Topic 4.6 Extratropical Transition

Rapporteur: J. D. Abraham
Meteorological Research Branch
2121 TransCanada Highway
Dorval, Quebec
H9P 1J3 Canada

e-mail: jim.abraham@ec.gc.ca
fax: 514-421-2106

Working Group: B. Hanstrum, N. Kitabatake, S. Ready, S. Jones, J. Evans, B. Hart

4.6.1 Introduction

Research scientists and forecasters at the Third and Fourth WMO/ICSU International Workshops on Tropical Cyclones (IWTC-III and IV) established tropical cyclone intensity and intensity change as a priority area requiring additional research effort. However, much less effort has been devoted to intensity changes after recurvature resulting from interactions with the baroclinic environment and cooler ocean in the middle latitudes.

This so-called extratropical transition (ET) has been raised as an issue by forecasters from all basins affected by tropical cyclones, including Australia, Canada, China, Japan, Korea, New Zealand, United States, and United Kingdom. At IWTC-IV in Haikou, China, an ad hoc meeting was called to discuss this forecast problem. Some 30 forecasters and researchers met, with about half of them making presentations on the work being done at their respective research or forecast center. Representatives from the University of Munich were so impressed by this level of interest, and the quality of the work being done, that they offered to host a special meeting on extratropical transition together with the support of the World Meteorological Organization (WMO) Commission on Atmospheric Science and the U.S. Office of Naval Research (ONR). The First International Workshop on the Extratropical Transition of Tropical Cyclones took place from 10-14 May 1999 in Kaufbeuren, Germany (WMO 1998a). Following this workshop, a number of scientists involved in the study of ET prepared a review-type paper to summarize the state of the science and the related issues. Much of this report is based on that effort (Jones et al. 2002).

4.6.2 Climatology and Case Studies

Extratropical transition of a tropical cyclone occurs in nearly every ocean basin that experiences tropical cyclones. The exception is the eastern North Pacific and the North Indian Ocean where synoptic conditions are not conducive to the ET of tropical cyclones. Overall, the number of ET events follows a distribution in time that is similar to the total number of tropical cyclone occurrences. The largest number of ET events occur in the western North Pacific with an average of 12 events during a typical June-October season. Since IWTC-IV, Hart and Evans (2001) have completed a comprehensive ET climatology of the North Atlantic basin, where the largest percentage of tropical cyclones undergo ET, with 45% of all tropical cyclones undergoing ET in the 30 period. They show that ET occurs at lower latitudes in the early and late hurricane season and at higher latitudes during the peak of the season. The highest percentage of ET events occurs in September and October. The increased probability that Atlantic ET will occur during these months can be explained by comparing the geographical location of the areas which support tropical and extratropical development (see Hart and Evans 2001 for details of the calculation). In September and October the distance that a tropical cyclone must travel from the region which supports tropical development to the region which supports extratropical development is shorter than in other months, implying that a tropical cyclone is more likely to reach the region of extratropical development and thus more likely to undergo ET.
They showed also that most of the storms that intensify after ET form in the deep tropics, and a large number are Cape Verde cyclones.

Tropical cyclones that have undergone ET have been tracked across the Atlantic or the Pacific (e.g., Thorncroft and Jones 2000). Such systems may re-intensify many days after ET and bring strong winds and heavy rain to the eastern side of the ocean basin (e.g., Hurricane Lili in 1996, Browning et al. 1998). Extratropical transition is, as its name suggests, a gradual process in which a tropical cyclone loses tropical characteristics and becomes more extratropical in nature. As a tropical cyclone moves poleward it experiences changes in its environment (Schnadt et al. 1998). These changes may include: increased baroclinicity and vertical shear, meridional humidity gradients, decreased sea-surface temperature (SST) or strong SST gradients (for example, those associated with the Gulf Stream), and an increased Coriolis parameter. The tropical cyclone may come into proximity with an upper-level trough or a mature extratropical system. If the tropical cyclone makes landfall, it will experience increased surface drag, a reduction of surface fluxes of latent and sensible heat and may encounter orography.

