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First THORPEX Scientific Symposium (5-9 December 2004, Montreal,
Canada)
Structure, Morphology and Energetics of Polar Lows: A Numerical
Experiment
Wataru Yanase and Hiroshi Niino (ORI, Univ. of Tokyo)
Contents1.Introduction2.Experimental design3.Results
1)Dependence of morphology on baroclinicity
2)Structure and developing mechanism
4.Summary and future subjects20 DEC 2003 (MOBIS)
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Meso-scale lows around Japan islands
Subtropical cyclone
Meso-α-scale low
Polar low
Pacific Ocean
JapanSea
25N
30N
35N
40N
East China Sea
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Polar low on 21 January 1997
Fu et al. (2004, MWR)Yanase et al. (2004, MWR)
(Courtesy of Japan Meteorological Society)
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(Courtesy of Japan Meteorological Society)
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Numerical simulation (00JST 21- 00JST 22 JAN 1997)
MRI-NHM
Horizontal grid size =2km
Yanase et al.(2002, GRL)
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1.Introduction A variety of cloud patterns of polar lowsAn eye
& spiral bands Comma-shaped cloud
Nordeng & Rasumusen(1992) Rasumussen(1985) Reed & Duncan
(1987)
Rasumussen(1981)Businger & Baik(1991) Shapiro et
al.(1987)1000km
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Development mechanism of polar lows・CISK
Rasmussen(1979),Bratseth(1985), Okland(1987):
linear theory
Emanuel and Rotunno(1989): axisymmetric numerical simulation
・WISHE
・Baroclinic instabilityHarold and
Browning(1969):observationMansfield(1974), Duncan(1977), Reed and
Duncan (1987), Tsuboki and Wakahama(1992): linear theory
・CISK+Baroclinic instabilitySardie and Warner(1983): linear
theory
No nonlinear, nearly cloud-resolving, three-dimensional model
study in an idealized configuration has been performed.
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Purpose of the present study:・To examine if polar lows having a
variety of cloud patterns are reproduced in a numerical experiment
for an observed range of the environmental parameters.
・To understand how the morphology of a polar low depends on the
environment.
・To clarify the development mechanism and structure of polar
lows having different cloud patterns.
Method
Idealized numerical experiment using a three-dimensional
non-hydrostatic model that marginally resolves cumulus
convection.
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2.Design of the numerical experiment(based on Yanase and Niino,
2005, GRL, Vol.32, in press)
Numerical model: MRI/NPD-NHM(Saito et al., 2001)
Horizontal grid size: 2km or 5km
Vertical grid size : 40m~780m
Boundary conditionsx-direction: cyclic y-direction: free-slip
z-direction:
top: free-slip bottom: bulk
f-plane (70N)(Yanase, 2004, Doctor Thesis, Dept. Earth and
Planet.
Sci , The University of Tokyo)
method
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Vs=7m/s, rmax =20km
Basic state
Westerly flow in a thermal wind balance
SST is 10K higher than the temperature at the lowest level
(Yanase, 2004)
Axisymmetric initial vortex(in thermal wind balance)
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3. Results1)Time evolution of eddy kinetic energy on
baroclinicity
KE
Mx: Moist exp. with a vertical shear of x×10-3s-1
(Yanase and Niino, 2005, GRL, in press)
∆x=∆y=5km
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t=20h t=50ht=30h t=60h
M0
M1
M3
Dependence of morphology on baroclinicity
1000km
(Yanase and Niino, 2005, GRL, Vol.32, in press)
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Variety of cloud patterns of polar lows
Businger & Baik(1991)
Nordeng & Rasumusen(1992)
Rasumussen(1985) Reed & Duncan (1987)
Rasumussen(1981) Shapiro et al.(1987)1000km
Uz=1-2×10-3s-1 Uz=2-4×10-3s-1
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2)Structure and development mechanismi) No baroclinicity
case(M0)
Horizontal grid size=2km
t=70hr
(Yanase and Niino, 2005, GRL, Vol.32, in press)
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Tangential velocity (contour)Potential temperature (shade)
Radial-vertical cross-section of azimuthally-averaged
quantities
Radial velocity (green;1m/s) Vertical velocity (red;0.2m/s)
Relative humidity (shade)
(60-70hrs)
50 km100 150 200
50 km100 150 200
(Yanase and Niino, 2005, GRL, Vol.32, in press)
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Kinetic energy
Pressure anomalyCNTLR50
V2
R100
(Yanase, 2004)
(Yanase, 2004)
Dependence on the initial disturbance
R50: 2.5 times larger in size
R100: 5 times larger in size
V2: 3.5 times weaker
Consistent with Emanuel & Rotunno(1989)
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ii)Strong baroclinicity case(M3)T=50hrT=30hr
T=70hr
T=20hr
T=60hr
(Yanase, 2004)
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Evolution for random white noiset=20hr t=30hr t=40hr
t=50hr t=60hr t=70hr
M3( 0.5)θ∆ =
(Yanase, 2004)
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Comparison of dry and moist experiments
Dry Moist
Ps(cont.) , w (z=2880m)
m/s m/s(Yanase, 2004)
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(cont.),
(cont.),
Dry MoistZonal-vertical cross-section
p′
p′
θ ′
w
(Yanase, 2004)
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4. Summary1)A cloud pattern of a polar low depends principally
on baroclinicity (consistent with observation).
2) For weak baroclinicity, a nearly axisymmetric vortex with a
cloud-free eye and spiral bands develops.
CISK/WISHE.
Strong dependence on the initial disturbance.
3) For strong baroclinicity, a polar low with a comma-shaped
cloud pattern develops.
Baroclinic instability modified by latent heating.
Initial disturbances are not crucial.
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5. Future subjects
1)Initiation processes
Upper disturbances, topography, and barotropic instability
2)Effect of surface fluxes
Uniform flow, reverse shear and so on.
3)Understanding wide spectrum of meso-scale cyclones
・CISK/WISHE vs. Baroclinic instability
Tropical cyclone, subtropical cyclone, polar low, and
meso-α-scale cyclone
Structure, Morphology and Energetics of Polar Lows: A Numerical
ExperimentMeso-scale lows around Japan islandsPolar low on 21
January 1997Numerical simulation1.IntroductionDevelopment mechanism
of polar lowsPurpose of the present study:2.Design of the numerical
experimentBasic state3. ResultsDependence of morphology on
baroclinicityVariety of cloud patterns of polar lowsi) No
baroclinicity case(M0)Radial-vertical cross-section of
azimuthally-averaged quantitiesDependence on the initial
disturbanceii)Strong baroclinicity case(M3)Evolution for random
white noiseComparison of dry and moist experiments4. Summary5.
Future subjects
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