Forecasting uncertainty: the ensemble solution

© Crown copyright 2006 ESWWIII, Royal Library of Belgium, Brussels, Nov 15th 2006

Forecasting uncertainty: the ensemble solution

Mike Keil,

Ken Mylne, Richard Swinbank and Camilla Mathison

Data Assimilation and Ensembles, Met R&D, Met Office

ESSWIII, 13-17 November 2006, Royal Library of Belgium, Brussels.


Outline

Introduction to Ensemble Forecasting

Perturbing analyses/models

Examples of probability forecastsApplication to space weather


Weather forecasting

Today’s NWP systems are one of the great scientific achievements of the 20th Century, but…

Forecasts still go wrong!

16-17 Oct '87 – still difficult with today’s systems

Less severe errors are much more common, especially in medium-range forecasts

What causes errors in forecasts? Analysis errors Model errors and approximations Unresolved processes


Ensemble Forecasts

Small errors grow and limit the useful forecast range.

By running an ensemble of many model forecasts with small differences in initial conditions and model formulation we can:

take account of uncertainty sample the distribution of forecast states estimate probabilities

Ensembles turn weather forecasts into Risk Management tools


Ensemble forecasts

time

Forecast uncertainty

Climatology

Initial Condition Uncertainty

X

Deterministic Forecast

Analysis X

Deterministic Forecast

Forecast uncertainty


Adding perturbations


IC perturbations: ensemble spread

Time

Deterministic forecast, with increments each analysis cycle

Ensemble forecast - spread increases, reflecting chaotic dynamics and model error

Ensemble spread is a measure of forecast error

After each analysis, spread is reduced, because of new information from observations

data assimilation creates a new analysis

Forecast phaseForecast phase

Forecast phase


The Met Office has three schemes to address different sources of model error: Error due to approximations in parameterisations

Random Parameters (RP)

Unresolved impact of organised convection Stochastic Convective Vorticity (SCV)

Excess dissipation of energy at small scales Stochastic Kinetic Energy Backscatter (SKEB)

Model perturbations: stochastic physics


Examples


Ensembles – estimating risk

By running models many times with small differences we can: take account of uncertainty estimate probabilities and risks

eg. 10 members out of 50 = 20%


Example: Early Warnings of Severe Weather

Met Office issues Early Warnings up to 5 days ahead - when probability 60% of disruption due to:

Severe Gales Heavy rain Heavy Snow

Forecasters provided with alerts and guidance from ensembles

Challenges: Severe events not fully

resolved

Few events so difficult to verify


Katrina – from “operational” system


Katrina – NHC warning


End-to End Outcome Forecasting

An ensemble weather forecast can be used to drive an ensemble of outcome models, eg: Wind power output Energy demand Hydrology – flood risk Ship or aircraft routes


Application to space weather


Application to SW: power supply

Forecasts of disruption to power distribution High degree of uncertainly

Longer timescales

Ensemble thinking can help! A variety of perturbations can be applied to

models

Inputs – the behaviour of the sun

Model parameters – known weaknesses


Power disruption probability

X aX aY

Y XZ bX Y

Z XY cZ

Information of this kind can be useful to customers

Critical thresholds can aid planning decisions:

•rescheduling grid maintenance

•load reduction


There’s a 50% prob of snow in London tomorrow

50% ? You mean you don’t know what will happen!

Probabilities in context - a warning

Probabilities need to be explained properly

Normally it only snows one day in 50 at this time of year - so 50% is a strong signal.

Probabilities must be unambiguous and relevant to the end user

When’s this talk going to

end?


Summary

Utilising ensembles is now a mature tool in

operational weather forecastingEnsembles provide extra information on

Uncertainty

Risks, particularly for high impact weather

We are learning how to use probability forecasts for

improved decision-makingThese ideas are being now considered in space

weather forecasting

Power supply disruption

Applicable to other areas


Questions


Met Office Operations Centre

Ops Centre forecaster uses the ensemble to assess the most probable outcome before creating the medium-range forecast charts…

…and assess risks

Forecasting uncertainty: the ensemble solution

Documents

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