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Results from a series of boundary layer measurements carried out at Ashchurch, Worcestershire during July 1976 are combined with those from the 1973 Minnesota experiment. This data set provides a more complete description of the behaviour of some turbulence statistics through the depth of the convective boundary layer and into the stable air of the free atmosphere. Although the two experimental regions differ quite markedly topographically, the two sets of data are found to merge together quite well in the middle of the boundary layer and do not reveal any systematic differences that might be attributable to surface effects. The vertical profiles of turbulence statistics are compared, where possible, with other results from numerical models and laboratory experiments.

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SUMMARYThe results of a field study of the evolution of radiation fog on three nights are presented. A parallel numerical model (Brown and Roach 1976 -referred to hereafter as 11) was also developed to test ideas suggested by the principal features observed during the field study. These were:(i) Periods of significant fog development appeared to occur when wind speeds dropped below 05-1 ms-l .(ii) The liquid water content of the fog was a small fraction of the total condensed out by cooling. The balance of water appears to have been deposited on the ground.(iii) Radiative cooling (deduced from radiative flux divergence measurements) was in general greater than the actual cooling.(iv) As the surface became radiatively shielded by developing fog, the radiation inversion migrated to the fog top, accompanied by the establishment of a convective regime with a slight superadiabatic lapse rate in the lower part of the fog.(v) Ammonium sulphate was a principal constituent of condensation nuclei. It is shown that these features are consistent with the suggestion that the development of radiation fog is primarily controlled by a balance between radiative cooling, which encourages fog, and turbulence, which inhibits it. Gravitational settling of fog droplets and soil heat flux also emerge as important factors. The role of cloud microphysics is not passive, but is less clearly defined as yet. The numerical model (11) reproduces most of the observed features, but not the stepwise growth or sharpness of the fog top.Some of the practical implications of this work for forecasting and for fog modification are briefly discussed.

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