Typhoon Noru (2017)


Typhoon Noru was the second-longest-lasting tropical cyclone of the Northwest Pacific Ocean on record—behind only 1986's Wayne and tied with 1972's Rita—and the second-most-intense tropical cyclone of the basin in 2017, tied with Talim. Forming as the fifth named storm of the annual typhoon season on July 20, Noru further intensified into the first typhoon of the year on July 23. However, Noru began to interact with nearby Tropical Storm Kulap on July 24, executing a counterclockwise loop southeast of Japan. Weakening to a severe tropical storm on July 28, Noru began to restrengthen as it turned sharply to the west on July 30. Amid favorable conditions, Noru rapidly intensified into the season's first super typhoon, and reached peak intensity with annular characteristics on July 31. In early August, Noru underwent a gradual weakening trend while curving northwestwards and then northwards. After stalling off the Satsunan Islands weakening to a severe tropical storm again on August 5, the system began to accelerate northeastwards towards the Kansai region of Japan, making landfall in Wakayama Prefecture on August 7. Noru became extratropical over the Sea of Japan on August 8, and dissipated one day later.
Noru was responsible for two deaths in Kagoshima Prefecture, and at least $100 million in damages in Japan.

Origins

A non-tropical low formed about 700 km north-northwest of Wake Island at around 00:00 UTC on July 18, 2017, which transitioned into a tropical depression about one day later. In the afternoon of July 19, the Japan Meteorological Agency began issuing tropical cyclone warnings on the system, expecting a tropical storm within 24 hours. Although outflow was weak for the partially organized system at that time, low vertical wind shear and sea surface temperature at 29 ºC were conducive to development. Originally retaining some subtropical characteristics, it began transitioning into a fully tropical and warm-core system; therefore, the Joint Typhoon Warning Center issued a Tropical Cyclone Formation Alert to the system early on July 20, when fragmented deep convective bands had wrapped around its broad low-level circulation center. A point source positioned over the center also improved outflow aloft. At 12:00 UTC, the system intensified into the fifth Northwest Pacific tropical storm of 2017, which was assigned the international name Noru half a day later by the JMA operationally. Besides, the JTWC upgraded the system to a tropical depression at 21:00 UTC. 12 hours later, the JTWC upgraded Noru to a tropical storm, despite its partially exposed LLCC; at the same time, it started tracking westward along the southwest periphery of the subtropical ridge positioned to the northeast.
Noru struggled to intensify significantly for two days owing to relatively low ocean heat content values and its asymmetric structure, until it developed into a severe tropical storm at around 18:00 UTC on July 22. Subsequently, Noru slowed down and remained stationary, due to a complex steering environment consisting of ridging on three sides. The steering flow also transitioned from a subtropical ridging to the west to a near equatorial ridge to the south of the system. Noru rapidly intensified into the first typhoon of 2017 around 12:00 UTC on July 23, marking the latest occurrence of the first typhoon since 1998. It featured a 20 km diameter pinhole eye that embedded in an extremely compact core located about 900 km east of the Bonin Islands, thanks to moderate eastward outflow, anomalously warm SSTs and low vertical wind shear. After that, the eye repeatedly reappeared and filled due to increasing vertical wind shear offset by improving poleward outflow into the upper-low associated with Tropical Storm Kulap. Noru also started to track east-southeastward under the steering influence of a near equatorial ridge to the south.

Interaction with Tropical Storm Kulap

On July 24, a Fujiwhara interaction between Noru and Kulap started. As a result, Kulap became directly located to the north of Noru early on July 25, and this situation allowed cold dry air to intrude along the western and southern periphery of the typhoon, making Noru weaken slightly with the significantly shallower convective core. The primary steering influence for Noru was still a near equatorial ridge to the south, but another ridge was also building in to the east, turning Noru northeastward and then northward. Although vertical wind shear relaxed somewhat with the robust eastward outflow channel on July 26, the reduction of mid-level relative humidity and slightly depressed SSTs at 26 °C to 27 °C tempered intensification and even eroded the convective structure. Having finished a cyclonic loop and tracking west-northwestward along the southern periphery of a deep-layered ridge to the northeast, Noru showed some signs of transitioning to a hybrid system, with an upper-level low nearby and a thermal structure more indicative of a subtropical storm. However, Noru reformed a 35 km diameter eye late on the same day, and while dry air was still entrenched across the southern portion of the typhoon, the extent of the dry air had shrunk and the moisture envelope was expanding, forming a hospitable cocoon for the system to reside in.
On July 27, Noru began to track westward along the southern periphery of a deep-layered ridge to the northeast. The upper-level environment was extremely complex, with a tropical upper tropospheric trough cell having developed just to the southwest, another upper-level low to the southeast and an induced anticyclone just to the south and extending over Noru. The aforementioned TUTT cell to the southwest was also providing a weak source of outflow. Moreover, the combination of the complex upper-level outflow and low vertical wind shear was helping Noru to maintain intensity. At around 00:00 UTC on July 28, Noru slightly weakened into a severe tropical storm when approaching the Bonin Islands. Dry air completely surrounded the system with weak convection wrapping around a ragged eye; moreover, vertical wind shear was high, but the shear vector was in general alignment with the storm motion, with resultant relative shear values in the low to moderate range. At the same time, the steering flow transitioned to the subtropical ridge centered over the East China Sea, causing Noru to track southwestward along the eastern boundary of this STR. Despite warmer SSTs at 28 °C to 29 °C which supported intensification for Noru in the area, the complex upper-level environment with a TUTT cell to the southeast and a ridge far to the north combining limit outflow, while dry air continued enveloping the typhoon and isolating it from any significant moisture.