In the southwest Pacific, ET is also triggered by the approach of a mid-latitude trough from the west (Sinclair 2002). This basin is unique in that tropical cyclones have an average eastward component of motion throughout most of their lives. This is because early in their lives they interact with the mid-latitude westerlies, which may extend to 15S during the Southern Hemisphere tropical cyclone season. As a consequence, tropical cyclones start acquiring the asymmetries characteristic of the onset of ET (section 4.6.3a) around 20S (closer to the equator than in any other ocean basin). On average, extratropical transformation is complete by 30S (Sinclair 2002). In contrast, Northern Hemisphere storms may preserve tropical characteristics as far north as 50N.

4.6.3 Physical Processes and Impacts

When a tropical cyclone begins to interact with the mid-latitude baroclinic environment, the characteristics of the cyclone change dramatically (Klein et al. 2000). In satellite imagery, the inner core of the tropical cyclone loses its symmetric appearance and gradually takes on the appearance of an extratropical cyclone. The nearly symmetric wind and precipitation distributions that are concentrated about the circulation center of the tropical cyclone evolve to produce strong and expansive asymmetric wind and precipitation distributions. Although the expanding cloud field associated with a poleward-moving tropical cyclone includes large amounts of high clouds due to the tropical cyclone outflow into the mid-latitude westerlies, regions of significant precipitation are typically embedded in the large cloud shield. This precipitation associated with an ET event can be substantial and result in severe flooding. For example, Japan, Korea, and China are at risk of torrential rains from ET events, with flooding and landslides being more of a threat than strong winds (e.g. Typhoon Seth in 1994, JTWC 1994; Tropical Storm Janis in 1995, JTWC 1995). Natural disasters such as the ET of Hurricane Floyd in 1999 are often associated with interactions of decaying tropical cyclones with the baroclinic environment in the mid-latitudes (e.g. Atallah and Bosart 2002), which results in extreme precipitation totals of 200-300 mm over a period as short as 18 hours. At the start of an ET event, heavy precipitation becomes embedded in the large cloud shield associated with the tropical cyclone outflow that extends poleward from the tropical cyclone center (Harr and Elsberry 2000; Kitabatake 2002). The heavy precipitation poleward of the tropical cyclone is not always anticipated as it begins far from the tropical cyclone center. Due to the expansion of the area covered by clouds and precipitation when the tropical cyclone moves poleward, heavy precipitation can occur over land without the tropical cyclone center making landfall. If the heavy precipitation associated with the central region of the tropical cyclone then falls in the same region as the pre-storm precipitation, the potential for flooding is increased.

Movement of a decaying tropical cyclone into the mid-latitude westerlies results in an increased translation speed, which contributes to the asymmetric distributions of severe weather elements. Over the ocean, high wind speeds and large translation speeds contribute to the generation of large ocean surface waves and swell (Bowyer 2000; Bigio 1996). This trapped-fetch phenomena resulted in peak waves of over 30 m in the case of Hurricane Luis (1995) that caused extensive damage to the luxury liner Queen Elizabeth II. Current work by MacAfee and Bowyer (2000a,b) suggests that the high forward speeds of ET events can result in a resonance between the ocean waves being generated and the wind system generating them. In essence, the storm moves with the fetch, the waves are “trapped” within the wind system, which thereby allows them to grow much larger than would be possible if the storm were stationary. The enhancement occurs only where the direction of storm motion is in the same sense as the wind, i.e., on the right (left) side of the storm track in the Northern (Southern) Hemisphere. Storm-wave resonance theory has been generally understood for some time (e.g., Suthons 1945). However, its relevance to ET has been recognized only recently. Accordingly, the waves associated with a tropical cyclone undergoing ET can pose a greater threat than those associated with a stronger tropical cyclone within the tropics.

