On the integral modelling of katabatic flows

The integral model for katabatic flows proposed by Manins and Sawford (1979) is solved numerically. It is shown that numerical solutions can be approximated by Ball's (1956) model in the upper part of the slope, while they tend toward Manins and Sawford's simplified solution farther downslope. The importance of entrainment and ambiant stable stratification is shown. Some limitations of Manins and Sawford's model are discussed.     

A simple model of drainage flow on a slope

The concept of a cold air ‘Parcel’ is introduced for describing the bulk properties of drainage flow. By means of a model based on the momentum and sensible heat transports under calm conditions, the thickness h and velocity u of the Parcel are derived in simple forms. It is shown that h and u correspond to the inversion height and maximum velocity of actual drainage flow. The governing parameters for h and u are the length and vertical drop of the slope, potential temperature difference between the ambient atmosphere and the Parcel, aerodynamic condition of the slope surface expressed by the mean bulk coefficients, and ambient stability. The mean bulk coefficients depend on the roughness lengths for the velocity and potential temperature profiles and are decreasing functions of the slope length. The Parcel Model agrees qualitatively with Manins and Sawford's (1979) model under neutral ambient stratification. But agreement is not so good under stable conditions. The thickness and velocity of drainage flow predicted by the Parcel Model agree with observations on slopes several tens of meters to several hundred kilometers long.     

Mean horizontal wind profiles measured in the atmospheric boundary layer about a simulated block building

Instrumented wind towers are used to measure the three components of wind about a simulated block building. The mean horizontal wind profiles over the building are compared with wind profiles measured in the absence of the building and the wind speed deficit in the wake of the building is correlated.Horizontal mean wind speeds measured in the natural atmospheric boundary layer with and without the presence of a simulated building show excellent reproducibility and agreement with fundamental concepts of fluid mechanics. The data possess all the characteristic features reported from wind-tunnel studies of building flows. In the present study the turbulence intensity is of the order of 20% in the undisturbed flow whereas the free stream turbulence intensity of wind-tunnel studies is generally not more than 5%. The effect of smaller averaging periods and the structure of the turbulence will be reported at a later time.The velocity profiles measured in the undisturbed flow zones support the representation of a neutrally stable atmospheric boundary layer with a logarithmic wind profile.     

Performance of the Wind Farm Parameterization Scheme Coupled with the Weather Research and Forecasting Model under Multiple Resolution Regimes for Simulating an Onshore Wind Farm

We use the Wind Farm Parameterization(WFP) scheme coupled with the Weather Research and Forecasting model under multiple resolution regimes to simulate turbulent wake dynamics generated by a real onshore wind farm and their influence at the local meteorological scale. The model outputs are compared with earlier modeling and observation studies. It is found that higher vertical and horizontal resolutions have great impacts on the simulated wake flow dynamics. The corresponding wind speed deficit and turbulent kinetic energy results match well with previous studies. In addition, the effect of horizontal resolution on near-surface meteorology is significantly higher than that of vertical resolution. The wake flow field extends from the start of the wind farm to downstream within 10 km, where the wind speed deficit may exceed 4%. For a height of 150 m or at a distance of about 25 km downstream, the wind speed deficit is around 2%. This indicates that, at a distance of more than 25 km downstream, the impact of the wind turbines can be ignored. Analysis of near-surface meteorology indicates a night and early morning warming near the surface, and increase in near-surface water vapor mixing ratio with decreasing surface sensible and latent heat fluxes. During daytime, a slight cooling near the surface and decrease in the near-surface water vapor mixing ratio with increasing surface sensible and latent heat fluxes is noticed over the wind farm area.     

