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)

Meso-scale lows around Japan islands

Subtropical cyclone

Meso-α-scale low

Polar low

Pacific Ocean

JapanSea

25N

30N

35N

40N

East China Sea

Polar low on 21 January 1997

Fu et al. (2004, MWR)Yanase et al. (2004, MWR)

(Courtesy of Japan Meteorological Society)

(Courtesy of Japan Meteorological Society)

Numerical simulation (00JST 21- 00JST 22 JAN 1997)

MRI-NHM

Horizontal grid size =2km

Yanase et al.(2002, GRL)

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

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.

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.

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

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）

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

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)

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

2)Structure and development mechanismi) No baroclinicity case（M0)

Horizontal grid size=2km

t=70hr

（Yanase and Niino, 2005, GRL, Vol.32, in press)

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)

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)

ii)Strong baroclinicity case（M３)T=50hrT=30hr

T=70hr

T=20hr

T=60hr

（Yanase, 2004)

Evolution for random white noiset=20hr t=30hr t=40hr

t=50hr t=60hr t=70hr

M3( 0.5)θ∆ =

（Yanase, 2004)

Comparison of dry and moist experiments

Dry Moist

Ps(cont.) , w (z=2880m)

m/s m/s（Yanase, 2004)

(cont.),

(cont.),

Dry MoistZonal-vertical cross-section

p′

p′

θ ′

w

（Yanase, 2004)

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.

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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