Radiation largely shapes temperature variations across continents, but evaporation and turbulent heat transfer also play their part. These are inherently complex processes. Following a new physical approach, the observed temperature patterns follow relatively simple and predictable rules.
A new study has been published Proceedings of the National Academy of Sciences Scientists at the Max-Planck-Institute for Biogeochemistry in Jena, Germany, used a new approach to determine the contribution of evaporation and turbulent heat transfer to temperatures across continents. Temperature on land reflects the balance of hot and cold. The surface is warmed by solar radiation and radiation emitted downward by the atmosphere, the latter also known as the atmospheric greenhouse effect. This warming is balanced by radiative cooling, evaporation of water, and transfer of heat to the atmosphere, accomplished by convective, turbulent motion.
While radiation is fairly well understood and commonly observed, the extent to which evaporation and movement cool the surface is controlled by many factors, and semi-empirical approaches are often used to describe them.
The authors took a different approach to describe these complex processes, drawing on basic physics: a power source is needed to drive motion, similar to how an engine drives a car. In the case of the atmosphere, surface heat powers the motion. But the consequences of movement must also be considered.
Sarosh Alam Ghousi, lead author of the study, explains, „With more movement, you cool the surface more. It’s like blowing on hot soup—the more you blow, the faster it cools.” But this cooling makes the power generation process less efficient, resulting in lower maximum power. This maximum can be used to calculate the cooling effects of evaporation and movement on land surface temperature.
The researchers used satellite-derived radiation datasets and implemented a mathematical maximum power approach to estimate rates of warming and cooling across continents and seasons. With this, they predicted temperatures, evaporation, and heat fluxes that matched observations remarkably well. They used this approach to understand why temperatures vary across continents, looking specifically at the role that water reserves play. But they did not get what they expected.
„I used to think deserts would get hotter because of the lack of water,” says Gousy, with a background as a hydrologist. „But we found that increasing energy is more important than water scarcity. The missing water is compensated by transferring more heat to the atmosphere.”
Warmer temperatures in deserts are due to two effects: Deserts have fewer clouds, so more sunlight heats the surface more strongly than in rainforests. The second effect is less trivial: Deserts are typically located in the subtropics, where the atmosphere transports heat horizontally through the so-called Hadley circulation. This heat is added not to the surface on which the engine is driven, but to the atmosphere above.
This makes the power generation process at the surface less efficient, resulting in less cooling and a hotter surface. With these two factors the authors were able to explain the temperature variations from rainforests to deserts.
Erwin Zehe, professor of hydrology at the Karlsruhe Institute of Technology and co-author of the study, sees great potential in this approach. „Our findings are really surprising because evaporation is generally considered the key to cooling the atmosphere. And I imagine this approach can move things forward by setting a new gold standard, improving today’s empirical approaches to model evaporation.”
Axel Gleidon, group leader at the Max-Planck-Institute for Biogeochemistry and senior author of the study, explains these results in a more general way. „Why this simple, yet unphysical approach works so well is not clear. One way to understand it is that these processes are so complex that the ultimate limit they face lies in the physics of power generation.”
The authors hope that their approach can be used more widely to identify the fundamental mechanisms that shape the climate around us and how they respond to global warming.
Sarosh Alam Ghousi et al., Radiative controls by clouds and thermodynamics shape surface temperature and turbulent fluxes over land, Proceedings of the National Academy of Sciences (2023) DOI: 10.1073/pnas.2220400120
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