cyclones-20

飓风 英语

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台风是怎么形成的?

"Hurricane" redirects here. For other uses, see Hurricane (disambiguation).。

Cyclone Catarina, a rare South Atlantic tropical cyclone, viewed from the International Space Station in March 2004 。

Tropical cyclones 。

Formation and naming 。

Development - Structure。

Naming - Seasonal lists - Full list 。

Effects

Effects

Watches and warnings。

Storm surge - Notable storms。

Retired names (Atlantic - Eastern Pacific - Western Pacific)。

Climatology and tracking 。

Basins - RSMCs - TCWCs - Scales。

Observation - Forecasting。

Rainfall forecasting。

Rainfall climatology 。

--------------------------------------------------------------------------------。

Part of the Nature series: Weather。

A tropical cyclone is a storm system characterized by a large low pressure center and numerous thunderstorms that produce strong winds and flooding rain. Tropical cyclones feed on heat released when moist air rises, resulting in condensation of water vapor contained in the moist air. They are fueled by a different heat mechanism than other cyclonic windstorms such as nor'easters, European windstorms, and polar lows, leading to their classification as "warm core" storm systems.。

The term "tropical" refers to both the geographic origin of these systems, which form almost exclusively in tropical regions of the globe, and their formation in Maritime Tropical air masses. The term "cyclone" refers to such storms' cyclonic nature, with counterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. Depending on its location and strength, a tropical cyclone is referred to by many other names, such as hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone.。

While tropical cyclones can produce extremely powerful winds and torrential rain, they are also able to produce high waves and damaging storm surge as well as spawning tornadoes. They develop over large bodies of warm water, and lose their strength if they move over land. This is the reason coastal regions can receive significant damage from a tropical cyclone, while inland regions are relatively safe from receiving strong winds. Heavy rains, however, can produce significant flooding inland, and storm surges can produce extensive coastal flooding up to 40 kilometres (25 mi) from the coastline. Although their effects on human populations can be devastating, tropical cyclones can also relieve drought conditions. They also carry heat and energy away from the tropics and transport it toward temperate latitudes, which makes them an important part of the global atmospheric circulation mechanism. As a result, tropical cyclones help to maintain equilibrium in the Earth's troposphere, and to maintain a relatively stable and warm temperature worldwide.。

Many tropical cyclones develop when the atmospheric conditions around a weak disturbance in the atmosphere are favorable. Others form when other types of cyclones acquire tropical characteristics. Tropical systems are then moved by steering winds in the troposphere; if the conditions remain favorable, the tropical disturbance intensifies, and can even develop an eye. On the other end of the spectrum, if the conditions around the system deteriorate or the tropical cyclone makes landfall, the system weakens and eventually dissipates. It is not possible to artificially induce the dissipation of these systems with current technology.。

All tropical cyclones are areas of low atmospheric pressure near the Earth's surface. The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth's surface at sea level.[1] Tropical cyclones are characterized and driven by the release of large amounts of latent heat of condensation, which occurs when moist air is carried upwards and its water vapor condenses. This heat is distributed vertically around the center of the storm. Thus, at any given altitude (except close to the surface, where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.[2]。

[edit] Eye and center。

A strong tropical cyclone will harbor an area of sinking air at the center of circulation. If this area is strong enough, it can develop into an eye. Weather in the eye is normally calm and free of clouds, although the sea may be extremely violent.[3] The eye is normally circular in shape, and may range in size from 3 kilometres (1.9 mi) to 370 kilometres (230 mi) in diameter.[4][5] Intense, mature tropical cyclones can sometimes exhibit an outward curving of the eyewall's top, making it resemble a football stadium; this phenomenon is thus sometimes referred to as the stadium effect.[6]。

