The physics of radiation fog: I – a field study

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.

On the formation of radiation fogs under heavily polluted conditions

“…Our preliminary cloud model calculations (Palonen, 2000) indicated that cooling rates on the order of 1 K/hour at 278.15 K and 1000 mbar are required for the unactivated cloud formation to occur due to the uptake of HNO 3 . Such cooling rates have been observed for radiation fogs (Roach et al, 1976).…”

Fog Spatial Distributions over the Central Namib Desert - An Isotope Approach

“…7). This fog may have been formed by radiative cooling of moisture derived from local waters as wind directions were variable among sites while wind speeds at each site were consistent with those associated with radiation fog (< 2.5 m s -1 ) (Roach et al, 1976;Meyer and Lala, 1990;Tardif and Rasmussen, 2007) (Table S3). Or, this fog was formed by the perpetuation of mixed fog inland with addition of local moisture along its trajectory similar to that proposed for the 18 th June 2016.…”

“…Therefore we did not characterize mixed fog because the Gobabeb fog sample was the only unevaporated mixed fog sample, d (+5.5‰). Both mixed fog and radiation fog require similar conditions for formation e.g., radiative cooling and calm winds (< 2.5 m s -1 ) (Roach et al, 1976;Meyer and Lala, 1990;Tardif and Rasmussen, 2007), conditions that were prevalent during the fog events at all sites (Table S3). However, there was a noticeable difference in the degree of radiative cooling and RH during the fog event between the mixed fog sites and the radiation fog sites.…”

Analysis of turbulence in fog episodes

“…An increase of the screen temperature after the onset of fog, attributed to the unstabilisation of the saturated layers and the movement of the thermal inversion from the ground to the top of the fog layer, is commonly observed (Roach et al, 1976). The particularity of the described case is that the fog layer deepens during several hours -at least until the sunrise -at a regular rate, causing a progressive increase of the low-level temperature that lasts for hours.…”

“…However, the role of turbulence in the evolution of radiation fog remains as one of the most controversial points. Whereas several authors, such as Roach et al (1976) state that turbulence is a factor that inhibits the onset of radiation fog, others (e.g. Welch and Welicki, 1986) support the theory that turbulence constitutes a contributing factor.…”