How basic physical constraints shape land-atmosphere interactions

Climate over land is strongly shaped by the conditions at the land surface, particularly regarding the partitioning of energy, the availability of water, and the presence of vegetation. What we show here is that a number of key climatological fluxes and variables can be estimated quite accurately simply by applying basic physical constraints. First, heat fluxes are associated mostly with convective motion, which requires work to be done in the form of buoyancy. The generation of this work is subject to a first, physical constraint, the thermodynamic limit of a heat engine. Second, on land, the large differences in solar heating over the course of the day are buffered within the lower atmosphere, and not below the surface as is the case over open water surfaces.  This sets a second constraint. Third, when hydrological aspects are involved, saturation, that is, the thermodynamic equilibrium state, sets another constraint to evaporation and the humidity of air.  We focus on diurnal variations of the surface energy balance, temperature, and humidity over land and compare these to observations to show that these three constraints dominantly shape climatological variations across regions.  What this implies is that physical constraints dominate the functioning of climate over land, and much of this is shaped by the prevalent radiative conditions, with secondary effects relating to soil water availability and advection.  This, in turn, should help us to better distinguish between the important drivers from mere responses in shaping land-atmosphere interactions.