Development of a longterm climatology of North Atlantic polar lows using a RCM
Matthias Zahn 1,2 , Hans v. Storch 1,2
(1) University of Hamburg, Meteorological Institute, Germany
(2) GKSS Research Centre, Institute for Coastal Research, Germany

Polar Lows mesoscale (< 1000 km) sized maritime storms
S p it zb er g en i ntense/ strong winds (>13.9 m/s) severe weather occur poleward the Polar Fronts in both hemispheres during winter typically induced by disturbances in the air flow typically driven by convective processes
~300 km
S ca n dina via
© Dundee Satellite Receiving Station

Climatology of Polar Lows
Comprehensive measurements are required to investigate long-term trends or changes of polar lows:
• 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
=> downscaling of reanalysis data with a RCM

Worksteps of my PhD
1 st : Can polar lows be reproduced by
RCMs ?
2 nd : Detect polar lows automatically in RCM output data
3 rd : Compilation of the Climatology

Ensemble Simulations for three
Polar Low cases in Climate Mode
NCEP/NCAR re1
(~200 km)
CLM 2.4.6
(~50 km)
Oct 1993, Dec 1993, Jan 1998
Initialised: approx. 2 week prior to polar low formation
With spectral Nudging (4x) and without (4x)
(v. Storch et al. 2000)

Oct 1993 case
S p itzb er g en
S ca n d in av ia
15 Oct 93, 05:24
© Dundee Satellite Receiving Station

Mean Sea level pressure (hPa) and
10m wind speed
15 Oct. 1993,
6:00
Dundee
15.10.93, 05:24
NCEP DWD

Mean Sea level pressure (hPa) and
10m wind speed
15 Oct. 1993,
6:00
NCEP DWD
Dundee
15.10.93, 05:24
CLM01nn CLM02nn CLM03nn CLM04nn
CLM01sn CLM02sn CLM03sn CLM04sn

Bandpass filterted mslp (hPa)
15 Oct. 1993,
6:00
Scales between
200-600 km are retained

Dec 1993 case
NCEP
Weatherchart, DWD
9 Dec 1993, 0:00
Greenland
Iceland
Dundee
9.12.93, 16:00
CLM22-nn CLM22-sn CLM22-sn, filtered

Jan 1998 case
NCEP
Berliner
Weatherchart
CLM01-nn
18 Jan 1998, 0:00
Dundee
18.1.98, 4:00
CLM01-sn
S ca nd in av ia
CLM01-sn, filtered

Detection algorithm
1 st : detection of all locations with a minimum in the filtered mslp field < -1hPa
(mesoscale signature)
2 nd : combine detected positions to individual
tracks
3 rd : checking further constraints along the tracks
=> application to longterm simulation (1948-2006)

Tracks of three Polar Lows
Reproduced and detected even after a simulation time of 40 years

Frequency of Polar Lows

Frequency of Polar Lows
Max:100
Mean:56
Min:36
σ=13

Climatological comparison
Number of polar lows per PLS.
Zahn and v.Storch (2008) (red) and observations (black) of MetNo (Noer, pers.comm)

Results
RCMs (CLM) principally are capable of reproducing polar lows without nudging the large scale, the formation of Polar Lows is subject to considerable ensemble variability a long-term trend in polar low activity could not be found strong inter annual variability

Thank you very much for your attention
Ensemble simulations: Zahn, M., H. von Storch, and S. Bakan (2008),
Climate mode simulation of North Atlantic Polar Lows in a limited area model, Tellus A,60,620 - 631, doi:10.1111/j.1600-0870.2008.00330.x
Tracking algorithm: Zahn, M.
and H. von Storch (2008) Tracking Polar
Lows in CLM, Meteorologische Zeitschrift,17(4), 445 - 453, doi:10.1127/0941-2948/2008/0317
Polar low climatology: Zahn, M.
, and H. von Storch (2008), A longterm climatology of North Atlantic polar lows, Geophys. Res. Lett., 35,
L22702, doi:10.1029/2008GL035769
Homepage: http://coast.gkss.de/staff/zahn/

Bracegirdle, T. J. and S. L. Gray, 2006: