Long-term persistence, invariant time scales and on-off intermittency of fog events

Abstract In this work we study different characteristics of fog long-term persistence, in events with different physical formation mechanisms. Specifically, we focus on the characterization of fog long-term persistence from observational data, by means of a Detrended Fluctuation Analysis (DFA) of its associated low-visibility time series. We analyze fog events with radiation and orographic underlying physical formation mechanisms, and identify a two-range pattern of long-term persistence. Our analysis leads to the emergence of a characteristic time, τ∗, at the crossover point between different scaling exponents in the DFA, independent of the time scale at which the fog event is studied. We also show that these scale-invariant characteristic times are related to the duration of fog/no-fog periods, what suggests that τ∗ is a good measure to study and compare fog events persistence, and also that fog events can be studied as on-off intermittent processes. We apply a binarizing process and confirm that the two-range shape structure in the DFA is maintained in the binary time series associated to fog events. We also show how τ∗ is related to the binary structure of the time series.