There are other features that either surround the eye, or cover it. The central dense overcast is the concentrated area of strong thunderstorm activity near the center of a tropical cyclone;[7] in weaker tropical cyclones, the CDO may cover the center completely.[8] The eyewall is a circle of strong thunderstorms that surrounds the eye; here is where the greatest wind speeds are found, where clouds reach the highest, and precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclone's eyewall passes over land.[3] Eyewall replacement cycles occur naturally in intense tropical cyclones. When cyclones reach peak intensity they usually have an eyewall and radius of maximum winds that contract to a very small size, around 10 kilometres (6.2 mi) to 25 kilometres (16 mi). Outer rainbands can organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum. When the inner eyewall weakens, the tropical cyclone weakens (in other words, the maximum sustained winds weaken and the central pressure rises.) The outer eyewall replaces the inner one completely at the end of the cycle. The storm can be of the same intensity as it was previously or even stronger after the eyewall replacement cycle finishes. The storm may strengthen again as it builds a new outer ring for the next eyewall replacement.[9]。

One measure of the size of a tropical cyclone is determined by measuring the distance from its center of circulation to its outermost closed isobar, also known as its ROCI. If the radius is less than two degrees of latitude or 222 kilometres (138 mi), then the cyclone is "very small" or a "midget". A Radius between 3 and 6 latitude degrees or 333 kilometres (207 mi) to 666 kilometres (414 mi) are considered "average sized". "Very large" tropical cyclones have a radius of greater than 8 degrees or 888 kilometres (552 mi).[10] Use of this measure has objectively determined that tropical cyclones in the northwest Pacific ocean are the largest on earth on average, with Atlantic tropical cyclones roughly half their size.[11] Other methods of determining a tropical cyclone's size include measuring the radius of gale force winds and measuring the radius at which its relative vorticity field decreases to 1×10-5 s-1 from its center.[12][13]。

A tropical cyclone's primary energy source is the release of the heat of condensation from water vapor condensing at high altitudes, with solar heating being the initial source for evaporation. Therefore, a tropical cyclone can be visualized as a giant vertical heat engine supported by mechanics driven by physical forces such as the rotation and gravity of the Earth.[15] In another way, tropical cyclones could be viewed as a special type of mesoscale convective complex, which continues to develop over a vast source of relative warmth and moisture. Condensation leads to higher wind speeds, as a tiny fraction of the released energy is converted into mechanical energy;[16] the faster winds and lower pressure associated with them in turn cause increased surface evaporation and thus even more condensation. Much of the released energy drives updrafts that increase the height of the storm clouds, speeding up condensation.[17] This positive feedback loop continues for as long as conditions are favorable for tropical cyclone development. Factors such as a continued lack of equilibrium in air mass distribution would also give supporting energy to the cyclone. The rotation of the Earth causes the system to spin, an effect known as the Coriolis effect,[18] giving it a cyclonic characteristic and affecting the trajectory of the storm.[19]。

What primarily distinguishes tropical cyclones from other meteorological phenomena is deep convection as a driving force.[20] Because convection is strongest in a tropical climate, it defines the initial domain of the tropical cyclone. By contrast, mid-latitude cyclones draw their energy mostly from pre-existing horizontal temperature gradients in the atmosphere.[20] To continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the needed atmospheric moisture to keep the positive feedback loop running. When a tropical cyclone passes over land, it is cut off from its heat source and its strength diminishes rapidly.[21]。

Chart displaying the drop in surface temperature in the Gulf of Mexico as Hurricanes Katrina and Rita passed overThe passage of a tropical cyclone over the ocean can cause the upper layers of the ocean to cool substantially, which can influence subsequent cyclone development. Cooling is primarily caused by upwelling of cold water from deeper in the ocean due to the wind. The cooler water causes the storm to weaken. This is a negative feedback process that causes the storms to weaken over sea because of their own effects. Additional cooling may come in the form of cold water from falling raindrops (this is because the atmosphere is cooler at higher altitudes). Cloud cover may also play a role in cooling the ocean, by shielding the ocean surface from direct sunlight before and slightly after the storm passage. All these effects can combine to produce a dramatic drop in sea surface temperature over a large area in just a few days.[22]。

Scientists at the US National Center for Atmospheric Research estimate that a tropical cyclone releases heat energy at the rate of 50 to 200 exajoules (1018 J) per day,[17] equivalent to about 1 PW (1015 watt). This rate of energy release is equivalent to 70 times the world energy consumption of humans and 200 times the worldwide electrical generating capacity,[17] or to exploding a 10-megaton nuclear bomb every 20 minutes.[23]。

