Researchers at UC Davis determine that the lightness of water vapor should not be excluded from climate models
By LILLY ACKERMAN — science@theaggie.org
A recent study from researchers in the UC Davis Department of Land, Air and Water Resources has determined that the lightness of water vapor in the atmosphere has significant impacts on clouds and temperature, and should not be overlooked in climate modeling.
Climate models are an important tool for simulating atmospheric conditions that help predict trends in climate change. However, not all climate models include the same variables in their equations.
Seth Seidel, a co-author of the study and a Ph.D. student in the Atmospheric Science Graduate Group at UC Davis, explained vapor buoyancy, a variable that was the focus of the study.
“Vapor buoyancy derives from the fact that the molecular weight of water vapor is a little bit less than that of dry air,” Seidel said. “So if you take air that’s otherwise dry, and you mix in some water vapor molecules […] it’s going to be less dense. It’s going to be lighter and more buoyant.”
This means that more humid areas of the atmosphere exhibit slightly lower air density than drier areas. According to Siedel, leading up to this study, some major climate models choose not to include vapor buoyancy as a variable in their governing equations because of its minimal effects on air density.
“Buoyancy governs vertical motions [in the atmosphere] and is thus among the most fundamental quantities in understanding atmospheric circulations, clouds and climate,” the study reads. “However, water vapor is a trace gas in Earth’s atmosphere, so [vapor buoyancy] has traditionally been considered small and often neglected in studies of climate dynamics.”
By using reanalysis data and atmospheric models, the authors determined that vapor buoyancy has effects beyond just small differences in air density. It also increases the atmospheric temperature in the tropical troposphere, the lowest layer of the atmosphere, and increases the low cloud fraction, or the amount of low cloud cover, in the subtropics.
“Both quantities [temperature and low cloud fraction] affect Earth’s energy balance and climate,” the study reads. “These biases may amplify with [global] warming due to increasing water vapor and then contribute to uncertainties in predicting future climate change.”
An increase in the amount of low cloud cover is a particularly crucial effect of vapor buoyancy that can heavily influence other climate factors, including lowering temperatures. This is why failure to accurately simulate clouds can lead to a cascade of other inaccuracies that may make a model unreliable.
“Clouds play a critical role in Earth’s energy balance […] Therefore, faithfully modeling clouds is necessary to improve our climate projections,” Seidel said via email. “[Vapor buoyancy] increases the amount of low clouds (‘low cloud fraction’), likely increasing their cooling effect on Earth’s surface.”
With this discovery of vapor buoyancy’s importance in climate modeling, the authors show that models that include vapor buoyancy may be more accurate and that those that do not should consider doing so to more accurately predict climate change trends.
Additionally, the study found that rising air in the tropics is colder than the surrounding air because of vapor buoyancy’s warming effects on the tropical atmosphere. This is an exception to the fact that warm air rises and speaks to the significance of vapor buoyancy’s effects on the atmosphere.
“We hope to build a body of evidence to help persuade the modeling community to include this in their models,” Seidel said. “What this hopefully will give us is a more physically faithful set of climate models in the future.”
Written by: Lilly Ackerman — science@theaggie.org
