An approach to detect North
Atlantic Polar Lows in
Climate Mode Simulations
Matthias Zahn1,2, Hans v. Storch1,2, Stephan Bakan3
(1) University 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
• Preliminary results of detection
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: detect 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 (approx. two weeks prior to polar low formation)
• 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/
5
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
9
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
10
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
11
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
12
PLows in
CLM-results
Pattern correlation coefficients (PCC) between windspeed
from HOAPS and CLM
Without spectral nudging
With spectral nudging
∑i ai −abi −b
P a b=
∑i ai −a2 ∑i b˙i −b2
HOAPS: http://www.hoaps.zmaw.de/
13
PLows in
CLM-results
Concept of scale separation to
cut off mesoscale features
Response function - corresponding to
wave lengths from ~200 km – ~600 km
Feser, F., and H. von Storch, A spatial two-dimensional discrete filter for limited area model evaluation purposes, Mon. Wea. Rev.,
133(6), 1774–1786, 2005
14
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.
15
PLows in
CLM-results
The 1993
case:
Dundee Satellite Receiving Station
CLM22-sn, filtered
0000 UTC 9 OCT 1993
weatherchart, DWD, 0000 UTC 9 Oct 1993
CLM22-sn, full field
0000 UTC 9 Oct 1993
Rasmussen and Turner, 2003: Polar Lows: Mesoscale Weather Systems in the Polar Regions
16
PLows in
CLM-results
The 1998
case:
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
17
PLows in
CLM-results
Intermediate result:
•Principally, 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
Zahn, M., H. von Storch, and S. Bakan, 2007: Climate mode simulation of North Atlantic Polar Lows in a
limited area model, submitted to TellusA , 18 June 2007, in Revision
18
PLows in
CLM-results
Intermediate result:
•Principally, 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
Application in longterm Simulation
(Oct 1993- Sept 1995, 2005)
Zahn, M., H. von Storch, and S. Bakan, 2007: Climate mode simulation of North Atlantic Polar Lows in a
limited area model, submitted to TellusA , 18 June 2007, in Revision
19
Application
Application of Results (preliminary):
Detection algorithm, criteria: strong minimum, or:
1. minimum in filtered MSLP field
2. surrounding wind speed above 13.9 m/s
3. no synoptic low
4. also local minimum in untreated MSLP field
5. instable atmosphere:  SST −700hPa  0
mslp −700 hPa
and detected at least twice in two consecutive time steps (3
hours)
20
Application
Detected features according to these criteria
Simulation period: October 1993 until September 1995,
2005
Winters (ONDJFM)
Summers (AMJJAS)
Application
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.
22
Wilhelmsen, K., 1985: Climatological Study Of Gale-Producing Polar Lows Near Norway. Tellus, 37A, 451–459
Application
Comparison of detected and
counted Polar Low ocurrences
Blechschmidt, A.-M.; pers. comm.
Outlook
Final results (of this presentation)
• Principally, 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
Application of intermediate results in longterm
simulation
Temporal frequency distributions are similar to those
expected
● Reproduction of single cases bares differences between
detection and counting by eye
●
24
Thank you very much
25