Ex-hurricane Ophelia

In a period of ten weeks, ten hurricanes including Maria, Irma, Harvey, and Jose have hit Central America, the Caribbean and U.S. Gulf Coast. This Atlantic Hurricane season has the dubious privilege of being the 7th most intense since 1850 to date. Furthermore, there is no precedent in the era of modern satellite imagery and only in 1878, 1886 and 1893 can we find a comparable situation.  So far this season there have been 357 deaths and an estimated $186.8 billion in damages, and as the hurricane season doesn’t finish until the end of November, what else can we expect?

In this context, Ophelia, the 15th named storm of the season (Humphries, 2017), has stood out as a stubborn hurricane with a trajectory bound for the mid-latitudes of Western Europe. Although it is not impossible for a hurricane to form in the region south of the Azores Islands, it is quite unusual. We have to look back to 1851 for such an event to happen (Henson, 2017). Even Michael in 2012 was 1.450 km further into the Atlantic (Henson, 2017). This makes Ophelia the hurricane with the furthest easterly position in the Atlantic and also the furthest north recorded since 1939 (Humphries, 2017).

Usually cyclones such as this one impact Ireland after following a long trajectory through the Atlantic, from west to east. However, if they are formed off western Africa they do not have to travel as long a distance to reach us as their American relatives, and therefore even when environmental conditions are not equal to those on the other side of the Atlantic, these cyclones can conserve most of their damaging characteristics (Haarsma et al., 2013).

For such a cyclone to develop the surface water temperature needs to be at around 27ºC. The warmer water 1.400 km off the coast of the Azores made possible the formation of “Tropical Depression Seventeen” (NHC, 2017), as it was called by then, on October 9th (Echo News, 2017). The depression strengthened and obtained the right to be given a name: Ophelia, although it wasn’t until the 11th that she became a hurricane with winds of 105 km/h.

On the 12th October the approach of Ophelia to Ireland gained media and public interest, since Met Éireann started to seriously warn about the characteristics of Ophelia and its potential for damage. Winds of 170 km/h were recorded at that stage and from 50 to 100 mm of rain was forecast for the Azores that weekend (NHC, 2017).

In the beginning of her journey, Ophelia moved to the southeast and veered later towards the northeast. After remaining Category 1 and being static between the trade winds and the westerlies (Fonseca, 2017), she started moving towards the northeast, with the maximum winds up to 170 km/h. On Saturday 14th she was considered Category 2 and was subsequently reclassified as Category 3 in the same day, when she passed the Azores.

During the night from Saturday to Sunday, the eye of the hurricane became indistinguishable. Nonetheless, the core of the storm was what impacted on Ireland, and not the tail. This, together with the fact that Ophelia was caught by a trough propelling her to move north and northeast, made Ireland more vulnerable to the high winds, precipitation and storm surge in the southwestern and southern areas. Furthermore, the upper jet streak helped to deepen the low pressure (NHC, 2017).

As the hurricane moved above colder waters, even despite the positive temperature anomaly of around 2ºC, she changed to an extra-tropical depression or post-tropical cyclone. As such, she transitioned from having a warm core and axial symmetric structure to being asymmetrical, with the low and mid-level centers separated (NHC, 2017), and lower core temperatures.

Sea Surface Temperature Anomaly in the Atlantic Ocean the 14th October 2017. Source: NCEP/NOAA.

Her movement was influenced by the Coriolis force and the trade winds from the west, and she was soon being driven by the westerly winds (Haarsma et al., 2013). Although now an ex-hurricane, the winds were still close to hurricane level once she landed, in particular in the sting jet, which is the frequent cause of windstorms. Furthermore, higher winds were present in the eastern and southern flank of the low, which changed direction as the extra-tropical cyclone passed through the country.

Air masses satellite image showing the position of the sting jet in ex-hurricane Ophelia. Source: @KeraunosObs.

It is true that positive anomalies in SST (Sea Surface Temperature) could slow the transition (see picture above). However, the baroclinic instability of mid-latitudes can promote a re-intensification of the depression (Haarsma et al., 2013). Indeed, this instability is the one which led to a previous formation of a similar storm in that area (Haarsma et al., 2013).

