Strong Downslope Winds at Boulder, Colorado

Brinkmann, W. A. R.

doi:10.1175/1520-0493(1974)102<0592:sdwabc>2.0.co;2

Cited by 110 publications

(73 citation statements)

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“…13, just east stable layer was present through most of the Santa Ana episode, similar to the situation studied by Vosper (2004). Elevated inversions also occur during highwind events at Boulder, Colorado (e.g., Brinkmann 1974;Klemp and Lilly 1975). The inversion had appeared prior to 0000 UTC 15 February, and the winds just east of the ridge had already acquired an easterly component, but the simulated winds at WSY (superposed for reference) did not rise until the inversion and cross-ridge flow both strengthened during the next 12 hours.…”

Section: A Verification Of the Standard Runsupporting

confidence: 60%

Downslope Windstorms of San Diego County. Part I: A Case Study

Yang

Fovell

2016

Monthly Weather Review

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The “Santa Ana” wind is an offshore flow that affects Southern California periodically during the winter half of the year, typically between September and May. The winds can be locally gusty, particularly in the complex terrain of San Diego County, where the winds have characteristics of downslope windstorms. These winds can cause and/or rapidly spread wildfires, the threat of which is particularly acute during the autumn season before the onset of winter rains. San Diego’s largest fires, including the Cedar fire of 2003 and Witch Creek fire of 2007, occurred during Santa Ana wind events. A case study of downslope flow during a moderately intense Santa Ana event during mid-February 2013 is presented. Motivated by the need to forecast winds impinging on electrical lines, the authors make use of an exceptionally dense network of near-surface observations in San Diego County to calibrate and verify simulations made utilizing the Advanced Research version of the Weather Research and Forecasting (WRF) Model, which in turn is employed to augment the observations. Results demonstrate that this particular Santa Ana episode consists of two pulses separated by a protracted lull. During the first pulse, the downslope flow is characterized by a prominent hydraulic jumplike feature, while during the second one the flow possesses a clear temporal progression of winds downslope. WRF has skill in capturing the evolution and magnitude of the event at most locations, although most model configurations overpredict the observed sustained wind and the forecast bias is itself biased.

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Section: A Verification Of the Standard Runsupporting

confidence: 60%

Downslope Windstorms of San Diego County. Part I: A Case Study

Yang

Fovell

2016

Monthly Weather Review

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“…Since the wind direction was westerly (wind direction not shown), these were likely associated with downsloping flow that frequently occurs in this area on the lee of the mountains, which transports momentum downward from higher in the troposphere (e.g. Brinkmann, 1974). Conditions predominantly supported weakly stable and neutral stability during the study period, as indicated within Table 2.…”

Section: Llnl Windcube V2mentioning

confidence: 99%

Improvement of vertical velocity statistics measured by a Doppler lidar through comparison with sonic anemometer observations

et al. 2016

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Abstract. Since turbulence measurements from Doppler lidars are being increasingly used within wind energy and boundary-layer meteorology, it is important to assess and improve the accuracy of these observations. While turbulent quantities are measured by Doppler lidars in several different ways, the simplest and most frequently used statistic is vertical velocity variance (w 2 ) from zenith stares. However, the competing effects of signal noise and resolution volume limitations, which respectively increase and decrease w 2 , reduce the accuracy of these measurements. Herein, an established method that utilises the autocovariance of the signal to remove noise is evaluated and its skill in correcting for volume-averaging effects in the calculation of w 2 is also assessed. Additionally, this autocovariance technique is further refined by defining the amount of lag time to use for the most accurate estimates of w 2 . Through comparison of observations from two Doppler lidars and sonic anemometers on a 300 m tower, the autocovariance technique is shown to generally improve estimates of w 2 . After the autocovariance technique is applied, values of w 2 from the Doppler lidars are generally in close agreement (R 2 ≈ 0.95 − 0.98) with those calculated from sonic anemometer measurements.

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“…While unstable stratification of the boundary layer weakens the mountain waves and reduces the surface drag by up to 90%, stable stratification may increase the drag several times compared to the hydrostatic wave drag without boundary-layer effects. Several other studies also found enhanced gravity-wave activity during nighttime when the surface heat flux was negative (Brinkmann 1974;Whiteman and Whiteman 1974;Doyle et al 2005;Valkonen et al 2010). In large wind farms, gravity waves impose significant pressure gradients in the boundary layer and play an important role in redistributing energy throughout the farm (Allaerts and Meyers 2017).…”

Section: Introductionmentioning

confidence: 99%

Gravity Waves and Wind-Farm Efficiency in Neutral and Stable Conditions

Allaerts

Meyers

2017

Boundary-Layer Meteorol

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We use large-eddy simulations (LES) to investigate the impact of stable stratification on gravity-wave excitation and energy extraction in a large wind farm. To this end, the development of an equilibrium conventionally neutral boundary layer into a stable boundary layer over a period of 8 h is considered, using two different cooling rates. We find that turbulence decay has considerable influence on the energy extraction at the beginning of the boundary-layer transition, but afterwards, energy extraction is dominated by geometrical and jet effects induced by an inertial oscillation. It is further shown that the inertial oscillation enhances gravity-wave excitation. By comparing LES results with a simple one-dimensional model, we show that this is related to an interplay between wind-farm drag, variations in the Froude number and the dispersive effects of vertically-propagating gravity waves. We further find that the pressure gradients induced by gravity waves lead to significant upstream flow deceleration, reducing the average turbine output compared to a turbine in isolated operation. This leads us to the definition of a non-local wind-farm efficiency, next to a more standard wind-farm wake efficiency, and we show that both can be of the same order of magnitude. Finally, an energy flux analysis is performed to further elucidate the effect of gravity waves on the flow in the wind farm.

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Strong Downslope Winds at Boulder, Colorado

Cited by 110 publications

References 0 publications

Downslope Windstorms of San Diego County. Part I: A Case Study

Downslope Windstorms of San Diego County. Part I: A Case Study

Improvement of vertical velocity statistics measured by a Doppler lidar through comparison with sonic anemometer observations

Gravity Waves and Wind-Farm Efficiency in Neutral and Stable Conditions

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