Dew, frost, fog and lifted temperature minima: Observations in southern England and implications for modelling

Given difficulties with modelling radiation fog and the similarity of meteorological conditions linked to dewfall and frost we investigated the formation of dew, frost and fog. For a site in the UK seven years of data were analysed representing high-resolution atmospheric profiles and dew meter measurements for radiation nights with stable conditions. Classical dewfall occurs by condensation when the surface is below the dew point and cooler than the air above. However, the profiles show that, in the absence of fog, typically dew and frost form with the surface warmer than the immediately overlying air due to lifted temperature minima (LTMs) at about 0.15 m. Observations of aerosol number density and average hydrated radii show that aerosol optical extinction (and hence their radiative effect) is weakly but significantly correlated with the intensity of LTMs. Low wind speed on stable nights allows settling of aerosols which radiatively cool the air near the ground more quickly than the surface cools - thus creating LTMs. In the presence of LTMs typically dew and frost form not by condensation, but by occult deposition of water droplets onto the canopy and ground. Among radiation fog observations, 91% are associated with light near-surface winds and LTMs. When the rate of removal of suspended water droplets by occult deposition generating dew or frost is too slow, then build-up of droplets in the air just above the surface leads to the formation of radiation fog. Future modelling should allow for the accumulation of near-surface aerosols and their radiative effects during stable nights to represent the formation of LTMs. Modelling of typical dew and frost will require representation of occult deposition. Assessing rates of occult deposition compared to rates of generation of suspended water droplets is needed to forecast the onset of radiation fog formed near the ground. In stable nocturnal conditions most radiation fog forms when the air near the surface is cooler than the grass canopy and soil, that is, during a lifted temperature minimum (LTM). Observations show LTMs result from low turbulence plus radiative cooling of the air by near-surface aerosols. Under LTMs most dewfall occurs by water droplet collision with the surface (occult deposition) rather than condensation. Radiation fog forms when occult deposition is too slow to prevent increasing water droplet concentration above the surface. image