Using Large-Aperture Scintillometry to Observe Path Integrated Fog Intensity During the C-FOG Campaign

Abstract:

Measurements within fog are non-trivial: water droplet accumulation on sensors and the general high water-vapor concentration can cause errant data. Devices operating in the near infra-red (NIR, 880 nm) and microwave (MW, 1.88 mm) regions of the electromagnetic spectrum have their signal attenuated by the excessive moisture and hydrometeors in the air. Scintillometers operating at those wavelengths have a degraded and sporadic signal during foggy conditions. With the addition of in-situ meteorological and particle-size distribution data, a two-wavelength scintillometer system can provide information on visibility and fog evolution at scales of approximately one kilometer. This scale coincides with the resolution currently used in many Numerical Weather Prediction (NWP) models, allowing scintillometers to bridge the resolution gap between ground-based point measurements and NWP model grid scales.

The data for this analysis comes from the C-FOG (Coastal-Fog prediction) project that was conducted along the eastern Canadian coast in Newfoundland during Aug 25-Oct 07 2018. The scintillometer system spanned 1.5 km across a bay between the Battery and The Downs sites at Ferryland, Newfoundland. Battery was at sea level while The Downs was ~ 20 m higher. Both sites had 15-m masts with five levels of turbulence measurements near the scintillometer receivers and transmitters. An array of optical particle counters at Battery measured aerosol and hydrometeor diameters from 0.3 μm to 1 cm, allowing for a robust analysis of the effects of hydrometeors on signal attenuation.

The focus of this study is on IOP 7 (16-17 September 2018). IOP 7 was a short, patchy-fog event during which the scintillometer signal intensity responded to visibility degradation indicated by multiple Present Weather Detectors (PWDs) stationed throughout the Ferryland site. A fixed optical camera pointed along the scintillometer path observed a low-level stratus blanketing The Downs site. The PWDs reported fog for only a short period of time while the scintillometer system indicated a longer period of reduced visibility. We explore the differences between point and path averaged measurements through turbulence and mean meteorological quantity variations between The Downs and Battery sites.