Prioritize... 优先考虑...
After completing this section, you should be able to describe the process by which a tropical cyclone intensifies, including the air flow through a tropical cyclone (known as the "secondary circulation"). You should also be able to describe why tropical cyclones weaken over land, and define "stadium effect."
完成本部分后，您应该能够描述热带气旋增强的过程，包括穿过热带气旋的气流（称为“二次环流”）。您还应该能够描述热带气旋在陆地上减弱的原因，并定义“体育场效应”。
Read... 读...
Over the past couple of sections, we've covered the ingredients needed for tropical cyclones to form and thrive. Here they are again, as a reminder:
在过去的几节中，我们介绍了热带气旋形成和发展所需的因素。以下是这些因素，供大家参考：
Tropical Cyclone Ingredients
热带气旋成分
The six basic ingredients for tropical cyclone formation are:
热带气旋形成的六个基本因素是：
- Sea-surface temperatures of at least 26.5 degrees Celsius or 80 degrees Fahrenheit (usually). A deep warm layer of water beneath the ocean surface (of at least 50 meters or so) can be helpful, but is not necessarily required.
海面温度至少为 26.5 摄氏度或 80 华氏度（通常）。海面下有一层深厚的暖水层（至少 50 米左右）可能会有所帮助，但并非必需。 - A location (usually) at least five degrees of latitude (roughly 300 nautical miles) away from the equator.
通常距离赤道至少五度纬度（约 300 海里）的位置。 - A pre-existing disturbance (cluster of showers and thunderstorms) with favorable low-level spin and convergence in the lower half of the troposphere.
预先存在的扰动（阵雨和雷暴的集群），具有有利的低空旋转和对流层下半部的辐合。 - Low values of vertical wind shear between roughly 5,000 and 38,000 feet (generally less than 10 meters per second, or 20 knots).
垂直风切变的低值大约在 5,000 至 38,000 英尺之间（通常低于 10 米/秒或 20 节）。 - A middle troposphere (roughly from about 10,000 to 20,000 feet) that is relatively moist (has moderate to high relative humidity).
中部对流层（大约 10,000 至 20,000 英尺）相对潮湿（相对湿度为中等至高）。 - A troposphere that is neutrally stable or unstable with respect to rising moist air parcels (environmental lapse rates must not be too stable such that thunderstorm development is greatly inhibited).
相对于上升的湿气团而言中性稳定或不稳定的对流层（环境递减率不能太稳定，以免雷暴发展受到极大抑制）。
So, what exactly happens when all of these ingredients get mixed together and the atmosphere cooks up a storm? In other words, how does a tropical cyclone go from a rather disorganized cluster of thunderstorms (like in the enhanced infrared image on the left below), to a highly organized, powerful hurricane (like in the image on the right below)?
那么，当所有这些成分混合在一起，大气中形成风暴时，究竟会发生什么？换句话说，热带气旋如何从一群相当混乱的雷暴（如下图左侧增强红外图像所示）转变为一个高度有序的强大飓风（如下图右侧所示）？
（左）：2017 年 8 月 28 日增强红外图像显示，热带扰动（阵雨和雷暴的混乱区域）最终发展成为飓风伊尔玛。 （右）一周后，伊尔玛发展成为组织良好、威力强大的 5 级飓风（9 月 5 日增强红外图像）。
图片来源：海军研究实验室
The image above on the left shows the tropical disturbance -- a somewhat disorganized area of showers and thunderstorms -- that would eventually develop into Hurricane Irma, as seen on enhanced infrared imagery on August 28, 2017. A week later (above on the right), Irma had developed into a well-organized, powerful Category 5 hurricane with a distinctive eye. How did Irma go from a cluster of showers and thunderstorms to a monster hurricane?
左上图显示的是热带扰动——一个有点混乱的阵雨和雷暴区域——最终发展成为飓风伊尔玛，如 2017 年 8 月 28 日增强红外图像所示。一周后（右上图），伊尔玛发展成为一场组织良好、强大的 5 级飓风，风眼清晰可见。伊尔玛是如何从一群阵雨和雷暴发展成为一场猛烈飓风的？
Well, for starters, the six ingredients listed above all were obviously present in a favorable fashion. But, let's explore how a tropical cyclone actually strengthens. I'll cover the process as a series of steps, and you'll see that the idea of feedback is very important. Assuming we're starting with a tropical disturbance over sufficiently warm water, far enough from the equator in a neutrally stable or unstable environment...
首先，上面列出的六种成分显然都是有利的。但是，让我们来探讨一下热带气旋实际上是如何增强的。我将以一系列步骤的形式介绍这个过程，你会发现反馈的概念非常重要。假设我们从足够温暖的水面上的热带扰动开始，在中性稳定或不稳定的环境中，距离赤道足够远……
- as low-level air converges into the region of thunderstorms, it flows over the warm ocean, which increases evaporation from the water below (spray from turbulent waves evaporates, etc.) and moistens the low-level air.
