Radiation largely contributes to variations of temperatures across continents, but evaporation and turbulent heat transfer also play a role. These are complex processes. Following new research, the observed temperature patterns follow relatively simple and predictable rules.
A new study by scientists from the Max-Planck-Institute for Biogeochemistry, used a novel approach to determine the contribution of evaporation and turbulent heat transfer to the temperatures across the continents. The surface is heated by solar radiation and by the radiation emitted downward by the atmosphere, the latter also reffered to as atmospheric greenhouse effect. This warming is balanced by the cooling of evaporating water, emitting radiation, and by transferring heat into the atmosphere, which is achieved by convective, turbulent motion.
Although radiation is very well understood and commonly observed, the degree to which evaporation and motion cool the surface is controlled by many factors, and semi-obersvational studies are often used to describe them.
The authors took a different approach to describing these complex processes, using basic physics: A power source is needed to drive motion, similar to how an engine powers the motion of a car. In the case of the atmosphere, the heating of the surface drives the motion. But the consequences of motion also have to be considered.
The researchers used satellite radiation data and implemented the maximum power approach mathematically to estimate heating and cooling rates across continents and seasons. Using this, they predicted temperatures, evaporation and heat fluxes that matched observations extremely well. Then they used this approach to understand why temperatures vary across continents, looking specifically at the role water availability plays and the results supriesed them.
“I thought that the lack of water would make deserts warmer,” said Sarosh Alam Ghausi, lead author of the study. “But we found that the maximization of power was more important than the lack of water. The missing water is compensated for in such a way that more heat was transferred into the atmosphere.”
The increased temperatures in deserts were then attributed to two effects: Desserts have fewer clouds, so that more sunshine heats the surface more strongly than in rainforests. The second effect is more relevant: deserts are typically located in the subtropics, where the atmosphere transports heat horizontally through the so-called Hadley circulation. This heat is not added to the surface where it could power the engine for motion, but to the atmosphere above.
This makes the power generation at the surface less efficient, resulting in decreased cooling and a warmer surface. With these two factors the study was able to explain the temperature variations from rainforests to deserts.
https://phys.org/news/2023-07-surface-temperatures-simple-physics-due.html