A THEORY OF THE LIFTED TEMPERATURE MINIMUM ON CALM CLEAR NIGHTS

Numerous reports from several parts of the world have confirmed that o n calm clear nights a minimum in air temperature can occur just above ground, at heights of the order of 1/2 m or less. This phenomenon, fir st observed by Ramdas & Atmanathan (1932), carries the associated para dox of an apparently unstable layer that sustains itself for several h ours, and has not so far been satisfactorily explained. We formulate h ere a theory that considers energy balance between radiation, conducti on and free or forced convection in humid air, with surface temperatur e, humidity and wind incorporated into an appropriate mathematical mod el as parameters. A complete numerical solution of the coupled air-soi l problem is used to validate an approach that specifies the surface t emperature boundary condition through a cooling rate parameter. Utiliz ing a flux-emissivity scheme for computing radiative transfer, the mod el is numerically solved for various values of turbulent friction velo city. It is shown that a lifted minimum is predicted by the model for values of ground emissivity not too close to unity, and for sufficient ly low surface cooling rates and eddy transport. Agreement with observ ation for reasonable values of the parameters is demonstrated. A heuri stic argument is offered to show that radiation substantially increase s the critical Rayleigh number for convection, thus circumventing or w eakening Rayleigh-Benard instability. The model highlights the key rol e played by two parameters generally ignored in explanations of the ph enomenon, namely surface emissivity and soil thermal conductivity, and shows that it is unnecessary to invoke the presence of such particula te constituents as haze to produce a lifted minimum.