On Sunday morning the National Emergency Coordination Group was in place to evaluate the situation and put into practice all the appropriate measures according to the red wind warning for the coastal areas and orange for the rest of Ireland issued by Met Éireann.

The predictions came true when on Sunday it was claimed to be the strongest storm in Ireland since Debbie (in that occasion, in 1961, 18 people died), or Charley in 1986, with 11 deaths (Evening Echo, 2017).

The initial warnings were finally extended to all the country from 9 am on Monday to 3 am on Tuesday as Ophelia approached.

Red warning for winds (a) and orange and yellow warning for rain (b) issued by Met Éireann due to the expected high probability of very strong winds directly impacting Ireland.

On that same day, Met Éireann informed the public about the approximate times of Ophelia’s arrival for different areas:

  • From 7 am: Coastal areas of counties Cork and Kerry.
  • From 9 am: Remaining parts of Munster.
  • From midday: South Leinster and Galway.
  • From 1 pm: Dublin and remaining Leinster.
  • From 3 pm: North Connacht and Ulster.

Once Ophelia made land she crossed the country causing damage on her way northwards. At 2 pm on Monday 16th the NHC (National Hurricane Center) stopped following Ophelia and Met Éireann published some of the strongest gusts and rainfalls recorded by that time:

  • 191 km/h at Fastnet Rock (6.5km SW of Cape Clear Co. Cork, at a height of 200ft).
  • 156 km/h at Roches Point.
  • 135 km/h at Sherkin Island.
  • 126 km/h at Cork Airport.
  • 122 km/h at Shannon Airport.

Rainfall:

  • 17 mm at Valentia, including 9mm in one hour.
  • 17 mm at Mace Head, including 8mm in one hour.

The warnings encouraged people to stay safe indoors while Ophelia was still impacting the country in order to avoid fallen trees or power lines, closed roads, debris or high waves. Moreover, some places were provided with sandbags. However, the response from the government has raised some criticism for its slow reaction (Daly, 2017).

Pictures of the aftermath of ex-hurricane Ophelia in College Road and high water levels in the south branch of the river Lee passing through the Brookfield bridge, Cork.

Authorities urged people in low coastal areas to be prepared in light of the 4 pm high tide. Fortunately, no significant tidal flooding was reported (Niamh et al., 2017).

However, there were accidents which ended tragically. A woman died in Aglish, Waterford, when a tree fell and hit her car, as wind gusts of more than 130 km/h reached the south (The Independent, 2017). Two more people died during the day and there were also reports of injured people. Intervention from the Coast Guard was needed to rescue kite surfers.

Numerous flights, more than 200, (The Independent, 2017) were cancelled on Monday, schools and institutions such as University College Cork (UCC) closed, public transport (bus, train, ferries) and other events were cancelled or rescheduled. Postal services, courts, government institutions, businesses, small and large shops,… all announced that they would remain closed in light of the high winds and the resulting risk for their employees. Also, Cork University Hospital (CUH) cancelled procedures. Measures were taken to offer assistance to elderly and homeless people. Hotels were recognised for their services and many farmers had to resort to generators for electricity.

The ESB (Electricity Supply Board) was updating the list of incidents as Ophelia advanced to the north of the country. Power outages increased continuously until an estimated figure of 360.000 customers were without power, 450.000 homes and businesses. They also warned that it could take up to 10 days before electricity was restored. Broadband and telephone services were interrupted too (The Independent, 2017). Water supply was also disrupted in some areas.

Service interruptions updates during storm Ophelia. Source: ESB.

The economic damage was forecast to be around 700 m euros, but this figure increased as the storm advanced up to 1.5bn euros (The Independent, 2017).

What about an attribution study of Ophelia?

The great majority of attribution studies focus on extreme temperatures and precipitation since these have greater reliability. Events such as tropical and extra-tropical cyclones, together with wildfires and severe convective storms (NASEM, 2016), experiment changes in their physical mechanisms due to climate change that are less understood.

