Yelena L. Pichugina

Turbulent Velocity-Variance Profiles in the Stable Boundary Layer Generated by a Nocturnal Low-Level Jet

Abstract Profiles of mean winds and turbulence were measured by the High Resolution Doppler lidar in the strong-wind stable boundary layer (SBL) with continuous turbulence. The turbulence quantity measured was the variance of the streamwise wind velocity component σ2u. This variance is a component of the turbulence kinetic energy (TKE), and it is shown to be numerically approximately equal to TKE for stable conditions—profiles of σ2u are therefore equivalent to profiles of TKE. Mean-wind profiles showed low-level jet (LLJ) structure for most of the profiles, which represented 10-min averages of mean and fluctuating quantities throughout each of the six nights studied. Heights were normalized by the height of the first LLJ maximum above the surface ZX, and the velocity scale used was the speed of the jet UX, which is shown to be superior to the friction velocity u* as a velocity scale. The major results were 1) the ratio of the maximum value of the streamwise standard deviation to the LLJ speed σu/UX was f...

Turbulence Regimes and Turbulence Intermittency in the Stable Boundary Layer during CASES-99

AbstractAn investigation of nocturnal intermittent turbulence during the Cooperative Atmosphere–Surface Exchange Study in 1999 (CASES-99) revealed three turbulence regimes at each observation height: 1) regime 1, a weak turbulence regime when the wind speed is less than a threshold value; 2) regime 2, a strong turbulence regime when the wind speed exceeds the threshold value; and 3) regime 3, a moderate turbulence regime when top-down turbulence sporadically bursts into the otherwise weak turbulence regime. For regime 1, the strength of small turbulence eddies is correlated with local shear and weakly related to local stratification. For regime 2, the turbulence strength increases systematically with wind speed as a result of turbulence generation by the bulk shear, which scales with the observation height. The threshold wind speed marks the transition above which the boundary layer approaches near-neutral conditions, where the turbulent mixing substantially reduces the stratification and temperature fluc...

Relationship between low-level jet properties and turbulence kinetic energy in the nocturnal stable boundary layer

Abstract In the nighttime stable boundary layer (SBL), shear and turbulence are generated in the layer between the maximum of the low-level jet (LLJ) and the earths surface. Here, it is investigated whether gross properties of the LLJ—its height and speed—could be used to diagnose turbulence intensities in this subjet layer. Data on the height and speed of the LLJ maximum were available at high vertical and temporal resolution using the high-resolution Doppler lidar (HRDL). These data were used to estimate a subjet layer shear, which was computed as the ratio of the speed to the height of the jet maximum, and a jet Richardson number RiJ, averaged at 15-min intervals for 10 nights when HRDL LLJ data were available for this study. The shear and RiJ values were compared with turbulence kinetic energy (TKE) values measured near the top of the 60-m tower at the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) main site. TKE values were small for RiJ greater than 0.4, but as RiJ decreased to less ...

The Very Stable Boundary Layer on Nights with Weak Low-Level Jets

Abstract The light-wind, clear-sky, very stable boundary layer (vSBL) is characterized by large values of bulk Richardson number. The light winds produce weak shear, turbulence, and mixing, and resulting strong temperature gradients near the surface. Here five nights with weak-wind, very stable boundary layers during the Cooperative Atmosphere–Surface Exchange Study (CASES-99) are investigated. Although the winds were light and variable near the surface, Doppler lidar profiles of wind speed often indicated persistent profile shapes and magnitudes for periods of an hour or more, sometimes exhibiting jetlike maxima. The near-surface structure of the boundary layer (BL) on the five nights all showed characteristics typical of the vSBL. These characteristics included a shallow traditional BL only 10–30 m deep with weak intermittent turbulence within the strong surface-based radiation inversion. Above this shallow BL sat a layer of very weak turbulence and negligible turbulent mixing. The focus of this paper i...

Doppler Lidar–Based Wind-Profile Measurement System for Offshore Wind-Energy and Other Marine Boundary Layer Applications

AbstractAccurate measurement of wind speed profiles aloft in the marine boundary layer is a difficult challenge. The development of offshore wind energy requires accurate information on wind speeds above the surface at least at the levels occupied by turbine blades. Few measured data are available at these heights, and the temporal and spatial behavior of near-surface winds is often unrepresentative of that at the required heights. As a consequence, numerical model data, another potential source of information, are essentially unverified at these levels of the atmosphere. In this paper, a motion-compensated, high-resolution Doppler lidar–based wind measurement system that is capable of providing needed information on offshore winds at several heights is described. The system has been evaluated and verified in several ways. A sampling of data from the 2004 New England Air Quality Study shows the kind of analyses and information available. Examples include time–height cross sections, time series, profiles, ...

