Detection of North Atlantic
Polar Lows in Climate Mode
Simulations
Matthias Zahn1,2, Hans v. Storch1,2, Stephan Bakan3
(1) University of Hamburg, Meteorological Institute, Germany
(2) GKSS Research Centre, Institute for Coastal Research, Germany
(3) Max-Planck-Institut für Meteorologie (MPI-M), Germany
• Background
• Quality of Polar Lows in LAMs
• Detection of Polar Lows
Background
My thesis: Trends and variability in extratropical
mesoscale cyclones
(VI EXTROP):
Long term aim: climatological description of polar low
occurrences (features)
EXTROP, http://www.extrop.zmaw.de/
Background
• Comprehensive measurements are required to
address such a question (long in time, high in
spatial detail, homogeneous)
• Use of numerical models in combination with
existing measurements to reconstruct the past
state of the atmosphere
• First part: capability of LAM's to reproduce polar lows
• Second part: detection of polar lows
• (Third part: derive climatology)
PLows in
CLM-data
Ensemble simulations (2x4) with CLM ( ~50km ) in climate mode for
polar low cases Oct. 1993 (and Dec. 1993, Jan. 1998)
• Driven by the NCEP reanalysis
• Initial times
Initialisation
PLow
2 weeks
• Spectral Nudging (4x) and without (4x)
• Different waves (above appr. 700 km)
Hans von Storch, Heike Langenberg, and Frauke Feser, A Spectral Nudging Technique for Dynamical Downscaling Purposes, Monthly
Weather Review 128(10) 3664-3673.
http://clm.gkss.de/
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PLows in
CLM-data
Le Cygne Oct. 1993
synoptic scale low, “the Swan”, shallow
baroclinic processes combined with upper
level vorticity advection
From Claud et.al
Claud, C., G. Heinemann, E. Raustein and L. McMurdie, 2004: Polar low le Cygne: Satellite observations and numerical simulations. Quarterly Journal Of The Royal
Meteorological Society, 130, 1075–1102
Nielsen,1997: An early Autumn polar low formation over the Norwegian Sea, J.Geophys. Res., 102, 13955-13973
PLows in
CLM-data
Le Cygne Oct. 1993
Mature state polar low: reaching peak
intensity, wind speeds above gale force,
closed small scale surface low
From Claud et.al
Claud, C., G. Heinemann, E. Raustein and L. McMurdie, 2004: Polar low le Cygne: Satellite observations and numerical simulations. Quarterly Journal Of The Royal
Meteorological Society, 130, 1075–1102
Nielsen,1997: An early Autumn polar low formation over the Norwegian Sea, J.Geophys. Res., 102, 13955-13973
PLows in
CLM-data
Le Cygne Oct. 1993
decaying quickly after landfall
From Claud et.al
Claud, C., G. Heinemann, E. Raustein and L. McMurdie, 2004: Polar low le Cygne: Satellite observations and numerical simulations. Quarterly Journal Of The Royal
Meteorological Society, 130, 1075–1102
Nielsen,1997: An early Autumn polar low formation over the Norwegian Sea, J.Geophys. Res., 102, 13955-13973
PLows in
CLM-results
0600 UTC
15 Oct 1993
NCEP
DWD
10m wind speed ≥ 13.9m/s and air pressure (at mean sea level) on 15 October 1993:
NCEP/NCAR analysis after interpolation onto the CLM grid, 0600 UTC, DWD analysis data,
0600 UTC, CLM ensemble run without (nn) and with(sn) spectral nudging , 0900 UTC
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PLows in
CLM-results
0600 UTC
15 Oct 1993
NCEP
CLM01-nn
CLM02-nn
DWD
CLM03-nn
CLM04-nn
10m wind speed ≥ 13.9m/s and air pressure (at mean sea level) on 15 October 1993:
NCEP/NCAR analysis after interpolation onto the CLM grid, 0600 UTC, DWD analysis data,
0600 UTC, CLM ensemble run without (nn) and with(sn) spectral nudging , 0900 UTC
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PLows in
CLM-results
0600 UTC
15 Oct 1993
NCEP
DWD
CLM01-nn
CLM02-nn
CLM03-nn
CLM04-nn
CLM01-sn
CLM02-sn
CLM03-sn
CLM04-sn
10m wind speed ≥ 13.9m/s and air pressure (at mean sea level) on 15 October 1993:
NCEP/NCAR analysis after interpolation onto the CLM grid, 0600 UTC, DWD analysis data,
0600 UTC, CLM ensemble run without (nn) and with(sn) spectral nudging , 0900 UTC
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PLows in
CLM-results
Track of polar low's local sea level pressure
minimum in CLM-sn
CLM01-sn
CLM02-sn
CLM03-sn
CLM04-sn
Start: 1500 UTC 14 Oct until 0000 UTC 16 Oct
Not possible in CLM-nn
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PLows in
CLM-results
0600 UTC
15 Oct 1993
(Response function:
wave lengths between
appr. 200 and 600 km
are retained)
NCEP
DWD
CLM01-nn
CLM02-nn
CLM03-nn
CLM04-nn
CLM01-sn
CLM02-sn
CLM03-sn
CLM04-sn
Band-pass filtered mslp (isolines; hPa) and 10m wind speed anomalies, NCEP/NCAR analysis after
interpolation onto the CLM grid, 0600 UTC, DWD analysis data, 0600 UTC, CLM ensemble run without
and with spectral nudging, 0900 UTC. Black dots indicate the positions of the polar low's pressure
minimum in the respective untreated field of the ensemble run.
