Microphysics and Optical Attenuation in Fog: Observations from Two Coastal Sites

Qing Wang,Oswaldo Alvarenga,Harindra Joseph Fernando,Alex Olson,John A Kalogiros,Denny P Alappattu,David G Ortiz-Suslow,Ryan T Yamaguchi,Zachary Daniels,Benjamin J Wauer,Haflidi Jonsson

doi:10.1007/s10546-021-00675-5

Abstract

A total of 15 fog events from two field campaigns are investigated: the High Energy Laser in Fog (HELFOG) project (central California) and the Toward Improving Coastal Fog Prediction (C-FOG) project (Ferryland Newfoundland). Nearly identical sensors were used in both projects to sample fog droplet-size spectra, wind, turbulence, and thermodynamic properties near the surface. Concurrent measurements of visibility were made by the present weather detector in both experiments, with the addition of a two-ended transmissometer in the HELFOG campaign. The analyses focused first on contrasting the observed fog microphysics and the associated thermodynamics from fog events in the two locations. The optical attenuation by fog was investigated using three methods: (1) derived from Mie theory using the measured droplet-size distribution, (2) parametrized as a function of fog liquid water content, and (3) parametrized in terms of total fog droplet number concentration. The consistency of these methods was investigated. The HELFOG data result in an empirical relationship between the meteorological range and liquid water content. Validation of such relationship is problematic using the C-FOG data due to the presence of rain and other factors. The parametrization with droplet number concentration only does not provide a robust visibility calculation since it cannot represent the effects of droplet size on visibility. Finally, a preliminary analysis of the mixed fog/rain case is presented to illustrate the nature of the problem to promote future research.

Highlights

Both fog and mist are water droplets suspended in the near-surface air that result in reduced visibility
Results of visibility variation with liquid water content (LWC) from in Coastal Fog Prediction (C-FOG) non-precipitating fog events are shown in Fig. 11
At LWC greater than 0.02 g cm−3, visibility was significantly reduced in most cases and appears to be consistent with the observations in High Energy Laser in Fog (HELFOG)

Summary

Introduction

Both fog and mist are water droplets suspended in the near-surface air that result in reduced visibility. The Glossary of Meteorology (AMS 2020) defines fog as when visibility is less than 1 km, while visibility in mist is greater than 1 km. An upper limit of visibility to define mist or light fog was set at 5 km (Meyer et al 1980), above which the reduction of visibility is attributed to haze with dry or activated aerosols. The commonly-used variables to describe the physical characteristics of fog/mist include the droplet size spectra, total droplet number concentration, liquid water content (LWC), and characteristics of droplet size parameters such as effective or mean droplet diameters. The thickness of the fog layer is used as one of the macroscopic properties of the hygrometer layer. The focus on the range of visibility is between 0 and 5 km

Methods

Results

Conclusion