No abstract

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.

Results from an investigation of the turbulence structure of the lower half of the convective boundary layer over the sea around the UK in a wide variety of meteorological conditions are presented. The data were obtained OR eight flights made by the Hercules aircraft of the Meteorological Research Flight.Differences in structure between boundary layers over sea and over land are emphasized, the most notable being the relatively increased importance of mechanically driven mixing over the sea. This means that a more general scaling scheme is required which retains both u* and I, as scaling parameters. Such a scheme orders the results quite effectively. Dimensionless profiles of momentum, sensible heat and latent heat fluxes are presented, together with dimensionless variance profiles. Excellent agreement is found between these and theoretical predictions from numerical models. Surface fluxes are estimated and compared with bulk aerodynamic formulae.The turbulence statistics are shown to be in good agreement with those published by other workers.

No abstract

This paper describes some of the results that were obtained in an experimental study of an inversion capping a convective boundary layer, during which a balloon-borne probe was flown through the actual interface at the same time as it was scanned by a high power 10-cm Doppler radar. The initial analysis has revealed several interesting features including the possible presence of small-scale Kelvin-Helmholtz billows at the crest of the 'hummocks' of the inversion and the absence of an inertial subrange. A model is proposed for the transfer of sensible heat downwards across the inversion. This is hypothesised as a two-stage process -sporadic thickening by the entrainment of air from above and below due to the action of the breaking billows, and a continual erosion of the lower surface by convective activity.