a) Classifications of extratropical transition

A number of different studies have attempted to classify the evolution of an ET event. Early case-studies of ET in the northwest Pacific used surface analyses to classify ET as complex when the tropical cyclone interacted with a surface baroclinic zone and compound when it interacted with a surface low-pressure system (Sekioka 1956, 1970, 1972 a, b; Matano and Sekioka 1971a, b; Mohr 1971; Brand and Guard 1979). A third group in this classification occurs when the tropical cyclone remnants dissipate while moving into the midlatitude environment. This classification was referred to as “straying”(Sekioka and Matano (1990) named this third type "straying” (into the cold airmass). The Japan Meteorological Agency (JMA) operationally classifies this third type(Kitabatake 2002). Foley and Hanstrum (1994) defined two types of ET over the southeast Indian Ocean as being either cradled or captured based on the interactions between the decaying tropical cyclone and the midlatitude circulation. Hart (2002) defined a cyclone phase space based on two parameters, one of which determines whether a cyclone has a warm or a cold core, and the other describes the magnitude of asymmetries in the thermal structure. Both parameters can be calculated from the three-dimensional height field, and thus the phase space can be obtained from numerical analyses and forecasts. Evans and Hart (2002) used these parameters to define the onset and completion of transition. Klein et al. (2000) examined ET events over the western North Pacific using infrared satellite imagery and observed that nearly all cases appeared to transform from a warm-core vortex into a baroclinic, extratropical cyclone in a similar manner. They labeled this stage of ET as the transformation stage. A second stage, labeled the re-intensification stage was added to define the completion of ET as a mature extratropical cyclone. Extratropical transition can also be assessed explicitly by means of classic synoptic tools such as thermal vorticity, the gradient of thermal vorticity, and the advection of absolute vorticity by the thermal wind, all ideas based on Sutcliffe (1939, 1947) and Sutcliffe and Forsdyke (1950). Darr (2002a,b,c) quantitatively analyzed ET in the Atlantic Basin using this approach. The onset of extratropical transition compares very well also with the phase space methodology of Hart (2002). Furthermore, in a qualitative sense, extratropical transition can be diagnosed by tracking the position of the thermal vorticity maximum (cold-core center) with respect to the low-level circulation center using a radius-azimuth plot as described by Sanders (1986a).

b) Vertical Shear

One factor that can lead to substantial modification of the structure and intensity of a tropical cyclone during the transformation stage is environmental vertical shear of the horizontal wind. As a tropical cyclone approaches the midlatitude westerlies the environmental vertical shear will increase. For example, Thorncroft and Jones (2000) calculated a vertical shear of over 5 m s-1 /(100 hPa) during the transformation stage of the ET of Hurricane Iris in 1995. When the ex-hurricane moved into the region of strong shear, the potential vorticity (PV) of the hurricane inner core became tilted strongly away from the vertical. A downshear tilt was seen also in the case study of Typhoon David (Klein et al. 2000) and Hurricane Irene (Agusti-Panareda 2002). Ritchie and Elsberry (2001) modeled the interaction of an idealized tropical cyclone with a vertical shear of over 3 m s-1 /(100 hPa) and showed that the vertical extent of the PV anomaly of the tropical cyclone decreased and the upper portion of the PV anomaly became tilted downshear.

c) Interactions with the mid-latitude circulation

Extratropical transition is sensitive to the interaction between the decaying tropical cyclone and the midlatitude circulation into which the tropical cyclone is moving. A recent study based on a large number of cases showed that ET in the western North Pacific is predominantly associated with one of the two synoptic-scale patterns shown in (Harr and Elsberry 2000, Harr et al. 2000) in which the primary mid-latitude circulation is either to the northeast or northwest of the tropical cyclone. The position of the primary midlatitude circulation relative to the poleward-moving tropical cyclone was found to influence frontogenesis and the energy budget during ET. Klein et al. (2000) found a statistically significant relationship between the central sea-level pressure of an ET event and the type of mid-latitude circulation pattern into which the tropical cyclone moved. Tropical cyclones that moved into the northwest pattern were deeper after 36 h of re-intensification than those that moved into the northeast pattern. Thorncroft et al. (1993) described two paradigms of baroclinic wave life cycle behavior, one of which is dominated by cyclonic Rossby wave breaking, the other by anticyclonic wave breaking. In the North Atlantic a number of ET events have occurred in association with the cyclonic wave breaking life cycle (Iris in 1995, Thorncroft and Jones 2000; Lili in 1996, Browning et al. 1998, 2000; Earl in 1998, McTaggart-Cowan et al. 2001; Irene in 1999, Agusti-Panareda et al., 2002; Gabrielle in 2001, Evans and Hart 2002). In the northwest Pacific ET is more frequently associated with the life cycle in which anticyclonic Rossby wave breaking dominates. An expansion of the study of Klein et al. (2000) should help to determine whether dominant large-scale circulation patterns associated with ET exist in other ocean basins and to identify dissimilarities among ET events in different ocean basins.

