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Learning Series: Extratropical Transition
      #74376 - Mon Mar 26 2007 12:43 PM

In this first installment of the Learning Series for 2007, we tackle the topic of extratropical transition. extratropical transition (or ET) is roughly defined as the process by which a tropical system becomes a midlatitude area of low pressure. It brings about many structural changes within the tropical system, can be tough to forecast (particularly after transition), and is one of the newer topics within the field of tropical meteorology. In this installment, we'll cover the changes brought about by ET, why those changes are brought about, and highlight the forecast concerns associated with ET events. The source for a lot of the general information provided here (and a good primer on the topic) is a 2003 paper by numerous scientists in the field called "The extratropical transition of tropical cyclones: forecasting challenges, current understanding and future directions" in the Weather and Forecasting meteorological journal. Other sources include my own work and some works of Dr. Bob Hart at FSU. I've tried to keep the technical language used here to a minimum, but please do send me a message if you have any questions!

As noted above, ET is the process by which a tropical cyclone - with a warm-core and symmetric rainfall and wind fields - acquires the characteristics of a midlatitude cyclone - initially cold-core with asymmetric rainfall and wind fields. During ET, the initially symmetric rainfall distribution becomes skewed to the north of the cyclone, the cyclone's outer wind field expands in size and radius of maximum winds moves away from the center of the storm, and warm and cold frontal structures develop to the east and south of the center of low pressure. The rapid speed of movement of storms undergoing ET often results in an abnormally high wave threat on the Equatorward side of the storm as well as an increased potential for brush fires over the dry lands of Australia. We won't be covering that last aspect here. Each of these changes can be said to be brought about by the transition from a low-shear, high sea surface temperature environment to one with strong horizontal and vertical temperature gradients that lead to the development of asymmetries across the storm.

Because of this, let's first discuss how ET occurs and how those asymmetries establish themselves, then expand into each of the specific changes noted above. While a tropical system, the tropical cyclone draws its energy from the underlying ocean. Shear disrupts the storm by causing it to be less efficient in how it uses this energy. extratropical/midlatitude storms, by contrast, draw their energy - to a first approximation - out of wind shear and sheared flows. As a tropical system moves north, it loses its heat source from the ocean but enters an environment of strong shear. If the separation between these two regions is too large, the storm will merely decay over colder waters. More often than not, however, these two regions are close to one another. After a period of adjustment, the previous tropical system can draw from the energy provided by shear; this is roughly when the storm is said to be extratropical.

The region of strong shear is accompanied by strong horizontal and vertical temperature gradients associated with both an oceanic current such as the Gulf Stream and another midlatitude area of low pressure. As the tropical system encounters these gradients, the flow around the system will rise over the gradients to the east and fall down them to the west of the storm. This serves to accomplish two things: one, these temperature gradients become rotated by the transitioning cyclone into the orientation that we see with cold and warm fronts, and two, these flows over and down these temperature gradients leads to contrasting areas of rising motion to the north and east and sinking motion to the south and west of the storm. As a result, convection (precipitation) associated with the storm takes on an asymmetric appearance, with the primary convection displaced to the north and east of the storm. Upper level winds associated with the midlatitude environment further amplify this effect plus help to accelerate the storm toward the poles. As the storm accelerates, on the side of the storm closest to the Equator, amplified wave growth can occur due to locally strong winds. Occasionally, this wave growth can become trapped within the storm's circulation, leading to waves that grow on top of one another to extreme heights (30+ feet). This poses a major problem for shipping interests! Finally, the wind field of the system expands outward as a result of cooling in the center of the storm and the development of temperature gradients across the storm, with asymmetries in the wind field developing as a result of storm motion and smaller-scale features. Altogether, these events comprise the changes we see during an ET event.

Forecasting an ET event was a tough task until about 10 years ago. Researchers and forecasters knew that ET occurred, they just didn't know enough about it to forecast it well. Finally, in the mid-1990s, forecasts of ET became viable due to pattern recognition (i.e. what did past events do as they became extratropical?) and analyses of the changes in the cloud patterns of the storm. This still wasn't ideal, however. A few years back, however, the cyclone phase space was developed based upon the two key changes noted between tropical and extratropical cyclones: warm- versus cold-core and symmetric versus asymmetric structures. A companion research paper provided many examples of ET events within the phase space and ultimately led to its use as the primary tool to identify ET events and the timing of changes noted during ET. For more information on the cyclone phase space, see its Learning Series topic from last offseason. Note that forecasting the storm does not end once it has completed ET; ET events change the pattern of midlatitude low pressure systems for thousands of miles away from where they undergo ET plus can change structures themselves during and after ET has completed. These topics are a bit beyond the scope of this work, however.

Next up in the Learning Series is a primer on seasonal tropical cyclone forecasting. Look for that later today or sometime this week.

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