Abstract. In the Atacama Desert, one of the driest places on earth, fog deposition plays an important role for the water balance and for the survival of vulnerable ecosystems. The eddy covariance method, previously applied for the quantification of fog deposition to forests in various parts of the world, was used for the first time to measure deposition of fog water to a desert. In this exploratory study we estimate the amount of water available for the ecosystem by deposition and determine the relevant processes driving fog deposition. This is especially important for the species Tillandsia landbecki living in coastal Atacama at the limit of plant existence with fog and dew being the only sources of water. Between 31 July and 19 August 2008 approximately 2.5 L m −2 of water were made available through deposition. Wholeyear deposition was estimated as 25 L m −2 . Turbulent upward fluxes occurred several times during the evenings and are explained by the formation of radiation fog. In connection with that, underestimates of the deposition are assumed. More detailed studies covering various seasons and all parameters and fluxes contributing to the local energy balance are suggested. This will help to further develop understanding about the processes of (i) deposition of water to the desert, and (ii) intensification of advection fog through additional formation of radiation fog.
Microphysical measurements of orographic fog were performed above a montane cloud forest in northeastern Taiwan (Chilan mountain site). The measured parameters include droplet size distribution (DSD), absolute humidity (AH), relative humidity (RH), air temperature, wind speed and direction, visibility, and solar short wave radiation. The scope of this work was to study the short term variations of DSD, temperature, and RH, with a temporal resolution of 3 Hz. The results show that orographic fog is randomly composed of various air volumes that are intrinsically rather homogeneous, but exhibit clear differences between each other with respect to their size, RH, LWC, and DSD. Three general types of air volumes have been identified via the recorded DSD. A statistical analysis of the characteristics of these volumes yielded large variabilities in persistence, RH, and LWC. Further, the data revealed an inverse relation between RH and LWC. In principle, this finding can be explained by the condensational growth theory for droplets containing soluble or insoluble material. Droplets with greater diameters can exist at lower ambient RH than smaller ones. However, condensational growth alone is not capable to explain the large observed differences in DSD and RH because the respective growth speeds are too slow to explain the observed phenomena. Other mechanisms play key roles as well. Possible processes leading to the large observed differences in RH and DSD include turbulence induced collision and coalescence, and heterogeneous mixing. More analyses including fog droplet chemistry and dynamic microphysical modeling are required to further study these processes. To our knowledge, this is the first experimental field observation of the anti-correlation between RH and LWC in fog.
Measurement of the turbulent fluxes of gases, momentum and heat can be biased by obstacles such as buildings or instrument platforms distorting the flow of air to the flux instruments. Standard methods have long been used to account for non-horizontal mean flows. Here we demonstrate a novel approach to correct for the effects of flow distortion which combines numerical flow modelling with eddy covariance measurements of the fluxes. This approach applies a flow distortion correction to the data prior to the application of the standard planar-fit and double-rotation methods. This new direction-dependent flow correction allows the determination of the correct orthogonal wind vector components and hence the vertical turbulent fluxes. We applied the method to a 10 Hz dataset of 3D wind components, temperature, and the concentrations of carbon dioxide and water vapour, as measured on top of a military tower above the city of Münster in northwest Germany during spring and summer 2007. Significant differences appeared between the fluxes that were calculated with the standard rotation methods alone and those that underwent flow distortion correction prior to the application of the rotation methods. The highest deviations of 27% were obtained for the momentum flux. Pronounced differences of 15% and 8% were found for the diurnal net fluxes of carbon dioxide and water vapour, respectively. The flow distortion correction for the carbon dioxide fluxes yielded the same magnitude as the WPL (Webb-Pearman-Leuning) correction for density fluctuations.
Size-resolved particle flux measurements were carried out in an urban area from April 2012 to April 2013. Together with a standard eddy covariance system, two fast optical particle counters have been employed on a 65-meter-high tower in Mu¨nster, Germany. Particle number fluxes were directly calculated for particles with diameters from 0.06 to 10 mm within 16 individual size-bins. Whereas particle number concentrations show a distinct yearly pattern with maxima in winter and minima in summer, the flux time series is more multifaceted. Average daily maxima of 3.0e'07 particles m (2 s (1 occurred during winter while minima of 2.0e'06 particles m (2 s (1 were observed in fall. The size-resolved measurements revealed that during spring and summer a considerable number of accumulation mode particles deposits while a simultaneous net particle emission occurred, which is mostly driven by particles smaller than 0.12 mm. These bi-directional fluxes lead to a net mass deposition of up to 13.5 mg m (2 d (1 . The tipping-point between the emission and deposition lay between 0.16 and 0.19 mm. In a comprehensive analysis of the flux and concentration time series, the degree of atmospheric stability, the seasons, and the type of source region have been identified as key influences for particle fluxes. Different responses between particle fluxes and concentrations have been found along these drivers.
From April 11 th to May 27 th , 2011, the turbulent exchange of sub-micrometer particles between the urban surface and the urban boundary-layer was measured above the city area of Münster (NW Germany). The scope of the study is to examine the contributions of particles of different size classes to the total measured fluxes. Eddy-covariance measurements were performed at 65 m above ground. The particle concentrations in 99 size bins with particle diameters ranging from 55 to 1000 nm were measured with an optical particle spectrometer. For flux calculations we grouped these 99 original bins into 18 wider channels with an upper cutoff of 320 nm, and a further rather coarse channel for particles up to 1 lm. The overall results reveal that Münster is a relevant source of about 2.8 AE 10 8 particles m À2 d À1 on weekdays and 1.8 AE 10 8 particles m À2 d À1 on Sundays within the indicated size range. These emissions are predominantly driven by secondary particles of the Aitken mode, which are most likely caused by traffic. Hence traffic hotspots are a major contribution to the net fluxes. On the other hand, considering the mass fluxes, Münster is a sink of 0.53 lg m À2 d À1 on weekdays and 0.08 lg m À2 d À1 on Sundays. Here, mainly particles of the accumulation mode with diameters above 167 nm lead to deposition fluxes. Number and mass fluxes exhibit distinct daily and weekly patterns.
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