© ECMWF 26/02/18

Polar lows in the ECMWF and Arome Arctic NWP modelsMatilda Hallerstig and Linus Magnusson

October 29, 2014

Who am I?

● PhD student at Uni Research●

● Graduate Trainee at ECMWF

● Forecaster at MET Norway, Tromsø

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Tromsø

October 29, 2014

What is a polar low?

● Small but intense maritime cyclone

● North of the polar front (in the northern hemisphere)

● Horizontal scale ca 150-600 km

● Near gale force or stronger

● “Arctic hurricane”

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Polar Low

Synoptic low

October 29, 2014

Weather in a polar low

● Sudden changes

● Heavy snow showers○ Poor visibility○ Icing on aircrafts○ Avalanche danger

● Strong, gusty winds○ Turbulence○ Large waves○ Vessel icing○ Avalanche danger

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Photo: Ronald JohansenPhoto: Morten I. Jensen

Photo: www.defenceimage.mod.uk Licence: https://creativecommons.org/licenses/by-sa/2.0/

October 29, 2014

Forecasting challenges

● Rapid development● Large contrasts● Few observations● Small scale

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Shear zone ~ 1 km

Tromsø mountain observation (Kjølen 800 moh):13:20z: 290 03kt (Calm)13:50z: 340 53kt gusting 69kt (Northwest storm 10)

L

October 29, 2014

Motivation

● Historically poor representation of Polar Lows in global NWP models

● After recent upgrade to 9 km resolution in ECMWF HRES polar lows can be resolved

● Knowing performance of HRES compared to the limited area model Arome Arctic is useful for forecasting Polar lows earlier ahead of the event.

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October 29, 2014

Polar low cases

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2016-03-31 (“Lofoten”)

2016-11-27 (“Barents I” and “Finnmark I”)

2016-12-08 (“Barents II” and Finnmark II”)

Semi-automatic tracking

● Developed for tropical hurricanes

● Starting point from satellite images

● Follow minimum mslp

● Find max wind speed within 150 km for each time stepCriteria:

● Operational runs both from Arome Arctic and ECMWF HRES available in archive.

● Within the Arome Arctic domain● Interesting from a meteorological

point of view

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Experiments

Model Resolution Domain Deep convection Hydrostatic?

Arome Arctic 2.5 km LAM Resolved no

ECMWF Ensemble 18 km Global Parameterized yes

ECMWF HRES 9 km Global Parameterized yes

Reference 9 km Global Parameterized yes

No convection 9 km Global Turned off yes

5 km 5 km Global Parameterized yes

No conv + 5 km 5 km Global Turned off yesArome Arctic domain

October 29, 2014

ReferenceReference

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5 km

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5 km

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Reference Reference

Aro

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ontro

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Scatterplots based on all cases

Reference

October 29, 2014

Observations at landfall

●

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BåtsfjordSluskfjellet

October 29, 2014

● 9 km and 5 km very similar

● More intense precipitation with convection off

● Convective cells becomes mini lows with convection off

Instantaneous precipitation

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Convection on

Linus Magnusson

Convection off

Example of model tendencies

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Conclusions

1. Deep convection is more important for wind speed than horizontal resolution

● Stronger winds in Arome Arctic than ECMWF HRES

● Minimum MSLP very similar between all models

● Small differences between 5 km and 9 km setup

● Turning off deep convection produces stronger winds, but weaker than Arome Arctic

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1.2. Far too weak winds in

ECMWF during landfalls

For the future:● Study tendencies● Verification against scatterometer winds● Effect on waves and ocean● Ensemble performance● Automatic tracking for HRES and ENS to create

long term statistics

1.2.

3. Polar lows are also present in ensemble

© ECMWF 26/02/18

Thank you for listening!Gamle-Erik

(Old Erik)

October 29, 2014

How do polar lows form?

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LW

ind

Svalbard

Norway

SwedenFinland

● Cold Air Outbreak (CAO)

● Warm sea surface

● Baroclinic zone

● Areas of high potential vorticity

● Upper through

● Upper cold core

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Blue: Arome Arctic domain

Red: study area

Svalbard

Tromsø

Oslo

Reading

Area of interest

Greenland

Iceland

October 29, 2014

Semi-automatic tracks

●

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October 29, 2014

Scatterplots based on all cases