Quantifying Wind Turbine Wake Characteristics from Scanning Remote Sensor Data

AbstractBecause of the dense arrays at most wind farms, the region of disturbed flow downstream of an individual turbine leads to reduced power production and increased structural loading for its leeward counterparts. Currently, wind farm wake modeling, and hence turbine layout optimization, suffers from an unacceptable degree of uncertainty, largely because of a lack of adequate experimental data for model validation. Accordingly, nearly 100 h of wake measurements were collected with long-range Doppler lidar at the National Wind Technology Center at the National Renewable Energy Laboratory in the Turbine Wake and Inflow Characterization Study (TWICS). This study presents quantitative procedures for determining critical parameters from this extensive dataset—such as the velocity deficit, the size of the wake boundary, and the location of the wake centerline—and categorizes the results by ambient wind speed, turbulence, and atmospheric stability. Despite specific reference to lidar, the methodology is gene...

3D Volumetric Analysis of Wind Turbine Wake Properties in the Atmosphere Using High-Resolution Doppler Lidar

AbstractWind turbine wakes in the atmosphere are three-dimensional (3D) and time dependent. An important question is how best to measure atmospheric wake properties, both for characterizing these properties observationally and for verification of numerical, conceptual, and physical (e.g., wind tunnel) models of wakes. Here a scanning, pulsed, coherent Doppler lidar is used to sample a turbine wake using 3D volume scan patterns that envelop the wake and simultaneously measure the inflow profile. The volume data are analyzed for quantities of interest, such as peak velocity deficit, downwind variability of the deficit, and downwind extent of the wake, in a manner that preserves the measured data. For the case study presented here, in which the wake was well defined in the lidar data, peak deficits of up to 80% were measured 0.6–2 rotor diameters (D) downwind of the turbine, and the wakes extended more than 11D downwind. Temporal wake variability over periods of minutes and the effects of atmospheric gusts a...

Stable Boundary Layer Depth from High-Resolution Measurements of the Mean Wind Profile

Abstract The depth h of the stable boundary layer (SBL) has long been an elusive measurement. In this diagnostic study the use of high-quality, high-resolution (Δz = 10 m) vertical profile data of the mean wind U(z) and streamwise variance σu2(z) is investigated to see whether mean-profile features alone can be equated with h. Three mean-profile diagnostics are identified: hJ, the height of maximum low-level-jet (LLJ) wind speed U in the SBL; h1, the height of the first zero crossing or minimum absolute value of the magnitude of the shear ∂U/∂z profile above the surface; and h2, the minimum in the curvature ∂2U/∂z2 profile. Boundary layer BL here is defined as the surface-based layer of significant turbulence, so the top of the BL was determined as the first significant minimum in the σu2(z) profile, designated as hσ. The height hσ was taken as a reference against which the three mean-profile diagnostics were tested. Mean-wind profiles smooth enough to calculate second derivatives were obtained by averagi...

Remote sensing of the nocturnal boundary layer for wind energy applications.

The fine temporal and spatial resolution of Doppler lidar observations has been highly effective in the study of wind and turbulence dynamic in the nocturnal boundary layer during Lamar Low-Level Project in 2003. The High-Resolution Doppler Lidar (HRDL), designed and developed at the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory (ESRL), measures range-resolved profiles of line-of sight (LOS) Doppler velocity and aerosol backscatter with a pulse repetition frequency of 200 Hz, velocity precision about 10 cm s-1, and a very narrow beam width. The majority of the lidar-measured wind speed and variance profiles were derived using a vertical-scan mode and the application of a vertical binning technique. The profile data were used to calculate quantities important for wind energy applications, including turbulence intensity, wind and directional shear through the layer of the turbine rotor. Profiles of all quantities show a strong variation with height. The mean wind fields, the turbulence, and turbulence intensities show a good agreement with sonic anemometer sodar high confidence (high SNR) measurements. The ability of HRDL to provide continuous information about wind and turbulence conditions at the turbine height and above the range of the tower measurements made HRDL as a powerful instrument for studies of the nighttime boundary layer features. Such information is needed as turbine rotors continue to rise higher into the boundary layer.

The POWER Experiment: Impact of Assimilation of a Network of Coastal Wind Profiling Radars on Simulating Offshore Winds in and above the Wind Turbine Layer

AbstractDuring the summer of 2004 a network of 11 wind profiling radars (WPRs) was deployed in New England as part of the New England Air Quality Study (NEAQS). Observations from this dataset are used to determine their impact on numerical weather prediction (NWP) model skill at simulating coastal and offshore winds through data-denial experiments. This study is a part of the Position of Offshore Wind Energy Resources (POWER) experiment, a Department of Energy (DOE) sponsored project that uses National Oceanic and Atmospheric Administration (NOAA) models for two 1-week periods to measure the impact of the assimilation of observations from 11 inland WPRs. Model simulations with and without assimilation of the WPR data are compared at the locations of the inland WPRs, as well as against observations from an additional WPR and a high-resolution Doppler lidar (HRDL) located on board the Research Vessel Ronald H. Brown (RHB), which cruised the Gulf of Maine during the NEAQS experiment. Model evaluation in the ...