Determining the radiative and hydrologic controls on the diurnal air-temperature range using the thermodynamic limit of maximum power

The diurnal air temperature range (DTR) is strongly shaped by solar radiation but is modulated by hydrologic cycling through changes in atmospheric (clouds) and land-surface (evaporation) characteristics. Here, we aim to determine the distinct patterns in DTR over dry and wet periods and identify their respective controls. To do this, we develop a simple energy balance model that constrains the land-atmosphere exchange using the thermodynamic limit of maximum power. In this framework, we explicitly account for changes in radiative conditions due to clouds and changes in boundary layer heat storage associated with surface water limitation, both of which affect the maximum power limit. Using observations of radiative forcings and surface evaporation, our model predicts DTR reasonably well across 81 FLUXNET sites in North America, Europe, and Australia. We show that DTR is primarily shaped by the trade-off between the heat gain due to solar absorption and heat lost at the surface due to evaporation. Radiation remains a primary control on DTR over very dry and wet conditions where evaporation is either close to zero or limited by available energy. Over these regions, changes in DTR are strongly modulated by clouds which alters the radiative conditions. DTR becomes coupled to the land surface during the transition regime where changes in surface water availability directly control the evaporation rates. Over these regions, increased soil moisture results in more evaporation and reduced DTR. These responses were consistent in both, observations and maximum power estimates. We then apply our framework to quantify the response of DTR to global warming. Our model projects a decrease in DTR by 0.18K for a 1K rise in global temperature, which is consistent with the current observed response. Our findings imply that the predominant controls on DTR are set by clouds and evaporation as they directly modulate the diurnal heating of the lower atmosphere and can be further altered by increased greenhouse forcing.