Abstract
Abstract. An improved version of the near-real-time decision tool PARAFOG (PFG2) is presented to retrieve pre-fog alert levels and to discriminate between radiation (RAD) and stratus lowering (STL) fog situations. PFG2 has two distinct modules to monitor the physical processes involved in RAD and STL fog formation and is evaluated at European sites. The modules are based on innovative fuzzy logic algorithms to retrieve fog alert levels (low, moderate, high) specific to RAD/STL conditions, minutes to hours prior to fog onset. The PFG2-RAD module assesses also the thickness of the fog. Both the PFG2-RAD and PFG2-STL modules rely on the combination of visibility observations and automatic lidar and ceilometer (ALC) measurements. The overall performance of the PFG2-RAD and PFG2-STL modules is evaluated based on 9 years of measurements at the SIRTA (Instrumented Site for Atmospheric Remote Sensing Research) observatory near Paris and up to two fog seasons at the Paris-Roissy, Vienna, Munich, and Zurich airports. At all sites, pre-fog alert levels retrieved by PFG2 are found to be consistent with the local weather analysis. The advanced PFG2 algorithm performs with a hit rate of about 100 % for both considered fog types and presents a false alarm ratio on the order of 10 % (30 %) for RAD (STL) fog situations. Finally, the first high alerts that result in a subsequent fog event are found to occur for periods of time ranging from −120 min to fog onset, with the first high alerts occurring earlier for RAD than STL cases.
Highlights
According to the World Meteorological Organization (2021), fog occurs when visibility at the Earth’s surface is lower than 1 km due to the presence of suspended water droplets
We only focus on fog events that correspond to RAD or stratus lowering (STL), which represent more than 90 % of cases regardless of the sites considered
A second version of PARAFOG (PFG2) has been developed to retrieve pre-fog alert levels and to discriminate between RAD and STL fog situations based on the analysis of ALC and meteorological station measurements in near real time
Summary
According to the World Meteorological Organization (2021), fog occurs when visibility at the Earth’s surface is lower than 1 km due to the presence of suspended water droplets. Kneringer et al (2019) and Dietz et al (2019) developed probabilistic fog nowcasting systems to forecast different low-visibility procedures from standard meteorological measurements available at Vienna international airport for lead times of +30 to +120 min While both the aforementioned satellite- and learn-based studies do not intend to track the evolution of particular physical processes driving fog formation, ground-based observations may provide valuable key information by monitoring their true values, complementing NWP models. Haeffelin et al (2016) developed the near-real-time fog analysis tool PARAFOG (hereafter referred to as PFG1), with the objective to predict radiation fog formation based on ALC measurements, together with classical meteorological observations.
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