While the most obvious motion of clouds is toward the center, tropical cyclones also develop an upper-level (high-altitude) outward flow of clouds. These originate from air that has released its moisture and is expelled at high altitude through the "chimney" of the storm engine.[15] This outflow produces high, thin cirrus clouds that spiral away from the center. The clouds are thin enough for the sun to be visible through them. These high cirrus clouds may be the first signs of an approaching tropical cyclone.[24]。

There are six Regional Specialized Meteorological Centres (RSMCs) worldwide. These organizations are designated by the World Meteorological Organization and are responsible for tracking and issuing bulletins, warnings, and advisories about tropical cyclones in their designated areas of responsibility. Additionally, there are six Tropical Cyclone Warning Centres (TCWCs) that provide information to smaller regions.[26] The RSMCs and TCWCs are not the only organizations that provide information about tropical cyclones to the public. The Joint Typhoon Warning Center (JTWC) issues advisories in all basins except the Northern Atlantic for the purposes of the United States Government.[27] The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) issues advisories and names for tropical cyclones that approach the Philippines in the Northwestern Pacific to protect the life and property of its citizens.[28] The Canadian Hurricane Centre (CHC) issues advisories on hurricanes and their remnants for Canadian citizens when they affect Canada.[29]。

On 26 March 2004, Cyclone Catarina became the first recorded South Atlantic cyclone and subsequently struck southern Brazil with winds equivalent to Category 2 on the Saffir-Simpson Hurricane Scale. As the cyclone formed outside the authority of another warning center, Brazilian meteorologists initially treated the system as an extratropical cyclone, although subsequently classified it as tropical.[30]。

Worldwide, tropical cyclone activity peaks in late summer, when the difference between temperatures aloft and sea surface temperatures is the greatest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month, while September is the most active.[31]。

In the Northern Atlantic Ocean, a distinct hurricane season occurs from June 1 to November 30, sharply peaking from late August through September.[31] The statistical peak of the Atlantic hurricane season is 10 September. The Northeast Pacific Ocean has a broader period of activity, but in a similar time frame to the Atlantic.[32] The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and March and a peak in early September. In the North Indian basin, storms are most common from April to December, with peaks in May and November.[31]。

In the Southern Hemisphere, tropical cyclone activity begins in late October and ends in May. Southern Hemisphere activity peaks in mid-February to early March.[31]。

The formation of tropical cyclones is the topic of extensive ongoing research and is still not fully understood.[34] While six factors appear to be generally necessary, tropical cyclones may occasionally form without meeting all of the following conditions. In most situations, water temperatures of at least 26.5 °C (79.7 °F) are needed down to a depth of at least 50 metres (160 ft);[35] waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms.[36] Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone.[35] High humidity is needed, especially in the lower-to-mid troposphere; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop.[35] Low amounts of wind shear are needed, as high shear is disruptive to the storm's circulation.[35] Tropical cyclones generally need to form more than 555 kilometres (345 mi) or 5 degrees of latitude away from the equator, allowing the Coriolis effect to deflect winds blowing towards the low pressure center and creating a circulation.[35] Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place.[35]。

Most tropical cyclones form in a worldwide band of thunderstorm activity called by several names: the Intertropical Front (ITF),[37] the Intertropical Convergence Zone (ITCZ),[38] or the monsoon trough.[39] Another important source of atmospheric instability is found in tropical waves, which cause about 85% of intense tropical cyclones in the Atlantic ocean,[40] and become most of the tropical cyclones in the Eastern Pacific basin.[41][42]。

Tropical cyclones move westward when equatorward of the subtropical ridge, intensifying as they move. Most of these systems form between 10 and 30 degrees away of the equator,[43] and 87% form no farther away than 20 degrees of latitude, north or south.[44] Because the Coriolis effect initiates and maintains tropical cyclone rotation, tropical cyclones rarely form or move within about 5 degrees of the equator, where the Coriolis effect is weakest.[43] However, it is possible for tropical cyclones to form within this boundary as Tropical Storm Vamei did in 2001 and Cyclone Agni in 2004.[45][46]。

