RAINS: Radar Applications in Northern Scotland

Storm Frank rainfall radar (2)

Caption: 24 hour quantitative precipitation estimates from radar across Scotland for Storm Frank (29th – 30th December 2015). Typical catchment average rainfall exceeded 80 mm in 24 hours for the Upper Dee, with some point estimates of 150 mm. All background radar data © Met Office.

The past two months have brought significant rainfall across much of Scotland resulting in some severe disruption due to the impacts of flooding.  Impacts have been seen in early December with Storm Desmond, through to Storm Frank which brought major flooding along the Dee Valley in Northern Scotland.

The use of radar is essential to increase our understanding of the spatial representation and real time estimation of rainfall for flood forecasting across Scotland.  However, whilst this is a vital tool there are some unique challenges with doing so in Scotland not least the effects of the upland environment and distance from the radar.  To this end the flood forecasting service are working with the National Centre for Atmospheric Science (NCAS) and the University of Leeds to deploy a mobile X-band radar to research the use of high resolution radar measurements for improved flood forecasting.

The RAINS project (Radar Applications In Northern Scotland) will run from January to July 2016 and will be making detailed measurements of the atmosphere from a radar based in Kinloss, Morayshire, specifically looking to measure clouds and rainfall as they form across Northern Scotland.  The NCAS radar is capable of measuring clouds and precipitation in high detail and is the only one of its type in the United Kingdom.

X Band deployment

The mobile X Band radar deployed at Kinloss Barracks in Moray (left) and initial quantitative precipitation estimates on the 2nd February (right) when flooding occurred across Ross-shire.

The collaborative project involving the Met Office and Scottish Water will specifically look to improve the understanding of rainfall and the prediction of floods through comparative hydrometeorological modelling using the quantitative precipitation estimates of rainfall generated by the Kinloss radar.

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The 2005 Teviot floods: then and now

As previously reported Storm Desmond caused severe flooding over the weekend of the 5th and 6th December in parts of the country including the Teviot Valley. The River Teviot in Hawick recorded its second highest river flow on record and close to the peak levels recorded in the damaging 2005 flood. However, levels at Ormiston Mill 20km downstream recorded their highest on a record which dates back to 1960. In this article we take a look at what has changed in our capabilities in flood forecasting in the intervening ten years between these two events.

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The River Teviot in Crailing in Roxburghshire recording its highest river level on a record spanning 55 years. Copyright Ali Graham Photography.

How do the flood predictions compare?

Meteorological forecasts supporting flooding operations ahead of the 2005 flood did provide early indications with national severe weather warnings activated several days in advance. However for hydrologists what has changed is the access to more detailed spatial and temporal meteorological forecasting tools to help in the decision making process, such as the use of probabilistic rainfall forecasts based on depth and duration. What this supports is the provision of guidance in terms of what is likely, but also what could be expected as a worst case scenario.

Meteorological forecasts then and now

An example of one of the 2005 severe weather warnings for the Scottish Borders, at this time the service was based on fixed rainfall triggers (left). By comparison tools such as MOGREPS-UK that are used now provide guidance on predicted likelihood of a range of rainfall depth-duration, in this case 40mm in 6 hours, with 70% likelihood across the Southern Borders for Storm Desmond (right). Copyright Met Office.

In hydrological terms, a review following the 2005 event noted that warning lead times for this flood on the Teviot were at or around 3 hours and it recommended the development of rainfall-runoff forecasting to increase future lead times. Since then systems have been developed on a catchment scale (PDM) and national scale (Grid-to-Grid) for coupling meteorological and hydrological forecasts and as reported previously this led to flood guidance being provided several days in advance of the recent event. More crucially in the 2015 flood these predictions supported community warnings being issued some 24-hours in advance of the main flooding impacts being witnessed.

Were we more prepared for the 2015 flood?

The 2007 Scottish Government study into the flooding in Hawick, Elgin, Glasgow and other communities looked at societal impacts of these events and specifically one aspect for us how communities respond to flood warnings. At the time the study found that only 33% of those that had previously been flooded used Floodline; only 39.7% moved a car on receipt of warning; and crucially the study found that the effectiveness of a warning largely depends on the time it provides to take appropriate action. The study also noted that although local automatic voice messaging systems were in place (through Scottish Borders Council), there were issues around take up, warning messaging being reliant on two systems and speed and efficiency of warning messaging.

Rainfall for the Teviot floods

Hourly rainfall recorded across the Southern Uplands. 48 hour accumulation for the 2005 flood (111mm) and the 2015 flood (96mm). Data courtesy of SEPA.