In the majority of ET events, an interaction occurs between the decaying tropical cyclone and an upper-level trough. Hanley (1999) constructed composites of tropical cyclones that interacted with upper-level troughs and underwent extratropical transition. The ET composite contains a set of 14 cases of tropical cyclones that have undergone a trough interaction (as defined by Hanley et al. 2001)and then intensified 10 hPa or more as an extratropical cyclone. In the ET composite, an upper-level PV anomaly approaches the storm center from the northwest. In this case, the upper-level PV anomaly is much deeper and wider than in cases when a trough interaction resulted in intensification of the tropical cyclone (Hanley et al. 2001). During the ET process, a region of high PV begins to wrap around the composite center. Associated with the PV anomaly is very strong vertical shear (up to 24.5 m s-1 between 850 hPa-200 hPa). The shear is observed to weaken in time in the ET composite. The shear in the ET composite is on the order of 2-3 times the shear observed in the case of a trough interaction that results in tropical cyclone intensification.

McTaggart-Cowan et al. (2001) used PV inversion techniques to demonstrate that the upper-level trough was crucial for the re-intensification of Hurricane Earl in the extratropics, whereas the circulation of the hurricane did not play such an important role. In a further study, McTaggart-Cowan et al. (2002) demonstrated that the ET of Earl was also sensitive to the structure of the downstream flow. They attributed this sensitivity to whether the hurricane remnants were in the right (equatorward) entrance or left (poleward) exit region of an upper-level jet. The combination of the ageostrophic circulation associated with the jet and the hurricane circulation led to enhanced cold advection to the west of the hurricane in the former case, and enhanced warm advection to the east of the hurricane in the latter case. A further example of the contribution of an upper-level trough to ET is given for Hurricane Floyd (1999) by Atallah and Bosart (2002).

4.6.4 Forecast Challenges

An ET can re-intensify into a larger and more powerful storm than the original tropical system (Thorncroft and Jones 2000; Hurricane Earl in1999, McTaggart-Cowan et al. 2001; Irene in 1999, Prater and Evans 2002, and Agusti-Panareda 2002). Alternately, a tropical cyclone may decay significantly on entering the mid-latitudes but the remnants of the tropical cyclone may interact with an extratropical system many days later. This kind of development, which can bring severe weather to western Europe and the North Pacific coast of North America, is often poorly forecast by numerical weather prediction models (e.g. Hurricane Floyd in 1993, Rabier et al. 1996). Over each ocean basin that experiences tropical cyclones (TCs), the poleward movement of a TC into the midlatitudes is normally associated with the weakening or decay stage of its lifecycle. However, these systems can develop into fast-moving and rapidly-developing hybrid or extratropical cyclones that contain gale-, storm-, or even tropical cyclone hurricane-force winds. These transforming TCs, which may accelerate from typical forward speeds of 5 m/s in the tropics to more than 20 m/s in the mid-latitudes, often pose a serious threat and forecast problem to maritime activities and shore locations over wide geographic regions. A common problem in all of these regions that experience ET is the difficult challenge of predicting accurately the behavior (track, intensity, and impacts) of these rapidly-changing systems. Forecasters in each center responsible for producing warnings and advisories during an ET event are faced with very similar problems. The most difficult forecast issues are associated with the potential large amounts of precipitation, continued high wind speeds, and generation of large ocean surface wave heights and swell during the ET event. These severe impacts can and do occur after the tropical cyclone has weakened, and is no longer being classified as a TC so that advisories have been discontinued by the TC Forecast Center.

In the western North Atlantic, the impact of a transformed tropical cyclone may bring hurricane-like conditions to latitudes that do not normally experience these types of conditions; in other words, winter-storm like conditions to midlatitude locations during a summer or autumn season. As well, the increased translation speed, which decreases the warning time for the small fishing and recreational vessels, presents an important forecast and warning problem during this season. The occurrence of an ET over offshore and coastal regions can produce extremely large surface wave fields due to the continued high wind speeds and increased translation speed of the entire system.