新疆克拉玛依强下坡风暴的机理研究

利用美国中尺度数值模式 WRF 对2013年3月7—8日克拉玛依强风进行了模拟,对下坡风发生、发展和结束3个阶段的三维结构特征进行了分析,并由此提出克拉玛依强下坡风的形成机制模型：上游地区出现中高层西南风、低层西北风并伴有强冷平流的配置,当风速不断增大时,气流能够翻越加依尔山在背风坡侧形成重力波,重力波相位向气流上游方向倾斜产生非线性效应,促进了波不稳定区域的形成并导致波破碎,形成湍流活跃层,不断把上层的能量向下传播;克拉玛依中低层形成三层夹心的大气层结稳定度分布,出现明显的过渡气流带从而导致强下坡风的形成;南北风分量在低层和中层符号相反,形成了临界层,不断吸收上层波能量并向地面传送,强下坡风暴不断维持发展。最后利用2006—2012年克拉玛依33个强下坡风过程中的探空观测资料对提出的形成机制进行了验证。     

On the impact of atmospheric thermal stability on the characteristics of nocturnal downslope flows

The impacts of background (or ambient) and local atmospheric thermal stabilities, and slope steepness, on nighttime thermally induced downslope flow in meso- domains (i.e., 20–200 km horizontal extent) have been investigated using analytical and numerical model approaches. Good agreement between the analytical and numerical evaluations was found. It was concluded that: (i) as anticipated, the intensity of the downslope flow increases with increased slope steepness, although the depth of the downslope flow was found to be insensitive to slope steepness in the studied situations; (ii) the intensity of the downslope flow is generally independent of background atmospheric thermal stability; (iii) for given integrated nighttime cooling across the nocturnal boundary layer (NBL), Q _s the local atmospheric thermal stability exerts a strong influence on downslope flow behavior: the downslope flow intensity increases when local atmospheric thermal stability increases; and (iv) the downslope flow intensity is proportional to Q _s ^1/2.     

Impact of land surface roughness on downslope windstorm modelling in the Arctic

The article considers the reasons for the underestimation of the wind speed by the WRF-ARW model when simulating downslope windstorms in the Russian Arctic. Simulation results for the Tiksi windstorm, for which sensitivity tests were carried out, appeared to be weakly dependent on the initial and boundary conditions, topography resolution, and boundary layer parameterization. Wind speed underestimation was mostly related to improper land use and the highly overestimated roughness length, which are used in the model. Reduction of the roughness length in accordance with the observations leads not only to a quantitative change in the wind speed in the boundary layer, but to qualitative changes in the dynamics of the flow. Wind underestimation in simulations with the overestimated roughness was caused by the jet stream unrealistically jumping over the lee slope and wake formation in the station area, while jet stream stayed near the surface and propagated to the station area in simulations with the modified roughness length. Modification of land use and roughness length in Tiksi and other regions where downslope windstorms are observed (Novaya Zemlya, Pevek, Wrangel Island) led to a decrease in wind speed modelling error by more than 2.5 times.     

下坡运动的分析解

本文采用二层模式,在一个斜坡地形上,求解大气波动方程,得出了下坡运动的普遍分析解,并对解在不同大气条件和地形条件下的物理意义做了分析。当下层大气厚而稳定、风速较强时,得到的是背风坡大风。而当下层大气强稳定、小风时,得到的是泄流风。对各类下坡运动近地面风速和大气层结、流场结构及Scorer参数之间的关系也做了讨论。     

A Simple Temporal and Spatial Analysis of Flow in Complex Terrain in the Context of Wind Energy Modelling

A simple temporal and spatial analysisis done on wind speed and direction data from a number ofmeteorological towers separated by distances between roughly 1 and 100 kilometres. The analysis is done in the context of expected model error in wind energy calculations. The study first uses single point statistics to show the evolution of mean values with time. It is shown that strong seasonal signals are present and that stable means are achieved only after averaging periods of a year or more. The study then uses discrete Fourier transforms to show that significant amounts of spectral energy reside in modes with periods of a few days to less than a day. Frequency dependent cross correlation values are then derived and used to show how correlation between towers diminishes with increasing frequency. The mechanism responsible for this diminished correlation is shown through the comparison of cross-correlation phase as a function of frequency and its relationship to distance between towers. Error in wind energy estimates are shown to be strongly related to correlation and therefore distance over which the prediction is made. In summary, much of the inaccuracy in modelling flow in the context of wind energy calculations is due to a lack of scale separation between the deterministic part of the flow, which is well modelled, and that part of the flow that is stochastic at the length and time scales modelled.     