Since 2005, SEPA has introduced a direct warning service of which over 300 individuals are now registered in Hawick, helped with the awareness of the flood risk and active community engagement through initiatives such as the Hawick Flood Group. Although the full extent of the flood is still to be assessed the response to the recent flood from Emergency Planners was that ‘we were much better prepared’ which aided the proactive evacuation of 600 people from the community. Notably whilst reports of the 2005 flood mention cars being washed away, amongst the 100 properties affected initial reports are that not one car was flooded in the 2015 event as a result of the precautionary actions.

What lessons can we take away from the 2015 floods?

Whilst the key points from the 2015 Teviot flood might be that forecasts were reasonably accurate, warnings were timely and the response was proportionate to the scale of the flood, the same might not be true about the neighbouring community of Newcastleton which was equally devastated in 2005 yet escaped major flooding this time.

Newcastleton G2G prediction for Storm Desmond

The Grid-to-Grid flow predictions for the Liddel at Newcastleton with a observed peak being recorded at 254 cumecs being closest to the minimum flow ensemble member in these forecasts.

Short range flood predictions for the Liddel Water (see figure) were similar to that of Hawick in that some levels were predicted to be close to those of 2005. However, spatial variation in the predicted and observed rainfall meant that Newcastleton narrowly missed the more severe rainfall accumulation observed across other parts of the southern uplands. This case highlights that flood forecasting continues to be challenging, however the changes over the past decade has enabled us to be more prepared for future flooding.

 

 

 

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Storm Desmond – Scotland’s first red flood risk

Over the weekend of 5 – 6 December 2015 Storm Desmond – the fourth UK named storm of the winter season – brought damaging winds and heavy rain across Scotland, which experienced its most severe flood event since the advent of the forecasting service nearly 5 years ago. Impacts were widespread, and particularly severe in the southern Borders area. We issued our first ever Flood Guidance Statement containing a red (high) risk area, and also issued the first severe flood warnings since the service began. This is the first of two articles looking at the Desmond flooding. This article looks at the forecasting for this event; the second one will look at how this compared with a similar event in 2005.

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Areas of Hawick affected by flooding during 5 – 6 December 2015                                                                             Photo credits clockwise from top left: bbc.co.uk; Hawick Councillor Ron Smith; The Southern Reporter; Hawick RFC

Exceptional rainfall totals were recorded in a 48hour period from the 4th to the 6th of December as its associated weather fronts wriggled southward across Scotland and northern England.

Forecasters at the Met Office Aberdeen watched the developments of Desmond closely. The ground was already saturated after a wet and gloomy November – so the challenge was to predict as accurately as possible the intensity and distribution of the rainfall. Forecast models tend to provide more reliable guidance for the distribution of rainfall that is dynamic (frontal) rather than locally generated (e.g. thundery showers).

chartsIt was recognised at an early stage that the system would have a moist ‘river’ or ‘conveyer’ of winds ahead of the cold front. It was also important to consider the orographic enhancement of rainfall totals over windward mountain ranges.

Whilst the frontal structure of Desmond was complex, the initial weather predictions supported the view that the cold weather front would clear southward into England; and that the primary rainfall totals would be associated with the leading weather fronts. As Desmond moved towards Iceland, subsequent weather calculations indicated a trend to a slower and more erratic clearance of the cold front southward over the weekend.

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24 hour rainfall totals forecast on Friday for Saturday

Then finally; the cold front almost became stationary across the Borders, as further weather disturbances ran along it bringing further pulses of heavy rain.

The greater persistence and intensity of the moist conveyor, as well as the slower clearance of the dynamic rain associated with the cold front – lead to the prediction of exceptional rainfall accumulations across NW England and the Scottish Borders.

High flows were forecast throughout Scotland, as can be seen on the Grid-to-Grid (G2G) river flow model spatial plot, below. The widespread and prolonged nature of the expected rainfall meant that almost unprecendented quantities of run off were expected.

g2gspatial

4/12/15 07:30 Grid-to-Grid forecast for 5/12/15 18:30

Of particular concern were the Scottish Borders and Tayside, the G2G forecasts for which can be seen below. The forecasts for Hawick in particular were very consistent from two days before the event and closely matched what did occur. The forecast flows were comparable with those experienced in the extreme event of 2005, when there was extensive property flooding. The river Tay forecasts, whilst showing quite a range, all consistently indicated an expected flow approaching at least as great as that seen during the flood event of 2005, and possibly even the event of 1993, which was the largest in recent history. In the event it is clear that the model overestimated the Tay flows somewhat, possibly due to the non-inclusion of hydro schemes upstream which may have attenuated the peak of the flow.

desmond flowsThe heightened flood risk for the weekend was first signalled on the Flood Guidance Statement on Thursday, with a very low likelihood of significant impacts indicated. As confidence increased, on Friday the likelihood was increased to high, giving amber status to a large part of the country. Weather forecast models and G2G were giving consistent forecasts of very high rainfall totals and exceptional river flows. During Saturday it became clear that areas of the Scottish Borders, particularly Hawick and Newcastleton, were expected to experience severe impacts. These areas are not very far from the parts of northern England that were also going through extreme conditions.

