Effect of Extratropical Transitioning on Pacific Typhoons

Introduction

Tropical cyclones are low pressure systems characterized by high winds and heavy convective rainfall. They have a warm core (that is, warmer than the surrounding environment) and surface temperatures across their width are relatively uniform. Importantly, tropical cyclones derive their energy from warm ocean water that moistens and destabilizes air so that it rises, cools, and condenses to forms clouds and precipitation.

Extratropical cyclones are also low pressure systems characterized by high winds and heavy, often steady rainfall. But extratropical cyclones differ from tropical cyclones in that they occur in middle and high latitudes, are cold-core (colder than the surrounding environment), and have large horizontal surface temperature differences (“baroclinic”). An important distinction from their tropical counterparts is that extratropical cyclones derive their energy primarily from horizontal temperature gradients in the atmosphere and not from warm ocean water; warm air is mechanically lifted over cold air to form clouds and precipitation.¹

While tropical cyclones and extratropical cyclones are distinctly different phenomena, it is not unusual for the former to evolve, or transition, into the latter.² At some point in its predominately east-to-west track in the tropics, a tropical cyclone may begin to move poleward where it encounters cooler sea surface temperatures and increased vertical wind shear associated with the mid-latitude westerlies. These and other factors cause the tropical cyclone to increase its forward speed, lose its symmetric cloud shield and, at times, reintensify rapidly into a powerful extratropical cyclone.

Extratropical transitioning cyclones occur in most tropical ocean basins. Indeed, approximately 45-50% of tropical cyclones in both the Northwest Pacific and North Atlantic basins undergo extratropical transition.^3,4 The resultant storm typically has a larger wind and precipitation footprint than it did as a tropical cyclone—and it can still pose a serious threat to land and sea interests.

Challenges for the Forecaster

Forecasting extratropical transition and its downstream impacts can be challenging and costly. While considerable progress has been made in recent years in forecasting the track, intensity and even wind field of tropical cyclones, transitioning cyclones are more difficult. Additionally, because extratropical cyclones typically move much faster and exist in a more dynamic environment, an error in the forecasted transition time by even a few hours can mean a dramatic error in forecasted location and speed of the system. Even sophisticated numerical weather prediction models have difficulty with accurately simulating extratropical transition.

Nevertheless, some basic tools are available to help forecast and identify when transition will happen and, retrospectively, when it occurred. These range from very subjective analyses of surface data and satellite imagery to a more objective “phase space” approach developed by Hart (2003). The amount of uncertainty is potentially large when comparing results from different agencies using different approaches. Typhoon Songda (2004) offers a good example. Satellite imagery in Figure 1a-c shows extratropical transition occurring around 1200 UTC on September 7th, while the phase space diagram in Figure 2 shows transition occurring around 0600 UTC on the 8th.⁵ The Japan Meteorological Agency (JMA) reported that transition had occurred at 0000 UTC on the 8th.

Clearly, the choice of technique will have implications for evaluating extratropical transition and where, when and, more importantly, the extent to which it will impact regions downstream. The nature of those impacts, in the context of a case study of two Japan typhoons, is discussed below.

Impact of Extratropical Transition on Precipitation

Mature tropical cyclones—at least those in low wind shear environments—typically exhibit a very symmetric precipitation shield. Convergence and convection—both associated with thunderstorm activity in the eyewall—are greatest near the radius of maximum winds (Rmax) and it is here that the most intense precipitation occurs. Outward from the Rmax, precipitation rates generally decay, with most of the precipitation falling within the radius of gale-force winds (Matyas 2006). During extratropical transition, increased vertical wind shear forces the precipitation shield to become highly asymmetric with most of the precipitation falling north and east of the storm center. Additionally, colder, drier air enters, or “is entrained,” into the southern and western parts of the storm, which erodes the precipitation over the center.

