Vertical profiles of liquid water content in fog layers during the SOFOG3D experiment
Costabloz, Théophane ; Burnet, Frédéric ; Lac, Christine ; Martinet, Pauline ; Delanoë, Julien ; Jorquera, Susana ; Fathalli, Maroua
A better understanding of the fog life cycle is required to improve forecasts by numerical weather prediction models and to reduce impacts of fog on human activities. However there are still many unknowns about the physical mechanisms driving fog variability. In particular, a main issue is that the transition from optically thin to thick fog is too sudden in numerical simulations. The liquid water content (LWC) profile is a key parameter, but observations in fog are sorely lacking. Here, we investigate observations from the SOuth west FOGs 3D experiment for processes study (SOFOG3D). In situ measurements collected under a tethered balloon provide 140 vertical profiles, which allow an exhaustive analysis of 8 thin fogs (thickness <50 m) and 4 developed layers. We estimate the thin-to-thick transition period using thresholds for longwave radiation flux, turbulent kinetic energy, vertical temperature gradient, fog top height and liquid water path. In situ data are used to compute the equivalent fog adiabaticity from closure, which is compared with the value derived using a one-column conceptual model of adiabatic fog, assuming that LWC linearly increases with height. We found that the reverse trend of the LWC profile (LWC maximal at the ground and decreasing with height) is ubiquitous in optically thin fogs under stable temperature conditions, while quasi-adiabatic features with increasing LWC values with height are mainly observed in well-mixed optically thick fogs under slightly unstable conditions. This study provides new insights into the evolution of LWC profile during the fog life cycle, to constrain numerical simulations.</p>
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