The Impact of Water Vapor on Afternoon Rainfall

Soil Moisture Active Passive Satellite

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Artist’s making of the Soil Moisture Active Passive satellite. The width of the area scanned on Earth’s surface area throughout each orbit has to do with 620 miles. Credit: NASA/JPL-Caltech

The function of inbound water vapor on rains has actually been a complicated location of research study, however brand-new research study reveals water vapor is a crucial piece of the puzzle.

The function that climatic water vapor plays in weather condition is complicated and not plainly comprehended. However, University of Arizona scientists have actually begun to tease out the relationship in between early morning soil wetness and afternoon rains under various climatic conditions in a brand-new research 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,” stated lead author Josh Welty, a UArizona doctoral trainee 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 group, which likewise consisted of scientists from the Desert Research Institute in Nevada and NASA’s Goddard Space Flight Center, utilized satellite-based observations of soil wetness and afternoon rains in the northern hemisphere from the last 5 years. The work was supported by NASA and is based upon NASA satellite information from the Global Precipitation Measurement objective and the Soil Moisture Active Passive satellite, in addition to climatic wetness and motion information from the Modern-Era Retrospective analysis for Research and Applications Version 2, or MERRA-2, design, which includes satellite observations.

The scientists discovered that on days when wind blows in little climatic wetness, afternoon rains is most likely to take place over wetter soils or greater relative humidity. On days when wind presents great deals of climatic wetness, afternoon rains is most likely over drier soils or lower relative humidity. The group likewise discovered that for both conditions, afternoon rains incident is most likely with warmer early morning soil or air temperature level.

The scientists concentrated on days in which afternoon rains took place and kept in mind the distinction in between the variety of rains days that took place over wetter-than-average soil and the variety of rains days that took place over drier-than-average soil. They then organized their outcomes into 3 classifications: high, mid and low climatic wetness transportation by wind.

This research study constructs on a 2018 research study that determined soil wetness’s function in afternoon rains quantity in the Southern Great Plains of Oklahoma. The brand-new findings reveal that the relationship in between soil wetness, afternoon rains and climatic wetness in Oklahoma doesn’t use throughout the whole 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 stated.

Understanding the function of water vapor in weather condition is very important due to the fact that its results are felt all over, states Welty’s thesis advisor 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,” Zeng stated. Atmospheric rivers are passages of focused water vapor that can rapidly speed up when striking a range of mountains, triggering mass flooding.

“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 stated.

Understanding this relationship is a lot more crucial as international warming modifications patterns of climatic wetness, soil wetness and more. Such modifications will not just have results on weather condition and natural catastrophes, however likewise on farming, Zeng stated.

“The results really show the complexity of the land’s influence on weather and climate,” stated physical researcher 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 enhance weather condition and environment designs. “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 action is to examine how these relationships play out in international environment and weather condition forecasting designs.

“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 stated. “We know it’s true, but we don’t quantitatively know why.”


“Increased Likelihood of Appreciable Afternoon Rainfall Over Wetter or Drier Soils Dependent Upon Atmospheric Dynamic Influence” by Josh Welty,
Susan Stillman, Xubin Zeng and Joseph Santanello Jr., 29 April 2020, Geophysical Research Letters.
DOI: 10.1029/2020GL087779

“Does Soil Moisture Affect Warm Season Precipitation Over the Southern Great Plains?” by J. Welty and X. Zeng, 25 July 2018, Geophysical Research Letters.
DOI: 10.1029/2018GL078598

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