No abstract

A semi‐empirical treatment of the vertical diffusion of matter in the lowest layers of the atmosphere is given in the following and shown to be in good agreement with the results of carefully conducted field experiments made over two different types of natural land surface. The present work extends the treatment of diffusion in an adiabatic atmosphere given by Calder (1948) by allowing for the effects of thermal stratification. The treatment is based on the results of an extensive investigation of vertical profiles of mean wind velocity in the lowest ten metres of the atmosphere (mainly carried out at the Chemical Defence Experimental Station, Porton, Salisbury Plain). A complete account of this study will probably be published as a Geophysical Memoir of the Meteorological Office.

A treatment based on Reichardt's formulation of the velocity profile in turbulent flow over a smooth plane surface is shown to give good agreement with published data on the boundary layer transfer of heat and mass over a wide range of Prandtl (or Schmidt) number, s̀. Applied to transfer to a water surface, agreement with published laboratory results is also good for low air speeds (smooth water). Comparison with observations for the sea shows there is little difference between the calculated evaporation coefficient and those reported for the sea with winds of ∼7 m s−1. This is consistent with sea trials having so far detected little increase of evaporation and heat transfer coefficients with wind speed over the range 4–10 m s−1. The air/water transfer of non‐reactive gas is governed by the resistance of the viscous sublayer of water, and the smooth surface treatment gives the transfer velocity (VL) on a liquid phase basis as VL = 0.082 (pa/pw)1/2s̀−2/3u* where papw is the density ratio, air/water, and u* = friction velocity of the surface air flow. The agreement between this formula and published wind tunnel results is good for the smooth water condition. At higher wind speeds transfer exceeds the calculated value and appears then to increase roughly in proportion to the square of the wind speed.

The rates of transfer of radon from sea to air estimated by Peng et al. (1979) from an extensive series of observations of radon protiles made during the Geosecs oceanographical cruises, are reexamined in relation to wind speed dependence. It is concluded that there is a significant increase with wind speed, but the extent of the increase is uncertain. At 7 m s -I, the transfer velocity is indicated to be some 33% greater than the BOMEX value of Broecker and Peng (1974) -a reasonably close agreement. These rates exceed the theoretical smooth-surface value by a factor of two or three. It is shown that little of this excess can be attributed to the surface dilation effect of capillary waves.

Data on the relationship of the surface wind to the geostrophic wind at Porton Down, Salisbury Plain, are presented for various stability conditions and analysed in the light of the Rossbynumber similarity theory. For near-neutral conditions, the geostrophic drag coefficients for geostrophic wind speeds 5 to 15 m s -l are close to those found by other workers but at higher speeds the values are low. Comparisons of geostrophic and radar wind speeds for 900-m height, suggest that undetectably small mean cyclonic curvatures of the trajectories of the air are responsible for this departure.A value of the geostrophic drag coefficient for the open sea at wind speeds around 8 m s -1 (neutral conditions) is deduced from recent observations of the drag in relation to the surface wind, combined with the ratios of 900-mb radar wind to surface wind obtained from the North Atlantic weather ship data tabulations of Findlater et al. (1966).