On August 15th, 1987, the 54th Weather Reconnaissance Squadron flew its final mission in Typhoon Cary, marking the end of an era. Since the 1940s, flying aircraft reconnaissance into typhoons was a regular occurrence all across the Western Pacific, but aside from the occasional research mission, they would be flown no longer. All was not lost though. Unlike when recon was first flown in the basin, typhoons could now be tracked by both geostationary and polar orbiting satellites, and their intensity could be estimated with reasonable confidence through application of the Dvorak Technique. As time went on, other ways to remotely estimate the intensity of a tropical cyclone were developed, including the automated objective Advanced Dvorak Technique (ADT) and the satellite consensus SATCON, which uses primarily microwave data provided by polar orbiting satellites.
There is no substitute for direct observation though. While remote sensing offered by the Dvorak Technique, ADT, SATCON, and others is often a reasonable placeholder, the true intensity of a system without direct observation will never be actually known. These intensity estimation methods can struggle with various extremes as well. Small overall TC size, small core size, large core size, and rapid fluctuations in intensity can all give these methods fits. The Tropical Western Pacific is a land of extremes. Aircraft reconnaissance investigated fourteen different typhoons with a minimum pressure of 900 mb or lower in the ten year span from 1978-1987, a number that exceeds all other recorded occurrences during observed history anywhere else in the world. Seven of the eight deepest observed tropical cyclones have called the confines of this basin home, including the deepest of them all: Super Typhoon Tip from 1979. Because of a lack of direct observations, surpassing the 870 mb measured minimum pressure from Tip is a tall order, whether it deserves to be or not.
Early in October 2019, a disturbance began to gain organization in the eastern portions of the Tropical Western Pacific, not too far removed west of the International Date Line, which separates the Pacific Ocean into eastern and western halves. As the disturbance traversed westwards, it encountered a region of low latitude westerly winds, aiding vorticity and helping to establish the disturbance’s identity amidst the background trade winds. While in a low vertical wind shear environment, an anticyclone aloft quickly developed in response to thunderstorm convection associated with the disturbance, hastening the development process along even further. On October 5th, it had become apparent that the disturbance had gained sufficient organization to be classified as a tropical cyclone. Not long afterwards, the Japan Meteorological Agency (JMA) gave the tropical cyclone the next name on the naming list: Hagibis.
Upon development, Hagibis wasted little time intensifying. As the system traversed westwards towards the Northern Mariana Islands, it was passing over the warmest and most heat laden waters anywhere in the world at the time, mostly undisturbed this season by abnormally low typhoon activity. Additionally, because of atmospheric changes associated with seasonal shifts, the upper troposphere had already begun to cool. This temperature difference between ocean waters and upper troposphere is what drives a tropical cyclone, and because a tropical cyclone is a Carnot Heat Engine, the bigger the temperature difference between the two results in a higher maximum potential intensity. In the case of Hagibis, maximum potential intensity was truly eye-popping. The system was already taking advantage of these favorable conditions due to basically non-existent vertical wind shear and a well-established upper level anticyclone with good outflow, but in order to actually approach the potential provided by the environment, Hagibis would need to establish an inner core, a chaotic process that currently is not fully understood.
During the course of October 6th, it became apparent that based on passive microwave data from polar orbiting satellites, an inner core was indeed developing and consolidating at a rapid pace. The first hints of a possible eye were observed early on October 6th, but was soon obscured by intense convection. A SSMIS F-16 pass at 0506Z corroborated the brief appearance of the eye feature on conventional imagery with a complete ring on 91H. Over the course of the following twelve or so hours, each following microwave pass showed continued organization overall and a decreasing eye diameter, and by the time the next F-16 pass came in at 1742Z on October 6th, the now developed core had morphed into a configuration supportive of a pinhole eye. Hagibis had already been strengthening rapidly, but the stage now appeared set for explosive intensification.
As the sun rose over Hagibis late on October 6th and early on October 7th UTC, the pinhole eye emerged in the center of the cold CDO the typhoon had developed the previous day. Eye diameter was extremely small, even for pinhole standards, and reminiscent of the eyes from Hurricane Wilma in 2005 and Super Typhoon Parma in 2009. The eye size was small enough to challenge even the spatially impressive 2 km resolution infrared data provided by the geostationary satellite Himawari-8. Impressively, Hagibis maintained a ring of very cold cloudtops of about -80ºC around the pinhole eye throughout the entirety of the daylight hours, when sunshine has a warming effect on cloud tops.
Hagibis maintained its pinhole eye through sunset and into the overnight hours. As the sun disappeared beneath the horizon, the region of -80ºC or colder cloud tops expanded within the CDO. Gravity waves could also clearly be seen emanating radially outwards from the eye across the CDO. The extremely rapid strengthening appeared to continue until roughly 12Z October 7, at which point satellite presentation appeared to level off or even slightly degrade. Microwave imagery would reveal that an outer eyewall developed at about this time, completely encircling the tiny pinhole eye feature and initiating the beginning of an eyewall replacement cycle and a weakening phase.
