Comments on ‘the influence of geostrophic shear on the cross-isobar angle of the surface wind’

Boundary-Layer Meteorology -     

The influence of the geostrophic wind advection approximation on a well-mixed layer

Numerical results indicate that advection of momentum in the boundary layer may significantly alter both the structure of the planetary boundary layer and its influence on the overlying free atmosphere. However, due to the nonlinearity of the inertial terms, it is always difficult to obtain the analytical solution of the boundary-layer model that retains the flow acceleration. In order to overcome this difficulty, the geostrophic momentum (hereafter GM) approximation has been introduced into boundary-layer models. By replacing the advected momentum with the geostrophic wind, the effect of the flow acceleration is partially considered and the original nonlinear partial differential equation set is converted to ordinary differential equations, the solutions of which can be obtained easily with standard techniques. However, the model employing GM fails to capture the features of the boundary layer when the spatio-temporal variation of the boundary-layer flow cannot be properly approximated by the geostrophic wind. In the present work, a modified boundary-layer model with the inertial acceleration in a different approximate form is proposed, in which the advecting wind instead of the advected momentum is approximated by the geostrophic wind (hereafter GAM).Comparing the horizontal velocity and boundary-layer pumping obtained from the classical Ekman theory, and the model incorporating (i) GM and (ii) GAM, it is found that the model with GAM describes most facets of the steady well-mixed layer beneath a north-westerly flow with embedded mesoscale perturbations that is considered in the present work. Inspection of the solution of the model with GAM shows that, within the limit of the validation of the model (i.e., the Rossby number R_O is not very large and the drag coefficient C_D is not too small), the horizontal convergence (divergence) is strengthened by the effect of the inertial acceleration in the region of maximum positive (negative) geostrophic vorticity. Consequently, the boundary-layer pumping there is intensified. It is found that the intensification is firstly strengthened and then weakened as R_O or C_D increases.     

地转动量近似下边界层中风场的调整

本文利用地转动量近似,并假设气压场为定常的圆形涡旋和初始风场不满足四力平衡(气压梯度力、科里奥利力、湍流粘性力和半地转惯性力)的条件下,求解了正压边界层中风场向气压场调整的初边值问题,得到了一些初步结论。本工作为利用四力平衡下的风速分布来诊断预报边界层风场提供了理论依据。     

风廓线雷达中风切变分析方法的初步研究

在风廓线雷达连续输出的风场时间高度显示图上,尝试进行风场的流线分析和风切变值等值线分析,以便用于识别强烈风切变区。在风场变化比较均匀时,分析出的线形分布比较平缓,而在风场变化比较剧烈的区域,风向等值线、垂直风切变等值线和流线三者一致表现出了汇聚特点,线形的分布也异常地密集,分析结果直观地反映出了风场分布的特征和危险性风切变区域,该结果有助于风切变自动识别方法的研究。     

The influence of sea waves on the wind profile

From wind profile and wave measurements performed during the JONSWAP II experiment, relations between the dimensionless profile slope and the significant wave height are derived. It is shown that the wind profile is distorted by the waves especially in the vicinity of the water surface. The wave influence on the profile seems to be restricted to heights below about three wave heights. Above this level, the dimensionless profile slope is an approximately constant value corresponding to a drag coefficient of about 1.15 × 10^–3.     

雷暴型风切变及其对飞机飞行影响的初步探讨

本文利用北京320米气象观测塔的实测资料和715测雨雷达回波图片.对1980年8月31日晚影响北京地区的两次雷暴天气过程对塔层各要素和参量的影响情况进行了综合分析.结果表明,尽管两次雷暴天气过程的主体路径偏北,但塔层资料仍然明显地反映出雷暴冷性外流气流的存在.冷外流前沿离雷暴云体20公里左右.两次雷暴天气过程的外流气流中都出现了大于0.2秒的严重风切变.这足以使飞机纵向运动发生不稳定而危及安全.为此提出,应把飞机远离雷暴云主体20—30公里飞行作为一个重要的安全措施,不要侥幸地去抢飞这一危险空域.     

