Much was made in the mainstream media that Windstorm Christian – that only had moderate impacts of the United Kingdom - was far removed from the infamous October Storm of 1987. However digging a little deeper into the lifecycle of these two events actually highlights that these two events were a lot closer than meets the eye and are both are good examples of a class of windstorm that has the potential to be particularly damaging.
A Sting in the Tail
Even though the UK only typically received inland winds of up to 70 mph – enough to start dislodging roof tiles and knocking over weaker trees – the satellite imagery gave clues that it was going to be quite a threat to the countries that lay in wake downwind of the system. Comparing the satellite imagery of Christian and the 1987 storm shows very interesting similarities. In the figure below we see an arrow marking the development of a “cloud head” that signifies the intensification of windstorm. By chance or otherwise, Christian’s cloud head was very reminiscent of that of the October Storm. The systems shown here were both around 9 hours away from their maximum intensity.
Left: October Storm. Right: Windstorm Christian. Arrow shows the flow of air that creates the "cloud head".
Left image: Dundee Satellite Receiving Station. Right image: WeatherOnline
This cloud feature is now very well recognised by forecasters; it wasn’t in 1987. It is a measure of the advancement of our understanding that we now understand these features better.
At both these times the systems were still developing. In 1987, the storm was over the sea at this point. Windstorm Christian was doing its development over land which will provide meteorological researchers with a rich seam of information to further understand how these storms develop and their attendant hazard. Strong winds were reported from the system in a band from South Wales through to North London – but some more significantly strong winds were felt in Eastern Counties of the UK at the time of the above image which turned out to be a sign of what was to come for Denmark and Sweden.
The enigmatic "sting jet"
What Eastern England experienced in Christian was the beginning of the most damaging phase of the system. As the cloud feature begins to wrap around the system it signals the development of one – and sometimes two flows that cause most of the damage we usually experience in a European windstorm that are simplified below. In the first phase of development, a “sting jet” can descend from the cloud head from around 3-4km above the earth’s surface in staccato bursts down to earth: but this doesn't always happen and is a very enigmatic feature of windstorms. In the second phase of development, a low-level flow called the “cold conveyor belt” becomes the dominant flow and causes most of the damage; this is more reliable that the sting jet! A very idealised schematic below shows what one might highest as two “phases” of the storm’s development as dictated by the existence of these two flows:
Schematic showing the position of the strongest winds at two stages of development of a damaging low pressure system. Top left picture shows the location of the sting jet, top right shows the location of the cold conveyor belt. Bottom chart shows the timing typically of when these features exist relative to the central pressure of the storm
The two charts at the top of the schematic are a simple example of the cloud pattern and the attendant strongest wind flows at the time. The left chart is intended to depict what we see in the satellite images earlier in the discussion as it is undergoing rapid development. The sting jet exists close to and ahead of the tip of the cloud hook. The right image shows the storm when it is fully-developed and at its most intense and position of the "cold conveyor" belt strongest winds which usually undercut the sting jet and become the dominant damaging flow of wind.
The sting jet doesn't always exist and doesn't always cause damage in a damaging windstorm; the cold conveyor belt is usually the cause of the strongest winds. The bottom chart shows simply plots the "L" from each of the schematics relative to how low the central pressure is in the system: the sting jet occurs as the pressure is still dropping rapidly and usually the cold conveyor belt occurs when the storm is at its deepest. It is worth also adding that this is highly idealised: not all storms behave the same but it picks up some main points of developing storms that are relevant to this study.
Two peas in a pod?
Many of the storms we’ve seen in history only have the cold conveyor belt causing damage. What is interesting for the October Storm is that the south-of England saw damage from the sting jet, and then the cold conveyor belt blew over the sea (and caused relatively lesser damage in Norway). What is currently being suggested (and researched) for Windstorm Christian is that both the damaging flows shown above were again present. There are hints that the sting jet caused patchy but quite localised damage in south-eastern England, and it is still up for discussion as to whether it also caused the damage in Holland, Germany and Denmark or whether by then the cold-conveyor belt was the key damaging flow.
