Ophelia - October 2017

All opinions conveyed in this paper are my own, and I’d welcome any constructive feedback. :)

Introduction

    Post-tropical cyclone (previously Hurricane) Ophelia was the second major storm of the 2017-2018 Winter in the Ireland and the UK. Ophelia developed as a hurricane over the marginally warm sea surface to the south-west of the Azores Islands in the mid-Atlantic from the 9th - 14th October. By the 14th, Ophelia became the easternmost major hurricane ever recorded in the Atlantic basin, with maximum sustained winds reaching 115mph. By the 15th, Ophelia raced to the north-east, accelerated by an approaching upper trough, and began extratropical transition. During this transition, the storm maintained sustained wind speeds equivalent to a category 1 hurricane, as confirmed by the last forecast discussion issued by the National Hurricane Center (Discussion no. 28). Ophelia made her only landfall on the south-west coast of Ireland on the morning of the 16th October, as a strong post-tropical cyclone. 

    Fig. 1 illustrates how the extratropical transition of Ophelia looked from a visible satellite perspective. The last remnants of the eye are just about visible in the top image, while the storm is fully extratropical with a likely fully-developed sting jet in the bottom image. Ophelia was still producing violent storm force 11 to hurricane force 12 winds by the time she made landfall in SW Ireland. The extratropical transition of Ophelia from the perspective of Met Office surface pressure charts is shown in Fig. 2.
 
Figure 1: The two faces of Ophelia - one of the last visible satellite images of Ophelia as a Hurriacane (Category 2) (top), compared with one of the first visible images of Ophelia as a post-tropical storm the next morning (bottom). Credit: Sat24/ EUMETSAT.
Figure 2: Met Office surface analyses from 00 UTC 15/10/2017 - 18 UTC 16/10/2017. Credit: Met Office. Archived by: wetter3.de.
Meteorological History

Hurricane Ophelia developed from a non-tropical low which formed about 800 miles south-west of the Azores on the 7th October. Over marginally warm sea surface temperatures, this storm gradually obtained subtropical characteristics. Tropical Storm Ophelia was eventually named on the 9th October, making her the 15th named storm of the season. Continued strengthening of the storm occurred, because cold upper-tropospheric temperatures of below -60C offset the marginal sea surface temperatures of 24-25˚C (below that typically required for tropical cyclone formation). Ophelia continued to gradually strengthen, eventually becoming a hurricane on October 11th. 24 hours later, the storm obtained Category 2 strength winds; this gradual strengthening was all aided by the storm remaining near-stationary in an environment with low wind shear, cold upper temperatures and marginal sea surface temperatures. 

    By the 14th October, Ophelia was upgraded to a category 3 hurricane, with winds estimated conservatively at 115mph. The near-symmetric structure of Hurricane Ophelia, with a well-defined eye and spiral cloud bands is shown in Fig. 3. By this point, an upper trough was approaching the system from the north-west, and the strengthening can be attributed to baroclinic processes; positive vorticity advection downstream from the trough axis and ageostropic ascent as a right jet entrance began to move over the storm. As a result, Ophelia became the easternmost major hurricane ever recorded in the Atlantic Ocean; a very rare hurricane for it’s position in the Atlantic, just 235 miles south-east of the Azores. 

    As the incoming trough from the north-west continued to dig south-east towards the storm, Ophelia began to move more quickly towards the north-east. However, a weakening of the storm began as Ophelia became embedded within the upper trough; wind shear increased and the sea surface temperatures cooled as the storm moved north-east. In addition, the surface cold front became enveloped within Ophelia, increasing surface wind shear and injecting cold, dry air into the storm. As a result, Ophelia began to weaken (from a category 3 hurricane), and undergo extratropical transition. By the 15th October, the warm-cored structure was deteriorating rapidly as shear began to rip the deep convection apart; this is shown nicely by Fig. 4. The final tropical advisory issued by the National Hurricane Center was at 21:00 UTC on the 15th October. By 04:00 UTC on the 16th, Ophelia was declared post-tropical by the National Hurricane Center, despite the Met Office unofficially declaring this before 21:00 UTC on the 15th. According to the final National Hurricane Center advisory, the storm’s initial post-tropical intensity was 75kts (86mph), although the storm weakened slightly before it made landfall in Ireland. By the 17th October, post-tropical cyclone Ophelia had dissipated over Scotland and the North Sea.