These features can be seen more clearly in Figure 1. Figure 1b shows the dry air entrainment (in greens and blues) south of the storm center, which is denoted by “X.” By Figure 1c, the precipitation shield has become even more asymmetric, with dry air entrainment to the north and south. This asymmetric precipitation shield is typically larger than it was when the system was tropical, but hourly precipitation rates are generally lower.⁶

Typhoons Etau (2003) and Tokage (2004) had similar tracks and intensities, but Tokage was already beginning to transition as it made landfall on Japan’s main island of Honshu, while Etau transitioned farther north over Hokkaido. Observed wind speeds were similar for the two events but Tokage caused substantially more flooding, which claimed nearly 100 lives. Figure 3 demonstrates that Tokage and Etau’s observed precipitation totals were similar over the mountains of the southern islands of Kyushu and Shikoku, but that Etau was noticeably drier across Honshu, especially the northern half.

Impact of Extratropical Transition on Losses

The economic impacts of tropical cyclones that have transitioned to extratropical can be dramatic.⁷ One reason is that buildings and their owners in the middle and high latitudes may be less well prepared for storm-force winds and heavy precipitation. Whether due to building codes that are less strict, a lack of evacuation strategies and emergency preparedness, or poor forecasting, a transitioned storm can have serious impacts.

Typhoons Tokage and Etau resulted in significantly different levels of damage and loss. The JMA (http://agora.ex.nii.ac.jp/digital-typhoon/) reported that Etau inundated 2,398 houses, while Tokage inundated 55,455 houses. Wind losses were also very disparate between the two storms. The General Insurance Agency of Japan (GIAJ) reported insured losses of 5.1 Bn JPY for Etau and 88.5 Bn JPY for Tokage.

There are numerous facts that support the notion that the extratropical transition of Tokage contributed more significantly to the greater damage than did any differences in intensity or storm path. Table 1 summarizes a comparison of key features at and post-landfall for the two typhoons. First, it is evident that both storms were nearly equally strong from a central pressure and gradient wind perspective. Second, maximum rainfall amounts⁸ (i.e., at any given station) were also very comparable, so this too cannot explain Tokage’s higher level of damage. Finally, while the two storms took slightly different inland paths, which can explain loss differences from a proximity-to-exposure standpoint, the path differences per se were not as important as what happened because of them. Specifically, because Etau traversed more land before transition, it had lost its primary source of energy and moisture—that is, the ocean. Tokage, on the other hand, had essentially completed transition as it made its way across Honshu and so was able to extract energy and moisture not from the ocean, but rather from the free atmosphere.

A closer comparison of the two typhoons reveals other differences related to extratropical transition that support the disparity in the losses. For example, note that Tokage’s precipitation was considerably higher across the northern half of Honshu, including the Tokyo region, at a time when it had already completed transition. The heavier precipitation is supported by stronger and more extensive warm advection (horizontal transport) north and east of its path. The presence of a stronger baroclinic zone (steep horizontal temperature gradients) surrounding Tokage (Figure 4), as well as stronger and broader (southerly) winds despite higher central pressure 12 hours after landfall, certainly contributed to the stronger warm advection. The extensive warm advection contributed, in turn, to precipitation amounts from Tokage that were heavier than Etau’s along the northern coast of Honshu even though Etau hugged the northern coast.

It is worth noting that Etau did ultimately transition while it was crossing Hokkaido. Then and only then did it regenerate heavier amounts of precipitation. Indeed, the heaviest damage and losses from Etau occurred primarily over Hokkaido.

Closing Thoughts

Extratropical transitioning is the process whereby tropical cyclones evolve from systems that derive their intensity from ocean heat energy to systems that derive their intensity from atmospheric potential energy.

The strength of the transitioned storm can be comparable to that from before. Importantly, however, the footprint of storm characteristics and hence their impact can be significantly larger. From a damage estimation perspective, therefore, understanding the process of extratropical transition and the ability to forecast when and where it will occur can help better anticipate and prepare for the losses that can result.