Preliminary results of the PreViBOSS project: description of the fog life cycle by ground-based and satellite observation

摘要

The instrument set-up designed by the PreViBOSS project for the ParisFog field campaign is suitable to sound microphysical properties of droplets and interstitial aerosols during developed fog in a semi-urban environment. Developed fog is defined as LWC ＜ 7 mg m~(-3) and the temperature vertical gradient over 30 m, AT, smaller than 0.04 K/m. Visibility averaged over November 2011 is 385±340 m (with rare values larger than 1000 m), and month average of LWC is 60±60 mg m~(-3). The droplet effective radius decreases from 14 to 4 μm when the number concentration increases from less than 10 to 220 cm~(-3). Particle extinction coefficient is computed by Mie theory applied on size distribution observed during developed fog in ambient conditions by both PALAS WELAS and DMT FM100. Comparison with particle extinction coefficient directly measured by the Degreanne DF20 visibilimeter demonstrates satisfying agreement, within combined uncertainties. Ratio of computed over measured particle extinction coefficient is 1.15±0.35. Visibility smaller than 1000 m at 3 m above ground level is observed not only during developed fog but also during shallow fog, which presents a significant vertical gradient, as ΔΤ ＞ 0.4 K/m. In this case, LWC is highly variable and may be observed below 7 mg m~(-3). The consequent month average of LWC is 30±80 mg m~(-3). The optical counters miss large droplets significantly contributing to extinction in shallow fogs. Consequently, it is not possible to reproduce with satisfaction the particle extinction coefficient in shallow fog. Fog type may be distinguished by association of ground-based visibilimeter and MSG/SEVIRI. When clear-sky is given by EUMETSAT/NWCSAF cloud type product while visibility is observed smaller than 1000 m at SIRTA, in 75% cases a shallow fog occurs, and in other cases, horizontal heterogeneity characterises the developed fog within the SIRTA pixel, as during the dissipation phase. Moreover, consistently, low and very low clouds are mostly detected by the satellite product when developed fog is observed by ground-based instrumentation.