polar lows in the ecmwf and arome arctic nwp models
TRANSCRIPT
© 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
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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
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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/
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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
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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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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
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ReferenceReference
Con
vect
ion
off
5 km
reso
lutio
n
5 km
reso
l, no
con
v
Reference Reference
Aro
me
Arc
tic
EN
S c
ontro
l
Scatterplots based on all cases
Reference
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Observations at landfall
●
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BåtsfjordSluskfjellet
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● 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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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
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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