Taiwan’s Effort on Meiyu Heavy Taiwan’s Effort on Meiyu Heavy Rainfall ProblemsRainfall Problems

George Tai-Jen ChenGeorge Tai-Jen Chen（陳泰然）（陳泰然）

Chair Professor/Distinguished Professor, Department of Atmospheric Sciences National Taiwan UniversityNational Taiwan University

June 28, 2012, 09:0009:00 –09:45 –09:45 University of Hawaii at ManoaUniversity of Hawaii at Manoa

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Ι. Heavy Rainfall Problems in a Changing Society of Taiwan

II. National Effort on Disaster Prevention Research III. Taiwan’s Research Effort on Meiyu Heavy Rainfall

IV. Recent Research Effort on Meiyu Frontal Systems

V. Climate Change and Taiwan’s Meiyu 2

Ι. Heavy Rainfall Problems in a Changing Society

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4

Heavy Rain/Flash Flood Event of May 28, 1981

0700 LST May 28

0800 LST May 28

Hourly and 3-h rainfall amounts at some stations in northern Taiwan for extremely heavy rain of May 28, 1981

Time Stations Rainfall Amount5 AM6 AM7 AM8 AM9 AM

10 AM11 AM

TaoyuanChungliChungliChungliChungliHsinchuMingteh

51.8 mm50.5 mm51.3 mm50.5 mm51.2 mm88.7 mm64.4 mm

4-6 AM7-9 AM

10-12 AM

TaoyuanChungliHsinchu

107.2 mm153.0 mm143.5 mm

Effect and size of meteorological disaster in a changing society: • agricultural era industrialized era• Heavy rainfalls and severe floods of May 28,1981 caused a loss of 300 M US $

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6Meiyu in southern China and Taiwan (mid-May – mid-June)

Meiyu in East Asia

7Japan Baiu (late-May – late-June)

8Meiyu in Yangtze River Valley (mid-June – mid-July)

9Korea Changma (mid-July – mid-August)

II. National Effort on Disaster Prevention Research

1980 1990 2000 2010

1982

1997

2003

2007

NSC Disaster Prevention Research Program

Disaster Prevention Research National S&T Program

National Center for Disaster Reduction Research(NCDR)

APEC Center for Typhoon and Society

Taiwan Typhoon and Flood Research Institute

10

F

C

CF

CT

S

　f

B

T

BTCPF

1

)1(

BTB

BT

F

CPA

)1(

)1(

Threat Score:

Bias:

Prefigurance:

Postagreement:

What is the current capability of heavy rain forecast ? Why?

The illustration of Threat Score. F is the forecast, is the observation, C is the correct forecast.

III. Taiwan’s Research Effort on Meiyu Heavy Rainfall

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TS PF PA

Typhoon 0.60 0.68 0.85

Meiyu 0.17 0.20 0.57

• Synoptic-scale process v.s. Mesoscale process

• Lack of basic understanding of the mesoscale process responsible for the low TS and PF of heavy rain in Meiyu season.

Heavy rain forecast capability of CWB

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1970 1980

TAMEX Field Phase

2000 20101990

National Conference on Disastrous Weathers in Taiwan

Post-TAMEX Forecast Experiment

TIMREXField Phase

1978

1987

1992 2008

Response of meteorological community to Meiyu heavy rainfall

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1.National Conference on Disastrous Weathers in Taiwan

Typhoon, drought, cold surge, and Meiyu were identified to be the 4 major disastrous weathers in Taiwan. Research focus on these phenomena was suggested and then became NSC policy.

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2. TAMEX (1983-1993)

• Goal: To improve, through better understanding , the forecasting of heavy precipitation events that leads to flash floods

• Scientific Objectives: 1) To study the mesoscale circulation associated with the Meiyu front; 2) To study the evolution of the mesoscale convective systems (MCSs) in the vicinity of the Meiyu front; 3) To study the effects of orography on the Meiyu front and on mesoscale convective systems.

