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- ItemHigh-resolution measurement of cloud microphysics and turbulence at a mountaintop station(München : European Geopyhsical Union, 2015) Siebert, H.; Shaw, R.A.; Ditas, J.; Schmeissner, T.; Malinowski, S.P.; Bodenschatz, E.; Xu, H.Mountain research stations are advantageous not only for long-term sampling of cloud properties but also for measurements that are prohibitively difficult to perform on airborne platforms due to the large true air speed or adverse factors such as weight and complexity of the equipment necessary. Some cloud–turbulence measurements, especially Lagrangian in nature, fall into this category. We report results from simultaneous, high-resolution and collocated measurements of cloud microphysical and turbulence properties during several warm cloud events at the Umweltforschungsstation Schneefernerhaus (UFS) on Zugspitze in the German Alps. The data gathered were found to be representative of observations made with similar instrumentation in free clouds. The observed turbulence shared all features known for high-Reynolds-number flows: it exhibited approximately Gaussian fluctuations for all three velocity components, a clearly defined inertial subrange following Kolmogorov scaling (power spectrum, and second- and third-order Eulerian structure functions), and highly intermittent velocity gradients, as well as approximately lognormal kinetic energy dissipation rates. The clouds were observed to have liquid water contents on the order of 1 g m−3 and size distributions typical of continental clouds, sometimes exhibiting long positive tails indicative of large drop production through turbulent mixing or coalescence growth. Dimensionless parameters relevant to cloud–turbulence interactions, the Stokes number and settling parameter are in the range typically observed in atmospheric clouds. Observed fluctuations in droplet number concentration and diameter suggest a preference for inhomogeneous mixing. Finally, enhanced variance in liquid water content fluctuations is observed at high frequencies, and the scale break occurs at a value consistent with the independently estimated phase relaxation time from microphysical measurements.
- ItemSchneefernerhaus as a mountain research station for clouds and turbulence(München : European Geopyhsical Union, 2015) Risius, S.; Xu, H.; Di Lorenzo, F.; Xi, H.; Siebert, H.; Shaw, R.A.; Bodenschatz, E.Cloud measurements are usually carried out with airborne campaigns, which are expensive and are limited by temporal duration and weather conditions. Ground-based measurements at high-altitude research stations therefore play a complementary role in cloud study. Using the meteorological data (wind speed, direction, temperature, humidity, visibility, etc.) collected by the German Weather Service (DWD) from 2000 to 2012 and turbulence measurements recorded by multiple ultrasonic sensors (sampled at 10 Hz) in 2010, we show that the Umweltforschungsstation Schneefernerhaus (UFS) located just below the peak of Zugspitze in the German Alps, at a height of 2650 m, is a well-suited station for cloud–turbulence research. The wind at UFS is dominantly in the east–west direction and nearly horizontal. During the summertime (July and August) the UFS is immersed in warm clouds about 25 % of the time. The clouds are either from convection originating in the valley in the east, or associated with synoptic-scale weather systems typically advected from the west. Air turbulence, as measured from the second- and third-order velocity structure functions that exhibit well-developed inertial ranges, possesses Taylor microscale Reynolds numbers up to 104, with the most probable value at ~ 3000. In spite of the complex topography, the turbulence appears to be nearly as isotropic as many laboratory flows when evaluated on the "Lumley triangle".
- ItemChemical characteristics of cloud water and the impacts on aerosol properties at a subtropical mountain site in Hong Kong SAR(Katlenburg-Lindau : EGU, 2020) Li, Tao; Wang, Zhe; Wang, Yaru; Wu, Chen; Liang, Yiheng; Xia, Men; Yu, Chuan; Yun, Hui; Wang, Weihao; Wang, Yan; Guo, Jia; Herrmann, Hartmut; Wang, TaoTo investigate the cloud water chemistry and the effects of cloud processing on aerosol properties, comprehensive field observations of cloud water, aerosols, and gasphase species were conducted at a mountaintop site in Hong Kong SAR in October and November 2016. The chemical composition of cloud water including water-soluble ions, dissolved organic matter (DOM), carbonyl compounds (refer to aldehydes and acetone), carboxylic acids, and trace metals was quantified. The measured cloud water was very acidic with a mean pH of 3.63, as the ammonium (174 μeq L-1) was insufficient for neutralizing the dominant sulfate (231 μeq L-1) and nitrate (160 μeq L-1). Substantial DOM (9.3 mgC L-1) was found in cloud water, with carbonyl compounds and carboxylic acids accounting for 18% and 6% in carbon molar concentrations, respectively. Different from previous observations, concentrations of methylglyoxal (19.1 μM; μM is equal to μmol L-1) and glyoxal (6.72 μM) were higher than that of formaldehyde (1.59 μM). The partitioning of carbonyls between cloud water and the gas phase was also investigated. The measured aqueous fractions of dicarbonyls were comparable to the theoretical estimations, while significant aqueous-phase supersaturation was found for less soluble monocarbonyls. Both organics and sulfate were significantly produced in cloud water, and the aqueous formation of organics was more enhanced by photochemistry and under less acidic conditions. Moreover, elevated sulfate and organics were measured in the cloudprocessed aerosols, and they were expected to contribute largely to the increase in droplet-mode aerosol mass fraction. This study demonstrates the significant role of clouds in altering the chemical compositions and physical properties of aerosols via scavenging and aqueous chemical processing, providing valuable information about gas-cloud-aerosol interactions in subtropical and coastal regions. © 2020 Author(s). This work is distributed under the Creative Commons Attribution 4.0 License.