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Risk maps from successive Flood Guidance Statements (left). Area of Concern map for peak of event (right)

A large amount of rainfall fell across Scotland from Friday through to Sunday. As can be seen below, the main areas affected in Scotland were the Borders and the centre of the country (Tayside area) plus the north west.

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Rainfall intensity from RADAR for the event

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Totals in mm recorded at SEPA raingauges during the event (note Teviothead is an underestimate due to the site flooding)

 

 

 

 

 

The impacts of this event were considerable. 15 flood alerts, 75 flood warnings and 2 severe flood warnings were issued across the country.  Over 150 properties were flooded in Hawick and other locations across the country. Significant travel disruption occurred. The Perth flood defence scheme was activated, entailing the closure of the main town bridge. Several railway lines were closed. Travel impacts were also exacerbated by the (unrelated) recent closure of the Forth Road Bridge for emergency maintenance.

This event proved a major challenge for the forecasting service. For the first time we had to decide on issuing a Flood Guidance Statement with an area on high (red) status. We also input to the decision to issue two severe flood warnings, also an unprecedented situation. Rainfall predictions, particularly the UKV 36 hour high resolution forecasts, proved to be very reliable for the most part. G2G forecasts were invaluable. They were particularly in demand for locations where no local model currently exists, and for lead times longer than that used for local models. Timely forecasts and warnings enabled mitigating actions to be taken. As will be shown in the next blog, improvements in forecasting and warning have led to a much improved outcome since the last time we saw this magnitude of flooding in Scotland.

By Richard Maxey and John Mitchell

Posted in Flood, Forecasting, G2G, Uncategorized, Weather prediction | 2 Comments

Decision making in times of uncertainty: the crying wolf

Nikéh Booister presents the second and final part of this series on decision making in times of uncertainty.

As part of her M.Sc. thesis research in Flood Risk Management Nikéh did a part of her research in SEPA. The research focussed on decision making under uncertainty with an outlook on improving decision making based on the cost-loss method. Nikéh continues working in the field of Flood Risk Management with her company FloodCom.

As part of her M.Sc. thesis research in Flood Risk Management Nikéh did a part of her research in SEPA. The research focussed on decision making under uncertainty with an outlook on improving decision making based on the cost-loss method. Nikéh continues working in the field of Flood Risk Management with her company FloodCom.

“The forecaster has to be the expert on knowing what uncertainties are and where they come from as we could have read in the previous post. The forecaster has to decide on what is relevant and not and to a certain extent already has to make a decision on the quantification of the uncertainty. Whether or not a forecaster understands and interprets correctly the uncertainty, the decision maker has a final call and responsibility to “pull a trigger” for action.

Currently most of the forecasts are presented in a deterministic way, which means a single valued number of a possible future. A part of the uncertainty is presented in a description of the future system state, nevertheless this is not quantified. A possibility is to present a forecast in a probabilistic way, where uncertainties are presented.

As mentioned in the blogpost of Paul Ryles; longer lead time makes the prediction of a future situation more challenging; the rate of uncertainty is higher. Probabilistic forecasts indicate possible future outcomes of a system and give the decision maker and responder more possibilities of responding (or at least the consideration of responding and with this more time to respond). This makes it possible to respond earlier in time and make decisions with a lower regret. Making a decision based on a deterministic forecast seems easier because it presents a binary solution (yes or no decisions). Nevertheless research has proven that decision making based on a probabilistic forecast gives better results (Ramos et al. 2013).

In possible flood situations Aberdeenshire Council can pull out the demountable defences to protect Stonehaven from flooding. Nevertheless at the moment a decision has to be made it is not sure if the event will actually materialise but due to time limitations the decision has to be made.  (Source: Aberdeenshire Council, 2014)

In possible flood situations Aberdeenshire Council can pull out the demountable defences to protect Stonehaven from flooding. Nevertheless at the moment a decision has to be made it is not sure if the event will actually materialise but due to time limitations the decision has to be made. (Source: Aberdeenshire Council, 2014)

When decision makers are introduced to the problem of uncertainty in forecasting it can appear to be too much; too complicated; and out of the scope of the decision maker to respond properly to this. This interacts with the fact that the decision makers and responders are the ones that have to deal with for example the uncertainty of citizen response. Thus, the responsibility is bigger. When a wrong decision is made, there is a threat of a “crying wolf” principle. Despite the efforts, there will always be dissatisfaction with the decision taken or damage occurred. And so, someone to be blamed. When a decision is based on a deterministic one, it is the model or the forecast to be blamed and not the decision maker – which makes the situation more acceptable.

