Ensemble Forecastingof High-Impact Weather

Richard Swinbankwith thanks to various, mainly Met Office, colleagues

High-Impact Weather THORPEX follow-on project meeting, Karlsruhe, March 2013

Ensemble forecasting of High-Impact Weather

Challenges of convective-scale ensembles

Ensemble-based warnings & products

Links with other post-THORPEX initiatives

Limits of Predictability

Following Lorenz (1984), errors grow fastest at smaller scales, eventually affecting largest scales.

Leads to challenges in high-resolution forecasting – in both making and using the predictions

Since the predictability limit is shorter for small scales, ensembles are key to high-resolution prediction.

An Ensemble-based future

For data assimilation, as we focus on higher resolution (convective scales), we cannot exploit Gaussian assumptions about the behaviour of error statistics, so need an ensemble-based approach.

For short-range high resolution forecasting, ensemble methods are needed to predict the risks of severe weather at close to the model grid scale.

For longer range global forecasts, ensemble methods are required to estimate the risks of high-impact weather and produce probabilistic forecasts beyond the limits of deterministic predictability.

Challenges of convective-scale:modelling

Operational centres are now starting to introduce convective-scale ensembles.

Gives the potential to produce much more detailed forecasting of storm systems, but… Grey zone – still cannot afford to truly resolve convective

processes, rather use “convection permitting” km-scale resolutions.

Limited to small, (sub?) national-scale domains.

During life of the HIW project, look forward to <1km grid scale and larger (regional) domain sizes.

Example: MOGREPS-UK system

Currently run as a downscaling ensemble, initial and boundary conditions driven by 33km MOGREPS-G (NB. No intermediate regional ensemble).

Challenges: Time to spin up small scales Use high-resolution analysis to initialise ensemble?

Ensemble Modelling challenges

Representing uncertainties Initial condition uncertainties - in MOGREPS, currently from

MOGREPS-G, but should use ensemble DA. Model errors – what stochastic physics is appropriate for

convective scales? Surface uncertainties – how to represent uncertainties in soil

moisture, surface roughness, sea surface, etc.?

Consistency with lateral boundary conditions – movie from Warrant Tennant

Tropical Cyclones Potential for improved prediction of structure & intensity

using high resolution nested ensembles.

High-resolution simulation, by Stu Webster (Met Office)

Challenges of convective-scale:post-processing

How to post-process when details are unreliable? Neighbourhood methods for displaying output at predictable scales

Threshold exceeded where squares are blue [thanks to Nigel Roberts]

observed forecast

Optimising smoothing for skill

MOGREPS-UK Heavy Rainfall forecast

17-18Z Torrential >16mm/hour 17-18Z Heavy >4mm/hour

Probability Torrential Rain >16mm/hourCT 2012/06/28 03Z VT 17-18Z

Probability Heavy Rain >4mm/hourCT 2012/06/28 03Z VT17-18Z

© Crown copyright Met Office

Warnings based on ensembles:EPS-W weather impact matrix

≥70mph ≥80mph ≥90mph

Example of EPS-W wind

gust thresholds used for the

“Highlands and Islands”

• Likelihoods of low, medium and high impact weather are presented as probability contour maps

• These are also combined to form overall warning colour maps…

High≥60%

Medium≥40%

Low≥20%

Very Low≥1%

Very Low Low Medium High

Lik

elih

oo

d

Impact

Thanks to Rob Neal, Met Office

© Crown copyright Met Office

MOGREPS-UK example – yellow warning for gales in Orkneys & Shetlands 14-15 Dec 2012

36hr forecast 30hr forecast

HIW project - links with other ensemble forecasting initiatives

A trio of complementary datasets: TIGGE project (global medium-range EPS), since October 2006. TIGGE-LAM project, limited area counterpart to TIGGE, will be an

additional resource for HIW project – European LAM-EPS data now starting to be archived at ECMWF.

Sub-seasonal to Seasonal archive to support S2S project – coming soon.

All planned to use similar GRIB2 format and conventions. A technical liaison group (representatives from data providers &

archive centres) could manage archive.

Proposed “Predictability and Ensemble Forecasting” working group, focusing on science of dynamics & predictability and ensemble forecasting.

WWRP-THORPEX

TIGGE dataset

UsersPredictability, dynamics, probabilistic forecasting

PDP working group

GIFS-TIGGE working group

TIGGE-LAM panel

TIGGE-LAMdataset

WWRP

TIGGE dataset

UsersSub-seasonal to seasonal and polar predictability,

high-impact weather, probabilistic forecasting, RDPs, FDPs

P&EF expert team

Datasetliaison group

TIGGE-LAMdataset

HIW project team

S2S projectteam

S2S dataset

WCRP

Summary Convective-scale ensembles give new challenges and

opportunities Opportunities

More realistic simulation of severe storms More detailed local forecasts Better warnings of severe weather Exploit TIGGE & TIGGE-LAM datasets for HIW research

Challenges Resolving convection? Representing uncertainties – initial and model error Balance between resolution, domain size & members Presentation of small-scale information Combine short-range detail & longer range warnings

Any Questions?