Detection and Longterm Climatology
of North Atlantic Polar Lows
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, Hamburg, Germany
Content
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What are Polar Lows (why is it difficult to
detect them) ?
How does my approach look like ?
How does the climatotology of Polar Lows look
like ?
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Polar Lows
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
develop extremely rapidly
(reaching peak intensity
within 24 hours)
Arctic hurricanes
Rasmussen and Turner (2003) state:
"A polar low is a small, but fairly intense maritime
cyclone that forms poleward of the main baroclinic zone
(the polar front or other major baroclinic zone). The
horizontal scale of the polar low is approximately
between 200 and 1000 kilometres and surface winds
near or above gale force."
http://www.eumetcal.org.uk/polarlow/cometplows/polarlows/1.2_typeso
fdistrubances.htm
Rasmussen and Turner, 2003: Polar Lows: Mesoscale Weather Systems in the Polar Regions
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Polar Lows
Polar Low development conditions
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form under cold upper level troughs
form in cold arctic air that flows over a warm
body of water/ Cold Air Outbreak
shallow baroclinic zones
along the main baroclinic zone
triggered by convection / sensible heat flux
instable atmospheric environments:
baroclinic instability
(barotropic instability)
CISK conditional instability of second kind
WISHE wind induced surface heat exchange
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Polar Lows
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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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
• 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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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-results
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) 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
Satellite image from http://www.sat.dundee.ac.uk/
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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
weatherchart, DWD, 0000 UTC 9 Dec 1993
CLM22-sn, filtered
0000 UTC 9 Dec 1993
Rasmussen and Turner, 2003: Polar Lows: Mesoscale Weather Systems in the Polar Regions
CLM22-sn, full field
0000 UTC 9 Dec 1993
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PLows in
CLM-results
The 1998
case:
NERC Dundee Satellite Receiving Station
Weather chart, 0100 UTC 18 Jan 1998
CLM01-sn, filtered
0000 UTC 18 Jan 1998
Woetmann Nielsen, N., Om forudsigelighed af polare lavtryk, Vejret, 20, 37–48, in Danish, 1998
CLM01-sn, full field
0000 UTC 18 Jan 1998
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PLows in
CLM-results
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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PLows in
CLM-results
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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Detectionalgorithm
Setup of the detection algorithm applied
1st step: detection of all locations with a filtered mslp
minimum < -1hPa
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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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 < ~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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Climatologymodel setup
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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Climatologytime series
Time series of the number of
Polar Lows per winter
• Strong interannual
variability
• No longterm trend visible
• Most active winter was
PLS 1981
• Fewest Polat Lows were
detected PLS 1964
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.
Zahn, M. and H. von Storch, 2008: A longterm climatology of North Atlantic Polar Lows, submitted to Geophysical Research Letters , 21 August,
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2008
Climatologysensitivity
Algorithm's
sensitivity
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Climatologyresemblance
Climatological comparison
C=0,72
C=0,58
Wilhelmsen, K., Climatological study of gale-producing polar lows near Norway, Tellus, 37A, 451–459, 1985.
Blechschmidt, A.-M., A 2-year climatology of polar low events over the nordic seas from satellite remote sensing, Geophys. Res. Lett.,
35 (L09815), doi:doi:10.1029/2008GL033706, 2008
Gunnar Noer, MetNo, Norwegian Meteorological Service, pers. comm.
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Climatologyresemblance
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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Climatologyregions
Regional aufgeteilt
Subregions, for which the number of detected polar lows were counted (R1-R14) and respective number of detected
mature state polar lows (see text). Difference in total number of polar lows (3313) and sum of polar lows in the
respective regions (3193) is due to polar lows occurring in the Southwest outside the investigated area. Also given
for each region are mean number of polar lows per year (Mean), standard deviation (sdev), slope of least squares fit
(lsqf) and polar lows per area unit (P/AU). The latter number is normalised by the smallest region in terms of surface
area, R6, and divided by the respective region's fraction of maritime boxes in the model.
Zahn, M. and H. von Storch, 2008: A longterm climatology of North Atlantic Polar Lows, submitted to Geophysical Research Letters , 21 August,
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2008
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
Y: gridded mean MSLP fields per PLS
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Climatologylarge scale
Links to large scale mean pattern
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Climatologylarge scale
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 southerly mean flow =>
more Polar Lows
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Thank you very much
Grazie per la vostra attenzione!!
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