Ensemble development for convective-scale forecasting

Floods in the UK are often caused by extreme rainfall events. At present, weather forecasts can give an indication of a threat of severe convection which might cause flash floods, but are unable to say precisely when and where the downpours will occur, due to the complex range of processes and space-time scales involved. This project is part of a collaborative effort to improve flood forecasting. The first stage is to predict the air motions leading to convergence and ascent at a certain location where the precipitation will be initiated, then the development of the precipitation needs to be forecast (our aspect!), and hydrological models used to produce accurate, quantitative, probabilistic flood predictions.

The forecast model used will is a new Met Office model that can be run with resolution of order 1-4km. The crucial point about these high resolutions is that convective motions can be explicitly represented, rather than treated as a sub-grid-scale effect.

Of course, there are important uncertainities inherent in a numerical weather forecast. Initialisation errors are one obvious source but not the only one. Models are often run only for a limited region of the world, and the data on the boundaries of this area provided from a larger-scale model. These data are known as lateral boundary conditions. Errors in these lateral boundary conditions and modelling errors also contribute to the errors in the forecast. Even if these errors were reduced, the nonlinear nature of the convective dynamics ensures that there is a predictability limit, beyond which the value of deterministic forecasts becomes questionable. After that point it becomes important to determine the uncertainties in the forecast precipitation, so an ensemble approach is required. The appropriate construction of a high-resolution ensemble is an open question. Techniques for generating ensembles at global scales cannot simply be extended to convective scales. We are attempting to design perturbations on the basis of physical insight into convective forcing mechanisms. The resulting probabilistic rainfall forecasts can be interfaced to hydrological models used for flood forecasting.

Some links for this work:

Publications:

1. Paper on the robust assessment of model uncertainties based on CSIP IOP8
2. Paper on model uncertainties in simulations of the Boscastle flood
3. Paper on perturbation growth on convective scales

Talks:

1. Overview of the project presented by Anthony Illingworth at a Royal Meteorological Society National Meeting, October 2010.
2. An informal and non-technical talk presented to a small group from industry as an introduction to the issues in forecasting the rainfall associated with flash flooding.
3. A talk presented by Giovanni Leoncini at the 9th European Conference on Applications of Meteorology, Toulouse, France, 28 September-2 October 2009.
4. A talk presented by Giovanni Leoncini at the FREE Ensembles Workshop, Reading, 23-24 September 2009. A summary of the workshop can be found here.
5. A talk presented by Giovanni Leoncini at the 4th SRNWP Workshop on Short-range Ensemble Prediction Systems, Exeter, 23-25 June 2009.
6. A talk presented by Giovanni Leoncini as a departmental seminar.

Posters:

1. A poster presented by Sue Gray at a Royal Meteorological Society National Meeting, October 2010.
2. A poster presented by Giovanni Leoncini at the Royal Meteorological Society Conference 2009.
3. A poster presented by Sue Gray on perturbation growth on convective scales, at the 2009 EGU conference in Vienna.
4. A poster presented by Giovanni Leoncini at the Second FREE Science Meeting, University of Reading, 12-13 November 2008.