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USA Taiwan

Universities

1. Colorado State U. 2. Florida State U.3.North Carolina State U.4. Oklahoma U.5. Purdue U.6. St. Louis U.7. Yale U.8. U. Alabama9. U. Hawaii10. U. Washington

Universitiesand Colleges

1. National Taiwan University2. National Central University3. Chinese Culture University4. School of Communication and Electronics, Air Force

Government Agencies

5. Central Weather Bureau (CWB)6. Civil Aeronautics Administration7. Air Force (Weather Wing, Weather Center)8. Navy (Weather Center)9. TaiPower10. Water Resources Agency/Provincial Government11. Shih-men Reservoir Administration Bureau/Provincial Government12. Tseng-wen Reservoir Administration Bureau/Provincial Government13. National Freeway Bureau/Ministry of Transportation and Communications14. Energy and Minerals Agency/Ministry of Economic Affairs15. Fishing Training Center/Council of Agriculture

Research Institutes

11. NCAR12. Naval Research Lab.13. NOAA

• Participants in TAMEX Field Phase （ May 1-June 30, 1987）

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USA ： 70 scholars and experts from 10 universities and 3 research institutions.Taiwan ： 80 scholars and experts and 1000 professional technicians from 4 universities /colleges and 11 government agencies.

• Human Resources Mobilized in Field Phase

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• Important Events Prior to the Field Phase of TAMEX

1981 1982 1983 1986

May 28, 1981 Heavy rainfalls and severe floods caused a loss of 300 M US $

Disaster Prevention Research Program was pushed by the NSC

TAMEX project was proposed to NSC

Planning Stage of TAMEX (1983–1986)Taiwan ： 40 experts and scholars from 5 academic institutions and 3 meteorological operational agencies (CWB, CAF, and CAA)USA ： 50 professors, scientists and experts

from 15 universities and 4 research institutions

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• Important Events of the Follow-up Basic Research of TAMEX from 1988

1993

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1988 1989 1990 1991 1992

9–11 Februarysymposium@NCAR

24–26 Septembersymposium@NCAR

22–30 Junesymposium@Taipei

November A special issue of TAMEX research was published in Mon. Wea. Rev.

3-6 December International Symposium on Mesoscale Meteorology and TAMEX@Taipei

December A special issue of TAMEX research was published in TAO

26–30 April A Retrospective Symposium on Mesoscale Research and TAMEX Project@Taipei

3. Post-TAMEX Forecast Experiment: Important Events of the Follow-up Applied and Operational Researches of TAMEX from 1988

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1989 1990 1991 1992

November 22 Planning group of Forecast Experiment was established.December 15 Project Office of Forecast Experiment was established.December 30 8 working groups of Forecast Experiment were established (60 professors/experts).

February 26 – March 3 Taiwan/USA Planning Meeting (I)@Taipei.May 14 working groups and training team were re-integrated.December 17 a 6-person advising team was established; working groups

were expanded to 10 (80 professors/experts).

May 1–June 30 Post-TAMEX Forecast Experiment was conducted using the Weather Integration and Now casting System(WINS) established by CWB

April 22–23 & May 1–3 Taiwan/USA Planning Meeting (II)@Taipei.May 19–June 20 Pilot experiment @Taipei.June 25 advising team meeting @NCARDecember 7–10 Taiwan/USA Planning Meeting (III)@Heng-chun

• Goal: Application of results obtained through basic and applied researches in TAMEX program, to improve the forecasting capability of the short-range and nowcasting of heavy rain.

• Objectives:1) To establish the new forecasting concept in the mesoscale

forecast system.2) Using the newly established WINS of the CWB and the new

forecasting techniques obtained through TAMEX, to improve the short-range forecast and nowcasting capabilities of heavy rain and quantitative precipitation.

3) Constructing the base line of nowcasting and short-range forecast, to provide the reference for the future forecast improvement.

4) To test the forecast capability of different forecasting methods for heavy rain and quantitative precipitation in the 0-24 h forecast period.

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4. Taiwan WRP (2000-2010): TIMREX (SoWMEX;TAMEXII) May-June 2008

• Goal:

To improve the capability and accuracy of the QPE and QPF (within 24-36 hours) in county city and/or watershed scales during the prevailing southwesterly monsoonal flow to meet the urgent need of disaster reduction in the Taiwan area

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• Scientific Objectives:1) Dynamic and thermodynamic characteristics of southwesterly monsoonal flow upstream of Taiwan and its relation with the Meiyu front and the formation, organization and maintenance of the MCSs and their downstream development (SW monsoon, Meiyu front, environmental characteristics)

2) Kinematic, thermodynamic, and microphysical precipitation characteristics of MCSs and the precipitation mechanisms for heavy rain (storm and cloud dynamics and microphysics)

3) Taiwan coastal and topographic effects on the impinging SW flows and on the intensifying and/or suppressing the development of MCSs (Topographic effect)

4) Radar data assimilation and short term QPF experiment (NWP development)

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• Participants in TIMREX

USA Others

1. U. Washington2. UCLA3. U. Utah4. U. Hawaii5. North Carolina State U.6. CSU7. U. Oklahoma8. NOAA9. NASA10.NCAR

Canada (U. McGill)Japan (Nagoya U.)Korea (Seoul National U.Puking U. Kingpei U.) Australia (Weather Bureau)Philippines (PAGASA)

