Hydrological modelling using convective scale rainfall modelling – phase 1

Hydrological models have the capability to provide useful river flow predictions and flood warnings. The aim of this project, ‘Hydrological Modelling using Convective Scale Rainfall Modelling’, is to investigate which models and associated computational methods would allow best use of the latest Met Office developments in numerical weather prediction (NWP). Two recent enhancements in particular offer interesting opportunities and open the door to the use of probabilistic flood forecasting. These two developments are: • operation of the nowcasting system STEPS (Short Term Ensemble Prediction System) at 2 km resolution; • a new system for longer term numerical weather prediction called MOGREPS (Met Office Global and Regional Ensemble Prediction System). The three-phase project is concerned primarily with: • how to use high resolution (convective scale) rainfall forecasts effectively for flood forecasting; • how to make operational the use of ensembles of numerical weatherprediction (MOGREPS) in flood forecasting and warning within the Environment Agency’s National Flood Forecasting System (NFFS). This report presents the results of Phase 1 (inventory and data collection) and forms the starting point for the rest of the project – Phase 2 (pilot case study). Phase 1 involved: • examining the suitability of three recent storm events as the pilot case study for Phase 2; • obtaining detailed numerical weather predictions from the Met Office’s Joint Centre for Mesoscale Meteorology (JCMM) in Reading; • compiling an inventory of hydrological models (primarily rainfall-runoff models) suitable for predicting runoff generated by intensive storm events; • configuring and calibrating four models (PRTF, PDM, G2G, REW) for transformation of high resolution rainfall predictions into accurate flood forecasts; • carrying out an analysis of the use of high resolution NWP forecasts and the generation of ‘pseudo’ ensembles; • determining the procedures and performance indicators for assessing model and ensemble performance (e.g. skill scores) when applied to the pilot case study; • selecting two Environment Agency Regions (North East and Thames) to act as pilots for ensemble forecasting in NFFS. The extreme weather event at Boscastle in north Cornwall on 16 August 2004 was selected as the pilot case study for Phase 2. At the end of Phase 3 (analysis and verification), overall conclusions will be drawn on the general benefits of using high resolution NWP as input into a hydrological model for flood forecasting. In addition, a possible approach using the hydrological models – and calibration and computation methods – will be formulated