当低空空气汇聚到雷暴区域时，它会流过温暖的海洋，从而增加下方水的蒸发（湍流波浪的水花蒸发等）并使低空空气变湿。 - In this way, the interaction between the atmosphere and ocean actually makes the environment more favorable for thunderstorms within the developing tropical cyclone by creating and sustaining an area where the low-level air is locally more moist than its surroundings. As air rises in thunderstorm updrafts near the center of the storm, it cools, but releases latent heat along the way. The release of latent heat makes the air near the core of the storm warmer than its surroundings.
这样，大气和海洋之间的相互作用实际上使正在形成的热带气旋中的环境更有利于雷暴的形成，因为它形成并维持了一个低空空气局部比周围更潮湿的区域。当空气在风暴中心附近的雷暴上升气流中上升时，它会冷却，但会在此过程中释放潜热。潜热的释放使风暴核心附近的空气比周围环境更温暖。 - As air parcels reach the tops of thunderstorms at the tropopause, most flow outward, creating upper-level divergence which reduces the weight of central air columns and reduces surface pressure. Some lofty air parcels also sink into the center and warm. This warming also works to reduce surface pressure because these warmer air columns are less dense than their surroundings.
当气团到达对流层顶的雷暴顶部时，大部分气团会向外流动，形成高空辐散，从而减轻中央气柱的重量并降低地表压力。一些高空气团也会下沉到中心并变暖。这种变暖也有助于降低地表压力，因为这些较暖的气柱比周围环境的密度小。 - Lower surface pressures at the center of the storm work to increase the pressure gradient across the storm, which increases surface wind speeds. Faster winds blowing over the ocean further increases evaporation rates of warm ocean waters (further moistening the low-level air toward the center of the storm), which favors more thunderstorms, and so on.
风暴中心较低的表面气压会增大风暴的气压梯度，从而增加表面风速。吹过海洋的风速越快，温暖海水的蒸发率就越高（使靠近风暴中心的低层空气更加湿润），这有利于形成更多的雷暴，等等。 - If thunderstorms can remain organized and flourish around the center of the storm for long enough, the sinking, warming air over the center of the storm evaporates clouds and the eye forms, signifying a healthy tropical cyclone. Surface pressures continue to fall as upper-level divergence and warming from sinking air over the center continue.
如果雷暴能够在风暴中心周围保持有序并持续足够长的时间，风暴中心上方下沉的暖空气将蒸发云层并形成风眼，这标志着热带气旋的健康发展。随着高空辐散和中心上方下沉空气的持续变暖，表面气压将继续下降。
To help you visualize this process, check out the short video (2:37) I created below, which shows a cross-section through a hurricane, and highlights way air parcels flow through a tropical cyclone, helping it to strengthen. Keep in mind that in reality, air spirals inward toward the center of a tropical cyclone in the lower troposphere (remember, air flows counterclockwise around low-pressure systems in the Northern Hemisphere), but the schematic shown in the video shows a simpler picture and emphasizes the storm's "secondary circulation," tracing air parcels as they flow in toward the center of the storm at low altitudes, then rise in thunderstorms, and then flow outward at the top of the storm (they ultimately sink around the periphery of the tropical cyclone).
为了帮助您直观地了解这一过程，请观看我在下面制作的短片（2:37），其中展示了飓风的横截面，并强调了气团如何流经热带气旋，从而帮助其增强。请记住，实际上，空气在低对流层向内盘旋向热带气旋的中心（请记住，空气在北半球的低压系统周围逆时针流动），但视频中显示的示意图显示了一个更简单的画面，并强调了风暴的“二次环流”，追踪气团在低海拔地区向风暴中心流动，然后在雷暴中上升，然后在风暴顶部向外流动（它们最终在热带气旋的外围下沉）。
飓风加剧
点击此处查看《飓风强化》的记录。
图片来源：宾夕法尼亚州立大学达顿研究所，根据 CC-BY-NC-4.0 许可使用
This basic sketch of how tropical cyclones strengthen should emphasize the importance of evaporation of warm ocean waters feeding organized thunderstorms around the center of the storm. The upper-level divergence that occurs at the top of these thunderstorms as air spreads out, along with the sinking, warming air over the center of the storm act to reduce surface pressures, intensifying the storm as feedback processes support the development of more thunderstorms. But, if the storm moves into an environment where one or more of the ingredients are unfavorable (say, vertical wind shear is strong, the middle troposphere has low relative humidity, or sea-surface temperatures are less than 80 degrees Fahrenheit), then the thunderstorms become less intense and / or less organized, which interferes with the feedback processes needed to strengthen the storm. In such cases, tropical cyclones typically weaken.
