Discriminating Fog and Rain at the Kilometre Scale Using the Extinction from Collocated Infrared and Microwave Scintillometers

We investigate the path-averaged visibility and discrimination of fog and rain events using a two-wavelength (near-infrared and microwave) scintillometer system. These systems are normally used to measure near-surface turbulent heat fluxes on scales of O(1 km). Fog attenuates electromagnetic radiation as a function of the wavelength and droplet-size spectra with a known refractive index. Near-infrared (0.88 μ m) radiation is highly attenuated by fog whereas fog is translucent to microwave (1860 μ m) radiation which propagates with minimal attenuation. During liquid precipitation events, both near-infrared and microwave radiation are attenuated to similar levels. Observations from the Coastal-Fog campaign, dubbed the C-FOG experiment, conducted along the east coast of Newfoundland, Canada, are used. Both near-infrared and microwave scintillometers are used to differentiate between fog, precipitation, and clear-sky conditions along a 1444-m path length. We lay the groundwork for using a two-wavelength scintillometer system for fog–rain discrimination and visibility measurements. The scintillometer provides path-averaged extinction values compared to typical point measurements of visibility. Results suggest that scintillometer data can effectively discriminate between rain and fog as well as provide path-averaged visibility, which was found to be ≈ 10% greater than point-visibility measurements. Measurements of attenuation in the near-infrared and microwave regimes can improve numerical weather prediction of fog visibility by providing ground-based data on the same length scales as typical grid resolutions.