Extratropical cyclone

A ficticious synoptic chart of an extratropical cyclone affecting the UK. The blue arrows between isobars indicate the direction of the wind, while the "L" symbol denotes the center of the "low". Note the occluded, cold and warm frontal boundaries.

Extratropical cyclone is a meteorological term referring to low pressure weather systems known as cyclones. They are connected with fronts, horizontal gradients in temperature and in dewpoint, which are otherwise known as "baroclinic zones".

The descriptor "extratropical" refers to the fact that these cylones generally occur in the middle latitudes of the planet, outside of the tropics. These systems are also known as "mid-lattitude cyclones" due to their area of formation, or "post-tropical cyclones" (depending on a given cyclone's origin).. They can also be referred to in forecasts and by the general public as "depressions" or "lows". It refers to the common everyday phenomenon which, along with anti-cyclones, drives the weather in much of the northern and southern hemispheres of the planet. Although extratropical cyclones are almost always classified as baroclinic, they can sometimes become barotropic.

Formation

Extratropical cyclones can and do form anywhere in the extratropical regions of planet, usually between thirty and sixty degrees lattitude either north or south of the equator. They generally form in one of two ways, being through either cyclogenesis or extratropical transition.

Cyclogenesis

Main article: Cyclogenesis

An upper level jet streak. DIV areas are regions of divergence aloft, which will lead to surface convergence and aid cyclogenesis.

Since extratropical cyclones form along linear bands of temperature/dewpoint gradient with significant wind shear, they are classified as baroclinic cyclones. Initially, Cyclogenesis occurs along frontal zones near a favorable quadrant of the upper level jet, and then propagates into the deepest pocket of cold air aloft as a cyclone matures. As the cyclone strengthens, the cold front sweeps equatorward around its western periphery due to the greater density of the cold air in its wake while its warm front progresses much more slowly, as a result of the fact that it is more difficult to dislodge (or erode) the cooler air ahead of the system due to its greater density. When cyclones begin this march into the colder air aloft, the cyclones occlude as the poleward portion of the cold front overtakes the western section of the warm front, forcing a tongue of warm air aloft, or trowal. Eventually, the cyclone will become barotropically cold and will begin to weaken.

The air pressure in these cyclones can fall rapidly, or bomb, to below 980 millibars (980 hectopascals) (hPa) (SI) under favorable conditions, such as near a natural temperature discontinuity like the Gulf Stream or at a preferred quadrant of an upper level jet streak. The lowest pressure measured from an extratropical cyclone in the United States was 951.7 hPa on March 1, 1914 in Bridgehampton, New York. Between January 4 and January 5, 1989, an extratropical cyclone south of Atlantic Canada deepened to 928 hPa.

Extratropical transition

Cyclone Florence in the north Atlantic after completing its transition to an extratropical cyclone from a hurricane.

Tropical cyclones often transit into extratropical cyclones at the end of their tropical existence, usually between 30° and 40° latitude when there is sufficient upper level forcing from an upper-level trough or a short wave riding the Westerlies. This process is known as extratropical transition. During extratropical transition, poleward displacement of the cyclone occurs and the cyclone's primary energy source converts from the release of latent heat of condensation to a baroclinic process, in essence losing its warm core and becoming a cold cored system. During this process, a cylone in extra tropical transition (in Canada known as the post-tropical stage) will invariably form fronts and/or troughs consistant with a baroclinic system, and the breadth/size of the overall depression will usually be seen to increase, while the core of the depression will usually weaken at first before re-stengthening to a depth that will depend on the environmental conditions it arrives in.

On rare occasions, an extratropical cyclone can transition into a tropical cyclone if it enters warmer waters and an environment with less vertical wind shear. The height of subtropical cyclogenesis, which is the midpoint of this transition, occurs in the months of September and October when the differences between the temperature of the air aloft and the sea surface temperatures are the greatest, leading to the most potential for instability.

Structure of extratropical cyclones

Surface Pressure/Wind Distribution

Quikscat image of typical extratropical cyclones over the ocean. Note the maximum winds are on the outside of the occlusion.

The center of a cyclone is defined as its area of lowest pressure. Near the center of the cyclone the pressure gradient force (from the pressure in the center of the cyclone compared the pressure outside the cyclone) and the Coriolis force must be in an approximate balance (or else the cyclone would collapse in on itself under the pressure gradient). In the beginning, the sea-level pressure is not very low; typically below 1013.2 mbar (hPa), which is the average sea level pressure for Earth. Nearly always called by the term low-pressure cells in North America, they often form quickly along stationary of cold fronts when the required upper level forcing exists such as an upper level jet streak or an upper level shortwave. Usually, the part of the front ahead of the cyclone will develop into a warm front, giving the frontal zone (as drawn on surface weather analyses a wave-like shape. Hence, early in their life cycles, extratropical cyclones can also be referred to as frontal waves. An old name for such a system is "warm wave" (in the United States). Intensification or "deepening" of the cyclone progresses slowly if forcing aloft is weak. In the Arctic, the average pressure for cyclones is 988 hPa during the winter, and 1000 hPa during the summer. Rapidly falling atmospheric pressure is possible due to strong upper level forcing, and such a cyclone is sometimes referred to as a bomb.