Re-intensification and peak intensity

Noru reformed an eye at around 09:00 JST on July 29 while situated 70 km to the east of Chichijima, the largest Ogasawara island. Based on the observations from Iwo To, the JTWC downgraded Noru to a tropical storm at around 18:00 UTC, at which time the system was tracking directly southward along the eastern periphery of an extension of the STR positioned to the west. Early on July 30, Noru began to improve its convective structure, making both of the JMA and the JTWC upgrade Noru to a typhoon again. Over the following 24 hours, Noru rapidly intensified, and the JTWC reported that it had become a Category 4 super typhoon about 225 km south of Iwo To at around 18:00 UTC, with one-minute sustained winds at 250 km/h, though operationally it was put at Category 5 strength with winds at 260 km/h. The typhoon took on annular characteristics, with a symmetric ring of deep convection surrounding a 30 km well-defined eye and fairly uniform cloud top temperatures. Environmental conditions were favorable with radial outflow, enhanced by a poleward outflow channel into the TUTT and a subtropical jet to the east-northeast. Noru's previous southward motion also led itself to the warmer area with SSTs at 29 °C to 30 °C, favoring intensification. At this time, Noru began to drift westward within a competing steering environment.

On July 31, Noru reached its peak intensity at around 00:00 UTC with ten-minute sustained winds at 175 km/h and the central pressure at 935 hPa. It remained an impressively annular system with a clear eye but lacking banding features, yet the JTWC downgraded it to a typhoon in the afternoon because of recently warming cloud tops. Traveling northwestward over an area of low ocean heat content and isolated from well-defined outflow, Noru's gradual weakening trend became more evident early on the next day, which is also indicated by the JMA. A microwave imagery depicted a weakened eyewall, while the eye was temporarily not as clear as before. Since late August 1, Noru presented a 95 km diameter large but ragged eye, yet the convective ring surrounding the eye gradually elongated, especially on the northwestern quadrant; meanwhile, microwave imageries even depicted a break from the northwestern portion of the eyewall. Noru was establishing a poleward outflow channel that tapped into a passing mid-latitude trough located to the north on August 2, and a strong near-equatorial ridge was steering the typhoon towards a weak area in the subtropical ridge to the northwest, being the result of that passing mid-latitude trough.

Landfall and dissipation

The ragged eye of Noru became enormous on August 3, with the diameter about 110 km. Despite maintaining a good poleward outflow channel into an upper-level low to the northeast, the mid-latitude trough just to the northwest caused subsidence along the northwest quadrant and offset the favorable conditions including low vertical wind shear and warm SSTs, making deep convective bands shallower more and more. Having tracked westward relatively faster for one day, Noru slowed down again and turned northwestward on August 4, along the southern periphery of a deep-layered subtropical ridge over the Korean Peninsula and western Japan. Deep convection in the northern portion was very shallow early on the same day that prompted the JTWC reporting a partially exposed system. In the afternoon, convective bands re-intensified and surrounded a 70 km eye, yet the JMA radar depicted a much larger eye, with the diameter about 95 km to 110 km. On August 5, deep convective banding presented predominantly in the southern semi-circle of Typhoon Noru, owing to strong equatorward outflow and the terrain of Kyushu as well as the Ōsumi Islands. In spite of moderate vertical wind shear, the very warm SSTs at 31 °C of the Kuroshio still helped sustain the strength of Noru. However, the system weakened into a severe tropical storm again about noon when the eye had completely deteriorated, as an extension of the subtropical ridge was suffocating the typhoon on the northern periphery, resulting in the deteriorating poleward outflow channel.
Noru remained almost stationary and looping around Yakushima until it accelerated northeastward off the southeastern coast of Kyushu on August 6, along the western boundary of a deep-layered ridge extension located to the southeast. While the eye was struggling to reform, a strong convective structure started to wrap around a growing ragged eye feature later, over the SSTs at 29 °C. On August 7, Noru passed near Muroto, Kōchi at 10:00 JST on August 7 and made landfall over the northern part of Wakayama Prefecture at 15:30 JST. Both of the JMA and the JTWC downgraded Noru to a tropical storm at around 24:00 JST, for the weakening system tracked over the Japanese Alps. Noru emerged over the Sea of Japan from Toyama Prefecture at around 05:00 JST on August 8 and then continued weakening, as deep convection had gradually deteriorated because of the mountainous terrain. Thus, the JTWC downgraded Noru to a tropical depression about five hours later and issued their final warning owing to the poorly-defined and disorganized LLCC in the afternoon. Noru subsequently became an extratropical cyclone at around 21:00 JST, before the system was declared dissipated by the JMA on August 9.

Preparation and impact

In Japan, hundreds of flights were cancelled ahead of the storm, and thousands of people were moved to evacuation zones.
As of August 7, two people had been killed by the passage of Typhoon Noru, both in Kagoshima Prefecture. One man died due to Noru's high winds. The other Noru victim was a fisherman. Total economic losses in Japan were counted to be US$100 million.