Kan latent varmefrigjøring ha negativ effekt på intensiteten av polare

Can latent heat release have a negative
effect on polar low intensity ?
Ivan Føre, Jon Egill Kristjansson, Erik W. Kolstad,
Thomas J. Bracegirdle and Øyvind Sætra
Institutt for Geofag, Avdeling for Meteorologi og Oseanografi
Polar lows:
are intense mesoscale cyclones (200-1000km)
are short-lived (<24 hours)
are associated with wind speed 15 m/s
develop in the cold air outbreak of Arctic air masses during
winter time
  occur mainly over ocean, polewards of the main mid-latitude
baroclinic zone
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o  Given low coverage of observations by the conventional
meteorological observation network in the Nordic seas polar
lows are often poorly predicted
Severe weather risk in polar and sub-polar waters
Institutt for Geofag, Avdeling for Meteorologi og Oseanografi
NOAA AVHRR Infra red (CH4 ) satellite image of our
polar low case at 0200utc 20 December 2002
Sea ice
Spitsbergen
Objective: To contribute to an improved
understanding of the role of condensational
heating
inAvdeling
“hurricane-like”
polar lows
Institutt for Geofag,
for Meteorologi og Oseanografi
Russia
Outline
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Model description
Control run
Sensitivity experiments
Summary and conclusions
Institutt for Geofag, Avdeling for Meteorologi og Oseanografi
Weather Research and Forecast (WRF) Version 2.2.1
•  National Center for Atmospheric Research (NCAR, USA)
•  51 vertical levels
•  9 and 3 km mesh grids
•  ECMWF (25km analysis) is used as initial and boundary condition
•  Run time: 00 UTC 17 to 12 UTC 21 December 2002
•  14 sensitivity experiments
Physical Process
Parameterization
Cloud microphysics
Thompson et al. scheme
Moist convection
Planetary Boundary Layer
processes
Betts-Miller-Janjic scheme
Land Surface
Noah Land Surface model
Surface layer processes
MM5 similarity
Longwave radiation
RRTM scheme
Shortwave radiation
Dudhia scheme
Yonsei University scheme
Institutt for Geofag, Avdeling for Meteorologi og Oseanografi
Sea level pressure (hPa, black contours), 10-m wind speed (ms-1)
and 2-m temperature (ºC, red contours) for the control run
Spin-up stage
975 hPa
Max wind speed ~20m s-1
Cold air outbreak
Institutt for Geofag, Avdeling for Meteorologi og Oseanografi
Mature stage
965 hPa
Max wind speed ~25m s-1
Hurricane-like appearance
SH fluxes (Wm-2)
Mature stage
extreme sensible heat fluxes
~ 1100 Wm-2
sensitive to variations in fluxes
Institutt for Geofag, Avdeling for Meteorologi og Oseanografi
LH fluxes (Wm-2)
Mature stage
extreme latent heat fluxes
~ 450 Wm-2
sensitive to variations in fluxes
East-West cross section of temperature (2ºC intervals,
red contours), condensational heating rate (Kh-1, black contours) and
wind speed (ms-1, shaded colour)
Institutt for Geofag, Avdeling for Meteorologi og Oseanografi
Sensitivity experiments on latent heat
release
Minimum surface pressure (hPa) versus time
NO condensational heating
Control run
NO condensational heating
Control run
Spin-up stage
Mature stage
Institutt for Geofag, Avdeling for Meteorologi og Oseanografi
North-South cross section of temperature (2ºC intervals,
red contours), condensational heating rate (Kh-1, black contours) and
wind speed (ms-1, shaded colour)
Control run
Upper level low at 4000-8000 m
Close to the sea ice edge
More stable air masses
Shallow development
Institutt for Geofag,
Avdeling
for Meteorologi
og Oseanografi
Larger distance
between
the lows
-LHatm
Upper level low at 3000-8000 m
Further south
Less stable air masses
Deeper development
Shorter distance between the lows
North-South cross section of potential vorticity
(PVU, shaded colour)
Control run
UPV anomaly (> 3 PVU) at 2000m height
Stratosphere (~2 PVU) at 3000m height
PV destruction above the eye
LPV anomaly (4 PVU)
Institutt for Geofag, Avdeling for Meteorologi og Oseanografi
-LHatm run
UPV anomaly (> 3-5 PVU) at 3000m height
Stratosphere (~2 PVU) at 3000m height
No PV destruction above the eye
Weaker LPV anomaly (~3 PVU)
Conceptual model showing phase lock between
upper level low (i.e. UPV anomaly) and the polar
low (i.e. LPV anomaly) with and without latent
heat release
Control run
(North)
-LHatm run
(South)
Upper level cold low (~ -41ºC at 500 hPa)
Upper level cold low (~ -43ºC at 500 hPa)
Polar low
Institutt for Geofag, Avdeling for Meteorologi og Oseanografi
Polar low
Summary and Conclusions
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What's different from other hurricane-like polar lows?
–  unusually strong UPV anomaly (i.e., upper level low)
–  continuous strong upper level forcing
–  extremely high surface fluxes
• 
Experiments omitting condensational heating (-LHatm) relative to
experiments including its effect (C):
–  their position further south deepens their development
–  vertical alignment between UPV anomaly and polar low (unnatural)
–  stronger upper level forcing present at all times
»  results in more intense polar low
• 
Condensational heating can be negative for the “hurricane-like” polar lows developing
close to the sea ice edge
–  Condensational heating moves polar low developments north
–  The positive effect of condensational heating is less than the weakening of net surface
fluxes entering the polar low system
Institutt for Geofag, Avdeling for Meteorologi og Oseanografi
Acknowledgement
•  Wrfhelp (WRF)
•  Gunnar Wollan, UIO, MetOs (WRF, matlab)
•  Greg Thompson, NCAR (Discussion, New MP
scheme, WRF)
•  Simen Gaure, UIO, USIT (WRF)
•  Bjørn Egil Nygård, UIO, met.no (Discussion, WRF,
matlab)
•  Øyvind Hodnebrog, UIO, MetOs (WRF, matlab)
•  Students and colleges, UIO, MetOs (Discussion)
Institutt for Geofag, Avdeling for Meteorologi og Oseanografi
• Thank you for your attention!
• Questions?
Institutt for Geofag, Avdeling for Meteorologi og Oseanografi