Missed Fog?

Abstract

Conventional in situ observations of meteorological variables are restricted to a limited number of levels near the surface, with the lowest observation often made around 1-m height. This can result in missed observations of both shallow fog, and 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 near-surface layer to accurately simulate the onset of fog; this requires correspondingly high-resolution observational data for validation. 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. Temperature and relative humidity were measured at centimetre resolution in the lowest 7 m using distributed temperature sensing. Further, a novel approach was employed to estimate visibility in the lowest 2.5 m using a camera and an extended light source. These observations were supplemented by the existing conventional sensors at the site, including those along a 200-m tall tower. Comparison between the increased-resolution observations and their conventional counterparts show the errors to be small, giving confidence in the reliability of the techniques. The increased resolution of the observations subsequently allows for detailed investigations of fog growth and evolution. This includes the observation of large temperature inversions in the lowest metre (up to 5 K) and corresponding regions of (super)saturation where the fog formed. Throughout the two-week observation period, fog was observed twice at the conventional sensor height of 2.0 m. Two additional low-visibility events were observed in the lowest 0–0.5 m using the camera-based observations, but were missed by the conventional sensors. The camera observations also showed the growth of shallow radiation fog, forming in the lowest 0.5 m as early as two hours before it was observed at the conventional height of 2 m.

Suitability of fibre-optic distributed temperature sensing for revealing mixing processes and higher-order moments at the forest–air interface

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Decoupling of a Douglas fir canopy: a look into the subcanopy with continuous vertical temperature profiles

“…In all analyses we only make use of the dry adiabatic lapse rate. Condensation of moisture can contribute to convection in forests (Jiménez-Rodríguez et al, 2020), but this effect has not been taken into account in this study due to lack of accurate data.…”

Direct Numerical Simulation of the Moist Stably Stratified Surface Layer: Turbulence and Fog Formation

“…For typical values of ∼ 10 −3 m 2 s −3 then L Oz is of the same order as L b . Recent high-resolution field observations have also noted substantial temperature gradients close to the ground during fog formation with N ≈ 0.4 s −1 (Izett et al 2019), suggesting L B and L Oz may be even smaller. Ultimately, this implies that LES, with horizontal grid spacings of several metres and at best vertical resolutions of 1 m (e.g.…”