    The intensity and central pressure of Ophelia, both as a tropical and post-tropical system, are shown particularly well in Fig. 5. Throughout its lifetime, Ophelia transitioned from a symmetric warm-cored major hurricane to an asymmetric warm-cored seclusion, dominated by baroclinic processes, and finally to an asymmetric cold-cored cyclone upon it’s dissipation. The UKMO surface analyses in Fig. 2 illustrate these transitions very nicely, with a weak cold front and strong warm front clearly plotted. At 00, 06 and 12 UTC on the 16th, it can be seen how the system is shown with a warm core seclusion, and the warm and cold fronts are not connected. Only by 18 UTC on the 16th is the system shown as a ‘classical’ occluding low according to the Norwegian cyclone model. NWP forecasts from the Arpege model of the 850 hPa temperature evolution of the storm reinforce this analysis, as shown in Fig. 6. The development of the warm-cored seclusion to the south-west of Ireland is perfect - as textbook as is possible!

Figure 3: Comparison of visible and infrared satellite images of Ophelia as a major Hurricane at 14:45 UTC on the 14th October. The Azores islands are visible in the north of the image. Credit: NOAA/ NHC.
Figure 4: IR image taken by the Sentinel-3A satellite during the early hours of the 16th October, as Ophelia was undergoing extratropical transition. The last of the deep convection is being sheared to the north of the newly-developing extratropical storm. A banded cloud head strongly suggests the presence of a sting jet. Credit: ESA.
Figure 5: The track and intensity of Ophelia, both as a hurricane and a post tropical storm. Credit: xmetman.
 
Figure 6: Animation of the 850 hPa temperature evolution throughout Ophelia’s extratropical transition from the French Arpege model. Credit: wxcharts.
    The storm made it’s only landfall on the south-west coast of Ireland during the morning of the 16th, bringing hurricane-force winds, large waves and a dangerous storm surge. The strongest recorded gust was 119mph at Fastnet lighthouse, about 8 miles south of County Cork, which also recorded a maximum average wind speed of 94mph. On mainland Ireland, the highest gust was 97mph at Roaches Point, where a maximum average of 69mph was recorded. According to Met Éireann, the highest individual wave was 26.1m, and the lowest recorded pressure on the mainland was 962.2 hPa, at Valentia in the extreme south-west of the country. The UKMO analysis in Fig. 2 suggests a minimum central pressure for post-tropical Ophelia of 958 hPa, although this is likely a conservative estimate given the intensity of the storm and sparsity of observations over the ocean. Met Éireann additionally issued a “status red” warning for wind (their highest warning) 48 hours in advance of the storm, an unprecedented event. This warning was eventually extended to all of the Republic of Ireland. All schools were closed in Ireland, and at least 360,000 customers were left without power in Ireland alone.

    Ophelia was an extremely dangerous storm upon landfall, and, overall, I feel that the various weather services did a sterling job forecasting what was an extremely volatile and dynamic storm. Fundamentally, I feel there was excellent communication to the public for this event, particularly through social media. Unusually, collaboration was required between the National Hurricane Center, Met Office and Met Éireann, to ensure a consistent message was delivered across all forecast platforms, which, on the whole, it was. However, on top of all of this, the final forecast issued by the National Hurricane Centre, No. 28, is almost comical (Fig. 7); the massive area of tropical storm-force and hurricane-force winds dwarfs the UK and Ireland, and it’s certainly got to go down in the record books for a tropical advisory to be issued so close to our shores!

Figure 7: The final advisory issued for Ophelia by the National Hurricane Center, deemed a post-tropical cyclone by this stage. Credit: NOAA/ NHC.
Meteorological conditions for development

In essence, the development and intensification of Ophelia can be broken down into two stages:

--> The development of Tropical Cyclone Ophelia.

--> The extratropical transition and development of post-tropical cyclone Ophelia. 