IV. Recent Research Effort on Meiyu Frontal Systems

Low Level Jet (LLJ)• Formation mechanism• Relationship with extremely heavy rainfall

Meiyu front

Frontogenesis Cyclogenesis

Deformation CISK

Deformation

Baroclinicity

Movement

Strong(large ▽T; large ζ, q, ) Weak(small ▽T; large ζ, q, )

Dynamically(propagation)

Kinematically(advection)▽T↑ζ↑q↑

CISK

yV

yV

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Case 1: 12-13 June 1990(Chen et al. 2003, Mon. Wea. Rev., 2680-2696)

(a)

850 hPa weather map and PV at 12Z 12 June

Wind shear and PV (10-2 PVU) accompanying the front.

Mei-Yu frontogenesis

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(b)

850 hPa weather map and PV at 00Z 13 June

PV along the front significantly increase (frontogenesis) with a LLJ formation to the south of the front during the 12 h.

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PV inversion techniques (Davis and Emanuel 1991, Mon. Wea. Rev.)

• PV: conserved property and invertibility.

• Nonlinear balance equation (Charney 1962, Proc. Symp. Numerical Weather Prediction, Tokyo)

• Given a known distribution of PV and specified boundary conditions, the system can be solved to give height and wind fields under nonlinear balanced relationship.

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2

2

2

2

242

cos

2

af

22

4

22

242

22 1

cos

1)(

aaf

p

gq

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• Piecewise inversion The PV anomalies can be divided into any number of

parts and the height and the wind field associated with each part can be obtained.

• Prognostic system q/t, /t, /t, , and under nonlinear

balanced condition can be obtained.

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Scheme for q’ partitioning and contributions to frontal intensity from all processes at 850 hPa

PV anomaly (109.125-117E; 29.25-30.375N ) associated with latent heat release (ms) were responsible for the

frontogenesis.

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GMS IR imagery and vertical motion as computed by PV prognostic system along AB at

00Z 13 June

A B

ww

Upward motion (cm s-1) computed by prognostic system was closely matching the position of deep convection on

cloud imagery.

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PV tendency and height tendency as computed by PV prognostic system along AB at 00Z 13 June

Positive PV tendency and negative height tendency (frontogenesis) at low level were related to the MCSs.

qq//tt //tt+

-

-

+

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Mei-Yu fronotogenesis by CISK

• q/t is directly proportional to both the vertical gradient of heating/cooling rate and the absolute vorticity.

• In a quasi-barotropic system, the vertical component of η is rather close to q.

• q/t is proportional to q → nonlinear interaction.

FηV

)(*

dt

d

p

gqq

t

qh

36

qq//ttw

Similar vertical motion pattern with much less PV generation at the low level.

If ms is reduced by ½ at 00Z 13 June

-

+

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• PV perturbations related to latent heat release from MCSs were responsible for the frontogenesis.

• CISK mechanism proposed by Cho and Chen (1995) was observed to be responsible for the Mei-Yu frontogenesis.

Conclusion

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Case 2 : 7-8 June 1998 (Chen et al. 2006, Mon. Wea. Rev., 874-896)

• Although this phenomenon is not rare, the mechanism has never been investigated.

Northward retreating Mei-Yu front

Formation of LLJ

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GMS IR images

Frontal cloud band with an organized MCS over the frontal

disturbance moved northeastward.

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Synoptic maps at 850 hPa between 12Z 7 and 06Z 8 June

Trough deepened in association with the organized MCS, and the southwesterly winds intensified (LLJ formation)

to the south of the MCS.

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Composite vorticity and divergence at 925 and 850 hPa normal to and across the MYF (at 0) during 12Z 7 - 06Z 8

June

Vorticity in phase with convergence.

Comparable values of vorticity at both levels.

Nearly no vertical tilt.

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Effect of horizontal vorticity advection (10-5 s-1(6h)-1) mainly caused the northward retreat of the front. (The vital role of the LLJ

to the southwest of the front.)

Retreat of the front

Time variations of vorticity budget

across the front at 850 hPa

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Scheme for q’ partitioning and contributions to frontal intensity at 850 hPa from all processes

PV anomaly associated with latent heat release (LLh) were mainly responsible for the frontogenesis.

