Differentiation of evolutionary stages in fog life cycles based on microphysical properties – implications for the operation of novel cloud radar profilers
Enlarged knowledge of the spatiotemporal distribution of fog and low stratus (FLS) is of great value in regards to traffic safety and air quality control. Not only the horizontal visibility in fog but also the dissolving power of harmful pollutants in boundary clouds depend on the prevailing small d...
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|Zusammenfassung:||Enlarged knowledge of the spatiotemporal distribution of fog and low stratus (FLS) is of great value in regards to traffic safety and air quality control. Not only the horizontal visibility in fog but also the dissolving power of harmful pollutants in boundary clouds depend on the prevailing small droplets. Since the drop size spectrum (DSD) of both phenomena varies spatially with the vertical extent of these clouds and temporally from formation to dissipation, nowcasting and forecasting of FLS is faced with difficult challenges. Present models have need of theoretical assumptions on vertical microphysical profiles and their evolution during fog life cycle for their computations since real-time data on these cannot be provided contemporaneously so far. According to COST actions 720 and 722 novel ground-based microwave FMCW cloud RADAR profilers possess the instrumental requirements for deriving microphysical properties such as liquid water content (LWC) from radar reflectivity (Z); but no implemented retrievals have been developed so far. Since for the derivation of vertical LWC-profiles from Z detailed information on prevailing DSD are required, the evolution of the latter as a function of the fog life cycle has to be considered. An accurate classification of fog evolutionary stages, accompanied with phase-specific DSD, is a necessary condition for a proper usage of the microwave RADAR profiler. Otherwise, the derivation of vertical LWC-profiles from Z would underlie too big inaccuracies. Hence, the major aim of the thesis was the investigation of the temporal dynamics of fog microphysics with emphasis on DSD over its whole life cycle. This intention was based on the hypothesis that it is possible to separate consecutive evolutionary stages temporally within fog life cycle on the basis of fog microphysics such as DSD at the ground as well as in vertical profiles. Novel findings of the current thesis are: 1. It is possible to derive vertical LWC-profiles in FLS directly from RADAR reflectivity of a novel 94 GH FMCW cloud RADAR profiler since a direct but non-linear relationship between Z and LWC could be approved whereby further information on the prevailing drop size distribution has to be presumed. 2. Fog occurrences can be separated in three consecutive phases during its life cycle by means of an innovative statistical approach that relies on measured microphysical fog properties or horizontal visibility at the ground. 3. According to balloon-borne measurements of vertical LWC-profiles it is legitimate to interpolate FLS life cycle phases from ground- based measurements of microphysical properties and horizontal visibility in their whole vertical extension. The results of the thesis have manifold benefits for climate research and operational FLS applications. The identification of cloud geometrical thickness and thus the distinction between fog and low stratus by means of optical satellite retrievals has to be improved with regards to their reliability. The introduced approach for the classification of evolutionary stages during fog life cycle based on microphysical properties is a valuable step towards the development of a method for the derivation of vertical LWC-profiles from novel FMCW microwave cloud RADAR profilers. These are notably suitable for the exploration of microphysical properties of FLS with high temporal resolution. The resultant findings about the dynamics of microphysical properties during FLS could be used to improve the implemented theoretical assumptions on LWC-profiles in satellite-based approaches for fog detection. This optimization could permit in turn an operational and continuous monitoring of LWC-profiles in FLS thanks to their high spatiotemporal resolution.|