Flight Center, used satellite-based observations of soil moisture and afternoon rainfall in the northern hemisphere from the last five years. The work was supported by NASA and is based on NASA satellite data from the Global Precipitation Measurement mission and the Soil Moisture Active Passive 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 researchers 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. On days when wind introduces lots of atmospheric moisture, afternoon rainfall is more likely over drier soils or lower relative humidity. The team also found that for both conditions, afternoon rainfall occurrence is more likely with warmer morning soil or air temperature.

The researchers focused 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. They then grouped their results into three categories: high, mid and low atmospheric moisture transport by wind.

This research builds on a 2018 study that identified soil moisture's role in afternoon rainfall amount in the Southern Great Plains of Oklahoma. The new findings show that the relationship between soil moisture, afternoon rainfall and atmospheric moisture in Oklahoma doesn't apply across 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," Welty said.

Understanding the role of water vapor in weather is important because its effects are felt everywhere, according to Welty's thesis adviser and paper co-author Xubin Zeng, Agnese N. Haury Endowed Chair in Environment and director of the Climate Dynamics and Hydrometeorology Center and Land-Atmosphere-Ocean Interaction Group.

"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," Zeng said.

"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," Zeng said.

Understanding this relationship is even more important as global warming changes patterns of atmospheric moisture, soil moisture and more. Such changes will not only have effects on weather and natural disasters, but also on agriculture, Zeng said.

"The results really show the complexity of the land's influence on weather and climate," said physical scientist and paper co-author Joe Santanello from NASA's Goddard Space Flight Center, who chairs the NASA-supported Local Land-Atmosphere Coupling working group to improve weather and climate models. "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 modeling to help us understand why drier or wetter soil could enhance rainfall likelihood," Zeng said. "We know it's true, but we don't quantitatively know why."

University of Arizona

Related Rainfall Articles:

Importance of rainfall highlighted for tropical animals
Imagine a tropical forest, and you might conjure up tall trees hung with vines, brightly colored birds, howling monkeys, and ... rain.
New study could help better predict rainfall during El Niño
Researchers at the University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science have uncovered a new connection between tropical weather events and US rainfall during El Niño years.
Mediterranean rainfall immediately affected by greenhouse gas changes
Mediterranean-type climates face immediate drops in rainfall when greenhouse gases rise, but this could be interrupted quickly if emissions are cut.
Future rainfall could far outweigh current climate predictions
Scientists from the University of Plymouth analysed rainfall records from the 1870s to the present day with their findings showing there could be large divergence in projected rainfall by the mid to late 21st century.
NASA estimates Imelda's extreme rainfall
NASA estimated extreme rainfall over eastern Texas from the remnants of Tropical Depression Imelda using a NASA satellite rainfall product that incorporates data from satellites and observations.
NASA estimates heavy rainfall in Hurricane Dorian
Hurricane Dorian is packing heavy rain as it moves toward the Bahamas as predicted by NOAA's NHC or National Hurricane Center.
NASA looks at Barry's rainfall rates
After Barry made landfall as a Category 1 hurricane, NASA's GPM core satellite analyzed the rate in which rain was falling throughout the storm.
NASA looks at Tropical Storm Barbara's heavy rainfall
Tropical Storm Barbara formed on Sunday, June 30 in the Eastern Pacific Ocean over 800 miles from the coast of western Mexico.
NASA looks at Tropical Storm Fani's rainfall rates
Tropical Storm Fani formed in the Northern Indian Ocean over the weekend of April 27 and 28, 2019.
Changes in rainfall and temperatures have already impacted water quality
Changes in temperature and precipitation have already impacted the amount of nitrogen introduced into US waterways.
More Rainfall News and Rainfall Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Listen Again: The Power Of Spaces
How do spaces shape the human experience? In what ways do our rooms, homes, and buildings give us meaning and purpose? This hour, TED speakers explore the power of the spaces we make and inhabit. Guests include architect Michael Murphy, musician David Byrne, artist Es Devlin, and architect Siamak Hariri.
Now Playing: Science for the People

#576 Science Communication in Creative Places
When you think of science communication, you might think of TED talks or museum talks or video talks, or... people giving lectures. It's a lot of people talking. But there's more to sci comm than that. This week host Bethany Brookshire talks to three people who have looked at science communication in places you might not expect it. We'll speak with Mauna Dasari, a graduate student at Notre Dame, about making mammals into a March Madness match. We'll talk with Sarah Garner, director of the Pathologists Assistant Program at Tulane University School of Medicine, who takes pathology instruction out of...
Now Playing: Radiolab

What If?
There's plenty of speculation about what Donald Trump might do in the wake of the election. Would he dispute the results if he loses? Would he simply refuse to leave office, or even try to use the military to maintain control? Last summer, Rosa Brooks got together a team of experts and political operatives from both sides of the aisle to ask a slightly different question. Rather than arguing about whether he'd do those things, they dug into what exactly would happen if he did. Part war game part choose your own adventure, Rosa's Transition Integrity Project doesn't give us any predictions, and it isn't a referendum on Trump. Instead, it's a deeply illuminating stress test on our laws, our institutions, and on the commitment to democracy written into the constitution. This episode was reported by Bethel Habte, with help from Tracie Hunte, and produced by Bethel Habte. Jeremy Bloom provided original music. Support Radiolab by becoming a member today at     You can read The Transition Integrity Project's report here.