Assessment of Surface-Layer Coherent Structure Detection in Dual-Doppler Lidar Data Based on Virtual Measurements

C Stawiarski,S Raasch,C Knigge,K Träumner,C Kottmeier

doi:10.1007/s10546-015-0039-3

Abstract

Dual-Doppler lidar has become a useful tool to investigate the wind-field structure in two-dimensional planes. However, lidar pulse width and scan duration entail significant and complex averaging in the resulting retrieved wind-field components. The effects of these processes on the wind-field structure remain difficult to investigate with in situ measurements. Based on high resolution large-eddy simulation (LES) data for the surface layer, we performed virtual dual-Doppler lidar measurements and two-dimensional data retrievals. Applying common techniques (integral length scale computation, wavelet analysis, two-dimensional clustering of low-speed streaks) to detect and quantify the length scales of the occurring coherent structures in both the LES and the virtual lidar wind fields, we found that, (i) dual-Doppler lidar measurements overestimate the correlation length due to inherent averaging processes, (ii) the wavelet analysis of lidar data produces reliable results, provided the length scales exceed a lower threshold as a function of the lidar resolution, and (iii) the low-speed streak clusters are too small to be detected directly by the dual-Doppler lidar. Furthermore, we developed and tested a method to correct the integral scale overestimation that, in addition to the dual-Doppler lidar, only requires high-resolution wind-speed variance measurements, e.g. at a tower or energy balance station.

Highlights

IntroductionThree kinds of structures have been observed (Agee 1984; Young et al 2002): surface-layer streaks comparable to flat-plate boundary layer (FPBL) streaks in shear-dominated flows (Hommema and Adrian 2003; Newsom et al 2008), horizontal rolls reaching through the whole boundary layer in moderately convective situations (Etling and Brown 1993; Hartmann et al 1997), and cell-shaped updrafts in very unstable situations (Feingold et al 2010)
It has long been known that turbulent fluid motion exhibits patterns of self-organization, socalled coherent structures. Robinson (1991) listed various shapes of structures occurring in Germany 2 Institute for Meteorology and Climatology, Leibniz-Universität Hannover, Hannover, Germany a flat-plate boundary layer (FPBL) at low Reynolds number: near-wall low-speed and highspeed streaks, i.e. alternating regions in which the streamwise fluid velocity is increased or decreased compared to its mean value and that are elongated in the direction of the mean flow; ejections and sweeps, i.e. intermittent, rapid movements of the fluid outward from the wall in the low-speed regions or towards the wall in high-speed regions, respectively; as well as various vortical structures shaped like quasi-streamwise vortices, arches, hairpins or horseshoes
While giving the most direct information on each coherent structure, the small-scale structures investigated here cannot be reliably detected with this method in dual-Doppler lidar data, because the lidar averages out the small scales

Summary

Introduction

Three kinds of structures have been observed (Agee 1984; Young et al 2002): surface-layer streaks comparable to FPBL streaks in shear-dominated flows (Hommema and Adrian 2003; Newsom et al 2008), horizontal rolls reaching through the whole boundary layer in moderately convective situations (Etling and Brown 1993; Hartmann et al 1997), and cell-shaped updrafts in very unstable situations (Feingold et al 2010) These structures appear as well in turbulence-resolving large-eddy simulations (LES, Moeng and Sullivan 1994; Khanna and Brasseur 1998; Kim and Park 2003; Hellsten and Zilitinkevich 2013)

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Results

Conclusion