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Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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Record number 431472
Title Short-time variability in scalar structure parameters
Author(s) Braam, M.; Moene, A.F.; Beyrich, F.
Source In: 20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction, American Meteorological Society (9-13 July 2012, Boston, MA). - Boston : American Meteorological Society - p. 13A.4 - 13A.4.
Event Boston : American Meteorological Society 20th Symposium on Boundary Layers and Turbulence/18th Conference on Air-Sea Interaction, Boston, 2012-07-09/2012-07-13
Department(s) Meteorology and Air Quality
WIMEK
Publication type Contribution in proceedings
Publication year 2012
Abstract The structure parameter of refractive index of air (Cn2) is the basic parameter derived from the scintillometer data. It can be linked via the structure parameter of temperature (CT2), humidity (Cq2) and the cross structure parameter of temperature and humidity (CTq) to a path averaged surface sensible and latent heat flux. Cx2 is a measure of the variation of a turbulent quantity x in space within the inertial subrange. The structure parameter is a local quantity of a turbulent field and is therefore variable along the scintillometer path. It has a log-normal distribution in time (Cheinet and Siebesma, 2009) indicating that the variability is strong. On top of the variability due to turbulence, the structure parameter may also vary due to spatial heterogeneity of the forcing of turbulence (i.e. the surface fluxes of heat and moisture). A scintillometer installed over heterogeneous terrain will ‘see' a combination of these two effects. To better understand this influence of surface heterogeneity along a scintillometer path on the spatial variability of Cx2 was one of the main objectives of the LITFASS-2009 field experiment. This experiment was performed around the Meteorological Observatory Lindenberg of the German Meteorological Service (DWD) in July 2009, and combined eddy-covariance techniques, scintillometer observations, unmanned aircraft measurements and large eddy simulation (LES) modeling (Beyrich et al., 2012). However, if spatial variability is investigated, one is interested in the influence of surface heterogeneity only rather than in the combined effect of turbulent variation and imposed spatial variations due to surface heterogeneity. Therefore, it is essential to decompose the two causes. A method to quantify the turbulent variation is to investigate the short time temporal variability of Cx2 at a given location (i.e. without any influence of spatial variability). In this research we will focus on the short-time temporal variability of scalar structure parameters obtained by eddy-covariance measurements performed in the atmospheric surface layer during the LITFASS-2009 experiment: First, we investigate the shape of the probability density functions of the structure parameters near the surface. We search for a uniform behavior of the variability under different atmospheric conditions. Second, since Cx2 is derived over a certain time window, we have to quantify how the variance in the structure parameter depends on the size of this window. Furthermore, we show how the dimensionless standard deviation of Cx2, a measure of the temporal variability, behaves under unstable conditions. Finally, we will discuss implications for the interpretation of path-integrated structure parameters.
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