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12Z 7 June12Z 7 June

00Z 8 June00Z 8 June

18Z 7 June18Z 7 June

06Z 8 June06Z 8 June

LLJ formed and intensified largely through the Coriolis acceleration of ageostrophic winds( z). ( // z shaded) T

tV

The formation of LLJ: ageostrophic wind analysis

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The formation of LLJ: PV perspective

• Front intensified through latent heat release.

• LLh caused the increase of southwesterly wind components to the southeast of the MCS.

PV anomaly due to latent heating (LLh) and the associated (inverted) balanced winds at 850 hPa

12Z 7 June12Z 7 June

00Z 8 June00Z 8 June

06Z 8 June06Z 8 June

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When southwesterlies associated with LLh are superimposed upon the background SW monsoonal flows

→ LLJ formation.

Wind vectors averaged over a hexagonal domain centered along the axis of the LLJ from different PV anomaly components at 00Z 8 June

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• Strong southwesterly flow (LLJ) led to rapid retreat of the front while the movement was dominated by horizontal advection.

• Enhanced gradient of height tendency induced ageostrophic winds, and the LLJ formed through Coriolis acceleration of these winds.

Conclusion

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Case 3: 8-14 June 2000 (Chen et al. 2007, Mon. Wea. Rev., 2588-2609)

(a) (b) (c)

Thick dashed lines indicate the position of 925-hPa Mei-Yu front based on temperature gradient and winds.

Mei-Yu frontogenesis and frontal movement

The thermal gradient of Mei-Yu front increased from 8 June to reach a maximum at 1200 UTC 10 June then remained quite

strong until after 12 June 2000.

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(a) 2000 Jun 8 00Z

(b) 2000 Jun 10 12Z

(c) 2000 Jun 13 00Z

Using 2-D frontogenetical function of Ninomiya (1984).

Formation stage

intensification stage

decaying stage

F: frontogenetical function (d|H |/dt)

Contributing terms:FG1: diabatic processes;FG2: horizontal convergence;FG3: deformation; GT: magnitude of horizontal potential temperature gradient (|H |)

The Mei-Yu frontogenesis and the maintenance of the front were

attributed to both deformation and convergence.

S N

S N

S N

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HHV

-

Movement of the Mei-Yu front

: GT, the distribution of frontal strengthH

: F, frontogenetical functiondtd H

: LT, local tendency of HtH

: ADV, horizontal advection of H

The total frontogenetical function (F) that peaked ahead of the frontal zone

contributing toward the southward propagation of the Meiyu front, in

addition to the transport by advection of the postfrontal cold air.

(a) 2000 Jun 8 00Z

(b) 2000 Jun 10 12Z

(c) 2000 Jun 13 00Z

S N

S N

S N

LT = F + ADV （ frontal motion: phase difference between LT and frontal zone）

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• The frontogenetical function calculation at 925-hPa indicated that the intensification and maintenance of the Mei-Yu front were attributed to both deformation and convergence, and the former was usually slightly stronger.

• Meiyu frontal movement was contributed by the southward frontal propagation due to frontogenetical processes in addition to the transport by advection of the postfrontal cold air.

Conclusion

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Case 4: 6-7 June 2003(Chen et al. 2008, Mon. Wea. Rev., 41-61)

(a) 00Z 6 June

(b) 12Z 6 June

(c) 18Z 6 June

(d) 00Z 7 June

850 hPa

Frontal cyclogenesis

A Mei-Yu front over southern China intensified with a development of frontal disturbance and an LLJ formation at 850

hPa within a 24-h period

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500 and 300 hPa at 00Z 6 June 500 hPa 300 hPa

No favorable synoptic-scale system at upper levels

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Was the CISK mechanism responsible for• Development of frontal disturbance?• Mei-Yu frontogenesis?• Formation of the LLJ?

Questions

Diagnosis using ECMWF 1.125 data and methods including the piecewise PVIT and vorticity budget analysis.

Data and methodology

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Wave-like structure of the frontal disturbances(studied by Kuo and Horng 1994, Terr. Atmos. Ocean; Du and Cho 1996, J. Meteor. Soc. Japan → Barotropic instability)

12Z 6 June 18Z 6 June 00Z 7 June

Individual vorticity maxima along the front about 400 km apart (wave-like), in good agreement with the MCSs

A A ABB BC C C

DD

Relative vorticity (10-5s-1) at 850 hPa and Satellite (GOES-9) imageries

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6-h sfc rainfall (mm), 12-18Z 6 June ω (Pa s-1) at 700 hPa

• Strongest 700-hPa vertical velocity and surface rainfall also along Meiyu front, consistent with vorticity centers

• Convection did occur along the front, as well as south of the front

Relative vorticity (10-5s-1) at 850 hPa

Some fields at 18Z 6 June

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Vorticity budget analyses (18Z 6 June)

