How much water evaporates from the ocean surface is an important factor in climate projections. Evaporation rates in existing climate models do not match measurements taken at the ocean surface. A study led by CSU Department of Atmospheric Science researcher Charlotte DeMott, and funded by the National Oceanic and Atmospheric Administration, aims to bridge the gap between observations and models, improving the accuracy of climate projections.
The evaporation rates calculated by climate models might not be far off from the true rate, but slight differences in evaporation can impact clouds significantly. DeMott and collaborator Carol Anne Clayson, a scientist with the Woods Hole Oceanographic Institution, will evaluate how small changes in the way we compute ocean surface evaporation in climate models affect our understanding of clouds – an important consideration in predicting climate.
Clouds are key to climate models. They cool the planet by reflecting sunlight back to space, preventing it from reaching Earth’s surface. They also insulate the Earth, preventing heat from escaping to space. Cloud cover is determined, in part, by the amount of water vapor in the air.
Ocean surface evaporation varies based on wind, temperature and humidity of the air over the surface. Climate models use slightly different methods, or algorithms, to estimate evaporation according to these environmental factors. These algorithm differences result in different evaporation rates.
“Our project seeks to understand how these algorithm differences contribute to differences in cloud patterns among climate models and the uncertainties surrounding how clouds regulate the Earth’s temperature, both today and in the future,” said DeMott, principal investigator on the project.
DeMott and Clayson will use surface evaporation measurements to determine where the models are going wrong in calculating evaporation rate. Once they have dialed in an accurate evaporation algorithm, the researchers will apply it to a climate model and see how it compares to the original algorithm in predicting cloud effects. Depending on which algorithm most accurately predicts cloud effects, they will know whether the improved evaporation algorithm helps reduce uncertainties in climate projections.
Slight differences in evaporation can affect how tall clouds become, how much they heat the air and how long they last, all of which can have a big impact on how much sunlight they reflect or planetary heat loss they prevent.
“We expect that changes to the frequency and vertical structure of clouds for different evaporation algorithms will yield significant differences in globally averaged cloud effects,” DeMott said.
This project brings together one scientist with expertise in evaluating model representations of ocean-atmosphere interactions (DeMott) and one scientist with expertise in surface evaporation measurements and algorithms (Clayson). By connecting the two perspectives, DeMott and Clayson will create a framework for comparing observed and modeled ocean surface evaporation that can be used to develop future models.
