The Structure of the Near-Neutral Atmospheric Surface Layer

Drobinski, Philippe; Carlotti, Pierre; Newsom, Rob; Banta, Robert M.; Foster, R. C.; Redelsperger, Jean‐Luc

doi:10.1175/1520-0469(2004)061<0699:tsotna>2.0.co;2

Cited by 123 publications

(133 citation statements)

References 56 publications

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“…In addition to in situ instruments, remote sensing instruments have been used. First attempts at using Doppler lidar to study coherent structures were made by Drobinski et al (1998) who examined boundary-layer rolls and Drobinski et al (2004) who showed streaks in the radial velocity field of a Doppler lidar. To gain a more complete picture of the individual wind components, dual Doppler lidar techniques are an option.…”

Section: Introductionsupporting

confidence: 78%

Turbulent Structures and Coherence in the Atmospheric Surface Layer

Träumner

Damian

Stawiarski

et al. 2014

Boundary-Layer Meteorol

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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.

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Section: Introductionsupporting

confidence: 78%

Turbulent Structures and Coherence in the Atmospheric Surface Layer

Träumner

Damian

Stawiarski

et al. 2014

Boundary-Layer Meteorol

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“…More than 20 years after the pioneering work by Lenschow, Greenhut, Khalsa, and Young, a first comprehensive lidar-based study of updraft and downdraft occurrence frequencies, occurrence durations, corresponding horizontal extents, and mean vertical velocities of updrafts and downdrafts is presented. In contrast to airborne in situ observations (Lenschow and Stephens, 1980;Greenhut and Khalsa, 1982;Khalsa and Greenhut, 1985;Godowitch, 1986;Young, 1988b;Williams and Hacker, 1992;Durand et al, 2000;Said et al, 2009), Doppler lidar allows us to monitor the entire mixed layer including the entrainment zone vertically resolved and continuously over long time periods so that a detailed study of the full evolution cycle of the ABL over the day is possible (Grund et al, 2001;Bösenberg and Linné, 2002;Drobinski et al, 2004;Wulfmeyer and Janjić, 2005;Lothon et al, 2006;Gibert et al, 2007;Engelmann et al, 2008;Hogan et al, 2009).…”

mentioning

confidence: 82%

Updraft and downdraft characterization with Doppler lidar: cloud-free versus cumuli-topped mixed layer

2010

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Abstract. For the first time, a comprehensive, heightresolved Doppler lidar study of updrafts and downdrafts in the mixing layer is presented. The Doppler lidar measurements were performed at Leipzig, Germany, in the summer half year of 2006. The conditional sampling method is applied to the measured vertical velocities to identify, count, and analyze significant updraft and downdraft events. Three cases of atmospheric boundary-layer (ABL) evolution with and without fair-weather cumuli formation are discussed. Updrafts occur with an average frequency of 1-2 per unit length z i (boundary-layer depth z i ), downdrafts 20-30% more frequently. In the case with cumuli formation, the draft occurrence frequency is enhanced by about 50% at cloud level or near cloud base. The counted updraft events cover 30-34%, downdrafts 53-57% of the velocity time series in the central part of the ABL (subcloud layer) during the main period of convective activity. By considering all drafts with horizontal extent >36 m in the analysis, the updraft mean horizontal extent ranges here from 200-420 m and is about 0.16 z i -0.18 z i in all three cases disregarding the occurrence of cumulus clouds. Downdraft extents are a factor of 1.3-1.5 larger. The average value of the updraft mean vertical velocities is 0.5-0.7 m/s or 0.40 w * -0.45 w * (convective velocity scale w * ), and the negative downdraft mean vertical velocities are weaker by roughly 10-20%. The analysis of the relationship between the size (horizontal extent) of the updrafts and downdrafts and their mean vertical velocity reveals a pronounced increase of the average vertical velocity in updrafts from 0.4-0.5 m/s for small thermals (100-200 m) to about 1.5 m/s for large updrafts (>600 m) in the subcloud layer in the case with fair-weather cumuli. At cloudless conditions, the updraft velocities were found to be 20% smaller for the large thermals.

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“…As additional information, it is worth noting that streamwiseelongated streaked patterns appearing in this figure closely resemble the flow patterns obtained by LES of the atmospheric turbulence over urban canopies (Kanda et al 2004;Kanda 2006). The streaked patterns are also similar to what has been observed in a near-surface region of planetary boundary layers using Doppler lidars (e.g., Drobinski et al 2004;Fujiyoshi et al 2006;Newsom et al 2008) or LES (e.g., Deardor¤ 1972;Drobinski et al 2007;Lin et al 1996;Moeng and Sullivan 1994) although the spatial scales are quite di¤erent.…”

Section: Threshold For Detectionmentioning

confidence: 99%

LES Study on the Structure of Coherent Eddies Inducing Predominant Perturbations in Velocities in the Roughness Sublayer over Plant Canopies

Watanabe

2009

Journal of the Meteorological Society of Japan

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Large-eddy simulations (LESs) were performed for neutrally-stratified turbulent flows within and above a homogeneous plant canopy. 100 realizations of a three-dimensional turbulence field obtained from each of four LES runs, which di¤er in the driving force of the flow and the size of computational domain, were used in the present study. A conditional sampling technique was used to construct ensemble-averaged images of coherent eddies that induce predominant perturbations in the streamwise and vertical velocities near the canopy top. To reduce subjectivity, wavelet analysis was adopted for triggering the conditional sampling. Synthesis of the present study and numerous previous studies indicated that, in the canopy turbulence, the spatial scale of eddies that induce predominant perturbations in the streamwise velocity is generally three times larger than that of eddies that induce predominant perturbations in the vertical velocity, irrespective of whether concerning field observations, windtunnel experiments or numerical simulations. Therefore, scales of both eddies are mostly determined by a mechanism inherent in the roughness sublayer. An analysis of the ensemble-averaged results and each realization revealed several findings: (1) the smaller eddies that cause predominant perturbations in the vertical velocity are vortices that accord with the so-called mixing-layer (ML) analogy, which is widely accepted as a mechanism of coherent eddies developing near the canopy top; however, (2) the larger eddies inducing predominant perturbations in the streamwise velocity are not vortices and are much larger than expected from the ML analogy; (3) these eddies are streamwise-elongated motions of high-speed downdraft and low-speed updraft, having characteristic features such that the high-speed downdraft penetrates into the canopy and cross-streamwise spreads inside the canopy thus inducing low-speed updraft to the sides of the downdraft and that the low-speed updraft produces a lifted (higher than the canopy top) shear zone beneath an overriding high-speed motion thereby enhancing the shear instability in that area; (4) the high-speed and low-speed motions aligning side-by-side bear a close resemblance to streaky patterns observed in a near-surface region of planetary boundary layers, although the spatial scales are quite di¤erent.

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The Structure of the Near-Neutral Atmospheric Surface Layer

Cited by 123 publications

References 56 publications

Turbulent Structures and Coherence in the Atmospheric Surface Layer

Turbulent Structures and Coherence in the Atmospheric Surface Layer

Updraft and downdraft characterization with Doppler lidar: cloud-free versus cumuli-topped mixed layer

LES Study on the Structure of Coherent Eddies Inducing Predominant Perturbations in Velocities in the Roughness Sublayer over Plant Canopies

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