The dynamical behaviour of the lifted temperature minimum

Abstract

This paper concerns a phenomenon called the Ramdas Paradox by Lettau, referring to the occurrence of a minimum in the temperature some decimetres above ground on calm clear nights. Recently, Vasudeva Murthy, Srinivasan and Narasimha have proposed a theory that successfully reproduces the observed minima. We extend that work here to investigate the time evolution of near-ground temperature distributions through numerical simulations. It is demonstrated that, if the surface emissivity is not too close to unity, a lifted temperature minimum can appear shortly after sunset, but its subsequent evolution, depending strongly on ground cooling rate, can lead to i) monotonic growth, ii) near-steady state, or iii) growth followed by collapse. Solutions of the model to an appropriately formulated turbulent transport episode reveal that the lifted temperature minimum disappears after the commencement and reappears after the cessation of the gust in times of order 10-20 s, in qualitative agreement with the observations of Raschke. However, full recovery to the no-gust state takes times of order 10³ s after the episode. This behaviour is identified with a two-time response of the cold layer, involving a quick radiative adjustment followed by a slow diffusive relaxation. It is concluded that the VSN model fully reproduces the observed dynamical behaviour of the lifted temperature minimum, and provides a satisfactory basis for the study of the near-ground thermal environment.