The impact of meridonal wind shear on baroclinic life cycle

In this study, initial-value calculations are performed with a primitive equation model to examine whether the stabilizing effect of the horizontal zonal wind shear in the background state can account for the observed variability in baroclinic life cycles. The life cycle calculations show that a greater maximum eddy energy is attained for the observed basic state with anomalously weak horizontal zonal wind shear, suggesting that the horizontal zonal wind shear indeed plays the dominant role in determining the eddy amplitude. In addition, under this weak shear, the life cycle produces a more pronounced poleward jet shift. Because model simulations of warmer climates tend to show both a poleward jet shift and more intense zonally localized tropical convection which tends to produce the weak shear state, the result of this study provides a mechanism whereby the strengthening of tropical convection can contribute toward the poleward jet shift in warm climates.     

垂直风切变对螺旋波不稳定的影响

通过建立一系列局地准二维直角坐标系,用以研究垂直风切变对热带气旋中螺旋波不稳定的影响,并计算了螺旋波不稳定增长率在螺旋线上的分布,得到了以下结论:不稳定增长率开始随热带气旋半径增大而增大,在最大风速圈处达最大;然后随半径减小,以后再随半径增大,出现一个局地极值,再以后又减小,即在螺旋线上有两个极值中心;这与实际热带气旋的情况相一致。不稳定螺旋波的增长率与基流的垂直切变和层结参数密切有关,基流垂直切变越大,层结参数越小,则增长率越大;也即Richardson数越小,则不稳定增长率越大。     

西北太平洋风速垂直切变异常对热带气旋活动年际变化的影响

应用中国《台风年鉴》资料、欧洲中心40年月平均再分析资料和NOAA的逐月海温资料,研究了西北太平洋(5°—30°N,110°E—180°)风速垂直切变异常对热带气旋(TC)活动年际变化的影响。研究发现,西北太平洋所有TC、风暴以上级别的TC(TSTY,即达到热带风暴级别及以上的所有TC)和所有台风(WTY,包括台风、强台风和超强台风)年频数与西北太平洋风速垂直切变都显著负相关。西北太平洋风速垂直切变大小对生成源地在南海(5°—30°N,110°—120°E)TC和西北太平洋西部海域(5°—30°N,120°—150°E)TC的影响较小,而对西北太平洋东部海域(5°—30°N,150°E—180°)生成的TC影响最大:即西北太平洋风速垂直切变负异常年,有利于西北太平洋东部海域TC生成发展,使得负异常年较正异常年TC频数偏多和源地平均位置偏东;并且风速垂直切变的变化对TC频数和生成源地影响的显著性,随着TC强度的增加而增加。对TSTY生成环境场的进一步分析表明,西北太平洋风速垂直切变偏小年,季风槽偏强位置偏东,它的东端位于宽阔的太平洋洋面,与弱风速垂直切变区相配合,暖的海温加上低层强烈的正涡度和强烈辐合,且相应的高层有强的气流辐散区,这些环境场都有利于TSTY在主要源地尤其是西北太平洋东部海域生成,这是风速垂直切变偏小年TSTY偏多和生成源地偏东的重要原因。     

低空风切变的分析与预报

低空风切变是飞机起飞和着陆阶段威胁飞行安全的主要危险天气,分为水平风的垂直切变、水平风的水平切变、垂直气流切变三种类型.低空风切变主要是由大气运动的变化所造成,强对流天气、锋面天气、低空急流天气都可能引起低空风切变;另外,特别的地理环境也是不容忽视的因素.     

Analysis of the vortivity and divergence of the thermal wind in pure geostrophic flow

By applying the horizontal del operator to the thermal wind equation in vectorial form and considering only horizontal geostrophic motion, expressions are found for the vorticity and divergence of thermal wind. It is shown that vorticity, in the case of pure geostrophic flow at all levels, depends upon four factors, namely: latitude, horizontal distribution of temperature, temperature lapse rate and divergence of horizontal temperature gradient. The divergence of the thermal wind, on the other hand, depends only upon the orientation of the mean isotherms of an isobaric layer relative to the parallel circles. These terms are evaluated and it is shown that the latitude term and the lapse rate term are far greater than the remaining terms of the vorticity equation for geostrophic flow. A criterion is then given which enables to determine the conditions for irrotational horizontal motion with vertical geostrophic wind shear.