The highest winds in extratropical cyclones are typically just on the cold/poleward side of warm fronts and occlusions, as well as just behind cold fronts. This is because these areas tend to have the greatest pressure gradient, which is the cause of wind.

Rotation

The wind flow around a large cyclone, generally referred due as cyclonic, is counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere, due to the Coriolis effect.

Vertical structure

Mid-Latitude cyclones slant back into colder air masses with height, and can exceed 30,000 feet (c. 10km) in depth. Above the surface of the earth, the air temperature near the center of the storm is increasingly colder than the surrounding environment. Extratropical cyclones strengthen with height, completely opposite to their tropical counterparts. Mid-latitude cyclones are called "cold-core lows" for these reasons. Various charts can be examined to check its characteristics with height, such as the 700 millibars (hectopascals) (hPa) (SI) chart, which is at about 10,000 feet or 3,000 meters in height. Cyclone phase space diagrams can tell you whether a cyclone is tropical, subtropical, or extratropical.

Cyclone evolution

File:Cyclonemodels.gif

Norwegian cyclone and Shapiro-Keyser model differences in frontal structure.

There are two main models (which can be roughly described as "descriptions" in a non-specialist sense) of cyclone development and lifecycles in common use - The Norwegian model and the Shapiro-Keyser Model.

Norwegian cyclone model

Of the two theories on extratropical cyclone structure and life cycle, the oldest is the Norwegian Cyclone Model, developed during World War I. In this theory, cyclones develop as they move up and along a frontal boundary, eventually occluding and ending up in a barotropically cold envirnoment. It was developed completely from surface-based weather observations, including descriptions of cloud character near frontal boundaries. This theory still retains much merit over continental landmasses.

Shapiro-Keyser model

A second competing theory for extratropical cyclone development over the oceans is the Shapiro-Keyser model, developed in 1990. Its main difference with the Norwegian Cyclone Model is the fracture of the cold front, treating warm-type occlusions and warm fronts as the same, and allowing the cold front to progress through the warm sector at a right angle to the warm front. This model was based on oceanic cyclones and their frontal structure, as seen in surface observations and in previous projects which used planes to determine the vertical structure of fronts across the northwest Atlantic.

Warm Seclusion

File:Medionalflowpattern.jpg

Meridional Flow pattern of October 23, 2003. Note the numerous high and low centers and the contorted look of the 500 hPa height pattern. In a zonal flow regime, the height lines would be circular in relation to the pole, with at most very broad troughs and ridges.

A warm seclusion is the mature phase of an extratropical cyclone according to the Shapiro-Keyser model. These systems can have eye-like features at their center reminiscent of tropical cyclones, significant pressure falls, hurricane force winds, and moderate to strong convection. The low-level thermal structure of a warm seclusion can develop a shallow warm core. On those rare occasions where a cyclone attains a deep warm core, it becomes known as a tropical cyclone.

A vast majority of warm seclusion events occur over the world's oceans and may impact coastal nations with hurricane force winds and torrential rain. Climatologically, the Northern Hemisphere sees warm seclusions during the cold season months. The Southern Hemisphere, on the other hand, may see a strong cyclone event such as this during all times of the year.

In all tropical basins, except the Northern Indian Ocean, the extratropical transition of a tropical cyclone may result in reintensification into a warm seclusion. For example, Hurricanes Irene and Maria of 2005 reintensified into strong baroclinic systems and achieved warm seclusion status at maturity (or lowest pressure).

Motion of Extratropical Cyclones

Extratropical cyclones are generally driven quickly by deep westerly winds in a general west to east motion across both hemispheres when the flow in zonal. A zonal flow regime is the meteorological term meaning that the general flow pattern is west to east allong the earths lattitude, with weak shortwaves embedded in the flow. When the general flow pattern buckles from a zonal pattern to the medional pattern, a slower movement in a poleward or equatorward direction is more likely. Meridional flow patterns feature strong, amplified troughs and ridges, with more north-south flow in the general pattern than west-to-east flow.

Effects of Extratropical Cyclones

Preferred region of snowfall in an extratropical cyclone.

Extratropical cyclones can bring mild weather with a little rain and surface winds of 7-15 knots, or they can be cold and dangerous with torrential rain and winds exceeding 64 knots, (sometimes referred to as windstorms in Europe.) The band of precipitation that is associated with the warm front is often extensive. Cyclones tend to move along a predictable path at a moderate rate of progress. During late fall, winter, and spring, the atmosphere over continents can be cold enough through the depth of the troposphere to cause snowfall.

Over maritime areas such as the UK, the precise effects of such systems can be particularly difficult to forecast, while over areas such as continental landmasses such as much of Europe, the Americas or much of Asia, forecasts can generally be made a little further into the future with a higher degree of certainty.

While most tropical cyclones that become extratropical quickly dissipate or are absorbed by another weather system, they can still retain winds of hurricane or gale force. In 1954, Hurricane Hazel became extratropical over North Carolina while a strong Category 3 storm. The Columbus Day Storm of 1962, which evolved from the remains of Typhoon Freda, caused heavy damage well north in Oregon and Washington states with widespread damage equivalent to a Category 3 or higher hurricane. More recently, Hurricane Wilma in 2005 began to lose tropical characteristics while still sporting Category 3-force winds (and became fully extratropical while still a Category 1 storm).

See also

• Surface weather analysis
• Cyclogenesis
• Nor'easter
• European windstorm

References