地形激發對流影響侵台颱風運動之...
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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
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.
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
PV tendency analysis on TC motion- DH component along track
T3N run
PV tendency analysis on TC motion- DH component along track
PV DH term
averaged over
z=1-11 km
T3N run
PV DH term
averaged over
z=1-11 km
PV tendency analysis on TC motion- DH component along track
T3N FAR OFFSHORE
Q (diabatic heating) precip & winds
Q is quite symmetric, therefore DH is small.
Similar with OC
T3N AFTER MOUNTAIN CROSSING
Very slow progress along W coast when DH strongly opposes motion;
Storm weakened but also expanded significantly
Q precip & winds
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.