Both flows being present in a windstorm could potentially extend the damaging lifecycle rather than just the one flow as they simply increase the area being affected by damaging winds. Until recently, the only definite example of the Sting Jet causing damage of land was in the October Storm; the storm in which it was discovered. By understanding better how common this type of storms is (once every 5 years? every 50?) we can get a better feel for how frequent we should expect such long-track storms with the potential for damage – as was the case in Christian – from south-west England all the way to western Sweden. This is obviously of interest to those of us who are interested in understanding catastrophic risk and what sort size and shape your typical long return period storm looks like - the sort of storm against which insurers and reinsurers alike want to protect themselves.
How strong were the two storms?
We can simply compare the strongest winds reported for these storms to get a feel for their relative strengths. If you recall the UK media spoke a lot about the storm being weaker than the October Storm, however this was only being couched from a UK perspective. In the October Storm, winds at Shoreham reached 120 mph before the anemometer failed and 122 mph in Gorleston in Norfolk. Pointe du Raz in France had gusts up to 134 mph.
The strongest gusts for Windstorm Christian, whilst reaching nowhere near this strength in the UK as the storm hadn’t reached its peak as it crossed this country, were relatively similar in Denmark: Kegnæs Fyr lighthouse close to the western coast of Denmark recorded 120 mph which turns out to be the strongest winds ever recorded in Denmark. In Germany, the same windspeed was also recorded at Borkum and Helgoland. It is telling that these systems that seemed to have a similar type of development both ended ultimately with extremely damaging gusts. As much as Denmark received its strongest recorded gust in Christian, the Shoreham gust recorded in the October Storm is quite likely one of the strongest (if not the strongest) wind gust measured in south-east England. This serves to underscore the parallels that these two storms have.
Forecasting the October Storm and Windstorm Christian
It is worth touching upon this as the public was warned five days in advance of the impending windstorm threat, which is an impressive lead time for a specific warning and much has been made of the advancement of science that enables this. In 1987, the October Storm was poorly forecast, leaving a rather embarrassing situation for the Met Office. Michael Fish could easily have pleaded “Don’t shoot the messenger” as, even then, Numerical Weather Prediction forecast models were heavily used to inform forecasters’ decisions – as it is now. Four things have happened since that unfortunate day that could well have improved forecast accuracy:
better understanding of how we simulate the myriad processes in the atmosphere
how and where we capture the observational data that informs forecasting models if the "initial state" of the atmosphere before it is projected forward in time
the higher resolution of forecasting models: i.e. they can resolve more detail of the atmosphere and that includes intensity of developing weather systems
the forecaster’s understanding and recognition of rapidly developing windstorm situations: as mentioned earlier, we now understand better the signatures of rapid development in satellite imagery
All these things add to more accuracy from models and forecasters alike, but we must not just use this as the cover-all reasons. Some storms simply are more “forecastable” and are less on a knife-edge such that models are able to predict them well. Christian was spotted some five days out and flagged as a possible storm. However in 1990, using practically the same technology that had mis-forecast the 1987 storm, the Burns Day Storm (otherwise known as “Daria” in insurance) was forecast extremely well, similarly well in advance.
As if we need a reminder of the fickle nature of forecasting severe events, in January 2012, forecasters had spotted Windstorm Andrea was going to hit Scotland with strong winds, but it was only until the morning of the event that they had to increase the warnings to the highest level when it became evident just how strong the storm was. With this in mind – we will have to accept that not all future windstorms will necessarily be perfectly forecast – there may still be another October Storm that takes us by surprise, but we do have the back-up that forecasters now have more tools and knowledge available to them to spot an "unforecast" event and minimise the surprise to us.
For now however, we now have another windstorm to research in order to improve our understanding of European windstorms - both from the perspective of the meteorological and catastrophe risk communities.
This article was first written in November 2013
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