A numerical study on severe downslope windstorms occurred on 5 April 2005 at Gangneung and Yangyang, Korea

Severe downslope windstorms occurred on 5 April 2005 in the Taebaek Mountain Range, located in the eastern coast of Korea, are examined using the Weather Research and Forecasting (WRF) model. Strong winds are observed at Gangneung and Yangyang during two separate periods with a rapidly decreasing period in between. These downslope windstorms are reproduced in the simulation reasonably well, although the rapidly decreasing surface wind speed after the second windstorm could not be captured at Yangyang. It is found that the generation mechanisms of the downslope windstorms in these two periods are somewhat different. The severe wind in the first period is likely due to the reflection of the mountain waves from a critical level that locates near z = 8–9 km. Upward-propagating waves and reflected downward-propagating waves interact constructively in a duct between the critical level and the surface, resulting in strong surface wind. In the second period, the hydraulic-jump theory can be applied in that the wave breaking above the downstream induces a well-mixed region, and severe downslope wind is developed beneath this turbulent region as the streamlines descend along the downstream. Simultaneous lee wave structure is also reproduced during the second windstorm period. The sensitivity of the downslope wind speed to the change in the land-cover map showed that the absorption of trapped lee waves in the boundary layer reduces the downslope wind speed significantly after the second windstorm at Gangneung, improving the model performance, although with no significant impact at Yangyang.     

Development and testing of a two-dimensional downslope wind model

A two-dimensional numerical model has been formulated, using a terrain-following coordinate system. By introducing an iterative procedure into the direct integration method, almost identical results to the more complicated implicit method were achieved. This allows much higher computing efficiency without compromising accuracy. By using the model, the effects of ground cooling rate, slope angle and limited slope length were determined. In order to test the robustness of the model, a field campaign to collect downslope wind data was conducted. Results from the two-dimensional model simulations agree well with the observed data. It is possible to derive some simple rules for the downslope wind, with ground cooling, slope angle and slope length as the principal inputs. These simple rules will undoubtedly be useful in practical situations when accurate numerical simulations are difficult to make.     

Influence of non-flat terrain and wind direction shear on canopy turbulence

We have analyzed eddy covariance data collected within open canopy to investigate the influence of non-flat terrain and wind direction shear on the canopy turbulence. The study site is located on non-flat terrain with slopes in both south-north and east-west directions. The surface elevation change is smaller than the height of roughness element such as building and tree at this site. A variety of turbulent statistics were examined as a function of wind direction in near-neutral conditions. Heterogeneous surface characteristics results in significant differences in measured turbulent statistics. Upwind trees on the flat and up-sloping terrains yield typical features of canopy turbulence while upwind elevated surface with trees yields significant wind direction shear, reduced u and w skewness, and negligible correlation between u and w. The directional dependence of turbulence statistics is due that strong wind blows more horizontally rather than following terrain, and hence combination of slope related momentum flux and canopy eddy motion decreases the magnitude of Sk _w and r _uw for the downslope flow while it enhances them for the upslope flow. Significant v skewness to the west indicates intermittent downdraft of northerly wind, possibly due to lateral shear of wind in the presence of significant wind direction shear. The effects of wind direction shear on turbulent statistics were also examined. The results showed that correlation coefficient between lateral velocities and vertical velocity show significant dependence on wind direction shear through change of lateral wind shear. Quadrant analysis shows increased outward interaction and reduced role of sweep motion for longitudinal momentum flux for the downslope flow. Multi-resolution analysis indicates that uw correlation shows peak at larger averaging time for the upslope flow than for the downslope flow, indicating that large eddy plays an active role in momentum transfer for the upslope flow. On the other hand, downslope flow shows larger velocity variances than other flows despite similar wind speed. These results suggest that non-flatness of terrain significantly influences on canopy-atmosphere exchange.     

