Flooding is one of the leading global natural hazards and can result in loss of life and devastation (e.g., [1
]). In 2007, the UK experienced some of its worst summer flooding leading to “the largest peacetime emergency since World War II” [2
]. Much of this flooding was due to large-scale storms; however, flash and surface-water flooding also occurred. That summer’s events caused an estimated £4 billion (8 billion USD) in damages [3
In the aftermath of the 2007 floods the Pitt Review [2
] was commissioned to examine the events that led to the flooding. The review considered many different perspectives of the flooding, e.g., forecasting, defence and response. The Pitt Review was set up with the premise of improving the services and warnings for those communities directly involved with the flooding, from the first responders to the flood victims. The review made 92 recommendations on improving all areas including flood forecasting, response and preparedness actions that local government and citizens can take. One of the key recommendation themes to come out of the review was the need for co-operation of meteorological and hydrological agencies in both research and practice.
The Flooding From Intense Rainfall (FFIR) (throughout this article the acronym is only used as a direct reference to the project and not to the phenomenon) programme arose (in part) out of that key recommendation. The FFIR programme is a partnership between university researchers and the UK Met Office, with the universities being funded by the Natural Environment Research Council (NERC), and ran from 2013 to 2019. The programme consists of multiple researchers across a number of universities and disciplines. The researchers and Met Office worked closely with the UK Environment Agency (EA), Scottish Environmental Protection Agency (SEPA), Natural Resources Wales (NRW), Flood Forecasting Centre (FFC), Scottish Flood Forecasting Service (SFFS), Health and Safety Laboratory, Centre for Ecology and Hydrology, British Geological Survey, JBA consulting, Jacobs, Public Health England and European Centre for Medium Range Weather Forecasting (ECMWF). The programme has also had strong advice and direction from international agencies including Météo France, Deutscher Wetterdienst (DWD, Germany), Norwegian Meteorological Institute, European Commission Joint Research Centre (JRC), High resolution limited area modelling consortium (HIRLAM), Swedish Meteorological and Hydrological Institute (SMHI), Centre de Recerca Aplicada en Hidrometeorologia (Spain), and Rijkswaterstaat (Netherlands).
The FFIR programme considered end-to-end flood forecasting to further our understanding of these types of events and how to predict and manage them. An end-to-end flood forecasting system is one in which forecasts of different components of the earth system are linked, e.g., hydrological and meteorological forecasts for flood forecasting (see Hill et al. [4
] Chapter 8). The FFIR programme considered end-to-end forecasting as each stage in the forecast chain from observations through to the flood warnings and impacts (Section 3.2
). The resulting actions to the warnings and the groups responsible for those actions are not considered within the scope of the FFIR programme. The FFIR programme’s view is but one of many views of what an end-to-end forecasting system is. The FFIR programme also aims to produce tools that assist the education of the public about flooding from intense rainfall, from looking at susceptibility of catchments in the past to helping identify the risk of susceptibility to floods in real-time, thus enhancing the end-to-end forecasting chain.
End-to-end forecasting has been around for some time and many meteorological or hydrological centres have some form of operational end-to-end flood forecast (e.g., [5
]). The more sophisticated end-to-end forecasts go further down the chain, i.e., they include hydraulic components either through a look-up library of static flood maps (e.g., [7
]) or by running hydraulic models (e.g., [4
]). Thus the FFIR programme develops the end-to-end forecasts by improving stages in the forecasting chain and the overall integration of such systems, with a specific focus on small-scale intense rainfall events that lead to flooding in high-resolution models.
The FFIR programme consists of three components. It first considers the meteorological (Forecasting Rainfall exploiting new data Assimilation and Novel observations of Convection: FRANC) and hydrological aspects (Susceptibility of catchments to INTense RAinfall and flooding: SINATRA) separately before combining the results (Towards END-to End flood forecasting and a tool for ReaL-time catchment susceptibilitY: TENDERLY) and considering how the improvements in FRANC and SINATRA feed into the end-to-end forecasting chain. In this manuscript we make recommendations for improving the integration of an end-to-end flood forecasting system based on the work completed during the FFIR programme. The works that are direct outputs from the FFIR programme are marked with an asterisk (*) to distinguish them from other referenced work. We begin by defining flooding from intense rainfall (Section 2
). We then discuss the UK operational end-to-end flood forecasting system prior to FFIR, which is more focused on large-scale and coastal flooding (Section 3.1
). The system proposed for flooding from intense rainfall is broadly discussed in Section 3.2
. We then consider if it is possible to have any large-scale indications of when the full high resolution (i.e., including hydraulic components) end-to-end forecasting system should be used several days in advance in Section 4
. The improvements from the FFIR programme are considered in the following two sections with improvements from radar observations, and data assimilation through to the weather forecasts being considered in Section 5
and downstream applications such as catchment susceptibility, hydraulic modelling, model connectivity (to allow for improved integration), and public engagement in Section 6
. The key recommendations from the FFIR programme are discussed in Section 7
before a summary is made in Section 8
2. Definition and Examples of Flooding From Intense Rainfall
We define flooding from intense rainfall as flooding caused by a short-duration extreme rainfall event. The intense rain is produced from convective storms (e.g., thunderstorms), and often lasts a few hours at most. Therefore the criterion required for a flood to be classed as a flooding from intense rainfall event is that it is produced by convective rainfall. This definition is intentionally broad. We do not specify a duration or intensity to distinguish convective rainfall from other types of rainfall as the specification of such thresholds is largely arbitrary and can vary between case studies depending on the models used. Flooding from intense rainfall typically describes two key types of flooding: surface-water flooding and rapid-rate-of-rise river flooding. The amount of rainfall required to produce a flood will depend on the catchment area, soil or drainage permeability and the antecedent conditions. Table 1
provides examples of flooding from intense rainfall events in the UK, illustrating the variety of durations and rainfall totals that fall within the typical range for these type of events.
