Development of a Long-term
Climatology of North Atlantic
Polar Lows
Matthias Zahn1,2, Hans v. Storch1,2, Stephan Bakan3
http://coast.gkss.de/staff/zahn/
(1) University of Hamburg, Meteorological Institute, Germany
(2) GKSS Research Centre, Institute for Coastal Research, Germany
(3) Max Planck Institut für Meteorologie, Hamburg, Germany
Polar Lows
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Very intense mesoscale storms
typically several hundred km in diameter
occurring poleward the Polar Front in both
hemispheres
strong winds (>15m/s), severe weather, heavy
precipitation
Arctic hurricanes
Further reading:
Rasmussen and Turner, 2003:
Polar Lows: Mesoscale Weather Systems
in the Polar Regions
http://www.eumetcal.org.uk/polarlow/cometplows/polarlows/1.2_typeso
fdistrubances.htm
2
Examples of Polar Lows
20.12.2002, 2:00
04.03.2008, 11:35
11.03.08, 15:25
16.1.1995, 9:00
IPY-Thorpex field campaign:http://www.ipy-thorpex.com/ , images from http://www.sat.dundee.ac.uk/
Kolstad, E. W. & T. J. Bracegirdle & I. A. Seierstad: Marine cold-air outbreaks in the North Atlantic: temporal distribution and associations with
large-scale atmospheric circulation. Climate Dynamics, published online 19 June, 2008. DOI:10.1007/s00382-008-0431-5
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• 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
• Polar Lows need to be automatically detectable in
such data!
• First part: capability of LAM's to reproduce polar lows
(Polar Lows in CLM)
• Second part: detection of polar lows
• Third part: derive climatology
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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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0600 UTC
15 Oct 1993
Dundee
NCEP
DWD
CLM01-nn
CLM02-nn
CLM03-nn
CLM04-nn
CLM01-sn
CLM02-sn
CLM03-sn
CLM04-sn
15.10.93, 05:24
10m wind speed ≥ 13.9m/s and air pressure
(at mean sea level)
Satellite image from http://www.sat.dundee.ac.uk/
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Two dimensional bandpass filter
isotropic filters are
able to separate
large, medium and
small spatial scales
in a limited
(regional) gridded
field.
Feser, F., and H. von Storch, 2005: Spatial two-dimensional discrete filters for limited area model evaluation purposes. Mon. Wea Rev. 133,
1774-1786
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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,
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Dec 1993
case:
NERC Dundee Satellite Receiving Station
weatherchart, DWD, 0000 UTC 9 Dec 1993
Response
function: wave
lengths
between appr.
200 - 600 km
are retained
CLM22-sn, band pass filtered
0000 UTC 9 Dec 1993
CLM22-sn, full field
0000 UTC 9 Dec 1993
9
Jan 1998
case:
NERC Dundee Satellite Receiving Station
Weather chart, 0100 UTC 18 Jan 1998
Response
function: wave
lengths
between appr.
200 - 600 km
are retained
CLM01-sn, band pass filtered
0000 UTC 18 Jan 1998
CLM01-sn, full mslp field
0000 UTC 18 Jan 1998
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Intermediate 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, 2008: Climate mode simulation of North
Atlantic Polar Lows in a limited area model, TellusA 60 (4)
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Intermediate 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
Development of a detection algorithm and
application to longterm simulations
Zahn, M., H. von Storch, and S. Bakan, 2008: Climate mode simulation of North
Atlantic Polar Lows in a limited area model, TellusA 60 (4)
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Setup of the detection algorithm applied
1st step: detection of all locations with a filtered mslp
minimum < -1hPa
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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 < ~200 km
3rd step: checking further constraints along the tracks:
• strength of the minimum ( ≤ −2hPa once along the track)
• wind speed ( ≥ 13.9 m/s once along the track)
• air-sea temperature difference ( SST − T500hPa ≥ 43K)
• north south direction of the track
• limits to allowable adjacent grid boxes
OR: strength of the minimum in the bandpass filtered MSLP field
decreases below −6hPa once
Zahn, M., and H. von Storch, Tracking Polar Lows in CLM, Meteorologische Zeitschrift, 17 (4),
445-453, doi:10.1127/0941- 2948/2008/0317, 2008
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Setup of long-term simulation:
CLM 2.4.6
initialised: 1.1.1948
finished : 28.2.2006
driven by NCEP/NCAR reanalysis 1
spectral nudging of scales > 700 km
together with the algorithm enables a long-term
climatology of Polar Lows
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Three cases from Zahn et al.
Oct 1993
Dec 1993
Jan 1998
Tracks detected even after
a simulation time of several
decades
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Time series of the number of
polar lows per winter
• Mean number of
polar lows: 56
• Most active winter
was PLS 1981
• Fewest polar lows
were detected in
PLS 1964
• Strong inter annual
variability,
σ = ± 13
• No longterm trend
visible
Number of detected polar lows per polar low season. One
polar low season is defined as the period starting 1 Jul
and ending 30 Jun the following year.
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Algorithm's
sensitivity
To leaving NS and
dtz out
To varied
ws criteria
To varied
dtz criteria
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Climatological comparison
Number of detected polar lows per polar low season. Our data (black)
and observations (red) by Wilhelmsen (1985)
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Climatological comparison
Number of detected polar lows per polar low season. Our data (black)
and observed (red) by MetNo, Noer, (pers. comm.)
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C=0,58
C=0,72
Monthly comparison of our
data (in black) with
observed data (in red)
u.r.: Norwegian Met.
Service
l.l.: Blechschmidt (2008)
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Spatial density distribution
Bracegirdle, T. J. and S. L. Gray, 2006: The role of convection in the intensification of polar
lows. Ph.d. thesis, 69 pp., The University of Reading, UK.
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Number of Polar Lows in various
subregions
Subregions, for which the number of detected polar lows were counted (R1-R14).
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Canonical Correlation Analysis
(CCA)
Method to study the correlation bewteen two (or
more) random vectors, e.g. X and Y
we used:
X: number of Polar Lows per PLS and subregion
Y: gridded mean MSLP fields per PLS
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Links to large scale mean pattern
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Links to large scale mean pattern
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Final results
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No long-term trend detectable
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Strong interannual variability
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No one to one similarity to other studies, but
qualitative similarity
Large scale link: more southward mean flow =>
more Polar Lows
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Ideas for future work
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Investigate atmospheric conditions during Polar
Low events
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Do the same work for future szenarios
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Statistics on different parameters ?
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Use the CCA results to assess Polar Low
behavior on timescales beyond NCEP/NCAR
(MSLP fields of Trenberth)
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Thank you very much
for your attention
http://coast.gkss.de/staff/zahn/
Zahn, M., and H. von Storch (2008), A long-term
climatology of North Atlantic polar lows, Geophys.
Res. Lett., 35, L22702, doi:10.1029/2008GL035769
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