Katabatic wind profiles over the Greenland ice sheet: observation and modelling

Profiles of wind and temperature have been observedabove the Greenland ice sheet, 90 km from its westernmargin, in July 1991. The terrain slopes downward tothe west. Measurements were performed with instrumentson a 30 m mast, combined with a Doppler SODAR and aRASS. Whereas the surface is usually at the meltingpoint, the temperatures in the free atmosphere areabove freezing. The depth of the boundary layer, in whichthe wind turns to the free atmosphere direction, is notmuch more than 100 m. The surface wind is always aboutfrom the southeast (hence with a downslope component),whereas winds from the southwest (with an upslopecomponent) often occur at the 100 m level.Mixing length profiles for momentum were estimatedby comparison of calculated and observed windprofiles. A good accordance between calculated andobserved wind speed was obtained. The neutralmixing length had a maximum of only a few metres, whichwas approached already at low height. The limiting valueis proportional to the 0.7-th power of the Froudenumber times a length scale obtained from thetemperature profile.     

Large-Eddy Simulation of Katabatic Flows

A large-eddy simulation model with rotated coordinates and an open boundary is used to simulate the characteristics of katabatic flows over simple terrain. Experiments examine the effects of cross winds on the development of the slope-flow boundary layer for a steep (20°) slope and the role of drainage winds in preventing turbulence collapse on a gentle slope (1°). For the steep flow cases, comparisons between model average boundary-layer velocity, temperature deficit, and turbulence kinetic energy budget terms and tower observations show reasonable agreement. Results for different cross slope winds show that as the cross slope winds increase, the slope flow deepens faster and behaves more like a weakly stratified, sheared boundary layer. Analysis of the momentum budget shows that near the surface the flow is maintained by a balance between downslope buoyancy forcing and vertical turbulence flux from surface drag. Above the downslope jet, the turbulence vertical momentum flux reverses sign and acceleration of the flow by buoyancy is controlled by horizontal advection of slower moving ambient air. The turbulence budget is dominated by a balance between shear production and eddy dissipation, however, buoyancy and pressure transport both are significant in reducing the strength of turbulence above the jet. Results from the gentle slope case show that even a slight terrain variation can lead to significant drainage winds. Comparison of the gentle slope case with a flat terrain simulation indicates that drainage winds can effectively prevent the formation of very stable boundary layers, at least near the top of sloping terrain.     

Relation of slope winds to the ambient flow over gentle terrain

Four years of continuous tower data collected at the Risø National Laboratory are analyzed to study the climatological influence of a gentle slope on surface winds. Under very stable nocturnal conditions, the surface air tends to flow down the slope, at least intermittently, regardless of the direction of the overlying ambient wind. With more significant upslope ambient wind and only modest stability, downslope gravity flow is normally prevented. However, the slope-buoyancy effect is still of importance in that it retards the upslope flow of cold air. This effect is of climatic importance for the data studied here.     

Numerical simulations of katabatic jumps in coats land,Antartica

A non-hydrostatic numerical model, the Regional Atmospheric Modeling System (RAMS), has been used to investigate the development of katabatic jumps in Coats Land, Antarctica. In the control run with a 5 m s^-1downslope directed initial wind, a katabatic jump develops near the foot of the idealized slope. The jump is manifested as a rapid deceleration of the downslope flow and a change from supercritical to subcritical flow, in a hydraulic sense, i.e., the Froude number (Fr) of the flow changes from Fr > 1 to Fr> 1. Results from sensitivity experiments show that an increase in the upstream flow rate strengthens the jump, while an increase in the downstream inversion-layer depth results in a retreat of the jump. Hydraulic theory and Bernoulli's theorem have been used to explain the surface pressure change across the jump. It is found that hydraulic theory always underestimates the surface pressure change, while Bernoulli's theorem provides a satisfactory estimation. An analysis of the downs balance for the katabatic jump indicates that the important forces are those related to the pressure gradient, advection and, to a lesser extent, the turbulent momentum divergence. The development of katabatic jumps can be divided into two phases. In phase I, the t gradient force is nearly balanced by advection, while in phase II, the pressure gradient force is counterbalanced by turbulent momentum divergence. The upslope pressure gradient force associated with a pool of cold air over the ice shelf facilitates the formation of the katabatic jump.     