Prediction of this kind of weather system is extremely difficult. First of all because there is a combination of various specific ingredients needed for a hurricane to develop and in order to determine its intensity and trajectory. Besides, these events lack continuous observational time series with the necessary quality and duration. Rather, we have discrete events whose fragmented time series hinder the application of a proper study of extreme values. Even more, when we use models it is important to be aware of their limitations in the reproduction of extra-tropical cyclones. Their performance is even worse when referring to tropical cyclones (NASEM, 2016). All those factors lead to a low confidence when performing attribution studies of anthropogenic climate change for extra-tropical cyclones.

Nonetheless, the ingredients necessary for a hurricane or an extra-tropical cyclone to develop can shed some light about their potential changes in a warming climate. Yet, there is an implicit assumption that the underlying physical mechanisms are well-understood and that current relationships between variables and mechanisms will be maintained in the future.

Even so, there is already some evidence that anthropogenic climate change has potential impacts in terms of the frequency and/or intensity of tropical and extra-tropical cyclones. First of all, the increase in the mean global SST will have multiple different consequences. On one hand, the thermal expansion of the water in seas and oceans together with the melting of the main ice cores constitutes an increased risk of coastal flooding due to higher sea levels and storm surges. Extreme sea levels during storm surges have already been observed since 1970 (IPCC, 2014). The SST difference between the Pacific and Atlantic oceans also affects the wind shear, although with considerable uncertainty (Haarsma et al., 2013). On the other hand, warmer water leads to an increase in the moisture holding capacity of the atmosphere, which increases exponentially according to the Clausius-Clapeyron relation (7% per degree temperature).

It is also necessary to take into consideration that an increase in mean global temperatures decreases the temperature gradient between the equator and the poles, creating the instability that drives air masses from south to north in the Northern Hemisphere. Therefore, a smaller  gradient would be expected to imply less instability. However, a warmer atmosphere expands, increasing the height of the tropopause and the latent heat and therefore contributing to the instability.

As a result of considering some of these various factors, longer sustained systems are expected to move through the Atlantic, and some authors have already obtained a decrease in the number of hurricanes although with a higher intensity.  According to Haarsma et al., (2013) an increasing number of tropical hurricanes arriving to western Europe is also probable in a future scenario.

REFERENCES

ClimAtt project flyer

We have been making some aesthetic changes to our web site and Twitter account @ClimAtt_Project. ClimAtt is a dynamic project and we are working constantly to study the human influence in the extreme weather events in Ireland.

We are increasing our media presence day by day, in order to reach scientists and also the general public. We hope to communicate the objectives of the project as well as the progress, results and current events and news.

We will be perfoming studies of the extent of anthropogenic climate change influence on extreme weather events, so that this information can be used for taking objective and effective adaptation and mitigation measures. This is even more important because islands are being particularly impacted by human warming.

The new ClimAtt project flyer offers an overview of the project. It can be downloaded from here: Flyer_ClimAtt_Project

The ClimAtt logo represents the identity of the project: a swirl combining green and grey colours. The message we want to transmit with this logo is dual. Firstly, the complexity of the climate system,  the chaotic nature of the atmosphere, and the combination of dynamic and thermodynamic factors all influence specific extreme weather events. Secondly, it is necessary to evaluate and interpret all the pieces of this complex world of interactions in order to obtain robust estimates of the human influence on extreme weather events. This is done by comparing the actual world (factual) with another world scenario without human impact (counterfactual).

This project has been made possible through a grant awarded by the Environmental Protection Agency.

If you would like printed copies of the flyer, please contact us . We particularly welcome enquiries from schools.

 

 

Donegal Extreme Rainfall and Floods of August 2017

By Adam Pasik, ClimAtt Master’s Student, Department of Geography, UCC

The Weather of August 2017

August 2017 in Ireland was rather cold and dull, yet for the most part well within its normal scope of variability. All twenty five principal weather stations recorded mean temperatures somewhat below their 1981-2010 long term average (LTA) and the number of recorded sunshine hours was also below the LTA at most stations. Overall, August was unexceptional in terms of precipitation with monthly totals ranging from 75% to 185% of the LTA across the country, and only one day with gale force winds was recorded (Met Éireann, 2017a).
However, one event of localised extreme rainfall took place in the north western part of the country, causing extensive and severe flooding and landslides (Donegal Now, 2017; Maguire, 2017a).

