AIR Currents

September 19, 2011

Editor's Note: This year marks the 20th anniversary of Typhoon Mireille. Though its track and intensity were similar to those of other typhoons in Japan's recent history, the damage it caused stands apart; it is still, by far, the country's costliest typhoon. AIR's Dr. Peter Sousounis explains why.

Twenty years ago, in September 1991, Typhoon Mireille whipped across Japan with winds in excess of 160 km/h. According to the General Insurance Association of Japan (GIAJ), the typhoon caused damage in 41 of Japan's 47 prefectures, destroyed more than 170,000 houses and resulted in total insured losses of JPY 573 billion (in 1991 currency). At the time, this was the largest insured loss claim ever paid for a typhoon-related loss in Japan. Two decades later, Mireille remains Japan's costliest typhoon; AIR estimates its recurrence today would result in insured losses of JPY 906.7 billion (Figure 1).

Although Mireille never achieved super typhoon status, and its track was not out of the ordinary, it did exhibit several distinctive meteorological features that contributed to its record-breaking losses. This article examines just what made Mireille unique.

Cat Bond Figure 1 Event 1
Figure 1.Insured losses along the track of Typhoon Mireille if it were to recur today. (Source AIR)
 
Dr. Peter Sousounis  

By: Dr. Peter Sousounis

Edited by Meagan Phelan

On a Collision Course with Japan

Typhoon Mireille formed as a tropical depression over the central Pacific on September 13, 1991. It reached tropical storm strength three days later, on the 16th, and achieved typhoon strength the same night. Over the next several days, Mireille continued to strengthen—reaching its highest intensity on September 23, at which point maximum sustained winds were 212 km/h (Category 4).

During its northwestward journey across open ocean, Mireille briefly interacted with Typhoon Nat, a storm farther east that would eventually impact Taiwan. This interaction may have turned Mireille north at such a time that when it eventually re-curved towards the northeast—as most typhoons in this region do—it was on a collision course with the Japanese island of Kyushu.

On September 27th, when Mireille finally made landfall in Kyushu's Nagasaki Prefecture, it was the third typhoon of the 1991 season to affect Japan in just two weeks. While interaction with Nat may have influenced Mireille's eventual track, it was not the only factor instrumental in Mireille's rise to the top of Japan's insured loss charts.

A Familiar Track, but Higher Losses

Mireille took a track typical of other typhoons that have caused major damage on Japanese soil. Furthermore, as is typical, its strongest winds at landfall were on the right, or eastern, side of the storm (which is a result of the combination of counterclockwise rotational winds and forward motion).

Other typhoons that have caused significant wind damage in Kyushu include Bart in 1999 and Songda in 2004, both of which took paths very similar to Mireille's. Indeed, Songda took a track that was almost identical all the way up to Hokkaido, though differences in landfall intensity were evident; Mireille made landfall with a central pressure of 940 mb and 200 km/h winds, while Songda's central pressure at landfall was 945 mb and winds were 160 km/h. Bart, although it did not follow Mireille's track quite as closely, was more similar to Mireille in terms of intensity at landfall. The figure below compares the tracks of all three storms.

Cat Bond Figure 1 Event 1
Figure 2. Storm tracks of Mireille, Bart and Songda. Also identified in this map are the locations of the wind observation stations referenced in Figure 5. (Source: AIR)

AIR estimates that a recurrence today of 1999's Typhoon Bart would exceed JPY 444 billion (roughly 5.8 billion USD) in insured losses, while a recurrence of 2004's Songda would exceed JPY 643 billion (about 8.4 billion USD).* According to the GIAJ, these are the third and second costliest typhoons in Japan's history, respectively, after Mireille, which would cause insured losses approaching JPY 907 billion (11.8 billion USD) given present day exposures. Mireille's estimated losses are more than double those of Bart, and roughly 30% higher than Songda's.

The large losses from Mireille—as compared to those from the second highest insured loss-causing storm, Songda—were in part a result of Mireille's lower central pressure at landfall (even though both storms exhibited identical central pressure values three hours later, and nearly identical radii of maximum winds). Processes related to extratropical transitioning (XTT) were also prominently at work in Mireille, much more so than they were in Songda (or in Bart). These processes—described in detail in the next section—changed Mireille's wind field, and in doing so, contributed to higher losses.