热带气旋如何增强的基本轮廓应该强调温暖海水蒸发的重要性，因为温暖海水为风暴中心周围的有组织的雷暴提供了养分。随着空气扩散，这些雷暴顶部发生的高空辐散，以及风暴中心上方下沉的暖空气，都起到了降低表面压力的作用，从而增强了风暴，因为反馈过程支持更多雷暴的发展。但是，如果风暴进入一个或多个不利因素的环境（例如，垂直风切变强、中对流层相对湿度低或海面温度低于 80 华氏度），那么雷暴就会变得不那么强烈和/或不那么有组织，这会干扰增强风暴所需的反馈过程。在这种情况下，热带气旋通常会减弱。
The fact that tropical cyclones rely on evaporation of warm ocean waters to fuel thunderstorms also helps explain why they typically weaken when they travel over land. Without the high evaporation rates offered by warm ocean water, thunderstorm activity inevitably weakens, and the feedback processes break down. Interestingly, some weaker tropical cyclones (tropical depressions and tropical storms, in particular) can actually sustain themselves for a time over land if they travel over a warm area with extremely wet soils (called the "Brown Ocean Effect(link is external)"). Still, eventually these storms fizzle out over land, too.
热带气旋依靠温暖海水的蒸发来形成雷暴，这一事实也有助于解释为什么它们在陆地上行进时通常会减弱。如果没有温暖海水的高蒸发率，雷暴活动不可避免地会减弱，反馈过程也会中断。有趣的是，一些较弱的热带气旋（尤其是热带低气压和热带风暴）如果经过土壤极其潮湿的温暖地区（称为“棕色海洋效应”），实际上可以在陆地上维持一段时间。不过，这些风暴最终也会在陆地上逐渐减弱。
But, if favorable ingredients come together for long periods of time over the ocean, the atmosphere can cook up some monster tropical cyclones. For example, check out this 10-day enhanced infrared satellite time-lapse(link is external) showing the development and intensification of Hurricane Irma from September 1 through September 10, 2017, as it raged through the Caribbean, Cuba, and eventually the United States. At its peak, Irma was a Category 5 hurricane with maximum sustained winds of 180 miles per hour. The time-lapse also shows Hurricane Jose developing right on Irma's heels. Jose peaked briefly as a Category 4 storm, but couldn't maintain that intensity for long because of less favorable environmental conditions.
但是，如果有利因素在海洋上空长时间聚集，大气就会形成一些强大的热带气旋。例如，看看这段为期 10 天的增强红外卫星延时摄影，它展示了飓风伊尔玛从 2017 年 9 月 1 日到 9 月 10 日的发展和增强过程，当时它席卷了加勒比海、古巴，最终袭击了美国。在顶峰时期，伊尔玛是一场五级飓风，最大持续风速为每小时 180 英里。这段延时摄影还显示，飓风何塞紧随伊尔玛之后发展。何塞一度达到四级风暴的顶峰，但由于环境条件不太有利，它无法长时间维持这一强度。
2017 年 9 月 5 日，太阳刚刚落下飓风伊尔玛，风眼周围的高云在风眼中投下阴影。伊尔玛的风眼顶部比底部略宽，形状类似于体育场。
图片来源：NASA/Dakota Smith
When tropical cyclones become very powerful, they can make for some visually stunning satellite imagery (to meteorologists, anyway). Occasionally, the eye of a powerful tropical cyclone actually takes on the shape of a stadium (credit: Steve Seman) in that it's wider at the top than at the bottom. This so-called stadium effect often signifies an extremely intense tropical cyclone. You can get a sense of the stadium effect from the zoomed-in loop of Hurricane Irma's eye (on the right) from the evening of September 5, 2017. The shadows cast on the eye from the tall clouds surrounding it created a remarkable effect as the sun was setting. The eye of Super Typhoon Lan (2017) gives another good example of the stadium effect.
当热带气旋变得非常强大时，它们可以形成一些视觉上令人惊叹的卫星图像（至少对气象学家而言）。有时，强大的热带气旋的风眼实际上会呈现体育场的形状 （图片来源：Steve Seman），即顶部比底部更宽。这种所谓的体育场效应通常意味着热带气旋非常强烈。从 2017 年 9 月 5 日晚上的飓风伊尔玛风眼（右侧）的放大环中，您可以感受到体育场效应。太阳落山时，周围高高的云层投射在风眼上的阴影形成了非凡的效果。 超级台风兰（2017 年）的风眼是体育场效应的另一个很好的例子。
The complex and turbulent processes around the eye of tropical cyclones are on full display in this loop of visible satellite images of Hurricane Maria's eye (credit: NASA / Dakota Smith) from September 21, 2017. Note the chaotic and turbulent motions going on around the eye of the storm in the eye wall. For what it's worth, these small-scale, turbulent processes in tall eye-wall thunderstorms may hold the keys to some aspects of tropical cyclone intensification that aren't yet well understood. Ultimately, the processes described earlier in this section give a good basic idea of how tropical cyclones intensify, but they can't fully explain the very rapid intensification that some tropical cyclones exhibit. Such rapidly intensifying storms pose huge challenges for forecasters, and recent research suggests the turbulent processes in eye-wall thunderstorms may hold the keys to explaining rapid intensification. Research is ongoing, and hopefully will lead to forecasting improvements.
While meteorologists are often in awe of powerful tropical cyclones, these storms are also very dangerous, and public safety relies on accurate forecasts. How do forecasters know whether tropical cyclones are headed for a particular town? We'll answer that question next!