The ultimate question in forecasting and decision making is who is responsible in the end? And who is to blame in the case of a wrong decision being taken? Don’t we all try as hard as possible?

Timing and level uncertainty in flood prediction makes it hard to decide. The longer the lead-time the higher the uncertainty (Source: N.Booister M.Sc. thesis)

Timing and level uncertainty in flood prediction makes it hard to decide. The longer the lead-time the higher the uncertainty (Source: N.Booister M.Sc. thesis)

There are approaches that provide an ‘answer’ and provide the forecaster or the decision maker with a decision making perspective, but when we would like to provide the decision maker with a longer lead-time, as mentioned above, probabilistic forecast might be the solution. In order to avoid blaming of the decision maker; a decision making tool or method needs to be in place to support the decision. One of these methods is the cost-loss method. This method compares the probability of an event to occur with the cost of certain mitigation measures and the losses that they avoid. Although it presents a solution method, it seems too static in time and due to its economic point of view, does not take into account intangible effects.

Models are improving, more lead-time is being provided, but uncertainty will always remain. Decision making stays a difficult and also a sensitive topic.”

 

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Decision making in times of uncertainty: the forecaster

In a two-part feature, we cover various roles involved in decision making at times of uncertainty. The first is provided by Paul Ryles, Senior Scientist and operational flood forecaster at SEPA.

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Paul manages SEPA’s flood risk management and operational hydrometeorology trainee programmes and has been in flood warning service development since 2002

“Flood prediction is a science, especially in challenging situations such as providing long lead times (for example through our five day flood guidance statement) or flash flooding (surface water alerting). One of the primary roles of the flood forecaster in these situations is in communicating uncertainty in a way that is useful for responders. The forecaster must quantify uncertainties in the weather forecast (in terms of location, timing, intensities and amounts of rainfall) and any potential uncertainties in the resultant flood forecasting predictions and hazard footprint.

Meteorological and hydrological tools such as MOGREPS and Grid-to-Grid provide the forecaster with the opportunity to quantify uncertainty in flood predictions. Image of MOGREPS-G and probability of greater than 25mm in 24 hours ahead of ex-Hurricane Kate (Copyright Met Office)

Meteorological and hydrological tools such as MOGREPS and Grid-to-Grid provide the forecaster with the opportunity to quantify uncertainty in flood predictions. Image of MOGREPS-G and probability of greater than 25mm in 24 hours ahead of ex-Hurricane Kate (Copyright Met Office)

Understanding what type of information customers require is vital, as is an appreciation of how this information will be used and what type of decision will be made. The flood forecast will often be used to make crucial ‘yes or no’ decisions, such as committing resources in responding to a flood event, evacuations or deploying temporary flood prevention measures. Understanding these customer requirements is as vital as knowledge of meteorology and hydrology in producing successful flood forecasts.

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Flood Forecasters participating in Exercise Big Water, testing response and preparedness to new coastal flood forecasting capabilities.

Having competent and experienced flood forecasters is essential in delivering this service. To this end, the Flood Forecasting Service, with support from the Met Office College, has developed a formal qualification in Operational Hydrometeorology. The aim is for all of our flood forecasters to achieve this qualification, which assesses all aspects of producing and communicating our flood forecasts. The forecasters, who will already have completed training in flood forecasting, warning and response, will undertake this qualification after a period of on-the-job training.

Forecasting some flood situations remains a challenge, however ensuring all flood forecasters are trained and competent (and assessed) in accounting for, quantifying and communicating risk at times of uncertainty should maximise the benefit of flood predictions for responders when making difficult decisions.”

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Flood forecasting: current challenges and future aspirations

How can flood forecasting best rise to the challenges and opportunities that communities and partners face; at what point does effort in flood forecasting become less effective than local adaptation; and what are the aspirations and capability of our future flood forecasting services – all questions posed for the speakers at this one-day conference in Birmingham.

