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

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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 φH and φW. Data for φH were available from all five expeditions, and for φW from two of these.It was found that φH = φW 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 φH and φW 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 δθ3/2 and E = 51.7 δq δθ1/2 where E and H are in mWcm−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, φH was found to vary as |z/L|−1/2. Insufficient data limited the value of the corresponding analysis for φW Excellent agreement was found between the φH and φW 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 φH and φW over the whole of the |z/L| range to within a few per cent, thus permitting numerical evaluation of φ.

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