Techniques for the Eduction of Coherent Structures from Flow Measurements in the Atmospheric Boundary Layer

Well known for its severity, the bora downslope windstorms have been extensively studied over the last several decades. This study focuses on the turbulence characteristics of bora at a topographically complex site near the eastern coast of the Adriatic Sea. For this purpose, a 3-month eddy-covariance dataset obtained at three levels (10, 22, 40 m) on a 60-m flux tower is used. After determining a suitable averaging time scale of 15 min using the fast Fourier transform and the ogive method, vertical fluxes of momentum and heat were calculated for 17 bora episodes. Up to a wind speed of 12 m s −1 , typical vertical profiles of momentum and heat were observed. However, for wind speeds >12 m s −1 , several interesting observations arose. First, the nighttime heat flux at the 10-m level was often found to be positive rather negative. Second, vertical profiles of the momentum flux were larger at the 22-m level than at 10-and 40-m levels, mostly during nearly neutral to weakly stable thermal stratification. Third, these momentum flux profiles showed a large dependence on wind direction, with virtually no vertical transport of momentum for the largest observed wind speeds. For the first time, bora coherent structures have been analyzed using the so-called variable-interval time averaging (VITA) method. The method detected coherent structures in all three wind-speed components, with structure topologies similar to those observed over forest canopies. The momentum flux increase at the 22-m level, relative to the 10-and 40-m levels, is further supported by the VITA findings.

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

On Turbulent Fluxes During Strong Winter Bora Wind Events

Babić

Večenaj

Kozmar

et al. 2015

“…Barthlott et al 2007;Zhang et al 2010;Segalini and Alfredsson 2012;Zeeman et al 2013). The sweep-and-ejection viewpoint of coherent structures in meteorology is determined by this technique.…”

Section: Introductionmentioning

confidence: 99%

Turbulent Structures and Coherence in the Atmospheric Surface Layer

Träumner

Damian

Stawiarski

et al. 2014

Organized structures in turbulent flow fields are a well-known and still fascinating phenomenon. Although these so-called coherent structures are obvious from visual inspection, quantitative assessment is a challenge and many aspects e.g., formation mechanisms and contribution to turbulent fluxes, are discussed controversially. During the "High Definition Clouds and Precipitation for Advancing Climate Prediction" Observational Prototype Experiment (HOPE) from April to May 2013, an advanced dual Doppler lidar technique was used to image the horizontal wind field near the surface for approximately 300 h. A visual inspection method, as well as a two-dimensional integral length scale analysis, were performed to characterize the observations qualitatively and quantitatively. During situations with forcing due to shear, the wind fields showed characteristic patterns in the form of clearly bordered, elongated areas of enhanced or reduced wind speed, which can be associated with near-surface streaks. During calm situations with strong buoyancy forcing, open cell patterns in the horizontal divergence field were observed. The measurement technique used enables the calculation of integral length scales of both horizontal wind components in the streamwise and cross-stream directions. The individual length scales varied considerably during the observation period but were on average shorter during situations with z/L < 0 compared to strongly stable situations. During unstable situations, which were dominated by wind fields with structures, the streamwise length scales increased with increasing wind speed, whereas the cross-stream length scales decreased. Consequently, the anisotropy increased from 1 for calm situations to values of 2-3 for wind speeds of 8-10 m s −1 . During neutral to stable situations, the eddies were on average quite isotropic in the horizontal plane.

“…9). The WAVE wavelet function closely resembles the ejectionsweep cycle that can be expected in the streamwise wind field u (Zhang et al 2011;Segalini and Alfredsson 2012). Subsequently, the wavelet spectrum or scalogram was determined,…”

Section: Wavelet Analysismentioning

confidence: 96%

“…Additionally, the virtual tower time series of u and v in the original LES dataset for 15-m height, which were already used in the integral scale analysis, were included in the evaluation. On all series in space and time, a wavelet analysis was performed following the methods described in Barthlott et al (2007) and Segalini and Alfredsson (2012). First, the signal was appropriately detrended and zero-padded (Thomas and Foken 2005), then the wavelet coefficients (Eq.…”

Section: Wavelet Analysismentioning

confidence: 99%

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

Stawiarski

Träumner

Kottmeier

et al. 2015

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, twodimensional 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.