MXSH最大风切变指数
2018年11月30日周五
MXSH最大风切变指数
Jeppesen飞行计划中，有个MXSH最大风切变指数，如下图所示：
最大风切变指数这个数值的实用意义，基本上是和飞机颠簸的联系，有人甚至直接把这个数值叫成颠簸指数，但这种叫法并不准确。
JetPlan calculates wind shear based on a formula that samples wind velocity and direction two thousand feet above and two thousand feet below the current cruise altitude. Presently, JetPlan does not make use of any ICAO tables to assign a relative intensity to shear.
杰普逊计算风切变基于现在高度层上下2000ft的风速和风向的样本，但是，杰普逊计划不使用任何ICAO的相关表来匹配风切变的强度。
The formula is as follows:
Where:
· WD1= wind direction 2,000 feet below cruise altitude (degrees)
· SP1= wind velocity 2,000 feet below cruise altitude (knots)
· WD2 = wind direction 2,000 feet above cruise altitude (degrees)
· SP2 = wind velocity 2,000 feet above cruise altitude (knots)
· x1= SP1 × COS(WD1)
· y1= SP1 × SIN(WD1)
· x2 = SP2 × COS(WD2)
· y2 = SP2 × SIN(WD2)
· ALT1= altitude at 2,000 feet below cruise altitude.
· ALT2= altitude at 2,000 feet above cruise altitude.
Shear Value Interpretation
The following statement was prepared to guide pilots and dispatchers in the interpretation of shear values as they relate to Clear Air Turbulence (CAT).
Vertical wind shear calculated on JetPlan takes a velocity difference in a 4,000 foot section centered on the cruise flight level. The numbers derived from this method can sometimes be misleading when trying to relate wind shear to CAT. For example, an aircraft could be flying in the center of a 100 knot jet stream and the wind shear could be zero if the wind velocity 2,000 feet above and 2,000 feet below the cruise altitude are the same. In contrast, an aircraft could be flying through light winds of approximately 20 knots and there may be a direction
difference above and below the cruise flight level that would cause a relatively high shear number, but little turbulence.
以下声明旨在指导飞行员和调度员解释与晴空湍流(CAT)有关的风切变值。在JetPlan上计算的垂直风切变在以巡航飞行高度为中心的4,000英尺的截面上取速度差。当试图将风切变与CAT联系起来时，这种方法得出的数字有时会产生误导。例如，一架飞机可能在100节射流的中心飞行，如果巡航高度上2000英尺和下2000英尺的风速相同，那么风切变可能为零。相反，飞机可以在大约20节的微风中飞行，并且可能有一个方向在巡航飞行高度上下的差异会造成较高的剪切数，但湍流较小。
Vertical wind shear is only one of the many factors that cause CAT. Other factors may include: position relative to upper level troughs and ridges, position relative to the tropopa use, position relative to jet streams, temperature gradients, mountain waves and other small scale phenomenon. Generally, only large scale CAT is forecast for aircraft operations.
垂直风切变只是引起CAT的众多因素之一。其他因素可能包括:相对于高层槽脊的位置、相对于对流层的位置、相对于射流的位置、温度梯度、山波等小尺度现象。一般来说，只有大型CAT才能预测飞机的运行。
In general, the higher the vertical wind shear, the more likely an encounter with CAT and strong turbulence. There is little aviation criteria for relating wind shear values to a probable turbulence threshold, except the suggestions that exist in AC 00-6A, p. 143, and AC 00-45D,pp. 13-1,2. No criteria is defined for relating wind shear values to the occurrence of light, moderate, or severe.
The only relation is that the likelihood of encountering CAT is increased when there is strong vertical wind shear. But vertical wind shear values are used for guidance only, not for determining aircraft operations. High Level Significant Weather Prognostic charts and pilot reports (PIREPs) are useful tools for determining the areas of CAT over a large region.
一般来说，垂直风切变越大，遇到CAT和强湍流的可能性越大。除了AC 00-6A、p. 143和AC 00-45D、pp.13-1,2中存在的建议外，几乎没有航空标准将风切变值与可能的湍流阈值联系起来。没有定义将风切变值与轻微、中度或严重风切变的发生联系起来的标准。唯一的关系是在垂直风切变较强时遇到CAT的可能性增大。但垂直风切变值仅用于制导，不用于确定飞机的操作。高水平重要天气预报图和飞行员报告(PIREPs)是确定大区域CAT区域的有用工具。
根据JetPlan User Manual中的说明和公式，风切变指数其实就是上下2000ft高度层的风速矢量差除以4，其实就是平均1000ft的风速矢量差。
风速矢量差越大，说明风向风速的变化越大，那越可能产生颠簸，或者颠簸的强度越大。但是这并不是绝对的，举个例子，假如飞行高度层之上2000ft的风速为100kt，实际飞行高度层和下2000ft都是静风，此时计算出的风切变指数应该是25，但是实际飞行高度层因为是静风，一点颠簸也没有。反之亦然，颠簸指数很小的时候，也可能遭遇强颠簸。说以将这个值说成是颠簸指数肯定是不准确的。
另外，貌似业内有个概念，风切变指数5以上应该要提醒机组注意有颠簸，甚至有的公司定义了1-5为轻度颠簸，6-10为中度颠簸，并未找到出处（Jeppesen User Mannual中其实也强调了他的算法没有对应任何标准，并且也无法直接反应对应的颠簸程度）。
唯一找到的是ICAO中定义的风切变指数对应的颠簸强度：
轻度对应的是1-4，中度对用的是5-8，但是ICAO推荐值是以100ft高度的风速变化矢量差作为判断标准，Jeppesen飞行计划中是以1000ft的平均风速矢量差来进行计算，所以并不能直接对应。
所以认为在实际运行中，这个值并不能直接反应颠簸的程度或者概率。但晴空颠簸本就没有很好的预判方式，该值可以作为参考项，放行时向机组进行提示。
Wind Shear –wind speed at certain flight levels and direction. Max shear is the highest value along route of flight. It is based on a scale of 1 to 10:
0-4 Light
5-7 Moderate
8 - 10 Severe (11 & 12 used in extreme cases).
The wind shear/max shear values in JetPlan do not coincide with a particular set of guidelines for standardized windshear values. The values presented by JetPlan are whole number values easily recognizable to pilots. For high shear values, above 10 or 12: there is not an ultimate high end limit, (although the documentation implies 12 as the highest). Abnormally high values, while relatively rare, can be the result of strong inversions, orographic phenomena, and other cases where additional factors cause high valuesto the equation used by JetPlan.
译文：
JetPlan中的风切变/最大切变值与标准化风切变值的一组特定准则不一致。JetPlan显示的值是飞行员容易识别的整数值。对于高于10或12的高风切变值，没有最终的上限，(尽管文档暗示12是最高的)。异常高的值虽然相对少见，但可能是强反转、地形现象和其他一些情况的结果，这些情况下，其他因素会导致JetPlan使用的方程的高的值。