Strong Winds Where Strong Winds Don't Often Form

As it tracked over Japan, Mireille underwent extratropical transitioning, a process experienced by more than half of all typhoons that impact the Japanese archipelago, including nine out of Japan's top ten loss-causing typhoons. Even though Songda and Bart also experienced XTT, Mireille's transitioning process was unique. Strong winds were present not only on its right, as is typical of typhoons, but during XTT, they also developed where strong winds don't normally form: behind it, and—in some cases—on the storm's left. (If a storm system is divided into four quadrants, winds from behind and from the left can be thought of as coming from the left, rear quadrant of the storm structure.)

Three weather effects—all related to cold air—pwere to blame. Firstly, because of the time of year (late September), unseasonably cold air was in place just to the northwest of the storm, while warm, moist air was positioned to the southwest. This configuration resulted in a strong upper-level jet, a region of exceptionally high wind speeds (green shaded region in the schematic shown in Figure 3). As air moved rapidly into the jet from the west, a strong circulation developed perpendicular to the jet, through the entire depth of the atmosphere.The result was strong westerly winds—that is, winds blowing from west to east, towards Japan—at the surface in the left and rear portions of Mireille.

Cat Bond Figure 1 Event 1
Figure 3. Schematic of Mireille being influenced by upper level winds. The "L" indicates the low pressure center of a storm undergoing XTT. Note that the system has transformed from roughly circular to highly elliptical. As air in the upper atmosphere flows along the dashed lines and enters the green jet region northwest of the storm, it gets deflected to the left. A complementary and opposite motion occurs at the surface of the typhoon. This cross-jet circulation enhanced winds in Mireille's left rear quadrant, as represented here by the light purple arrow. (Source: AIR)

The second reason for the strong asymmetric winds was that as the size of the typhoon circulation expanded—which is common for storms undergoing XTT—and as the winds strengthened asymmetrically, a cold high pressure (see the "H" in Figure 4) moved towards Japan from the northwest to tighten the sea level pressure gradient between Mireille and the high. The increased gradient resulted in stronger winds behind and to the left of the typhoon as it exited Japan.

The third reason for the strong asymmetric wind field of Mireille was that the aforementioned contrast in air masses (warm and cool) created an upper-level cold front on the western edge of the typhoon. The front resulted in air descending towards Mireille's center. As it descended, it warmed, creating a low pressure region—or pressure dip (as indicated by the star in Figure 4)—extending towards the storm's southwest. The lower pressure tightened the horizontal pressure gradient on the left side of the storm still further. Much like a narrowing riverbed forces the river water moving through it to flow faster, the tighter gradient further sped up and strengthened the winds on the left side and behind Mireille.

Cat Bond Figure 1 Event 1
Figure 4. Schematic of Mireille being influenced by cold surface high pressure and pressure dip. The L in this schematic is the low pressure center of a storm undergoing XTT. In the case of Mireille, descending air behind the storm resulted in a pressure dip, shown here by the enhanced curvature of the two black isobars and highlighted by the green star in-between them. The pressure dip, in conjunction with cold high pressure moving southeastward, enhanced the storm's sea-level pressure gradient, and hence, the winds across the storm's left rear quadrant. (Source: AIR)

The result of the three processes described above—the upper-level jet streak that served to strengthen left-hand and backside winds, the cold high pressure moving in from the northwest, and the sinking and warming air that created a pressure dip—was that, despite Mireille's distance from Japan as it tracked north, strong westerly winds behind the storm center were able to wreak havoc on onshore properties. In fact, nearly two-thirds of the Japanese wind observing stations monitoring Mireille reported the strongest winds coming from the west (or left) onto Japan, rather than from the east (or right). Data from two Japanese wind observation stations—located in Imari, on the island of Kyushu, and in Tobishima, on northwestern Honshu—reveal the strengthening winds on the back and left of Mireille as the storm underwent XTT (Figure 5).

