Critical assessment of meteorological conditions and airflow connectivity during HCCT-2010

Abstract

Abstract. This study presents a comprehensive and critical assessment of the meteorological conditions and atmospheric flow during the Lagrangian-type "Hill Cap Cloud Thuringia 2010" experiment (HCCT-2010). HCCT-2010 was performed in September and October 2010 at Mt. Schmücke in the Thuringian forest, Germany, applying three measurements sites (upwind, in-cloud, downwind) to study physical and chemical aerosol-cloud-interactions. A Lagrangian-type hill cap cloud experiment requires suitable cloud and particularly connected airflow conditions, i.e. representative air masses at the different measurement sites. Therefore, the present study aimed at the identification of time periods during the 6-weeks duration of the campaign, where such conditions were fulfilled and which can be used in further data examinations. The following topics were studied in detail: (i) the general synoptic weather situations including the mesoscale flow conditions by means of a classification of advected air masses and calculation of non-dimensional flow parameters (e.g. Froude number), (ii) local meteorological conditions, including synoptic front passages, the presence of orographic or frontal cloudiness, cloud base heights and vertical stratification, and (iii) local flow conditions by means of statistical analyses using the quasi-inert trace gas ozone and selected size bins of particle number size distributions as well as SF6 tracer experiments in the campaign area. A comprehensive analyses using statistical measures such as the COD (Coefficient Of Divergence) and cross-correlation have been carried out for the first time in the context of a Lagrangian-type hill cap cloud experiment. Suitable criteria for the aimed statistical analyses were thus developed and applied in the present study to characterise the local flow connectivity in detail. The comprehensive examination resulted in a total of 14 so-called "Full Cloud Events" (FCE), which are shown to conform to the Lagrange-type experimental philosophy of HCCT-2010. In addition, 15 so-called "Non-Cloud Events" (NCEs) could be established, which can be used as reference cases as they provide similarly suitable flow conditions but no cloud at the summit site. Orographic cloudiness was identified for approx. one third of the FCE periods, while about two thirds were associated to synoptic fronts. The statistical flow analyses indicate the existence of a strong link between the sites during the events, particularly under constant south-westerly flow conditions, high wind speeds and slightly stable stratification. The COD analyses using continuously measured concentrations of ozone and the 49 nm diameter particle bin revealed particularly for COD values below 0.1 very consistent time series, i.e. closely linked air masses between the different sites. The cross-correlation analysis revealed under connected flow conditions typical overflow times of about 15 to 30 min between the two valley sites. Additionally, the performed SF6 tracer experiments during the campaign clearly demonstrate that under appropriate meteorological conditions a Lagrangian-type approach is valid and that the connected flow validation procedure developed in this work is suitable for identifying such conditions. Finally, an overall evaluation of the identified FCEs is presented, which provides the basis for subsequent investigations of the measured chemical and physical data during HCCT-2010.

In-cloud sulfate addition to single particles resolved with sulfur isotope analysis during HCCT-2010

“…In addition, connected flow between the sites was periodically measured with tracer experiments following the release of an inert gas (SF 6 ) at Goldlauter, with measurements at 5 min intervals at nine sites including the in-cloud and downwind stations. The connected flow analyses are discussed in detail in a companion paper in this special issue of Atmospheric Chemistry and Physics (Tilgner et al, 2014).…”

“…Connected flow between sites was investigated with ozone concentration profiles, ozone cross-correlations and hydrodynamic flow analysis, as ozone is quasi-chemically inert and relatively insoluble in water with no significant primary sources (Tilgner et al, 2014). The coefficients of divergence (COD) for several aerosol particle bins and ozone concentrations were also calculated to characterise connected flow conditions.…”

“…The coefficients of divergence (COD) for several aerosol particle bins and ozone concentrations were also calculated to characterise connected flow conditions. The COD is a statistical measure of temporal similarities between the concentrations measured at the different stations; lower COD values indicate very similar concentration profiles, and a COD of < 0.1-0.2 can be used as an indication of homogeneity between sites (Tilgner et al, 2014;USEPA, 2004). In addition, connected flow between the sites was periodically measured with tracer experiments following the release of an inert gas (SF 6 ) at Goldlauter, with measurements at 5 min intervals at nine sites including the in-cloud and downwind stations.…”

Influence of cloud processing on CCN activation behaviour in the Thuringian Forest, Germany during HCCT-2010

“…All measurement equipment was placed inside an air-conditioned container. On top of the container a PM 10 inlet followed by a self-regenerating diffusion drier was placed (Tuch et al, 2009), maintaining the relative humidity of the aerosol flow below 20 %. Inside the container -besides other instrumentation -a Mobility Particle Size Spectrometer (MPSS-type TROPOS; details of this instrument in Wiedensohler et al, 2012) was used to determine the particle number size distributions between 10 and 850 nm, and a Cloud Condensation Nucleus counter (CCNc, CCN-100, DMT Boulder, Roberts and Nenes, 2005) in combination with a differential mobility analyser (DMA) was used to measure CCN distributions between 25 and 300 nm.…”

Temporal evolution of stable water isotopologues in cloud droplets during HCCT-2010

“…5. Synoptic weather charts: We used synoptic charts from the German Weather Service (DWD) with a time resolution of 6 h for the air mass classification and identification of frontal passages (for details see Tilgner et al, 2012).…”