Pilot capability for a risk-based coastal decision support tool. (source: Environment Agency)

Pilot capability for a risk-based coastal decision support tool. (source: Environment Agency)

The growth of flood forecasting or hydrometeorology as a science and its associated services is evident given the range of presentations. Products such as the five-day flood guidance statement – ground breaking when introduced 5 years ago – is now firmly a part of the flood response environment with a new appetite for longer lead times (Turner) and more challenging local applications such as wave overtopping forecasting at the community and asset scale (Wall; Lawless). So what are the key challenges and aspirations:

Coastal decider tool providing a 32-day look at the probability of coastal flooding related weather types. (source: Met Office)

Coastal decider tool providing a 32-day look at the probability of coastal flooding related weather types. (source: Met Office)

Who are the customers of our forecasts?
Forecasts are no longer there to purely support the issuing of flood warnings. The type and range of forecasts vary and agencies are grappling with the idea of what to forecast, where and for who given the growing desire for forecast data. Policies in England are now focused on making forecasts more accessible and usable (Anspoks), whilst at a community level these need to sit alongside local tool-kits and support the potentially powerful use of social media by flood groups (Barnes). The desire to receive ‘everything’ (Young) should also be offset with the need to facilitate and support users of the forecast information (Rowlands and Williams).

How good are the forecasts?
Investment in our services is under increasing scrutiny especially with continued funding pressures in the public sector, therefore prioritising effort where it’s most needed for improvement is important. An example was presented of a framework to integrate flood model forecast assessment studies for local and national scale models to provide an evidence base for flood forecasting model development (Moore) – one to assist in future decision making.

Are they understood?
The case for the difficulties in effectively communicating flood forecasts was made by a number of speakers with the recent severe flooding in the south of France being highlighted (Mitchell). When the Somerset levels flooded in 2014, the impacts were felt for several months with the Environment Agency incident room being staffed continuously for 11 weeks and 3 days. One response from someone affected was that ‘the water came so quickly’; however it’s interesting to note that the first Flood Alert predicting the impacts was issued some 16 days before the first property was impacted (Rowlands and Williams).

Prototype fluvial flood risk forecast maps using G2G hydrological ensembles in conjunction with NFRA flood impact data presented for flooding affecting Prestwick Airport in 2012. (Photos left source: Prestwick Airport; Maps right source: CEH Wallingford)

Prototype fluvial flood risk forecast maps using G2G hydrological ensembles in conjunction with NFRA flood impact data presented for flooding affecting Prestwick Airport in 2012. (Photos left source: Prestwick Airport; Maps right source: CEH Wallingford)

Do they reflect the impacts well enough?
One speaker referred to this area as ‘making the forecasts come to life’. Effort is now being put into translating flood forecasts into risk-based flood impact assessments, with frameworks being developed for inundation mapping (Whiting), coastal flood risk decision support (Anspoks) and our own prototype forecast river hazard mapping in Scotland (above).

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Heavy Rainfall Alerts – latest advances

In order to provide probabilistic forecasts of flooding from surface water the forecasting service has been using a Heavy Rainfall Alert (HRA) tool since 2013, as discussed in a previous article. The tool consists of a spreadsheet that is updated four times a day and emailed to the forecasting duty officer, and a series of maps. After two years of use we have reviewed the tool and made some improvements to the spreadsheet, adding new functionality and making it more user friendly.

As before, the tool uses MOGREPS-UK forecasts subjected to neighbourhood sampling (over 60km) to calculate the probability of exceeding depth duration thresholds (that have been defined in terms of expected impact), within specified operational areas. The new tool now has two key innovations.

hra pix

Areas assessed by the tool. Point maxima (left) and catchment averages (right)

  1. Urban areas

In the new tool an assessment of depth-duration threshold exceedence is made for each of seven urban areas. These areas are each around 1000km2, which is believed to be the optimum size for trading off event probability and usefulness of forecast. Now, by looking at the spreadsheet, the forecaster can see at a glance which urban areas might be affected. Previously, she would only have had information on exceedences within a larger area, and there might not have been any information immediately to hand to confirm whether or not the urban area was at risk.

2. Fast responding catchments

The other innovation relates to a new assessment of 16 catchments. These are all small, rapidly responding, flood prone catchments, where early warning of potential river flooding is required. Catchment average depth duration thresholds for various levels of impact have been calculated. The tool calculates exceedence of these thresholds by the areal average of these catchments, as opposed to the point maxima used previously. This is quite an innovation, and is the first time that the HRA process has been used in this way within the UK.

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Example excerpt from the tool

  1. Other improvements

In addition, a first guess risk tool has been built in to the spreadsheet, combining impact and likelihood. Maps, lists of thresholds and user guidance are also included to make the tool user friendly.

As has been seen more than once this summer, the ability to focus risk based forecasts for surface water and rapid response catchments is critical to enable effective emergency response, and so the improved tool should go a long way towards helping us to meet that challenge.

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