A significant contributor to numerical forecast errors during ET is the uncertainty in the initial conditions. Although mid-ocean regions may contain large amounts of single-level data such as satellite winds (e.g., Velden et al. 1997), there is a lack of conventional multi-level data. Thus a major challenge for numerical forecasts of ET is the optimal use of the available observations. A number of studies have demonstrated the sensitivity of numerical forecasts of ET to the specification of the initial conditions. Evans et al. (2000) obtained an improved forecast of the ET of Hurricane Floyd (1999) by assimilating satellite winds into the initial conditions. Rabier et al. (1996) used an adjoint method based on the 48-h model forecast error to show that changes in the initial conditions could have lead to an improved forecast of the explosive extratropical cyclogenesis during the ET of Hurricane Floyd (1993). Hello et al. (2000) demonstrated how variants in the use of the same set of observations could dramatically change the forecast of the ET of Hurricane Iris (1995) over the North Atlantic. Their study suggests that variational data assimilation techniques are only optimal relative to a number of constraints, e.g., the specification of background errors.

To provide the necessary warning time for a tropical cyclone about to undergo ET that may be moving rapidly into the mid-latitudes, it is essential to achieve improved accuracy of numerical forecasts on the medium range. Thus, it is necessary to fully diagnose model tendencies and potential sources of model error during the ET of a tropical cyclone. Such a diagnosis requires both increased understanding of the physical aspects of ET and identification of sensitivities to the initial conditions and physical parameterizations of the forecast model. Browning et al. (2000) applied singular vector techniques to diagnose the forecast error during the ET of Hurricane Lili (1996) and showed that a comparison of water vapor imagery with model-generated imagery could be used to diagnose the position error of a mesoscale tropopause depression in the numerical model, and the depression subsequently played an important role in the re-intensification. In addition, multi-model ensembles, such as have been applied to tropical cyclone forecasting (Krishnamurti et al. 2000; Weber 2002) may prove useful in the forecast of ET.

More recently, piecewise PV inversion has been used to assess the importance of particular features, such as an upper-level trough or the tropical cyclone, during ET (Browning et al. 2000; McTaggart-Cowan et al. 2001, 2002; Agusti-Panareda et al. 2002).

4.6.5 Summary and recommendations for future directions

Extratropical transition is a complex four-dimensional evolutionary process that involves interactions over a variety of horizontal and vertical scales. Although much is yet to be learned about the thermodynamic and dynamic characteristics of a mature tropical cyclone, viable research programs are in place to examine these issues, as well as to examine the processes responsible for extratropical cyclogenesis. In contrast, the transition from a tropical cyclone into an extratropical cyclone is poorly understood and incompletely researched.

A universal definition of ET does not exist. The evolutionary nature of ET and the time interval between successive observations/synoptic analyses makes it difficult, if not impossible, to specify a precise time at which a tropical cyclone has become extratropical. From an operational viewpoint, a definition that is based on tools available to a forecaster would be desirable. A possible framework for a definition is the two-stage description of ET (Sect. 3a, Klein et al. 2000) and the phase space diagram of Hart (2002).

Because the societal impacts of an ET event are due to specific physical conditions (e.g., precipitation coverage and amount, wind speeds, and wave heights), research needs to be focused on improved understanding of the evolution and prediction of these impact variables throughout the ET process. As a tropical cyclone moves poleward, the impact variables change in magnitude and distribution. Why and at what rate these changes occur remain important unanswered questions.

Other outstanding questions concern the changes in structure and intensity of a tropical cyclone during ET. Significant gaps exist in our understanding of tropical cyclone intensity change, both with regard to rapid intensification and rapid decay. The latter plays an important role in ET. Additional questions are related to the roles of complex terrain, convection, surface roughness, and the air-sea exchange of heat, momentum, and mass, and moisture. Very little is known about the boundary layer environment at high wind speeds, and it is obvious that boundary layer processes will have important impacts on the evolution of precipitation, wind, and wave fields during ET.