Flooding from intense rainfall events tend to happen more frequently in the summer (June, July and August; e.g., [14
]) but can occur at any time of the year (e.g., [15
])*. For more classifications and a historic record of flooding from intense rainfall events refer to [16
]*. Due to the nature of flooding from intense rainfall events (being caused by convective storms which are short-lived and localised events) these floods are hard to predict from all aspects of the forecast (e.g., [17
]). Therefore, an end-to-end forecasting system that takes into account the uncertainty would be a useful (and necessary) tool for end users of flood forecasts (see Section 7
4. Improving the Lead Time for Flooding From Intense Rainfall Forecasts
One of the challenges of forecasting flooding from intense rainfall is the short lead time between identifying a convective feature in the NWP model that could lead to flooding, and the flooding occurring. Being able to identify weather systems further in advance that have the potential to result in flooding from intense rainfall would enable increased preparedness. This could be achieved through raising the awareness of a potential event to flood forecasters in advance, increasing the number of staff on operational duty, or running the more computationally demanding urban flood models. Therefore the FFIR programme looked for indications of whether flooding from intense rainfall was likely to occur days in advance.
The main drivers of winter rainfall are extra-tropical cyclones (e.g., [32
]) and UK winter flooding has been linked with Atmospheric Rivers, bands of intense moisture transport within the warm sector of these cyclones (e.g., [33
]). However, the most intense summer rainfall events based on an assessment of daily rain gauges are not found to be associated with these larger-scale atmospheric features [34
]*. Therefore, different drivers such as moisture convergence [35
] and stability [36
] are more likely to have an influence on the large-scale weather conditions leading to flooding from intense rainfall.
Investigations within the FFIR programme looked at the atmospheric conditions that would lead to a potentially unstable atmosphere, thus increasing the chances of convective processes being triggered. The focus was on large-scale processes that could be identified days in advance rather than the small-scale processes that can only be identified a few hours in advance.
The results of the study, which uses the quality controlled rain gauge observations from [15
], are discussed in detail in [37
]*. The results showed that the processes that lead to extreme rainfall events depend on the location of the event. Over the North-West of England orographic effects were more important, and the rainfall events could be associated with the presence of a high in the geopotential height at 200 hPa over the UK and Western Europe. This “high” was seen up to five days prior to the rain event. However, for the South-East of England the presence of locally high moisture anomalies were required and no large-scale precursor was found.
Despite the lack of indicators five days in advance (on large scales) for some forms of flooding from intense rainfall, there may be some indication of convection present in forecasts up to 48 h in advance. If this indication is present then it may still be worth running a high-resolution end-to-end forecast (i.e., with hydraulic component) as an early indicator, as opposed to fully relying upon it with 6–12 h notice.
Flooding from intense rainfall is a short-lived, localised natural hazard that is hard to predict accurately and is the main focus of the joint NERC and Met Office FFIR programme. The technical contributions of the FFIR programme to improving end-to-end forecasting for intense rainfall as outline in this paper can be summarised under four themes: improved knowledge of vulnerability to flash flooding, improved observations during events, improved forecasting of convection (which includes the data assimilation improvements), and real time flood inundation modelling (Figure 5
). Through consideration of how these components link together in an end-to-end framework, the programme has demonstrated the benefits of interdisciplinary collaboration and laid strong foundations for future integrated research. These achievements will support improvements in the logistics of delivering forecasts and warnings for flash flooding and will ultimately improve flood forecast and warnings (Figure 5
). We conclude that the current state of the science is such that further integration is possible and a first attempt at an improved-integrated platform for flash flooding can be made for demonstration purposes. However, there are still improvements, required before all the new research components presented here can be used operationally. These improvements are not only linked to the recommendations mentioned within this manuscript but also the real-time running of hydraulic models on supercomputer platforms to ensure consistency with the NWP models (to allow for its use in an operational context), and testing and validation of the complete system for multiple cases across the country. Given these improvements required to make this system operational it is likely to be many years before the proposed FFIR end-to-end system becomes a fully operational reality.
We recommend that these improvements to the operational end-to-end forecasting system, particularly focused on intense rainfall, should be a policy priority. Such a system would unify all the components into a fully automated, rapidly updating framework, and this streamlined system would provide better information to improve decision making by end users. The present warning chain relies upon the manual transfer of information between agencies, and the automated end-to-end system would removed these potential points of failure. There are scientific and logistical barriers to such a system, and we have proposed a way forward in bringing each of the components together, including the following key recommendations
Research into the propagation of uncertainty through the end-to-end modelling chain.
Build closer relationships with and between users throughout the chain to better understand their requirements from an end-to-end system to inform their response to flood events.
Real-time demonstration of the end-to-end system through testing on detailed case studies of flooding from intense rainfall events.
Further advances could be made through the integration and pull-through of ideas presented in the FFIR programme, and this article, into current projects considering earth system forecasting such as UK Environmental Prediction (e.g., [74
]) or impact modelling of hazards (e.g., [76
Through combined research across multiple disciplines the FFIR programme has not only helped provide ways to improve the forecasts and representation of flooding from intense rainfall it has also encouraged stronger links and greater understanding between all the communities interested in flooding from intense rainfall (i.e., researchers, practitioners, policy-makers, the general public) thus enhancing research, output and understanding for all involved.