Short-time variability in scalar structure parameters

The structure parameter of refractive index of air (C_n²) is the basic parameter derived from the scintillometer data. It can be linked via the structure parameter of temperature (C_T²), humidity (C_q²) and the cross structure parameter of temperature and humidity (C_Tq) to a path averaged surface sensible and latent heat flux. C_x² 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 C_x² 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 C_x² 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 C_x² 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 C_x², a measure of the temporal variability, behaves under unstable conditions. Finally, we will discuss implications for the interpretation of path-integrated structure parameters.

Cheinet, S and Siebesma, AP; 2009: Variability of Local Structure Parameter in the Convective Boundary Layer, Journal of the Atmospheric Sciences 66, 1002-1017

Beyrich, F, Bange, J, Hartogensis, OK, Raasch, S. and Braam, M, van Dinther, D, Gräf, D, van Kesteren, B, van den Kroonenberg, AC, Maronga, B, Martin, S and Moene, AF, 2012: Towards a validation of scintillometer measurements: The LITFASS-2009 experiment, Boundary Layer Meteorology DOI 10.1007/s10546-012-9715-8