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 • Polar Lows in LAMs
• Data
• Results
• Conclusions
• Example for application
Polar Lows
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Polar Lows
intense maritime mesoscale cyclones
typically several hundred kilometers in diameter
occurring poleward the Polar Front, October - March
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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instable atmospheric conditions:
baroclinic instability (shallow baroclinic zones)
(CISK) conditional instability of second kind
air-sea interaction instability
(barotropic instability)
form under cold upper level lows when cold arctic air
flows over a warm body of water (delta T > 35 °C)
cold air outbreaks
triggered by convection/sensible heat flux
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 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 CLM­data
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) 3664­3673. http://clm.gkss.de/
PLows in CLM­data
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
PLows in CLM­data
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
PLows in CLM­data
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
PLows in CLM­results
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
PLows in CLM­results
15.10., 6:00
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
PLows in CLM­results
15.10., 6:00
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
PLows in CLM­results
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
PLows in CLM­results
Pattern correlation coefficients (PCC) between HOAPS(b) and CLM(a)
1993 Oct
solid: sn , dashed: nn
P a b =
∑i  ai −a bi −b
  ∑i a i− a2 ∑i b˙i −b2
1998 Jan
1993 Dec
HOAPS, Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data, http://www.hoaps.zmaw.de/
PLows in CLM­results
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 two­dimensional discrete filter for limited area
model evaluation purposes, Mon. Wea. Rev., 133(6), 1774–1786, 2005
PLows in CLM­results
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.
PLows in CLM­results
The 1993 case:
Weather chart, DWD, 9.12.1993, 0:00
CLM22­sn, filtered
CLM22­sn, untreated
Rasmussen and Turner, 2003: Polar Lows: Mesoscale Weather Systems in the Polar Regions
PLows in CLM­results
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
PLows in CLM­results
CAPE in these Cases:
1993, Oct
1998, Jan
1993, Dec
PLows in CLM­conclusion
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
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. surrounding CAPE above 100 J/kg
and detected at least twice
Application
Detected features according to these criteria
Simulation period: October 1993 until September 1995
Winters (ONDJFM)
Summers (AMJJAS)
Application
Jan. 2002
Red dots: Norwegian Meteorological Service,
green dots: detected
Application
?
4.1.02, 4:00
5.1.02, 15:00
Application
29.1.02, 5:00
29.1.02, 15:00
Application
30.1.02, 4:00
31.1.02, 14:00
Outlook
Outlook / future plans
• More cases (differentiation of dynamical development/ forcing mechanisms/ size/ other parameters)
• [...]
• Simulation over 50 years
• Climatology
Thank you very much