60 th Anniversary Department of Atmospheric Sciences National Taiwan University George Tai-Jen Chen...
-
Upload
reynard-allison -
Category
Documents
-
view
231 -
download
0
Transcript of 60 th Anniversary Department of Atmospheric Sciences National Taiwan University George Tai-Jen Chen...
60th Anniversary Department of Atmospheric Sciences
National Taiwan University
George Tai-Jen ChenDepartment of Atmospheric Sciences
National Taiwan UniversityMay 15, 2015
1
Research and Perspective of Mei-Yu in Taiwan
Ι. Heavy Rainfall Problems in a Changing Society of Taiwan
II. Taiwan’s Research Effort on Mei-yu Heavy Rainfall
III. Recent Research Effort on Mei-yu Frontal Systems
IV. Climate Change and Taiwan’s Mei-yu
2
Ι. Heavy Rainfall Problems in a Changing Society
3
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 $
5
6Mei-yu in southern China and Taiwan (mid-May – mid-June)
Mei-yu in East Asia
7Japan Baiu (late-May – late-June)
8Mei-yu in Yangtze River Valley (mid-June – mid-July)
9Korea Changma (mid-July – mid-August)
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.
II. Taiwan’s Research Effort on Mei-yu Heavy Rainfall
10
TS PF PA
Typhoon 0.60 0.68 0.85
Mei-yu 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 Mei-yu season.
Heavy rain forecast capability of CWB
11
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 Mei-yu heavy rainfall
12
1.National Conference on Disastrous Weathers in Taiwan
Typhoon, drought, cold surge, and Mei-yu were identified to be the 4 major disastrous weathers in Taiwan. Research focus on these phenomena was
suggested and then became NSC policy.
13
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 Mei-yu front; 2) To study the evolution of the mesoscale convective systems (MCSs) in the vicinity of the Mei-yu front; 3) To study the effects of orography on the Mei-yu front and on mesoscale convective systems.
14
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)
15
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
16
• 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
17
• Important Events of the Follow-up Basic Research of TAMEX from 1988
1993
18
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
19
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.
• Objectives1) 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.
20
21
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 SW monsoonal flow to meet the urgent need of disaster reduction
in the Taiwan area
22
• Scientific Objectives
1) Dynamic and thermodynamic characteristics of SW monsoonal flow upstream of Taiwan and its relation with the Mei-yu front and the formation, organization and maintenance of the MCSs.
2) Kinematic, thermodynamic, and microphysical characteristics of MCSs and the precipitation mechanisms for heavy rain.3) Taiwan coastal and topographic effects on the impinging SW flows and on the intensity change of MCSs.
4) Radar data assimilation in NWP and short range QPF.
23
24
25
• 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)
III. Recent Research Effort on Mei-yu Frontal Systems
Low Level Jet (LLJ)• Formation mechanism• Relationship with extremely heavy rainfall
Mei-yu 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
26
27
Case 1: 12-13 June 1990(Chen et al. 2003, Mon. Wea. Rev., 2680-2696)
850 hPa weather map and PV at 12Z 12 June
Wind shear and PV (10-2 PVU) accompanying the front.
Mei-Yu frontogenesis
(a)
28
(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.
29
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.
22
2
2
2
2
242
cos
2
af
22
4
22
242
22 1
cos
1)(
aaf
p
gq
30
• 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.
31
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.
32
GMS IR imagery and vertical motion as computed by PV prognostic system along AB at
00Z 13 June
A B
w
Upward motion (cm s-1) computed by prognostic system was closely matching the position of deep convection on cloud
imagery.
33
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.
q/t /t+
-
-
+
34
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
35
Similar vertical motion pattern with much less PV generation at the low level.
If ms is reduced by ½ at 00Z 13 June
w q/t
-
+
36
• 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.
Conclusions
37
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
38
GMS IR images
Frontal cloud band with an organized MCS over the frontal
disturbance moved northeastward.
39
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.
40
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.
41
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
42
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.
43
12Z 7 June
00Z 8 June
18Z 7 June
06Z 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
44
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 June
00Z 8 June
06Z 8 June
45
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
46
• 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.
Conclusions
47
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.
48
(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
49
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 Mei-yu 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)
50
• 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.
• Mei-yu 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.
Conclusions
51
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
52
500 and 300 hPa at 00Z 6 June
500 hPa 300 hPa
No favorable synoptic-scale system at upper levels
53
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
54
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 Mei-yu 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
55
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
56
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
57
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
58
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
59
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
60
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
Pressure (hP
a)
Heating rate (C per 6 h)
12Z 6 June
18Z 6 June
00Z 7 June
61
• 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
62
Conclusions
• 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.
63
• 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.
IV. Climate Change and Taiwan’s Mei-yu Posterior period (2001-2010)- anterior period (1993-2000)
64
• 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. 65
• The distribution of the 72 rainfall stations in southwestern Taiwan.
66
The yellow and gray boxes represent the influence brought by the Mei-yu 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 Mei-yu season of
1993-2010.
67
68
(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.
Mei-yu in southwestern Taiwan
69