Radiative controls by clouds and thermodynamics shape surface temperatures and turbulent fluxes over land

• Published in: Proceedings of the National Academy of Sciences - PNAS 2023-07, Vol.120 (29), p.e2220400120-e2220400120
• Main Authors: Ghausi, Sarosh Alam, Tian, Yinglin, Zehe, Erwin, Kleidon, Axel
• Format: Article
• Language: English
• Subjects: Arid regions; Arid zones; Boundary layers; Climate system; Clouds; Constraints; Enthalpy; Evaporation; Evaporative cooling; Heat flux; Heating; Hydrology; Land surface temperature; Latent heat; Physical Sciences; Radiation; Satellite observation; Sensible heat; Solar radiation; Surface temperature; Thermodynamics; Water vapor
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.2220400120

Land surface temperatures (LSTs) are strongly shaped by radiation but are modulated by turbulent fluxes and hydrologic cycling as the presence of water vapor in the atmosphere (clouds) and at the surface (evaporation) affects temperatures across regions. Here, we used a thermodynamic systems framework forced with independent observations to show that the climatological variations in LSTs across dry and humid regions are mainly mediated through radiative effects. We first show that the turbulent fluxes of sensible and latent heat are constrained by thermodynamics and the local radiative conditions. This constraint arises from the ability of radiative heating at the surface to perform work to maintain turbulent fluxes and sustain vertical mixing within the convective boundary layer. This implies that reduced evaporative cooling in dry regions is then compensated for by an increased sensible heat flux and buoyancy, which is consistent with observations. We show that the mean temperature variation across dry and humid regions is mainly controlled by clouds that reduce surface heating by solar radiation. Using satellite observations for cloudy and clear-sky conditions, we show that clouds cool the land surface over humid regions by up to 7 K, while in arid regions, this effect is absent due to the lack of clouds. We conclude that radiation and thermodynamic limits are the primary controls on LSTs and turbulent flux exchange which leads to an emergent simplicity in the observed climatological patterns within the complex climate system.