Climate mode simulation of North Atlantic Polar Lows in a limited area model
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) Max­Planck­Institut für Meteorologie (MPI­M), Germany
• Polar Lows
• Background/ topic of PhD • Paper/ results of 1st part
• Data
• Results
• Conclusions
• Outlook/ future plans
• (Further work)
Polar Lows
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Polar Lows
intense maritime mesoscale cyclones
typically several hundred kilometers in diameter
occurring poleward the Polar Front
strong winds (>13,9m/s), severe weather, heavy
precipitation
develop extremely rapidly
(reaching peak intensity within 24 hours)
Arctic Hurricanes
Rasmussen and Turner, 2003: Polar Lows: Mesoscale Weather Systems in the Polar Regions
Nordeng, T., and E. Rasmussen, A most beautiful polar low. A case study of a polar low development in the bear island region., Tellus, 44A, 81–99, 1991
Polar Lows
Polar Low
development
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form under cold upper level lows when cold arctic air
flows over a warm body of water
(delta T > 35 °C)
(shallow) baroclinic zones
triggered by convection/sensible heat flux
instable atmospheric conditions:
baroclinic instability
(barotropic instability)
(CISK) conditional instability of second kind
air-sea interaction instability
http://meted.ucar.edu/norlat/snow/polarlows/,
Rasmussen and Turner, 2003: Polar Lows: Mesoscale Weather Systems in the Polar Regions
Background
My thesis: Trends and variability in extratropical mesoscale cyclones
(VI EXTROP):
Long term aim: climatological description of polar low features
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 of my PhD: capability of LAM's to reproduce polar lows
1st Paper­data
Ensemble simulations (2x4) with CLM ( ~50km ) in climate mode for polar low cases Oct. 1993 (and Dec.1993, Jan. 1998)
• Initial times (approx. two weeks prior to polar low formation)
• Spectral Nudging (4x) and without (4x)
• Different waves (above appr. 700 km)
• Different strength
Hans von Storch, Heike Langenberg, and Frauke Feser, A Spectral Nudging Technique for Dynamical Downscaling Purposes, Monthly Weather Review 128(10) 3664­3673.
1st Paper­data
Spectral nudging
NCEP large scale > 850 hPa
CLM
Spatial scale Separation in regional climate modelling, PhD thesis Frauke Feser
1st Paper­results
Nielsens (1997) case, Oct. 1993 synoptic scale low, “the Swan”, shallow baroclinic processes combined with upper level vorticity advection
Nielsen,1997: An early Autumn polar low formation over the Norwegian Sea, J.Geophys. Res., 102, 13955­13973
1st Paper­results
Nielsens (1997) case, Oct. 1993 Mature state polar low: reaching peak intensity, wind speeds above gale force, closed small scale surface low Nielsen,1997: An early Autumn polar low formation over the Norwegian Sea, J.Geophys. Res., 102, 13955­13973
1st Paper­results
Nielsens (1997) case, Oct. 1993 decaying quickly after landfall
Nielsen,1997: An early Autumn polar low formation over the Norwegian Sea, J.Geophys. Res., 102, 13955­13973
1st Paper­results
15.10., 6:00
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, 6:00 UTC, DWD analysis data, 6:00 UTC, CLM ensemble run without (nn) and with(sn) spectral nudging , 9:00 UTC
1st Paper­results
NCEP
CLM01­nn
CLM02­nn
DWD
CLM03­nn
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, 6:00 UTC, DWD analysis data, 6:00 UTC, CLM ensemble run without (nn) and with(sn) spectral nudging , 9:00 UTC
CLM04­nn
1st Paper­results
NCEP
DWD
CLM01­nn
CLM02­nn
CLM03­nn
CLM04­nn
CLM01­sn
CLM02­sn
CLM03­sn
CLM04­sn
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, 6:00 UTC, DWD analysis data, 6:00 UTC, CLM ensemble run without (nn) and with(sn) spectral nudging , 9:00 UTC
1st Paper­results
Track of polar low's local sea level pressure minimum in CLM­sn
CLM01­sn
CLM02­sn
CLM03­sn
Start: 14.10 15:00 until 18.10. 0:00
CLM04­sn
1st Paper­results
CLM­HOAPS
CLM01­sn
Mean diff = ­1.96 m/s
std. dev = 3.27 m/s
Difference maps for 10m wind speed as provided by CLM with spectral nudging (CLM01­sn) and without spectral nudging (CLM01­nn and CLM02­nn) compared to HOAPS­estimates of surface wind speed on 15 October 1993, 0­12 UTC.
1st Paper­results
CLM­HOAPS
CLM01­sn
Mean diff = ­1.96 m/s
std. dev = 3.27 m/s
CLM01­nn
Mean diff = 0.02 m/s
std. dev = 4.02 m/s
CLM02­nn
Mean diff = ­0.44 m/s
std. dev = 3.51 m/s
Difference maps for 10m wind speed as provided by CLM with spectral nudging (CLM01­sn) and without spectral nudging (CLM01­nn and CLM02­nn) compared to HOAPS­estimates of surface wind speed on 15 October 1993, 0­12 UTC.
1st Paper­results
Pattern correlation coefficients (PCC) between HOAPS(b) and CLM(a)
∑  a −a b −b
P b =
  ∑ a −a ∑ b˙ −b 
1993 Oct
1998 Jan
1993 Dec
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1st Paper­results
Concept of scale separation to cut off the meso scale Response function ­ corresponding to wave lengths from ~200 km – ~600 km
Feser, F., and H. von Storch, A spatial two­dimensional discrete filter for limited area
model evaluation purposes, Mon. Wea. Rev., 133(6), 1774–1786, 2005
1st Paper­results
15.10., 6:00
NCEP
DWD
CLM01­nn
CLM02­nn
CLM03­nn
CLM04­nn
CLM01­sn
CLM02­sn
CLM03­sn
CLM04­sn
Band­pass filtered mslp (isolines; hPa) and 10m wind speed anomalies, NCEP/NCAR analysis after interpolation onto the CLM grid, 6:00 UTC, DWD analysis data, 6:00 UTC, CLM ensemble run without and with spectral nudging, 9:00 UTC. Black dots indicate the positions of the polar lows pressure minimum in the respective untreated field of the ensemble run.
1st Paper­results
The 1993 case:
Weather chart, 9.12.1993, 0:00
CLM22­sn, filtered
CLM22­sn, untreated
Rasmussen and Turner, 2003: Polar Lows: Mesoscale Weather Systems in the Polar Regions
1st Paper­results
The 1998 case:
Weather chart, 18.1.1998, 1:00
CLM01­sn, filtered, at 0:00
CLM01­sn, untreated, 0:00
Woetmann Nielsen, N., Om forudsigelighed af polare lavtryk, Vejret, 20, 37–48, in Danish, 1998
1st Paper­results
CONCLUSION:
• 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
Outlook
Outlook / future plans
• (More cases)/ differentiation of dynamical development/ forcing mechanisms • Longer (1 year) simulation time
• How many Polar Lows are reproduced
• Comparison with HOAPS/ KLEPP (pers. comm.)
• [...]
• Simulation over 50 years
• Climatology
Thank you very much