Observing Shallow Fog at Very High Resolution

Conventional in situ observations of meteorological variables are restricted to a limited number of heights near the surface, with the lowest observation often made above 1 m. This can result in missed observations of shallow fog as well as the initial growth stage of thicker fog layers. At the same time, numerical experiments have demonstrated the need for high vertical grid resolution in the surface layer to accurately simulate the onset of fog; this requires correspondingly high-resolution observational data for validation.

Distributed temperature sensing (DTS), which has seen recent technological improvements that allow it to be used for atmospheric studies, has the ability to provide such high-resolution observations in this shallow layer. Using DTS to measure temperature in the lowest 7 m, a two-week field campaign was conducted in November 2017 at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands. The aim was to observe the growth of shallow fog layers and assess the possibility of obtaining very high-resolution observations near the surface during fog events. In addition to the high-resolution observations of temperature a further novel approach was employed to estimate visibility in the lowest 2 m using a camera and an extended light source in the form of an LED strip. These observations were supplemented by the existing observations at the site, including those along a 200 m tall tower.

Comparison between the high-resolution observations and their conventional counterparts show the errors to be small, giving confidence to the reliability of the techniques. The high resolution of the observations subsequently allows for detailed investigations of near surface processes. The growth of fog layers from the ground up was observed with very strong temperature inversions in the lowest metre (up to 5 K), and corresponding region of (super)saturation where the fog formed and grew. Throughout the two-week observation period, fog was observed twice at the conventional sensor height of 1.5 m, but up to four times in the lowest 0-0.5 m using the camera estimates.