Meteorological Background

In the early morning of the 13th of August the United States National Hurricane Center (NHC) issued its first public advisory notice on the tropical depression no. 8. The NHC continued to issue updates on the storm four times daily until the evening of the 17th of August, when the storm moved away and no longer endangered the East Coast of the United States (NHC, 2017).

The initial tropical depression developed east of the Bahamas and began to travel north-northwest towards the United States. It had an estimated minimum central pressure of 1,011mb and sustained winds of up to 35mph. With the decreasing pressure and strengthening winds, the depression evolved into a tropical storm, and received the name Gert on the evening of August 13th (NHC, 2017). Gert attained hurricane force in the early morning hours of August 15th, and began to veer northeast. Now travelling away from the continent, Gert continued to increase in strength and attained its maximum strength around 3am on August 17th, reaching 105mph in sustained winds with stronger gusts and a minimum central pressure of 967mb. From there on, the hurricane began to weaken quickly as it continued to move northeast into the colder waters of the North Atlantic, and was reduced to a post-tropical storm by the evening of August 17th (NHC, 2017).

The remnants of Gert became absorbed by another low pressure system travelling across the Atlantic, before making landfall in the northwest of Ireland in the afternoon of the 22nd of August. The low pressure weather system brought extremely heavy, although very localised, rainfall yet no significant winds. North Co. Donegal was most affected with an extremely high 77.2mm of rain being recorded at Malin Head weather station, most of which fell in the space of just 8 hours (Fleming, 2017). This was the second wettest day (and the wettest August day) recorded at Malin Head since 1955. The only wetter day recorded was December 5th 2015 with 80.6mm of rain. However, on that day the precipitation was more evenly spread over a 24 hour period (Met Éireann, 2017b). At Malin Head, August 2017 as a whole received 185% of the LTA rainfall, where the above mentioned event was responsible for 83% of this total (Met Éireann 2017c).

Impacts: Flooding in the Northwest

This downpour resulted in flash flooding in the eastern part of Co. Donegal, Co. Tyrone and Co. Derry/Londonderry. Flood waters caused severe structural damage to major roads and destroyed bridges (McClements, 2017; McClements et al., 2017). Many homes and businesses were damaged and local farmers reported losing farm animals to the flood waters (Highland Radio, 2017a). Tens of families registered as misplaced and worked with the local council to avail of temporary accommodation due to their homes being inundated (Maguire, 2017b). The city of Derry was virtually inaccessible by road and its airport had to temporarily shut down and cancel all flights (Highland Radio, 2017b).

Severe flood damage to the Old Mill bridge, Buncarna, Co. Donegal. Photograph courtesy of Pat Colhoun https://500px.com/paddyc

Worst affected however was the Inishowen Peninsula in Co. Donegal, where the damages included collapsed bridges and some roads being simply washed away. Some 1,500km of the road network were affected by the disaster on the Peninsula alone, parts of which are expected to remain impassable for weeks (Maguire, 2017b).

Restoration works at the Cockhill Bridge, Buncarna, Co. Donegal.
Photograph courtesy of Pat Colhoun https://500px.com/paddyc

There were power shortages following the rain, caused by the flooding as well as lightning strikes. The Electricity Supply Board (ESB) estimated that at the height of the storm some 25,000 dwellings were without power throughout the country. On Inishowen 1,600 homes were still without power the following afternoon. In many cases it was deemed unsafe to restore the power until the flood waters have receded (McNeice, 2017). Irish Water has announced several burst mains and damages to wastewater infrastructure due to flooding, causing shortages in freshwater supply on the Peninsula (McNeice, 2017). At least two instances of landslides were also reported occurring at Grainne’s Gap, near Muff, and a smaller one in Urris (Donegal Now, 2017; Maguire, 2017a).

Flood damage to the football pitch of Cockhill Celtic, Buncarna, Co. Donegal. Photograph courtesy of Pat Colhoun https://500px.com/paddyc

With more than 100 people having to be rescued by the emergency services from their stranded cars or flooded properties, it is surprising that no serious injuries or deaths resulting from this event were recorded (McClements et al., 2017).

