Atmospheric visibility inferred from continuous-wave Doppler wind lidar

Atmospheric visibility, or meteorological optical range (MOR), is governed by light extinction by aerosols. State-of-the-art visibility sensors, such as employed in meteorological observatories and airports, infer MOR by measuring either  transmittance or scattering. While these sensors yield robust measurements with reasonable accuracy (10% to 20 %), they measure in situ. MOR from these sensors may thus not be representative of MOR further away, for example, under conditions with stratified aerosol types. This includes off-shore sites near the sea surface during conditions with advection fog, sea spray or mist. Elastic backscatter lidar can be used to measure light extinction and has previously  demonstrated to be a powerful method to infer visibility. Lidar can measure visibility not just near the instrument but also further away (remotely) and single-ended whilst also being capable of measuring profiles of MOR along atmospheric slant paths. Continuous-wave (CW) Doppler wind lidar systems make up one of the most widespread type of elastic backscatter lidar and are typically used in wind resource assessment. Using these existing platforms for remote and single-ended measurement of MOR profiles could allow for new and valuable applications. However, the low-light extinction associated with this type of lidar excludes the use of the extinction coefficient for MOR retrieval but leaves the backscatter
coefficient as a possible proxy for MOR, though with an accuracy expected to be inferior to the former method. We analysed backscatter data from CW wind lidar and comeasured MOR from visibility sensors from two campaigns (Cabauw, Netherlands, and Pershore, United Kingdom) and found backscatter from CW wind lidar to be a viable proxy of MOR if calibrated against a visibility sensor. The expected accuracy of the method is low and of the order of few kilometres. This means MOR from CW wind lidar could be used in safety-uncritical problems, such as assessment of visibility of manmade objects, including wind turbines.