LANFEX: A Field and Modeling Study to Improve Our Understanding and Forecasting of Radiation Fog

Abstract

Fog is a high-impact weather phenomenon affecting human activity, including aviation, transport, and health. Its prediction is a longstanding issue for weather forecast models. The success of a forecast depends on complex interactions among various meteorological and topographical parameters; even very small changes in some of these can determine the difference between thick fog and good visibility. This makes prediction of fog one of the most challenging goals for numerical weather prediction. The Local and Nonlocal Fog Experiment (LANFEX) is an attempt to improve our understanding of radiation fog formation through a combined field and numerical study. The 18-month field trial was deployed in the United Kingdom with an extensive range of equipment, including some novel measurements (e.g., dew measurement and thermal imaging). In a hilly area we instrumented flux towers in four adjacent valleys to observe the evolution of similar, but crucially different, meteorological conditions at the different sites. We correlated these with the formation and evolution of fog. The results indicate new quantitative insight into the subtle turbulent conditions required for the formation of radiation fog within a stable boundary layer. Modeling studies have also been conducted, concentrating on high-resolution forecast models and research models from 1.5-km to 100-m resolution. Early results show that models with a resolution of around 100 m are capable of reproducing the local-scale variability that can lead to the onset and development of radiation fog, and also have identified deficiencies in aerosol activation, turbulence, and cloud micro- and macrophysics, in model parameterizations.

Numerical modelling of the evolution of the boundary layer during a radiation fog event

“…The humidity perturbation experiments were performed by perturbing the relative humidity (RH) by ±3% above the stable surface layer at 42m at 1700 utc . This falls within the 2–5% RH uncertainty of the radiosonde relative humidity measurements during the LANFEX campaign (Price et al ., ). Finally, a combination of both the humidity and wind perturbations was performed to assess the interaction between the two.…”

Aerosol-fog interaction and the transition to well-mixed radiation fog

“…The LES also provides the opportunity to explore the mechanisms leading to cloud droplet activation (or lack thereof) in the fog, and therefore suggest improvements to the NWP model. suggested in Price et al (2017) for the threshold value above which fog will not form. What this implies is that the peak updraft speed driving aerosol activation is very low, typically <0.1 ms…”

How important are aerosol–fog interactions for the successful modelling of nocturnal radiation fog?

“…The result is that the fog layer becomes well‐mixed (with a constant temperature profile within the layer) through convection, increasing its optical thickness. However, the turbulence levels and the humidity profile during fog formation can result in the fog remaining optically thin (Price et al , ).…”