Radiation fog formation alerts using attenuated backscatter power from automatic Lidars and ceilometers

Abstract

Abstract. Radiation fog occurs over many locations around the world in stable atmospheric conditions. Air traffic at busy airports can be significantly disrupted because low visibility at the ground makes it unsafe to take off, land and taxi on the ground. Current numerical weather prediction forecasts are able to predict general conditions favorable for fog formation, but not the exact time or location of fog occurrence. A selected set of observations available in near realtime at strategic locations could also be useful to track the evolution of key processes and key parameters that drive fog formation. Such observations could complement the information predicted by NWP models that is made available to airport forecasters in support of their fog forecast. This paper presents an experimental setup based on collocated automatic lidar and ceilometer measurements, relative humidity measurements and horizontal visibility measurements to study hygroscopic growth of fog condensation nuclei. This process can take several minutes to hours and can be tracked using lidar or ceilometer attenuated backscatter profiles. Based on hygroscopic growth laws we derive a set of parameters that can be used to provide alerts minutes to hours prior to formation of radiation fog. We present an algorithm that uses the temporal evolution of attenuated backscatter measurements to derive pre-fog formation alerts. The performance of the algorithm is tested on 45 independent pre-fog situations at two locations (near Paris, France and Brussels, Belgium). We find that pre-fog alerts occur predominantly 10–50 min prior to fog formation at an altitude ranging 0 to 100 m above ground. In a few cases, alerts can occur up to 100 min prior to fog formation. Alert durations are found to be sensitive to relative humidity conditions found a few hours prior to the fog.

Cross-evaluating WRF-Chem v4.1.2, TROPOMI, APEX, and in situ NO2 measurements over Antwerp, Belgium

“…There are four automatic lidar (light detection and ranging) ceilometer (ALC) monitoring stations in Belgium that have been set up by the RMI to measure the cloud base height and mixing layer height (https://ozone.meteo.be/ instruments-and-observation-techniques/lidar-ceilometer, last access: 9 January 2023). The Vaisala CL51 instrument emits a single-wavelength pulse vertically into the atmosphere and measures the time taken for a backscattered signal to return, proportional to the size and scattering cross section of molecules and particles (Haeffelin et al, 2016). The planetary boundary layer height (PBLH) is retrieved from ceilometer measurements by applying a similar approach to that used in Pal et al (2013).…”

Polarized Laser Backscattering of Atmospheric Cloud Distribution Based on Simulated Annealing Algorithm

“…e algorithm of prefog early warning evolved over time. Experiments show that the fog warning mainly occurs 10 to 50 minutes before the fog is formed, and the height is in the range of 0 to 100 m from the ground (Haeffelin et al) [10]. Gebru et al used the kilohertz optical remote sensing system to perform in situ determination of melanization and backscattering cross sections of near-infrared and short-wave infrared.…”

Observed aerosol characteristics to improve forward-modelled attenuated backscatter in urban areas

“…As RH increases, hydrophilic aerosol particles swell (i.e. increase in size) with water, and the complex index of refraction changes, reaching some state between that of a dry particle and a water droplet (Haeffelin et al, 2016). For hydrophilic species, the parameterised physical growth of each binned diameter follows the CLASSIC aerosol scheme (Bellouin et al, 2011) such that the swollen diameters D wet,aer and dry diameters D dry,aer are calculated using ambient RH.…”