The development of Tropical Cyclone Ophelia

--> The very initial development of Ophelia can be traced back to a non-tropical low which moved over the warm waters of the Central Atlantic Ocean around the 7th October. This provided the pre-existing convergence and vorticity for a sub-tropical and then tropical disturbance to develop.
Ophelia developed into a major hurricane over marginal sea surface temperatures of 24-26˚C. As shown in Fig. 8, these temperatures were 2-3˚C above the climatological average for this part of the Atlantic. 


--> Anomalously cold upper tropospheric temperatures to the south-west of the Azores (Fig. 9) more than offset for the marginal sea surface temperatures, enabling the tropospheric temperature gradient to sustain deep moist convection. 


--> Ophelia’s slow forward movement in a convectively unstable environment with low vertical wind shear meant the storm could gradually intensify over several days to the south-west and south of the Azores.


--> Hurricane Ophelia continued to intensify, at least for a time, as an upper-level trough approached from the north-west. Enhanced upper divergence due to positive vorticity advection and warm air advection downstream of the upper trough, as well as ageostropic ascent as a right jet entrance neared the hurricane, and from the curvature around the trough, helped to ventilate and intensify the storm, before increase upper-level shear could tilt the storm and initiate extratropical transition. This additional ventilation effect from the approaching trough likely helped to strengthen Ophelia into a major hurricane; this enhanced divergence is shown in Fig. 10. 


Figure 7: The final advisory issued for Ophelia by the National Hurricane Center, deemed a post-tropical cyclone by this stage. Credit: NOAA/ NHC.
Figure 9: 200 hPa geopotential height and temperature, displayed from GFS analysis. The anomalously cold upper air is circled in red to the south-west of the Azores. Archived by: wetter3.de.
The extratropical transition and development of post-tropical cyclone Ophelia

The extratropical transition of Hurricane Ophelia began on the night of the 14th - 15th October, as the storm began to interact increasingly with the aforementioned upper-level trough. 

--> At the surface, Ophelia became entangled with the polar front, which provided a preexisting surface thermal gradient and baroclinic zone (as if the hurricane itself wasn’t enough). 

--> Ophelia remained in the favourable environment for baroclinic cyclogenesis until just before the storm made landfall in south-west Ireland. This was because south-westerly winds around the upper trough drove the storm to the north-east, allowing the storm remain collocated with the 300 hPa right jet entrance.


--> A strong north-east/ south-west oriented jet stream developed downstream of the trough axis. This exceeded 150 knots according to the GFS forecast. 


--> During extratropical transition, coupled jet streaks also evolved around the storm. As shown in Fig. 11, Ophelia became colocated within an enhanced region of upper-level ageostrophic ascent, as both the right jet entrance of the upstream jet streak and the left jet exit of the downstream jet streak become located above the cyclone. 


--> At the same time, intense warm air advection and positive vorticity advection fuelled strong ascent just ahead of the storm. This is shown in Figures 12a and 12b. 


--> In addition, an impressive potential vorticity streamer was advected into the rear of the cyclone as it was undergoing extratropical transition, shown in Fig. 13. This enhanced the potential for a surface response in the form of a rapidly developing cyclone. 
 
Figure 10: 300 hPa geopotential height, wind speed and horizontal divergence for 12 UTC on the 14th October, displayed using GFS analysis. Two areas of divergence are highlighted: one is located above Hurricane Ophelia, and the other is found downstream of the upper-level trough.
Figure 11: 300 hPa wind speed, geopotential height and horizontal divergence, for 00 UTC on the 16th October. Ophelia, by this point located to the west of the Bay of Biscay, is located under an exceptionally favourable area of cyclogenesis, both downstream from the increasingly negatively-titled trough, and between coupled jet streaks. Both the right jet entrance of the strong upstream jet streak, and the left jet exit of the downstream jet streak, are contributing to the strong agestrophic ascent.
Figure 12: a) - 850 hPa temperature advection and geopotential height. b) - 300 hPa absolute vorticity advection and geopotential height. Both images are taken from the GFS analysis for 00 UTC 16/10/2017.
Figure 13: GFS potential vorticity on the 320K isentropic surface analysis for 00 UTC on the 16th October. Credit: wetter3.de.
Was there a sting jet?