克拉玛依大风数值预报中的重力波拖曳方案应用研究

新疆克拉玛依地区位于背风坡,长期受翻山气流形成的局地大风影响。该地区日常大风预报业务主要依托克拉玛依气象局的精细化数值预报系统,为进一步完善该系统的预报效果,本文开展了重力波拖曳参数化方案应用试验,对比分析了三组试验方案:不开启重力波拖曳、只开启外层区域重力波拖曳以及两重嵌套设置下均开启重力波拖曳的结果。试验结果表明:在克拉玛依大风预报个例中,对于山区地形背风坡处的克拉玛依站,不启动重力波拖曳会导致风速偏大和起风时间较早;开启外层重力波拖曳方案可以延迟克拉玛依站起风时间,两重区域均开启可以进一步延迟该站的起风时间,使起风时间更接近实况;对于非山区地形背风坡处的站点,在内层开启重力波拖曳会导致一定的负效果;对内层区域的统计检验结果表明不开启重力波拖曳,会使地形复杂区域风场预报存在一定的正偏差,在外层开启重力波拖曳会使正偏差有一定改善,而两重嵌套均开启重力波拖曳方案会导致非地形陡峭区域的负偏差增加。综合全区域内大风预报检验结果表明,只在外层开启重力波拖曳会获得最小的误差和较好的预报效果。     

Cross-Spectrum of Wind Speed for Meso-Gamma Scales in the Upper Surface Layer over South-Eastern Australia

Analytical expressions for the cross-spectrum of wind speed are developed for the stochastic simulation of wind power in south-eastern Australia. The expressions are valid for heights above the ground in the range 40–80 m, site separations of 1–30 km, and frequencies of (1/6)–3 cycles h⁻¹. The influence of site separation distance is taken into account, as are variables that are defined for blocks of time. These variables include the mean and standard deviation of wind speed and the mean wind direction. The parameters of the model equations are determined by non-linear least-squares regression with cross-validation over 10 years of wind measurements from 84 towers in south-eastern Australia.     

ANALYTICAL SOLUTIONS OF DOWNSLOPE MOTIONS

The solutions of downslope motions over a sloping terrain are solved analytically in terms of the atmos-pheric wave equations with a two-layer model.The physical meanings of the solutions are discussed.As thelower layer of the atmosphere is stable and deep with strong wind the solution represents strong downslopewind,while as the lower layer is strong stable with light wind the drainage flow is obtained.The dependenceof the strength of downslope motion on the atmospheric stratification,wind field structure as well as Scorerparameter is also examined.     

A study of the diurnal behavior of boundary-layer winds at the Boulder Atmospheric Observatory

Diurnal wind variations are examined at the Boulder Atmospheric Observatory which is located 25 km east of the foothills of the Rocky Mountains. Data were obtained from a 300-m tower which was instrumented at eight levels and operated almost continuously for three weeks during September 1978. Observations on clear days, for which the diurnal heating and cooling of the local terrain slopes can be expected to affect the winds, show that daytime winds tend to be easterly (upslope) throughout the 300-m depth. At night, a temperature inversion typically develops to about 100 m. Below this level, the nocturnal flow tends to be downslope; above the inversion, a distinctly different regime of flow develops. A diurnal wind oscillation, characterized by strong southerly flow beginning near sunset and ending near midnight, occurred in the upper layer on 25% of the days during the study period. Rapid clockwise rotation of the wind vector occurred during the period of increased wind speed. This oscillation occurred only on days when the synoptic-scale geostrophic wind was southerly. It is suggested that this non-steady state behavior is an inertial oscillation affected by the diurnally varying temperature gradients and local topography.