© Crown copyright 2007 A fully resolved stratosphere: Impact on seasonal forecasting Alberto Arribas Monthly-to-Decadal area, Met Office Hadley Centre.

© Crown copyright 2007

A fully resolved stratosphere:Impact on seasonal forecasting

Alberto Arribas Monthly-to-Decadal area, Met Office Hadley Centre

Reading, April 2013

- Ensemble forecast

- Dealing with model drift

obs. climatology

Forecastsimulations

How do we make seasonal forecasts?

obs

All Hindcast runs:~ 12 members14 years (96-09)

Hindcast mean

How do we make seasonal forecasts?

obs

Hindcast mean

Forecastmembers

How do we make seasonal forecasts?

Bias corrected forecast

obs

How do we make seasonal forecasts?

Skill of seasonal forecasts

Between 1990s and late 2000s, skill had barely improved, still minimal outside tropics

Hindcasts are expensive and can leave no space for model development ... i.e.

Important sources of predictability not properly explored:

- Stratosphere- Sea-ice- Ocean-atmosphere coupling….

Bigger not always better

Change of strategy

Hindcast length

Frequency of system upgrades

Centre's priorities

JMA ~ 30 yr ? ?

NCEP ~ 40 yr 8 yr Link to re-analysis

ECWMF ~ 25 yr 5 yr Med-range

UK Met Office * 14 yr 1 yr Link to model

development

* Arribas et al., 2011: GloSea4 ensemble prediction system for seasonal forecasting. MWR. 139, 1891-1910

GloSea history

- Summer 2009: GloSea4 becomes operational- Nov. 2010:

- Vertical high-res (L85 strat. / L75 ocean)- Sea-ice assimilation (concentrations)- Updated hcst (1996-2009)

- May 2011:- Monthly system

- Nov. 2012: GloSea5!- Horizontal high-res (N216 atm. / 0.25 ocn)- NEMOVAR ODA

Improved GloSea5

Range: 7monthsModel: HadGEM3

Resolution: N216L85O(0.25)L75Initialisation: daily, NWP state + NEMOVAR 0.25deg

Method: Full field + hindcast bias correctionEnsembles: SKEB2 stoch. physics, lagged initialisation

Forecasts: Seasonal 2 day-1: 42 members updated weekly

Monthly 2 day-1: 28 members updated daily Reference: MacLachlan et al., 2013, in prep.

Atmos & land surf: ERA-i

Ocean: Seasonal ODA reanalysis

Atmos & land surf: NWP anal

Ocean/sea-ice: NEMOVAR

A coupled system for med-range and ocean forecasting

60 days 6 months

6th D

ec 2

012

9th J

an 9

6-09

Fore

cast

sH

ind

cast

s

15 days

- 24 members - IC perts. from MOGREPS-G- Coupled model as in GloSea5- Run once a day- Comparison PS32 – PS34

- Operational end’13 or 14?

New suite:

GloSea5:Skillful NAO predictions!

Scaife et al., 2013Source: MetOffice.gov.uk

R=0.60

NAO: November forecast for Dec-Jan-Feb

observations

forecast

PMSL anomalies

from Nov for DJF

05/06 to 09/10winters

Reasons for skilful NAO forecasts

Gulf Stream Bias

Wly wind bias

=> Blocking Deficit

No Gulf Stream Bias

No Wly wind bias

=> Good Blocking in N. Atl

New Model

Scaife et al., Geophys. Res. Lett., 2012.

Low-res:

1 deg ocean

High-res:

0.25 deg ocean

- Courtesy of Warwick Norton -

Not December!

As found by Warwick Norton, there is very little skill in December

Not a simple persistence of anomalies over

the timescale of a few weeks

Hints at a possible mechanism in late winter: ENSO/stratosphere => strongest NAO

(Ineson and Scaife 2008)

From Nick Dunstone

Looking at stratosphere: Data available

L38 L85

GloSea4: N96

GloSea5: N216

89-02 89-09

96-09

SSWs in L38 and L85

From David Fereday

Obs

NoStrat

Strat

Moderate Nino Strong Nino

From Anna Maidens

A quick diversion:

How good are ENSO forecasts in GloSea5?

Improving ENSO forecasts

Obs The westward extension of Nino is a common error in many climate models. It affects remote regions.

High-res model has better ENSO

pattern and teleconnections

Low resolution

High resolution

Nino 3.4 SST:ACC / RMSE&Spread

ACC higher (good)

RMSE

reduced (also good)

May JJA Nov DJF

GloSea5 (red)

GloSea4 (blue)

Forecast Observed

ENSO Teleconnection: Prec. Nino - Nina

DJF

JJA

Back to the stratosphere

Importance of stratosphere.Winter 2012-2013

Strat. Winds Observations (winter 2012/2013)

From Jeff Knight

Forecasts

Easterly winds appear at high altitude first

First signs in forecasts made on 21 December…

Strat. Winds Observations

Monthly Forecasts: January 2013

From Jeff Knight

© Crown copyright Met Office

First significant Snow Fall ~18th Jan

Forecasters notified from late Dec => increased risk ACTIONABLE

DfT notified on 4th January: “…there is now an increasing risk of cold conditions returning later in January and an increased chance of wintry conditions starting later this month…”

“Wintry weather brings disruption to airports, road and rail networks across the UK, with more snow and ice on the way”

The Guardian 20th Jan

Metro 18th Jan

Conclusions

- GloSea5: High skill predicting winter NAO

- Why? High horizontal res.; sea-ice; stratosphere

- Skill highest in late winter, role of stratosphere?

- Consistent with ENSO/Europe teleconnection through stratosphere and better SSW

Thanks

Any questions?

Tropical storm tracks: impact of resolution

1990s ~240 km horizontalresolution

2000s ~120 km horizontalresolution

Observations

Today: GloSea5 ~53 km horizontal resolution Observations

Tropical storm tracks: impact of resolution