At the apparent intensity maximum at 12Z October 7th, JMA and the Joint Typhoon Warning Center (JTWC), the two main agencies in the Western Pacific, finally assigned Hagibis their T7.0/category 5 equivalent intensity. This intensity is based mostly on fairly sound subjective Dvorak analyses that adheres to some degree of constraints, but tropical cyclones that undergo extreme rapid intensification don’t nicely follow these constraints that help accurately assess most normal systems. SATCON and other microwave estimates are often particularly useful in assessing the intensity of high end systems, but the pinhole eye and associated small radius of maximum winds plays havoc on their resolution and ability to produce an accurate estimate. The objective ADT is also a useful tool, but has automated constraints that limit intensity estimates in extreme rapid intensification scenarios like the subjective Dvorak Technique. Additionally, the extremely small pinhole eye of Super Typhoon Hagibis was able to challenge even the very good resolution of Himawari-8, and it is possible that the actual warmth of the eye (which is used in the ADT intensity calculation) isn’t fully captured. With all these limitations in more traditional intensity estimation techniques, perhaps a better intensity estimate can be derived for this case of extreme rapid intensification based on past data from similar systems.
Using aircraft reconnaissance along with satellite and other data dating back to 1970, four suitable analog tropical cyclones based on eye size, structure, and environment were found. These four tropical cyclones in chronological order are Super Typhoon Irma (1971), Super Typhoon Forrest (1983), Hurricane Wilma (2005), and Hurricane Patricia (2015). Several other tropical cyclones were considered but eliminated for various reasons. Super Typhoon Nora (1973), Super Typhoon Tip (1979), Super Typhoon Wynne (1980), and Super Typhoon Vanessa (1984) had impressive extreme rapid intensification episodes as well, but none of these systems had an eye that reached as small as 5 nm in diameter. Eye size appears to be an important determinant in deepening rate with extreme rapid intensification. Super Typhoon Megi (2010) met eye size criteria, but the early appearance of an outer eyewall while deepening eliminated it on structural grounds. Super Typhoon Vera (1979) and Typhoon Agnes (1984) were also eliminated structurally due to small overall storm and anticyclone size. Super Typhoon Kim (1980) and Hurricane Maria (2017) were crossed off on an environmental basis due to land interaction. Super Typhoon Judy (1979) also failed environmentally due to moderate northerly shear impinging upon the system. Two notable omissions include Super Typhoon June (1975) and Hurricane Gilbert (1988), a pair of storms whose recon data is difficult to find.
With four reasonable analog tropical cyclones with recon data selected, their deepening rates can be examined. All four tropical cyclones had about an 18 hour period where deepening rates approached or even exceeded an average of 5 mb/hr. Geostationary imagery was not available in 1971 to view Super Typhoon Irma, but at the beginning of the period of extreme deepening rates, Super Typhoon Forrest, Hurricane Wilma, and Hurricane Patricia all featured a cloud pattern that would be classified with the embedded screening when assessing a tropical cyclone with the Dvorak Technique. The same can be said for Typhoon Hagibis at 18Z October 6th, 2019, which appears to be when it began a similar 18 hour period of extreme rapid intensification. Based on a center embedded at least 30 nm in a shade black or colder on BD imagery, Typhoon Hagibis is estimated to have an intensity of 90 kt at 18Z October 6th. Minimum pressure at the same time is estimated at 965 mb using the Knaff-Zehr-Courtney Wind/Pressure Relationship (KZC) with JTWC operational best track data. This intensity estimate appears reasonable based on a SATCON estimate of 87 kt/968 mb at about the same time.
The average deepening rate of the four analog tropical cyclones was then applied to Typhoon Hagibis with 18Z October 6th used as the initial point of extreme rapid intensification. The average 88 mb drop from the four tropical cyclone average applied to the estimated 965 mb pressure results in a minimum pressure estimate of 877 mb at 12Z October 7th. Using JTWC data in KZC to solve from wind from pressure results in somewhat disparate estimates of 170 kt when using the radius of tropical storm force winds (r34) of 204 nm as the size parameter and 179 kt when using the radius of the outermost closed isobar (rOCI) of 210 nm as the size parameter instead. This required further investigating.
When examining ASCAT data from near the time of maximum intensity, JTWC’s r34 estimate of 204 nm appears to be reasonable. However, after cross-referencing surface analysis data at the time, the rOCI of 210 nm at 1002 mb appears far too small and at a pressure too low. Based on JMA surface analysis, it is estimated that the rOCI is actually closer to 420 nm at a pressure of 1006 mb. Re-running KZC with the modified data resulted in a new estimate of 173 kt using r34 as the size parameter and 175 kt with the new rOCI data. With much better agreement between the two KZC methods, the maximum intensity of Super Typhoon Hagibis is estimated to be 175 kt and 877 mb at 12Z October 7th, 2019.
Due to the lack of direct data observed with Super Typhoon Hagibis, such an intensity estimate is of low confidence. Many assumptions and extrapolations had to be made to arrive at this estimate, and this estimate is not official. However, the author feels that this estimate is reasonable. Given limitations with other intensity estimation methods previously discussed, this estimate may provide a better idea of Hagibis’s true intensity.
Following peak intensity, Super Typhoon Hagibis would weaken some due to eyewall replacement, briefly weakening to category 4 before once again regaining category 5 intensity. The typhoon would then move north and make landfall in Japan as a significantly weaker typhoon, but still as one of the two most impactful typhoons (along with Typhoon Jebi in 2018) in Japan in over a decade.