形成台风的原因是什么

台风的形成原因：

台风发源于热带海面，那里温度高，大量的海水被蒸发到了空中，形成一个低气压中心。随着气压的变化和地球自身的运动，流入的空气也旋转起来，形成一个逆时针旋转的空气漩涡，这就是热带气旋。只要气温不下降，这个热带气旋就会越来越强大，最后形成了台风。

延伸：

1、台风

台风(Typhoon)，指形成于热带或副热带26℃以上广阔海面上的热带气旋。

世界气象组织定义:中心持续风速在12级至13级(即32.7~41.4m/s)的热带气旋为台风(typhoon)或飓风(hurricane)。

日本气象厅定义:中心持续风速118~156km/h(32.8~43.3m/s) 称之为台风。

北太平洋西部(赤道以北，国际日期线以西，东经100度以东)地区通常称其为台风，而北大西洋及东太平洋地区则普遍称之为飓风。每年的夏秋季节，我国毗邻的西北太平洋上会生成不少名为台风的猛烈风暴，有的消散于海上，有的则登上陆地，带来狂风暴雨，是自然灾害的一种。

2、台风源地

台风源地，指经常发生台风的海区，全球台风主要发生于8个海区。其中北半球有北太平洋西部和东部、北大西洋西部、孟加拉湾和阿拉伯海5个海区，而南半球有南太平洋西部、南印度洋西部和东部3个海区。从每年台风发生数及其占全球台风总数的百分率的区域分布图中可以看到，全球每年平均可发生62个台风，大洋西部发生的台风比大洋东部发生的台风多得多。其中以西北太平洋海区为最多（占36%以上），而南大西洋和东南太平洋至今尚未发现有台风生成。西北太平洋台风的源地又分三个相对集中区：菲律宾以东的洋面、关岛附近洋面和南海中部。在南海形成的台风，对我国华南一带影响重大。

台风大多数发生在南、北纬度的5°~20°，尤其是在10°~20°占到了总数的65%。而在20°以外的较高纬度发生的台风只占13%，发生在5°以内赤道附近的台风极少，但偶尔还是有的，如福建省气象台就发现1970~1971这两年中，西北太平洋共有3个台风发生在5°N以南区域。据近十多年来卫星资料的分析，发展成台风的扰动云团，在几天前即可发现，所以实际上扰动的初始位置比以前发现的位置偏东。如北大西洋上，以前认为发展成台风的初始扰动大多数产生在大洋的中部，而有人根据云图分析，认为每年有三分之二台风的扰动起源于非洲大陆。这些扰动一般表现为倒V形或旋涡状云型，它们沿东风气流向西移动，到达北大西洋中部和加勒比海时，便发展成台风。北太平洋西部和南海台风的初始扰动位置，也要比以前发现的位置偏东。

3、台风分类

过去我国习惯称形成于26℃以上热带洋面上的热带气旋(Tropical cyclones)为台风，按照其强度，分为六个等级：热带低压、热带风暴、强热带风暴、台风、强台风和超强台风。自1989年起，我国采用国际热带气旋名称和等级划分标准。

国际惯例依据其中心附近最大风力分为：

热带低压(Tropicaldepression)，最大风速6~7 级，(10.8-17.1 m/s)；

热带风暴(Tropicalstorm)，最大风速8～9 级，(17.2～24.4m/s)；

强热带风暴(Severe tropical storm)，最大风速10 ～11 级，(24.5 ～32.6m/s)；

台风(Ty-phoon)，最大风速12 ~13级，(32.7m/s~41.4m/s)；

强台风（severe typhoon），最大风速14~15级（41.5m/s~50.9m/s）；

超强台风（Super Typhoon），最大风速≥16级（≥51.0m/s）。

FPGA各型号最多有几个锁相环

形成台风的原因是海面水温在26.5℃以上；一定的正涡度初始扰动；环境风在垂直方向上的切变小；低压或云团扰动至少离赤道几个纬度。

台风的初始阶段为热带低压，从最初的低压环流到中心附近最大平均风力达八级，一般需要2天左右，慢的要三四天，快的只要几个小时。在发展阶段，台风不断吸收能量，直到中心气压达到最低值，风速达到最大值。而台风登陆陆地后，受到地面摩擦和能量供应不足的共同影响，台风会迅速减弱消亡。