Fxp

v

yp

uVf

pfV

t

local horizontal vertical convergence/ tilting residualtendency advection advection stretching

Eastward movements of vorticity centers

Eastward movements of frontal disturbances

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Fxp

v

yp

uVf

pfV

t

local horizontal vertical convergence/ tilting residualtendency advection advection stretching

Major contributor toward the generation of frontal vorticity

Southward movements of the front at later stage

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Piecewise PV inversion

Mean field: One-month mean from 15 May to 15 June, 2003

125 hPa ’150 hPa q’

200 hPa q’

250 hPa q’

300 hPa q’

400 hPa q’

500 hPa q’

700 hPa q’

850 hPa q’

925 hPa q’

962 hPa ’

ms muRH 70% RH < 70%and q’ 0 or q’ < 0

ul

lb

12Z 6 June 06Z 6 June 18Z 6 June 06Z 6 Jun 00Z 7 June

18Z 6 June 00Z 7 June

Domain of PV inversion

Area of interest

Front

LLJ

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Frontogenesis and cyclogenesis

PV anomaly associated with latent heat release (ms) were responsible for the frontogenesis and cyclogenesis.

Height (gpm), wind (ms-1), and vorticity (10-5s-1, shaded) associated with ms at 00Z 7 June

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Northward-directed ageostrophic flow at 850 hPa to the south of developing MCSs, producing local wave like LLJ maxima

through Coriolis torque

ABC

D

Ageostrophic flow

18Z 6 June 00Z 7 June

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Intense heating throughout troposphere and strongest at 400 hPa, reaching 30C per day at 18Z June 6

Apparent heat source (Q1) computed for the frontal zone

Pre

ssure

(hP

a)

Heating rate (C per 6 h)

12Z 6 June

18Z 6 June

00Z 7 June

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• Heating efficiency related to horizontal and time scale of convection:

Rossby radius of deformation (LR):

N ~ 1.3 102 s1, h ~ 7 km, ~ 1.8 104 s1

LR = N h / ~ 500 km

Horizontal scale of MCSs L LR

• Latent energy released inside the frontal MCSs could heat the atmosphere effectively

→ Wind field adjusted toward the mass field

→ Cyclogenesis

Heating efficiency

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Conclusion

• Frontal strength was maintained by stretching (convergence) effect. Eastward development was due to horizontal advection, and slowly southward migration at later stages was due to tilting effect.

• The CISK mechanism (cyclone-cumulus feedback) was responsible for development of the wave-like disturbances (cyclogenesis) along the Mei-Yu front.

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• Northward-directed ageostrophic flow at 850 hPa to the south of developing MCSs produced local LLJ maxima through Coriolis torque.

• Both frontal strengthening and LLJ development were largely attributed to PV perturbations associated with latent heat release (“ms”), and minimum effects were from adiabatic processes.

• The average hourly intensity of precipitation in southwestern Taiwan increased significantly in posterior period.

• The daily intensity of precipitation in southwestern Taiwan increased significantly in posterior period.

V. Climate Change and Taiwan’s Meiyu Posterior period (2001-2010)- anterior period (1993-2000)

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• The frequency of extremely heavy rainfall in southwestern Taiwan increased significantly in posterior period.

• The frequency of extremely heavy and heavy rainfall in southwestern Taiwan increased significantly in posterior period.

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• The distribution of the 72 rainfall stations in southwestern Taiwan.

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The yellow and gray boxes represent the influence brought by the Meiyu fronts and typhoons, respectively.

• There was a tendency of increase of ultra extremely heavy rainfalls (both timewise and spacewise) during the posterior period.

The time and number of observation stations at which ultra extremely heavy rainfalls ( 200 mmd≧ -1) occurred in southwestern Taiwan during the Meiyu season of 1993-

2010.

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(a) 1991-2000

(b) 2001-2010

(c) (2001-2010)-(1991-2000)

• Less frontal passages over southwestern Taiwan in posterior period.

• The average hourly intensity of precipitation increased significantly in posterior period.• The daily intensity of precipitation increased significantly in posterior period. • The frequency of heavy rainfall increased significantly in posterior period. • The frequency of extremely-heavy and heavy rainfall increased significantly in posterior period. • The size (both timewise and spacewise) of ultra extremely heavy rainfall increased significantly in posterior period. • Less frontal systems with greater rainfall intensity and higher frequency of heavy, extremely-heavy and ultra extremely heavy rainfall in posterior period.

Meiyu in southwestern Taiwan

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