The turbulent transport of heat and water vapour in an unstable atmosphere

Abstract

Abstract Micro‐meteorological data acquired during five expeditions to carefully chosen sites are analysed to determine the flux‐gradient relation for the transfer of heat and water vapour in the lower atmosphere. The analysis takes the form of a direct assessment of the Monin‐Obukhov universal functions φ H and φ W . Data for φ H were available from all five expeditions, and for φ W from two of these. It was found that φ H = φ W over the whole of the z / L range available, indicating an identity in mechanism for the turbulent transport of heat and water vapour over a freely evaporating surface. Over the range 0.02 < | z / L | < 0.6, both φ H and φ W were found to vary approximately as | z / L | −1/3 . In this range of | z / L |, Priestley's H * was assessed as 1.15·0.07 and Crawford's E * as 1.10·0.12. This value of H * leads to the formulae H = 21.2 δθ 3/2 and E = 51.7 δq δθ 1/2 where E and H are in m Wcm −2 , δθ is the potential temperature difference (°C) and δq the specific humidity difference (gm kg −1 ) between 1 and 4 metres height. For | z / L | > 0.2, φ H was found to vary as | z / L | −1/2 . Insufficient data limited the value of the corresponding analysis for φ W Excellent agreement was found between the φ H and φ W data, and φ‐curves assessed from a previous shape‐function analysis (Swinbank and Dyer 1967). The empirical relation φ = (1–15 z / L ) −0·55 agrees with the experimental values of φ H and φ W over the whole of the | z / L | range to within a few per cent, thus permitting numerical evaluation of φ.