Development and application of a method for the objective differentiation of fog life cycle phases

Abstract

A B S T R A C T An objective classification of radiation fog in distinct evolutionary stages during its life cycle based on reliable criteria is essential for various applications, for example for numerical fog modelling and fog forecasting. However, there have been Á up to now Á merely qualitative approaches for the distinction of different evolutionary stages in radiation fog. Measurements of the microphysical fog properties with an optical particle counter obtained from droplet measurement technologies together with recordings of the horizontal visibility (VIS) are statistically analyzed to determine individual evolutionary stages of radiation fog with consistent microphysical properties. The developed three-stage approach is based on a statistical change point analysis of the double sum curves of the VIS, the liquid water content, the droplet concentration and the mean radius of the drop size distributions. It could be shown that each of the three recorded radiation fog occurrences could be split into three consecutive phases from formation to dissipation, regardless whether the VIS or the microphysical properties were considered. Having featured consistent microphysical patterns, it could be assumed that the three separated phases of the single fog occurrence could be aggregated for radiation fog. Although this classification is statistically reliable, the dataset still has to be extended for a generalization concerning the separated evolutionary stages.

Size-dependent particle activation properties in fog during the ParisFog 2012/13 field campaign

“…The range of dry activation diameters observed in this study overlaps with the range of typical D act values reported in other fog studies (∼ 300 to ∼ 500 nm; e.g., Noone et al, 1992a;Ming and Russell, 2004). However, the dry activation diameters observed in the fog are considerably larger than those observed and/or inferred within stratus or cumulus clouds (e.g., Ditas et al, 2012;Hammer et al, 2014;Hudson and Rogers, 1986;Krüger et al, 2014;McFiggans et al, 2006) due to the lower SS peak encountered in fog.…”

“…Cloud condensation nuclei (CCN) activity can be defined by the particles' critical supersaturation (SS crit ), that is, the lowest supersaturation at which the particles can activate to a fog or cloud droplet. The critical supersaturation depends on the size and chemical composition of the particle (McFiggans et al, 2006). Cooling of an air parcel to temperatures below its dew point will result in the formation of a cloud or fog, and those particles with a critical supersaturation below the peak supersaturation in the cloud are activated to form cloud droplets.…”

Optical Thickness and Effective Radius Retrievals of Low Stratus and Fog from MTSAT Daytime Data as a Prerequisite for Yellow Sea Fog Detection

“…This time difference does not exist between Aqua MODIS products and CloudSat because they are delayed by just 1 min in the A-train constellation [38]. It should be kept in mind that because cloud development is a highly dynamic process, even small time differences can be critical and of great importance for considerable oscillation in the microphysical properties of LST [39]. Second, the CloudSat and MYD06 products are calculated using vertical atmospheric profiles, whereas the case-adapted CAPCOM retrievals from JAMI and MODIS use real-time radiosonde data in this study.…”

The Influence of Drop Size Distributions on the Relationship between Liquid Water Content and Radar Reflectivity in Radiation Fogs

“…Furthermore, it would be difficult to relate the differing Z-LWC relationship directly to the high temporal DSD dynamics during fog development. By using the derived MGD parameters, the comparison with other studies, e.g., Maier et al [4], Harris [40], Tampieri and Tomasi [48], Tomasi and Tampieri [49] and Maier et al [30] is facilitated. Nevertheless, the difference between both procedures should be investigated in future studies, using more continuous DSD measurements during different radiation fog events.…”

“…While in some cases the fog dissipated due to evaporation, in other cases, it rose from the ground to form low stratus layers. The fog events were classified into three life cycle stages-"formation", "mature fog" and "dissipation"-by applying the breakpoint analysis presented in Maier et al [30] to the 2-m visibility data. The method is based on a double sum curve analysis and consists of three steps: First, a Mann-Whitney U-test [31] is conducted to find the most possible points in the time series where changes occurred.…”