    The strongest winds occurred on the southern flank of the storm, where a sting jet is generally expected. However, this is often the case for any strong extratropical cyclone, as the synoptic pressure gradient is usually tighter on the southern flank of the storm. And yet the strongest measured gust - 119 mph at Fastnet Lighthouse, with a mean wind speed of 94mph would suggest a much tighter pressure gradient and lower central pressure for the storm to sustain such a wind speed. To the west of the Bay of Biscay, post-tropical cyclone Ophelia was likely at its most intense; although there are a lack of observations, NWP data from the main global weather models, as well as higher resolution limited-area models, were both consistent in showing gusts of around 140mph. Thus, it is likely that the winds were supergeostrophic - i.e. the synoptic pressure gradient is unlikely to have been able to sustain such strong winds. 

    With this in mind, looking back at Fig. 6 illustrates how Ophelia transitioned into a mature Shapiro-Keyser type cyclone with a warm seclusion. The thermal gradient along the warm front was much greater than along the weak and frontolysing cold front. This is backed up not only by multiple NWP forecasts, but also by radar and satellite observations, which showed the cold front producing very little precipitation. It is unsurprising that Hurricane Ophelia developed a warm seclusion in her post-tropical state, as Shapiro-Keyser cyclones develop in environments with bountiful latent heat (usually over the ocean). The warm-seclusion nature of the storm enhanced the potential for convective elements to develop in the cloud head, long after the original convective core of the tropical cyclone had dissipated. This, as we shall see in a moment, is important. 

    The potential vorticity streamer illustrated in Fig. 13 was advected into the rear of the cyclone. The isentropic layer measured (320K) suggests that the streamer of high PV (thus dry, stratospheric) air, penetrated at least into the mid-troposphere, and likely lower. Fig. 14 excellently demonstrates how this cold, dry air rapidly wrapped around the rear of the cyclone during extratropical transition. 

    Forecast GFS frontogenesis charts show a distinct region of divergent Q-vectors at the end of the warm seclusion, between the warm and cold fronts, as the storm was nearing maximum intensity to the west of the Bay of Biscay. Based on this analysis, it is likely that descending dry stratospheric air dissolved the back-bent warm front in this region. 

    The warm seclusion likely increased the potential for convective elements to develop in the cloud head, as discussed above. The convective energy could be released either in the form of conditional symmetric instability (also known as slantwise convection), or the more traditional upright convection, depending on how unstable the atmosphere was near the cyclone centre. Fig. 4 shows several convective bands present in the cloud head of post-tropical Ophelia - close to maximum intensity. Downdrafts associated with these convective elements may well have been enhanced by evaporative cooling as they encountered the dry, stratospheric air in the mid-levels. These would have helped to transfer high-momentum air down to the surface in the form of a sting jet, which could explain the incredibly high wind gusts predicted by the main NWP models (and ultimately, observed).

    In reality, determining whether Ophelia really did produce a sting jet requires complicated reanalysis of the cyclone and airmass back trajectories that is beyond the scope of this report. However, all the evidence complied so far indicates that a sting jet was likely.

Figure 14: IR loop showing Ophelia’s transition from a hurricane into a post-tropical storm, overlaid with the airmass type (red/ purple - cold and dry (high PV) air, green - warm and moist air). Credit: EUMETSAT/ Wetterzentrale.
Conclusions

    The lifecycle of Ophelia is fascinating, both from a thermodynamic and dynamic standpoint, and no doubt will be a case study found in text books and scientific papers for years to come! Ophelia obtained a minimum central pressure as a post-tropical storm dominated by baroclinic processes at least equivalent to, if not lower than, that reached when Ophelia was a major category 3 hurricane. Ophelia’s lifecycle shares many similarities with Sandy, which pummelled New Jersey and New York in October 2012. Both were major category 3 hurricanes which transitioned into massive extratropical cyclones before landfall, as a result of strong dynamical forcing and coupled jet streaks. 

    Through Ophelia’s lifecycle up to her landfall in Ireland, sea surface temperatures were 1.5-3˚C above the climatological average. This raises several fundamental questions: how would Ophelia have evolved if temperatures were around average - would there even have been a hurricane in the first place? And, in a warming world with rising sea temperatures, will Ireland, the UK and Western Europe see similar (and more intense) storms of tropical origin like Ophelia in the future?