Cat Bond Figure 1 Event 1
Figure 5. Data from observation stations in Imari and Tobishima shows that wind speeds from Mireille increased steadily from the east of the storm as it approached Japan (here, easterly—meaning from the east—winds are in red; westerly—that is, winds from the west—winds are in blue). Then, as Mireille passed by the stations, wind speeds dropped (at the times indicated by the dashed lines) before increasing again—but this time from the west, as the blocks of blue bars indicate. The westerly wind speeds actually surpassed the easterly ones. Note: time on the horizontal axis is in Japan Standard Time (JST), 9 hours ahead of UTC. (Source: AIR)

Because both observation stations were located to the right of Mireille's track, and thus positioned closer to the approaching storm's right-hand side, it is no surprise that the strong winds first recorded at these locations were from the southeast (or right-hand side) of the storm, as is the norm. After Mireille passed by, however, both locations experienced strong winds from the west (or left-hand side) of Mireille. Significantly, as the taller blue bars in Figure 5 show, these winds were actually stronger than the winds from the southeast had been.

The three regional-scale processes that caused strong left-hand winds in Mireille did not occur to the same degree in Bart or Songda. In the case of Songda, for example, there was a jet streak, but it was weaker and less well-defined. There was cold advection, but it was also less intense. And there was stratospheric air entering from the southwest, but it was less dramatic. As a result, Songda not only exhibited more relative symmetry as it tracked across Japan, but the majority of its strong winds were from its right, as is the case for most typhoons impacting countries in the Northwest Pacific. Thus, partly because Songda failed to deliver the additional damage-causing winds from its left and from behind, it was a less damaging storm than Mireille, its 1991 predecessor.

A Simulation Showing the Distribution of Insured Losses

A simulation of Mireille using the AIR Northwest Pacific basinwide typhoon model results in 40 prefectures experiencing damaging winds (those above tropical storm strength), which is quite consistent with the GIAJ report indicating that 41 of 47 prefectures reported damage. In Figure 6, which shows observed wind speeds, the hatched shading indicates the prefectures whose strongest winds were those blowing from west to east, or from behind the storm center—toward Japan.

Cat Bond Figure 1 Event 1
Figure 6. Map showing distribution of strong observed winds by prefecture; the hatched shading indicates winds from behind and/or from the left of the storm. Even many east coast prefectures experienced such winds. (Source: AIR)

As Figure 6 illustrates, nearly all west coast prefectures in Japan experienced high wind speeds due to the storm's asymmetric wind field—despite the fact that the storm's center was offshore for much of its journey up the Sea of Japan; indeed even many east coast prefectures experienced relatively strong winds due to this asymmetry, although damage from these winds was more limited. And it is worth noting that Mireille could have been far more damaging had its track up the Sea of Japan been closer to land.

Conclusion

While tropical and extratropical cyclones are distinctly different phenomena, it is not unusual for the former to evolve, or transition, into the latter. As a tropical cyclone begins to move poleward, it encounters cooler sea surface temperatures and increased vertical wind shear associated with mid-latitude westerlies. These and other factors cause the tropical cyclone to increase its forward speed, lose its symmetric characteristics, and, at times, reintensify rapidly into a powerful extratropical cyclone.

Twenty years ago, Typhoon Mireille's transition was accompanied by the simultaneous occurrence of these and other meteorological factors that enhanced its damaging winds—ultimately making it Japan's costliest typhoon on record. Extratropical transition—which is captured in the AIR Northwest Pacific basinwide typhoon model—has long been an active area of research at AIR and, because of its obvious complexities, will undoubtedly remain so.


* JPY = 0.013 USD; this exchange rate is used for all three conversions in this paragraph.


Fudeyasu, H., S. Iizuka, and T. Hayashi, 2007: Meso-β-scale pressure dips associated with typhoons. Mon. Wea. Rev., 135, 1225—1250.

Fumiaki F., N. Kitabatake, K. Bessho, and S. Hoshino, 2005: Features of the wind fields associated with Typhoon 0418 (Songda) compared with those of Typhoon 9119 (Mireille). Available online from http://www.pwri.go.jp/eng/ujnr/joint/37/paper/43fujibe.pdf.

Sousounis, P., and M. Desflots 2010, Evaluating the Impacts of Extratropical Transitioning on Typhoon Losses via Synoptic Case Studies. Twenty-Ninth Conference on Hurricanes and Tropical Meteorology, Tucson, Arizona, American Meteorological Society, May 10-14, 2010.

 

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