Abstract
Abstract. This study presents a comprehensive assessment of the meteorological conditions and atmospheric flow during the Lagrangian-type "Hill Cap Cloud Thuringia 2010" experiment (HCCT-2010), which was performed in September and October 2010 at Mt. Schmücke in the Thuringian Forest, Germany and which used observations at three measurement sites (upwind, in-cloud, and downwind) to study physical and chemical aerosol–cloud interactions. A Lagrangian-type hill cap cloud experiment requires not only suitable cloud conditions but also connected airflow conditions (i.e. representative air masses at the different measurement sites). The primary goal of the present study was to identify time periods during the 6-week duration of the experiment in which these conditions were fulfilled and therefore which are suitable for use in further data examinations. The following topics were studied in detail: (i) the general synoptic weather situations, including the mesoscale flow conditions, (ii) local meteorological conditions and (iii) local flow conditions. The latter were investigated by means of statistical analyses using best-available quasi-inert tracers, SF6 tracer experiments in the experiment area, and regional modelling. This study represents the first application of comprehensive analyses using statistical measures such as the coefficient of divergence (COD) and the cross-correlation in the context of a Lagrangian-type hill cap cloud experiment. This comprehensive examination of local flow connectivity yielded a total of 14 full-cloud events (FCEs), which are defined as periods during which all connected flow and cloud criteria for a suitable Lagrangian-type experiment were fulfilled, and 15 non-cloud events (NCEs), which are defined as periods with connected flow but no cloud at the summit site, and which can be used as reference cases. The overall evaluation of the identified FCEs provides the basis for subsequent investigations of the measured chemical and physical data during HCCT-2010 (see https://www.atmos-chem-phys.net/special_issue287.html). Results obtained from the statistical flow analyses and regional-scale modelling performed in this study indicate the existence of a strong link between the three measurement sites during the FCEs and NCEs, particularly under conditions of constant southwesterly flow, high wind speeds and slightly stable stratification. COD analyses performed using continuous measurements of ozone and particle (49 nm diameter size bin) concentrations at the three sites revealed, particularly for COD values < 0.1, very consistent time series (i.e. close links between air masses at the different sites). The regional-scale model simulations provided support for the findings of the other flow condition analyses. Cross-correlation analyses revealed typical overflow times of ~15–30 min between the upwind and downwind valley sites under connected flow conditions. The results described here, together with those obtained from the SF6 tracer experiments performed during the experiment, clearly demonstrate that (a) under appropriate meteorological conditions a Lagrangian-type approach is valid and (b) the connected flow validation procedure developed in this work is suitable for identifying such conditions. Overall, it is anticipated that the methods and tools developed and applied in the present study will prove useful in the identification of suitable meteorological and connected airflow conditions during future Lagrangian-type hill cap cloud experiments.
Highlights
Clouds occupy on average ∼ 15% of the volume of the lower troposphere (Pruppacher and Jaenicke, 1995) and play a crucial role in the various physical and chemical processes occurring there (Heintzenberg and Charlson, 2009; Möller, 2010; Ravishankara, 1997)
The main goal of the present study was to provide a comprehensive evaluation of the meteorological and connected flow conditions present during the ground-based Lagrangian-type experiment HCCT-2010, in order to provide a set of suitable measurement time periods for detailed investigations
The entire HCCT-2010 measurement period was analysed with respect to flow connectivity between the three measurement sites and the presence or non-presence of a cloud at the sites
Summary
Clouds occupy on average ∼ 15% of the volume of the lower troposphere (Pruppacher and Jaenicke, 1995) and play a crucial role in the various physical and chemical processes occurring there (Heintzenberg and Charlson, 2009; Möller, 2010; Ravishankara, 1997). Physical and chemical cloud processes influence large-scale environmental issues such as climate change and, by extension, have a variety of societal implications (Boucher et al, 2013). Since both the spatial and temporal occurrence and the altitude of clouds are highly variable, investigations of physical and chemical interactions between gases, aerosol particles and cloud droplets are quite challenging. These interactions are much less understood than pure gas-phase processes. Germany, in 2001 and 2002 (Herrmann et al, 2005) – have shown that ground-based Lagrangian-type experiments, where an orographic cloud is used as a natural flow-through reactor, provide a valuable opportunity to study cloud processes in detail
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