地形激發對流影響侵台颱風運動之...

地形激發對流影響侵台颱風運動之位渦趨勢診斷分析

徐理寰郭鴻基

國研院台灣颱風與洪水研究中心

國立台灣大學大氣科學系

COLLABORATOR

R. G. FOVELL

UCLA

102年天氣分析與預報研討會

MOTIVATION

Slow moving typhoons with heavy rainfall often cause serious disaster to Taiwan.

Land rainfall amount is roughly proportional to the inverse of TCs’ translation speed over land. (Chien and Kuo 2011)

Chien and Kuo (2011)

Chang et al. (1993)

Cheung et al. (2008)

TC rainfall climatology over TaiwanRainfall phase locked with topography

82 typhoons, 22 surface stations

62 typhoons, 371 rain gauges

Maximum in windward side and

central mountain area郭等 (2001)

Wang et al. 2012

Topography phase locked convection and diabatic heating may affect the translation speed and modified typhoon tracks.

Slowdown of typhoon motion for northern landfall typhoons.

TC motion affected by DH

8/60 00 ★8/60 12 ▲8/70 00 ●8/70 12 ▼8/80 00 ■8/80 12 ◆8/90 00 ★8/90 12 ▲8/10 00 ●

● EC-YOTC

● JTWC

● CTRL

8/6 008/7 00

8/8 00

8/9 00

8/10 00

8/8 125 km/h or 1.4 m/s

CReSS

Influence of Latent heating to storm motion of Morakot (2009)

Difference in mean motion vectors

between CTRL and 25% qv due to LH

effect

Units: m s1

CTRLqv =25%

motion difference(LH effect)

Wang et al. (2012)

00-12 Z 8 Aug (leaving)

Morakot (2009) simulation

1960-2010 westward landfall typhoons (61) with continuous track

Hourly typhoon position from Typhoon database of CWB

Rainfall data from 21 CWB surface stations

LandfallDepartured0, Overland distance

080715 UTC080806 UTC

2009 Morakot

Estimation of overland translation speed

OBSERVATION RESULTS

51 years data 1960-2010

V0-1

T0

Rainfall

amount

Rainfall

amount Slow “northern landfall” typhoons are with heaviest rainfall amount

Speed criteria :

61 continuous track

typhoons mean translation

speed (6.2 m/s)± 1 std. (2.9

m/s)

77 % of slow moving

storms making landfall on

northern Taiwan (10/13).

60 % of fast cases making

landfall on southern

Taiwan (6/10).

Asymmetric distribution of typhoon

translation speed overland

Slow

Fast

Composite Rainfall

3-hourly mean translation speed variations

Discontinuous cases

Northern landfall

cases

Southern landfall

cases

Fast cases

Slow cases

Large rainfall cases

All subgroups speed up

before landfall

Speed up after landfall

Discontinuous cases

Southern landfall cases

Fast cases

Slow down after landfall

Northern landfall cases

Slow cases

Large rainfall casesN

S

The large scale

mean flow still

important

Experiment design :

1500 km x 1500 km domain

5 km horizontal resolution, 35 vertical levels

Uniform 3 m/s easterly flow

Lin et al. microphysics scheme

Jordan’s (1958) Caribbeanhurricane season sounding with fixed SST=29°C

Bogused Rankine Vortex Initial vortex Vm=50 m/s Rm=50 km

Taiwan topography (land free) water-crafted mountain

Modified WRF Ver. 3.1.1 experiment (Fovell and Su, 2007; Fovell et al., 2009,2010; Cao et al., 2011)

Rain difference(green)

TO

small part of domain

NUMERICAL EXPERIMENTS

Wu and Wang (2000), etcWavenumber 1

Symmetric PV

advected by WN1

PV tendency

Obtained by the least squares method

HA VA

SymmetricWavenumber 1 Wavenumber 2

DH

PV tendency

PV tendency equation of baroclinicand diabatic TC motion

t

t

t

t

t

DH ( Q/ z) is not necessarily positive where Q is positive, because we consider the WN1 component.

The level or vertical averaging depth makes a big difference.

turns &

speeds up

turns &

slows down

OC run

DH small because

heating not asymmetric

PV DH term

averaged over 3 h

and z=1-11 km

PV tendency analysis on TC motion- DH component along track

T3N run

PV DH term

averaged over

z=1-11 km


T3N run


PV DH term

averaged over

z=1-11 km

T3N run

PV DH term

averaged over

z=1-11 km


T3N FAR OFFSHORE

Q (diabatic heating) precip & winds

Q is quite symmetric, therefore DH is small.

Similar with OC

T3N APPROACH TO LANDFALL

Note DH points away from Q and subsidence in SW quadrant

Q precip & winds

T3N AFTER MOUNTAIN CROSSING

Very slow progress along W coast when DH strongly opposes motion;

Storm weakened but also expanded significantly

Q precip & winds

Vertical cross-section of Q and vertical velocity

Vertical cross-section of Q…and DH

DH

Fast Northern landfall Experiment (-5 m/s)

precip & windsQ

C= 4.0 m/s

DH=1.4 m/s

NORTHERN VS. SOUTHERN LANDFALL

DH speeds up southern

landfall typhoon… while over land

DIABATIC VS. PHYSICAL FORCING

DH test

startDiabatic

heating Q = 0

with DH

(control)

without DH

L = landfall

DH test

start


with DH

(control)

L = landfall

DH test

start

without DH


Summary

Rainfall is phase-locked with topography in Taiwan. Slow storms with very large rainfall.

77% (60%) of slow (fast) typhoons making landfall at northern (southern) Taiwan. The slow (fast) cases often with a slower (faster) pre-landfall speed and decelerate (accelerate) further during the landfall period.

A positive feedback of rainfall and typhoon overland translation speed.

Topography induced asymmetric diabatic forcing influences TC motionsignificantly when the mean flow is weak.

Slow down northwestward TC motion after mountain crossing

Speed up and turns the TC to south when approach to landfall

REFERENCE

Hsu, L.-H., H.-C. Kuo, and R. G. Fovell, 2013: On the geographic asymmetry of

typhoon translation speed across the mountainous island of Taiwan. J. Atmos. Sci.,

70,1006-1022.

~END~THANKS FOR YOUR ATTENTION !

地形激發對流影響侵台颱風運動之...

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