Further research is needed to quantify whether changes in the nature of the tropical cyclone during the transformation stage influence the subsequent development during the extratropical stage. Klein et al. (2000) examined statistical relationships between the minimum central sea-level pressure associated with the resulting extratropical cyclone and several measures of tropical cyclone characteristics such as intensity and size. Although their analysis was based on a limited sample of 33 ET cases over the western North Pacific, they could not identify a statistically significant relationship between the original tropical cyclone characteristics and the minimum central pressure of the extratropical system. However, their study considered only tropical cyclone characteristics before the start of the transformation stage. Tropical cyclone structure and intensity at the end of the transformation stage may have a larger impact on the re-intensification stage. In addition, their study did not consider whether or not the mid-latitude environment was favorable for extratropical development.

As a tropical cyclone moves poleward the baroclinity of its environment increases and interactions with other synoptic features (e.g., fronts, upper-level troughs, mature extratropical systems) become more probable. The two-way interaction between synoptic-scale systems of tropical and extratropical origin and its influence on the primary variables that determine the impact of an ET event needs to be investigated. A key question here is predicting the outcome when a mid-latitude trough interacts with a tropical cyclone. Sometimes the vertical wind shear associated with the trough causes the cyclone to dissipate in the tropics. Sometimes the trough can actually invigorate the tropical cyclone in the tropics (without ET occurring). Sometimes the trough can “capture” the tropical cyclone, so that ET with or without re-intensification occurs.

Following the transformation stage, re-intensification as an extratropical cyclone may occur. Several studies have identified periods of re-intensification that meet the criterion of rapid extratropical cyclogenesis (Sanders and Gyakum 1980). During extratropical re-intensification, the relative roles of the remaining tropical cyclone features and mid-latitude characteristics in defining the re-intensification as an extratropical cyclone need to be investigated. During this extratropical stage, it is anticipated that the mid-latitude characteristics gradually dominate over the remaining tropical cyclone features.

Improved understanding of ET can be obtained through the use of a hierarchy of idealized models, for example with simplified physical representations or simplified initial conditions. Such an approach allows for particular processes to be studied first in isolation. Then by gradually increasing the complexity of the model or of the situation under study insight can be gained into the feedbacks among various processes. In addition, the further use of PV thinking to diagnose ET promises to be of great value.

Predictions of ET by numerical forecast models often do not accurately depict the characteristics of ET and subsequent evolution of the resulting extratropical cyclone. Outstanding questions concern the representation of various physical processes that play an extremely important role during ET and are typically parameterized in numerical models. Also, increased understanding of the impact of data assimilation and of the inclusion of synthetic observations on numerical predictions of ET is required. Finally, investigation is required to establish the limits of predictability using ensemble prediction techniques. Since ET involves many different atmospheric processes, the inherent predictability of an ET event might be less than that of a pure tropical or mid-latitude cyclone.

With regard to improved operational forecasts and warnings, questions remain as to how best to assimilate research results so that a forecaster can apply them to a specific ET scenario. Research is required to provide forecasters with conceptual models of ET and better diagnostic tools that would enable them to provide more effective advisories and warnings.

To further both the understanding and forecasting of ET, it is essential that we improve our knowledge of real ET events both by making better use of existing observations and by exploiting new observational capabilities. Investigations into ways of making better use of existing observations should consider: the documentation, validation and development of multi-channel passive and active satellite products, especially Advanced Microwave Sounding Unit (AMSU), TRMM, SSM/I and scatterometer data; improved data assimilation, including enhanced use of existing observations and efforts to incorporate special observations. Improved knowledge of the detailed structure of ET systems requires more in-situ observations of the decaying tropical cyclone and its environment. The need for such observations was demonstrated by the discovery of a low-level jet with maximum winds up to 72 m s-1 during Canadian reconnaissance of the ET of Hurricane Michael in 2000 (Abraham et al. 2002). Further in-situ observations could be obtained as part of a field experiment with Intensive Observation Periods. Scientific objectives would need to be developed for such an experiment and the possibility of using existing resources considered. The participation of both forecasters and researchers is essential for the success of any field program.


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