REFERENCES

 

 

Analogies for attribution of extreme events

Unlike other research fields where the work is performed with tangible samples and materials, attribution studies of extreme weather events don’t offer that possibility, which make them sometimes quite difficult to understand.

The analyses carried out by meteorologists and climatologists usually involve huge amounts of data which seem abstract until you make sense of them, identify what you are looking for, and present the results in a visual and comprehensible way.

Through the review of the literature of attribution studies of extreme weather events, it is possible to find analogies to make the basis and methodology of theses studies easier to understand by comparing them with possible everyday  activities or actions.

Maybe the first and most well known analogy is the one of Professor Myles Allen of the loaded dice (you can watch the seminar Loading the dice: climate change and extreme weather in our former post). If you repeatedly roll several dice, it can happen that initially everything seems normal, nothing extraordinary. However, if you look twice, more carefully, you will soon realize that the number six seems to occur frequently. Of course, this could be by chance, but it could also be associated to other factor/s which would explain this apparently extraordinary number of sixes. But in order to discover that, you would have to roll the dice quite a few times to get to know the chances of obtaining so many sixes from all the rolls of the dice. So, in attribution of extreme events, it is also needed to run the model over and over again in order to obtain multiple results (ensembles) that can be analyzed in order to be able to draw some conclusions.

Another analogy appeared in the Bulletin of the American Meteorological Society (BAMS) of 2014 (Herring et al., 2014), whose vision of how this new research field works is reflected in a video entitled Steroids, baseball, and climate change. Before you watch it, here is the outline. Imagine a baseball player who starts taking steroids. After that, he hits an average of 20% more home runs than before and so probably you would attribute this amazing improvement to the steroids. But was this the only factor which changed during that period? Maybe, he was able to spend more hours training, hired a new trainer, or changed his diet. But if all those factors didn’t change, we can say that the steroids are the responsible for that increase in the probability of him hitting more home runs.

(©UCAR. Video by Noah Besser, produced by UCAR Communications for AtmosNews: NCAR & UCAR Science. This video is freely available for media and nonprofit use.)

The BAMS issue of 2013 (Peterson et al., 2013) echoed another analogy from the University Corporation for Atmospheric Research (UCAR, 2012). In this case they use a car instead of a baseball player. If we go every day to work by car, but we increase the speed of our journey, the greater the speed, the more the likelihood of us having an accident. However, if this event unfortunately happens, it may not be due to the speed. It could be related to bad weather, another driver speaking by phone, an obstacle in the road… In this case atmospheric greenhouse gases are analogous to the speed. We can study the odds of having such an accident at a particular speed but also taking into account some of the other factors. That’s to say, maybe the speed was a key factor in the accident, but some other aspect could have played a role too (this is akin to natural climatic variability).

In these two cases is clear that greenhouse gases could have being playing a role (steroids, speed), but that there are many other factors which could be influencing the result to some extent. However, understanding how greenhouse gases affect the likelihood or magnitude of an extreme weather event is a key step in the decision making process and the adaptation and mitigation measures for the long term, which should be still effective in 50 years’ time or more.

The third and last analogy is that of the cookies. This more recent analogy is based on the book Attribution of Extreme Weather Events in the Context of Climate Change from The National Academies of Sciences, Engineering and Medicine, about which we have already talked in our post Getting to know attribution studies: basic concepts and references. Dave Titley, Committee Chair of the Board on Atmospheric Science and Climate, gave a presentation entitled: Attribution of Extreme Weather Events in the Context of Climate Change within the framework of the Report Release Briefing on the 11th March 2016. You can read and download Dave Titley’s  presentation in the following link:Also, here is the fragment of Dave’s talk so you can understand better the slides:

One of the slides it is called A Banking Analogy. In this case one cookie is the event, similar to the home run or the accident by car in the other cases. All the factors that are involved in this case are the ingredients of the recipe, and they may or may not be responsible to some extent for the output, which can be different according with the proportions of the different constituents involved: quantities of the ingredients, temperature of the oven,… This analogy is fully explained in the following video:

Even though climate models may be intangible, I hope you can have a better understanding of attribution science of extreme weather events. So, here is a video where you can apply what you have learned so far and put all the pieces together to become aware of the importance of this methodology and the outcomes it can offer us in order to be prepared for a changing climate.

