Radiation fog occurs over many locations around the world in stableatmospheric conditions. Air traffic at busy airports can be significantlydisrupted because low visibility at the ground makes it unsafe to take off,land and taxi on the ground. Current numerical weather prediction forecastsare able to predict general conditions favorable for fog formation, but notthe exact time or location of fog occurrence. A selected set of observationsavailable in near-real time at strategic locations could also be useful totrack the evolution of key processes and key parameters that drive fogformation. Such observations could complement the information predicted bynumerical weather prediction (NWP) models that is made available to airport forecasters in support of theirfog forecast. This paper presents an experimental setup based on collocatedautomatic lidar and ceilometer measurements, relative humidity measurementsand horizontal visibility measurements to study hygroscopic growth of fogcondensation nuclei. This process can take several minutes to hours, and canbe tracked using lidar- or ceilometer-attenuated backscatter profiles. Basedon hygroscopic growth laws we derive a set of parameters that can be used toprovide alerts minutes to hours prior to formation of radiation fog. Wepresent an algorithm that uses the temporal evolution of attenuatedbackscatter measurements to derive pre-fog formation alerts. The performanceof the algorithm is tested on 45 independent pre-fog situations at twolocations (near Paris, France, and Brussels, Belgium). We find that an alertfor pre-fog conditions predominantly occurs 10–50 min prior to fog formationat an altitude ranging 0 to 100 m above ground. In a few cases, alerts canoccur up to 100 min prior to fog formation. Alert durations are found to besensitive to the relative humidity conditions found a few hours prior to thefog.
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