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Zusammenfassung Durch Anwendung des horizontalenNabla-Operators auf die thermische Windgleichung in vektorieller Form und bei ausschließlicher Berücksichtigung der geostrophischen Horizontalbewegung lassen sich Ausdrücke für die vorticity und die Divergenz des thermischen Windes herleiten. Es wird gezeigt, daß die vorticity im Falle einer rein geostrophischen Strömung in allen Niveaus von folgenden vier Faktoren bedingt ist: Breite, horizontale Temperaturverteilung, vertikaler Temperaturgradient und Divergenz des horizontalen Temperaturgradienten. Andrerseits hängt die Divergenz des thermischen Windes nur von der Orientierung der mittleren Isothermen einer in Bezug auf die Parallelkreise isobaren Schicht ab. Diese vier Terme werden ausgewertet und es wird gezeigt, daß das Breitenglied und der Term für den Vertikalgradienten viel größer sind als die übrigen Terme der vorticity-Gleichung für geostrophische Strömung. Schließlich wird ein Kriterium entwickelt, das gestattet, die Bedingungen für rotorfreie Horizontalbewegung mit vertikaler geostrophischer Windscherung zu bestimmen.

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Résumé On peut déduire des équations sur la rotationelle et la divergence du vent thermique en appliquant l'opérateur nabla horizontal à l'équation du vent thermique sous une forme vectorielle et en ne considérant que le mouvement géostrophique horizontal. On démontre que la rotationelle, dans le cas d'un courant géostrophique pur, dépend dans tous les niveaux des quatre facteurs suivants: latitude, répartition horizontale de la température, gradient vertical de la température et divergence du gradient horizontal de température. D'autre part, la divergence du vent thermique ne dépend que de l'orientation des isothermes moyennes d'une couche isobare par rapport aux parallèles. En évaluant ces quatre termes, on montre que les termes relatifs à la latitude et au gradient vertical sont beaucoup plus grands que les autres termes de l'équation de la rotationelle pour un courant géostrophique. Pour terminer, l'auteur développe un critère qui permet de déterminer les conditions pour un mouvement horizontal irrotationel mais avec un cisaillement vertical géostrophique du vent.

     

低空风切变的分析与预报

低空风切变是飞机起飞和着陆阶段威胁飞行安全的主要危险天气，分为水平风的垂直切变、水平风的水平切变、垂直气流切变三种类型。低空风切变主要是由大气运动的变化所造成，强对流天气、锋面天气、低空急流天气都可能引起低空风切变；另外，特别的地理环境也是不容忽视的因素。     

洛阳一次早春雷暴天气过程的初步分析

本文通过对洛阳机场2001年2月22日傍晚发生的一次早春雷暴天气过程进行分析，讨论了导致该过程的有利环流形势、与能量场的配置关系以及温湿因素，对洛阳地区的早春雷暴预报线索做一探讨。     

阳一次早春雷暴天气过程的初步分析

本文通过对洛阳机场2001年2月22日傍晚发生的一次早春雷暴天气过程进行分析,讨论了导致该过程的有利环流形势、与能量场的配置关系以及温湿因素,对洛阳地区的早春雷暴预报线索做一探讨.     

Regional shear stress of broken forest from radiosonde wind profiles in the unstable surface layer

Mean wind speed profiles were measured by tracking radiosondes in the unstable atmospheric boundary layer (ABL) over the forested Landes region in southwestern France. New Monin-Obukhov stability correction functions, recently proposed following an, analysis by Kader and Yaglom, as well as the Businger-Dyer stability formulation were tested, with wind speeds in the surface sublayer to calculate the regional shear stress. These profile-derived shear stresses were compared with eddy correlation measurements gathered above a mature forest stand, at a location roughly, 4.5 km from the radiosonde launch site. The shear stress values obtained by means of the newly proposed stability function were in slightly better agreement with the eddy correlation values than those obtained by means of a Businger-Dyer type stability function. The general robustness of the profile method can be attributed in part to prior knowledge of the regional surface roughness (z ₀=1.2 m) and the momentum displacement height (d ₀=6.0 m), which were determined from neutral wind profile analysis. The 100 m drag coefficient for the unstable conditions above this broken forest surface was found to beu _* ² /V ₁₀₀ ² =0.0173.     