热带海洋上低层大气的温度和湿度，主要决定于海表面水温(SST)，SST越高，则低层大气的气温越高、湿度越大，位势不稳定越明显。台风形成于SST≥26～27℃的暖洋面上，一般来说，全球热带海洋面上全年都满足此条件，只有赤道东南太平洋全年SST≤26.5℃。

扩展资料

台风都生成于距赤道5个纬距以外的热带海洋上，只有西北太平洋有个别台风形成于3°N附近。但在赤道附近3个纬距以内鲜有发现台风形成。

对给定的辐合值，涡度随时间的变化正比于绝对涡度的大小。在赤道上f=0，如果扰动的相对涡度也为零，则无论辐否有多大，扰动的涡度也不会增加。在离开赤道一定纬度的地区f≠0，辐合能引起涡度的增大，并且对相同的辐合，离开赤道越远涡度的产生率越高。

对流层风速垂直切变的大小，决定着一个初始热带扰动中分散的对流释放的潜热，能否集中在一个有限的空间之内。如果垂直切变小，上下层空气相对运动很小，则凝结释放的潜热始终加热一个有限范围内的同一些气柱。

而使之很快增暖形成暖中心结构，初始扰动能迅速发展形成台风。反之，如果上下切变大，潜热将被很快输送出扰动区的上空，不能形成暖性结构，也不可能形成台风。

需要强调的是这些条件仅是必要条件，不是充分条件。

参考资料来源：百度百科-台风

FPGA锁相环 pll 进行倍频时 有没有最高倍频的限制 cyclone 2系列的FPGA

能有几个PLL（锁相环）这个在你建立工程的时候就能看得到啊，如果你用的是Altera系列的FPGA，在Quartus II 中用New Project Wizard建立的话，第三步（Page 3 of 5）即Family & Device Settings 的 Available devices 栏目里每款器件都有"PLL"栏目，下面写着数字几就是最多有几个锁相环啊。如果是已经建立好的工程，可以点击菜单栏下拉的”Assignments -> Devices...“，也可以看得到啊。

从FPGA芯片型号也可以大致知道它最多锁相环数目的范围，以Altera公司的最常见的Cyclone系列为例，EP1C3T最多有1个PLL，EP1C4F~EP1C20F最多有2个PLL；Cyclone II 系列中EP2C5A~EP2C8T 最多有2个PLL，EP2C15A~EP2C70F最多PLL为4个；Cyclone III 系列中EP3C5E~EP3C10U 最多有2个PLL，EP3C16E~EP3C120F最多有4个PLL。

不行就去官网看产品目录，下面网址是Altera公司的product catalog：

http://www.altera.com.cn/literature/sg/product-catalog.pdf。

Xilinx公司的FPGA我不太了解，不敢乱说，但我估计在开发工具里或者官网上也很容易查询得到的呀~ 。

补充：

Altera的命名规则与PLL数目并无直接关联。但目前可以说 cyclone II 和cyclone III 、cyclone IV E FPGA系列中最多有4个PLL的芯片，而cyclone IV GX FPGA系列中就有最多有8个PLL的芯片（其中4个是通用PLL，即GPLL，位于管芯的每个角上；另4个是多用途PLL，即MPLL，可以供收发器使用，也可以由FPGA架构使用）。

cyclone IV GX FPGA系列简介：

http://www.altera.com.cn/products/devices/cyclone-iv/overview/cyiv-overview.html。

其他型号的信息你也可以点击上面网页中左侧菜单链接~。

附：http://www.altera.com/support/kdb/solutions/rd07152010_131.html。

How can I select which type of PLL will be used in a Cyclone IV GX design?。

Altera的命名规则如下：

工艺+版本+型号+LE数量+封装+器件速度。

举例：

EP2C20F484C6

EP 工艺

2C cyclone2 （S代表stratix。A代表arria)。

20 2wLE数量

F484 FBGA484pin 封装。

C6 八速 数字越小速度越快。

原文地址：http://www.qianchusai.com/cyclones-20.html