The 2013/14 Thames Basin Floods: Do Improved Meteorological Forecasts Lead to More Skillful Hydrological Forecasts at Seasonal Time Scales?

Hannah Cloke,Jessica Neumann,Louise Arnal,Linus Magnusson

doi:10.1175/jhm-d-17-0182.1

Hannah Cloke, Jessica Neumann + Show 2 more

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https://doi.org/10.1175/jhm-d-17-0182.1

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Abstract

Abstract The Thames basin experienced 12 major Atlantic depressions in winter 2013/14, leading to extensive and prolonged fluvial and groundwater flooding. This exceptional weather coincided with highly anomalous meteorological conditions across the globe. Atmospheric relaxation experiments, whereby conditions within specified regions are relaxed toward a reanalysis, have been used to investigate teleconnection patterns. However, no studies have examined whether improvements to seasonal meteorological forecasts translate into more skillful seasonal hydrological forecasts. This study applied relaxation experiments to reforecast the 2013/14 floods for three Thames basin catchments with different hydrogeological characteristics. The tropics played an important role in the development of extreme conditions over the Thames basin. The greatest hydrological forecasting skill was associated with the tropical Atlantic and less with the tropical Pacific, although both captured seasonal meteorological flow anomalies. Relaxation applied over the northeastern Atlantic produced confident ensemble forecasts, but hydrological extremes were underpredicted; this was unexpected with relaxation applied so close to the United Kingdom. Streamflow was most skillfully forecast for the catchment representing a large drainage area with high peak flow. Permeable lithology and antecedent conditions were important for skillfully forecasting groundwater levels. Atmospheric relaxation experiments can improve our understanding of extratropical anomalies and the potential predictability of extreme events such as the Thames 2013/14 floods. Seasonal hydrological forecasts differed from what was expected from the meteorology alone, and thus knowledge is gained by considering both components. In the densely populated Thames basin, considering the local hydrogeological context can provide an effective early alert of potential high-impact events, allowing for better preparedness.

Highlights

IntroductionSkillful seasonal hydrological forecasts (SHFs) of river and groundwater levels have the potential to provide an indication of possible flood events weeks or months in advance, allowing for more optimal and consistent decisions to be made (Arnal et al 2017)
Severe weather conditions did not originate from a single event, but from a number of events between late December 2013 and the end of February 2014, as supported by the negative seasonal average anomaly of the 500-hPa geopotential height (z500) over the northeastern Atlantic, with the United Kingdom located at the southeastern edge (Fig. 2a)
For a seasonal forecasting system, capturing this structure was key to predicting the wet anomaly over the United Kingdom, but no anomaly was present in the ensemble mean averaged over the whole season for the System 4 (S4) forecast (Fig. 2b)

Summary

Introduction

Skillful seasonal hydrological forecasts (SHFs) of river and groundwater levels have the potential to provide an indication of possible flood events weeks or months in advance, allowing for more optimal and consistent decisions to be made (Arnal et al 2017). Across the United Kingdom and Europe, seasonal streamflow and groundwater forecast methods are currently being developed for application, for example, the U.K. Met Office Global Seasonal Forecast System (GloSea5), the Hydrological Outlook UK, and the Copernicus European Flood Awareness System (MacLachlan et al 2015; Mackay et al 2015; Svensson 2016), supported by Copernicus projects including Service for Water Indicators in Climate Change Adaptation (SWICCA) and End-toEnd Demonstrator for Improved Decision-Making in the Water Sector in Europe (EDgE; Copernicus 2017a,b). The Thames River basin (southeast United Kingdom) received more than half a year’s typical rainfall during DJF 2014 (Lewis et al 2015), which led to concurrent fluvial, pluvial (surface water/flash), and groundwater flooding—so-called compound or coincident flood events (Thorne 2014)

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