 This video is presented by the Climate & Development Knowledge Network (CDKN), who has an initiative to raise risk awareness in the developing countries. This Raising Risk Awareness initiative uses “state-of-the-art science to help Asian and African societies to understand the role of climate change in extreme weather events and prepare for future ones“.

If you feel now that you can be a part of the development of this exciting new field of research and want to contribute to the objective of ClimAtt, you can do it through the World Weather Attribution (WWA) project. Please, just click on the following image:

This project depends upon enthusiastic volunteers, citizen scientists who contribute to improvements in the science of attribution of extreme weather events and make a positive impact in societies around the world by offering some computing time.

 REFERENCES

Getting to know attribution studies: basic concepts and references

Attribution is considered the process of evaluating the relative contribution of multiple causal factors to a change or event with an assignment of statistical confidence (Hergerl et al., 2010).

You can learn some basic concepts about attribution and on how it works with this short but very instructive video produced by Professor Myles Allen (Oxford Martin School).

In 2012 the Bulletin of the American Meteorological Society (BAMS) published its first annual report on climate change attribution of extreme weather events. The authors submitted their works on attribution studies of extreme events which occurred across the world during the previous year. Although some skew can be perceived in relation to the areas studied or the interests of scientists, the number of papers has been increasing since then, as well as the interest due to applications of the findings in areas such as the management of land use or the development of improved forecast systems.

The book ATTRIBUTION OF EXTREME WEATHER EVENTS IN THE CONTEXT OF CLIMATE CHANGE, published in 2016 by the National Academies of Sciences, Engineering and Medicine, integrates all the basic concepts and approaches used in this relatively new field of research, Probabilistic Event Attribution (PEA)*. It offers guidance on how to carry out an end-to-end attribution study. For example, it addresses the importance of correctly framing the attribution question in order to clarify whether the frequency and/or magnitude of the event that we are going to study is experiencing changes due to a specific driver and to what extent. It also provides full information of the different methods that can be applied, as well as an overview for various event types, along with some results, such as extreme heat, extreme cold, droughts and extreme rainfall. These events, as they highlight, are also those which offer the highest confidence in attribution studies (as you can see in the figure on the right, below, reproduced from the book), but also the events which attract the most media and public attention.

It is therefore, a good point to start to go into greater depth on attribution studies and how is possible to probabilistically determine the contribution of a driver, such as the human greenhouse gases emissions, to an extreme event.

Today, a new book about the impact of climate change on weather and climate events has been launched by the American Geophysical Union, CLIMATE EXTREMES: PATTERNS AND MECHANISMS. To accompany the launch, the editors offered an interview. They spoke about the importance of attribution studies in the forecasting of weather and climate extremes, the regions of the world most vulnerable to extremes, and the benefits of increasing the resolution of the models, which could help to better understand the events from a physical point of view. However, they share their concerns about the continuing decrease in the number of weather stations and other observational data, which are necessary to validate climate models.

Here you will find the interview: https://eos.org/editors-vox/how-does-changing-climate-bring-more-extreme-events

We hope you enjoy these (maybe summer) readings.

 * Also called Attribution of Climate-Related Events (ACE).

REFERENCES

  • Hegerl, G.C., Hoegh-Guldberg, O., Casassa, G., Hoerling, M.P., Kovats, R.S., Parmesan, C., Pierce, D.W. and Stott, P.A., 2010. Good practice guidance paper on detection and attribution related to anthropogenic climate change. In Meeting Report of the Intergovernmental Panel on Climate Change Expert Meeting on Detection and Attribution of Anthropogenic Climate Change. IPCC Working Group I Technical Support Unit, University of Bern, Bern, Switzerland.
  • National Academies of Sciences, Engineering, and Medicine, 2016. Attribution of Extreme Weather Events in the Context of Climate Change. Washington, DC: The National Academies Press.
  • S.-Y. Simon Wang (Editor), Jin-Ho Yoon (Editor), Christopher C. Funk (Editor), Robert R. Gillies (Editor), 2017. Climate Extremes: Patterns and Mechanisms. American Geophysical Union. ISBN: 978-1-119-06784-9. 400 pages.