Generation mechanism of strong winds in the left-rear quadrant of Typhoon MA-ON (2004) during its passage

over the southern Kanto district, eastern Japan

Wataru Mashiko

(Meteorological Research Institute)

Copyright of this photo; Yokohama Observatory (2004)

Typhoon MA-ON (2004)

Rapid traslation speed; 70 km/h 　　　 Central pressure at landfall; 950 hPa　Stationary front existed to the south of eastern Japan

Surface synoptic weather chart at 15 LST on 9 October 2004

Best track analysis Japan Meteorological Agency (JMA)Numerals on the right show MSLP (hPa)

Topography and geographical locations around the Kanto Plain, Eastern Japan

Mt.Tanzawa

Sagami Bay

Tokyo

Hiratsuka

Kanto Plain

Kan

to

Mo

un

tains

The solid contour interval of elevation is 500 m, with dotted lines of 50 m and 250 m.

The Kanto Mountains with more than 1000 m elevation run from north to south on the western side of Kanto Plain.

Surface wind and temperature during the passage of MA-ON

Wind barb and flag represent 5m/s and 25m/s, respectively (10 min average). Solid line shows the track of MA-ON. Broken line shows the elevation of 250 m.

18:00

16:00 17:00 17:30

19:00

Soundings at Tsukuba

•The strong wind of 38.4 ms-1 (1-min average) was recorded when the wind direction started changing from north to northwest, which indicates that location of Hiratsuka was shifted to the rear side of MA-ON.

•The cold air with north-easterly winds was below 950 hPa.

Surface time series of 1-min-average wind speeds, wind direction and pressure

05

1015

2025

3035

4045

14:00 16:00 18:00J ST［ ］

m/s

［］

975

980

985

990

995

1000

1005

hPa

［］

Wind SpeedSurface Pressure

270290310330350370390410430450

14:00 16:00 18:00

Win

d di

rect

ion

[deg

ree］

38.4m/s

17:00

N

E

W

Hiratsuka observatory (NIED)

What is the structure of landfalling MA-ON ?

What is the generation mechanism of the strong winds in the left-rear quadrant of MA-ON in spite of its rapid translation ?

Numerical simulation

Numerical model

2-way 2-nested mesh

Relationship of nestingDesign of model domains

Domain A B CDimension (x,y) 501x401 451x529 514x514Area coverage (km2) 3000x2400 900x1056 342x342Vertical levels 50 50 50Horizontal grid size (km) 6 2 0.667Time step (s) 18 9 3

2-way multi-nested movable mesh model (Mashiko and Muroi 2003) This model is based on operational nonhydrostatic model of Japan Meteorological Agency (JMA). Cold-rain explicit cloud microphysics ( no cumulous parameterization)

A

B Regional Fcst

A 24h

10/812UTC

2way

1way

12UTC00

B 18h

Regional Anal

10/9

Wind speed before and after landfall250 m Height 1000 m Height

14:36 LST

Before landfall

959 hPa

17:24 LST

After landfall

964 hPa

Strong winds on the left-rear side occurred at the low level after landfall.

Vertical cross section C-A

ＡＣ

Ａ

Ｃ

Ｃ

Ｂ

Vertical cross section A-B

Wind Speed 　（ Arrows show horizontal wind ）

EPT 　（ Arrows show horizontal wind ）　　

Ａ

Ｂ

Ａ

Ａ

Ｂ

Structure of landfalling MA-ON

•The flow of the cold air formed narrow channel between typhoon center and the mountain range.•The typhoon with high EPT moved over the cold air at the low level in the Kanto plain. •The strong wind area on the western side corresponds to the cold air quite well.

EPT (Equivalent potential temperature) at a height of 250 m

When the storm center moved to the Sagami bay, the strong winds on the left-rear side occurred. The strong winds correspond to the outflow of the cold air quite well.

Transition of wind velocity and potential temperature at a height of 250 m

Potential TemperatureWind Speed

Strong winds in the left-rear quadrant of the typhoon 　 (1724LST)

The strong winds occurred over the Sagami bay, which corresponds to the outflow from the channel-like cold flow formed by the mountain range and typhoon center.

Wind speed with SLP (contour) at a height of 250 m

Potential temperature at a height of 250 m

Sagami Bay

Trajectory analysisBackward; 48min, Forward; 12min, Starting time; 1724 LST

Marker interval 　３ min

OriginStarting at 240 m above the sea surfaceShaded in color is model orography.