Estimation of persistence and trends in geostrophic wind speed for the assessment of wind energy yields in Northwest Europe

Wind climate in Northwest Europe is subject to long-term persistence (LTP), also called the Hurst phenomenon. Ignorance of LTP causes underestimation of climatic variability. The quantification of multi-year variability is important for the assessment of the uncertainty of future multi-year wind yields. Estimating LTP requires long homogeneous time series. Such series of wind observations are rare, but annual mean geostrophic wind speed (U) can be used instead. This study demonstrates a method to estimate the 10-year aggregated mean U for the near and the far future and its uncertainty in Northwest Europe. Time series of U were derived from daily sea level pressure from the European Climate Assessment Dataset. Minor inhomogeneities cannot be ruled out, but were shown to hardly affect the estimated Hurst exponent $(\hat{H})$ . A maximum likelihood method was adjusted to remove the biases in $\hat{H}$ . The geostrophic wind speed over the North Sea, the British Isles and along the Scandinavian coast are characterised by statistically significant H between 0.58 and 0.74, (H?=?0.5 implies no LTP). The additional affect of the parameter uncertainty is estimated in a Bayesian way and is highly dependent on the record length. The assessment of structural changes in future wind fields requires general circulation models. An ensemble of seventeen simulations (ESSENCE) with one single climate model (ECHAM5/MPI-OM) was used to evaluate structural trends and LTP. The estimated $\hat{H}$ in the ESSENCE simulations are generally close to 0.5 and not significant. Significant trends in U are found over large parts of the investigated domain, but the trends are small compared to the multi-year variability. Large decreasing trends are found in the vicinity of Iceland and increasing trends near the Greenland coast. This is likely related to the sea ice retreat within the ESSENCE simulations and the associated change in surface temperature gradients.     

The effect of rainfall on the wind in the surface layer

Raindrops falling into a wind-shear zone do not adjust their horizontal speeds immediately to the new wind. Thus, stresses develop which accelerate the air. The effect of raindrops falling into a layer with a logarithmic wind profile is investigated numerically. We find the adjustment of the wind to this situation to be quite small and only barely detectable, even in heavy rain conditions. The effect on the stress distribution is greater. Stress variations of 10–20 % and greater in the lowest 10 m are possible under certain conditions.     

垂直风切变中的CISK

天气分析已得到,“对流层通风”是热带气旋形成和发展的基本条件之一.本文基于第二类条件不稳定(CISK)概念,从动力学角度研究了这个问题.两层模式的分析和计算都表明,垂直切变基本气流的存在对热带扰动的发展有阻尼作用,同天气分析结果相一致.     

基于不同时间尺度风切变指数推算风资源的对比分析

利用2014—2018年辽宁省探空资料分析了水平风速的垂直风廓线分布特征。用2座代表性测风塔逐时梯度风观测分析了采用不同高度组合方案计算出风切变指数的月、日变化特征, 分别用月、小时、年风切变指数推算高层风速和风功率密度, 并与实测对比。结果表明: 沈阳相较于大连地区风速随高度增加较快, 180 m高度以上风速基本保持不变, 而大连因其纬度低且靠近海洋, 300 m以下风速均匀上升。在非复杂地形情况下, 距地面10 m高度以上间隔一定高度设立4层风观测, 基本可以满足近地层风资源评估需求。受太阳辐射、下垫面、海陆热力性质差异等影响, 辽宁省风切变指数日变化特征比月变化更显著。利用小时风切变指数推算高层风速和风功率密度的方案优于采用月、年风切变指数方案。风切变指数日变化越显著, 采用逐时风切变指数推算方案越优于其他计算方案。