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PLows in
CLM-results
The 1993
case:
NERC Dundee Satellite Receiving Station
CLM22-sn, filtered
0000 UTC 9 Dec 1993
weatherchart, DWD, 0000 UTC 9 Dec 1993
CLM22-sn, full field
0000 UTC 9 Dec 1993
Rasmussen and Turner, 2003: Polar Lows: Mesoscale Weather Systems in the Polar Regions
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PLows in
CLM-results
The 1998
case:
NERC Dundee Satellite Receiving Station
CLM01-sn, filtered
0000 UTC 18 Jan 1998
Weather chart, 0100 UTC 18 Jan 1998
CLM01-sn, full field
0000 UTC 18 Jan 1998
Woetmann Nielsen, N., Om forudsigelighed af polare lavtryk, Vejret, 20, 37–48, in Danish, 1998
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PLows in
CLM-results
Results:
•In principle, Polar Lows can be reproduced with CLM run in
climate mode
•Though, there may be deviations in location and amount of
pressure minima
•Without nudging the large scale, the formation of Polar Lows
is subject to considerable ensemble variability
•A digital filter could be useful for an automatic detection
Zahn, M., H. von Storch, and S. Bakan, 2007: Climate mode simulation of North Atlantic Polar Lows in a
limited area model, revised manuscript resubmitted to TellusA, 15 November 2007
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PLows in
CLM-results
Results:
•In principle, Polar Lows can be reproduced with CLM run in
climate mode
•Though, there may be deviations in location and amount of
pressure minima
•Without nudging the large scale, the formation of Polar Lows
is subject to considerable ensemble variability
•A digital filter could be useful for an automatic detection
Application in longterm Simulations (2*sn and 2*nn)
Oct 1993- Sept 1995
Zahn, M., H. von Storch, and S. Bakan, 2007: Climate mode simulation of North Atlantic Polar Lows in a
limited area model, revised manuscript resubmitted to TellusA, 15 November 2007
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Detectionalgorithm
Setup of the detection algorithm applied
1st step: detection of all locations with a filtered mslp
minimum < 1hPa
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Detectionalgorithm
Setup of the detection algorithm applied
1st step: detection of all locations with a filtered mslp
minimum < 1hPa
2nd step: combine detected positions to individual tracks,
distance to next (3h) pos < ~150 km
3rd step: checking further constraints along the tracks:
1. minimum in the filtered mslp field
2. wind speed >= 13,5 m/s
3. no synoptic low
4. a local minimum or trough in the untreated mslp field
5. atmospheric instability (decreasing vertical potential temperature
between sea level and 700 hPa)

SST −700hPa 
0
mslp −700 hPa
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Detectionresults
Detected tracks according to these criteria
Simulation period: Oct 1993 until Sep 1995, CLMsn
Winters (ONDJFM)
Summers (AMJJAS)
Detectionresults
Example: October tracks
CLMsn
CLMnn
Detectionresults
Monthly frequency distributions:
von:http://www.eumetcal.org.uk/polarlow/
Lystad, M., 1986: Polar lows in the Norwegian, Greenland and Barents Sea - Final Report. Norwegian Meteorological Institute, Oslo,
Norway, 196 pp.
Noer, G. and M. Ovhed, 2003: Forecasting of polar lows in the Norwegian and the Barents Sea. Proc. of the 9th meeting of the EGS Polar
Lows Working Group, Cambridge, UK, 2003.
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Wilhelmsen, K., 1985: Climatological Study Of Gale-Producing Polar Lows Near Norway. Tellus, 37A, 451–459
Detectionresults
Number of medial positions of detected tracks in
CLMsn and CLMnn:
Zahn, M.,
M. and H. von Storch, 2008: Climatological differences of reproduced North Atlantic Polar Lows in long term
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simulations with CLM, submitted to Meteorologische Zeitschrift
Final results
Final results (of this presentation)
• In principle, Polar Lows can be reproduced with CLM run in climate mode
• Though, there may be deviations in location and amount of pressure minima
• A digital filter could be useful for an automatic detection
Results of the application of the detection algorithm
Spatial digital bandpass filters applied to LAM output
fields represent a simple method to automatically
identify and track meso-scale disturbances
● Temporal frequency distributions qualitatively are
similar to those expected
● Smaller number of detected Polar Low tracks in CLMsn
compared to CLMnn
●
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Outlook
Outlook
Paper describing (and enhancing)
the algorithm
Comparison with observed Cases
(A.Blechschmidt)
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Thank you very much
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