The parcels moved along the eastern side of the Kanto Mountains and accelerated southward. After the parcels passed near Mt. Tanzawa, they descended with diffluent flows toward the Sagami Bay.

Mt. Tanzawa

Sagami Bay

TR-4

Time evolution of the height of isentropic (θ= 296 K ) with the trajectory of TR-4

When TR-4 accelerated descending and passed near Mt. Tanzawa at 1718 LST, large southward decline of isentropic exists there. (large southward PGF)

1712LST 1718LST 1724LST Dashed lines denote SLP with a contour interval of 4 hPa

Mt. Tanzawa

Generation mechanism of the strong winds

Horizontal momentum equation

)(1)(

cossin PGF others

amb

PGs

p

s

hdgdg

Fr Vk P1V

f Dt

D

　 　　

Acceleration Pressure gradient Corioli Friction (PGF)

PGslope PGdepth PGlarge d; potential temperature deficit, α; terrain slope, h; depth of the cold layer, θ0; average PT for cold layer

h

zdz

hd )(

10

Parish and Cassano (2003), Mahrt (1982)

PGslope; Cold, stably stratified air over sloping terrain

PGdepth; Variation in cold-layer thickness PGlarge; Large scale pressure gradient

Schematics of PGF effects

PGdepth

PGlarge

PGslope

Ground

Cold layerZ

GroundCold layerZ

Ground

Cold layer

θ1 ＜ θ2Z

or

Mou

ntain

s

L

H

Strait

PGslope; Cold, stably stratified air over sloping terrain

PGdepth; Variation in cold-layer thickness PGlarge; Large scale pressure gradient

Along wind components of the momentum equation of TR-4

- 4.00E- 02- 3.00E- 02- 2.00E- 02- 1.00E- 020.00E+001.00E- 022.00E- 023.00E- 024.00E- 025.00E- 02

16:36 16:48 17:00 17:12 17:24 17:36

TIME (J ST)

CO

MPO

NEN

TS (

m/s

**2) PGF

DV/ DtFriction

Shaded in color is model orography.

Mt. Tanzawa

Sagami Bay

Pre

ssur

e G

radi

ent

- 4.00E- 02- 3.00E- 02- 2.00E- 02- 1.00E- 020.00E+001.00E- 022.00E- 023.00E- 024.00E- 025.00E- 02

16:36 16:48 17:00 17:12 17:24 17:36

TIME (J ST)

CO

MPO

NEN

TS (

m/s

**2) Large

DepthSlope

PGF = PGlarge + PGdepth + PGslope TR-4 starts at 17:24 LST originating at 240 m above sea level.

Summary and Conclusion

The strong winds on the left-rear side of the storm occurred over the Sagami Bay, where is the exit of the channel-like cold flow at low level formed by the Mountain range and the typhoon center.

The strong winds are not only supported by the large-scale circulation, but also locally generated mesoscale forcing due to the variation in cold-layer thickness.

The dynamics and structure of the strong winds can be identified as those of “gap flow”.

Comparison between simulation results and observation at Hiratsuka

Hiratsuka observatory （ 139.346

E ， 35.306N ）

Model Result 　　　　（ 139.352E ， 35.282N

）

0

510

1520

2530

3540

45

15:30 16:00 16:30 17:00 17:30 18:00［J ST］

［m/s

］

965

970

975

980

985

990

995

1000

［hPa］

Wind SpeedCentral Pressure

270290310330350370390410430450

15:30 16:00 16:30 17:00 17:30 18:00

Win

d Direc

tion

[de

gree

］

05

10

15202530

354045

16:00 16:30 17:00 17:30 18:00 18:30[J ST]

m/s

［］

965

970

975

980

985

990

995

1000

hPa

［］

270290310330350370390410430450

16:00 16:30 17:00 17:30 18:00 18:30

Win

d D

irection [

degr

ee]

Sensitivity experiment ①; Orographic effectRemoving eastern part of the Kanto mountains

Model topography

Control Run

Wind speed at 250 m height( 17:24 JST )

SLP 963.6hPa

SLP 964.7hPa

The strong winds on the left-rear side decreased. (This experiment means that PGdepth and PGslope does not work.)

Sensitivity experiment

②; Effect of low-level cold air

The strong winds on the left-rear side decreased, and wind direction changed. (This experiment means that PGdepth, PGslope , PGlarge does not work.)

Changing the surface temperature to 26 ℃

PT

Wind speed

963.0 hPa963.6 hPa

Control Run

Vertical cross-section of wind speed and PT along the flow

The cold flow becomes shallower and accelerated to more than 60 m/s descending downward.