Hiroyuki Yamada (Japan Agency for Marine-Earth Science and Technology) With thanks to: Tomoe Nasuno,...
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Transcript of Hiroyuki Yamada (Japan Agency for Marine-Earth Science and Technology) With thanks to: Tomoe Nasuno,...
Hiroyuki Yamada
(Japan Agency for Marine-Earth Science and Technology)
With thanks to:Tomoe Nasuno, Wataru Yanase, Masaki Satoh,
Kunio Yoneyama, and Ryuichi Shirooka
Genesis of Typhoon Fengshen (2008) from vortex superposition:
PALAU field experiment andglobal cloud-resolving simulation
Typhoon Formation from Preexisting Disturbances
• Understanding of the process of tropical cyclogenesis is one of the
greatest remaining challenges in meteorology.
• It has been recognized that the key to understanding this process is to
explain the transformation of preexisting tropical disturbances into a
tropical cyclone (e.g. Zehr 1992).
• In the western tropical Pacific, synoptic-scale disturbance, such as
“easterly wave” or “tropical depression (TD) type disturbance” prevail
mainly in the lower troposphere (Reed and Recker 1971; Takayabu and
Nitta 1993; Tam and Li 2006).
Tam and Li (2006, MWR)
• Not all of cyclonic disturbance
can evolve into a tropical
cyclone.
Non-developing v.s. developing disturbancesStreamlines at 200hPa
Tangential wind in radial-vertical domain
McBride and Zehr (1981, JAS):
Differences between non-developing disturbances (N1) and pre-typhoon disturbances (D1) in
•flow in the upper troposphere, •strength of tangential wind.
Hypothsis:A deep axisymmetric disturbance is preferable to a shallow or asymmetric one for tropical cyclogenesis
Question:How such deep symmetric disturbance can be formed?
Possible Processes of Vortex Transformation
Main focus of idealized numerical studies:
Bister and Emanuel (1997), Montgomery et al. (2006), Nolan (2007)
Assuming the axisymmetric structure of an initial vortex
Support from many observational studies: Harr et al. (1996a,b), Ritchie and Holland (1997), Simpson et al. (1997), Raymond and Lopez (2011)
Insufficient support from numerical studies: Hogsett and Zhang (2010): vortex tilting
Focus of the present study: superposition of two separated synoptic-scale vortices leading to the genesis of Typhoon Fengshen (2008)
A developing tropical cyclone has coherent vertical structure
JAMSTEC’s Research on Typhoon Formation
Observation in East Philippine SeaObservation in East Philippine Sea
• PALAU field experiment in early summer season (June-July) of 2005, 2008, and 2010
• Using ground-based and ship-borne Doppler radars, upper-air sounding arrays, oceanic buoys
• To capture the structure and evolution of mesoscale convective systems embedded in a pre-typhoon vortex
Global cloud-resolving simulationGlobal cloud-resolving simulation
• Using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM), developed at JAMSTEC
• Explicit cloud physics, no cumulus parameterization, with horizontal resolution of 3.5 km
• To understand the key process of typhoon formation, under influences of synoptic- and large-scale waves and disturbances (e.g., MJO)
JAMSTECEarth Simulater 2
IcosahedralIcosahedralCoordinatesCoordinates
What did I do during PALAU-2008 Experiment?
Woleai Atoll, Micronesia
Typhoon Fengshen (2008)
Flood in Panay Is., Philippines at 27 June(image obtained from Wikipedia)
Observed and Simulated Tracks
• Category 3 typhoon, formed in the PALAU observation area
• Disasters with death of more than 1,300 people in Philippines
• Erroneous northward track before landfall, predicted by all operational centers (JTWC, JMA, ECMWF etc.)
Synoptic-scale processes
Synoptic Situation (ECMWF-YOTC analysis)
Two synoptic-scale disturbances in different vertical levels:
• A closed circulation near 5oN at 850 hPa
• A trough in a easterly flow near 15oN at 500 hPa
Height, Wind, Relative Vorticity(15 June, 3 days before genesis)
Vortex
Trough
Change in the Vertical Structure
Zonal-vertical sections of relative vorticity (7.5-12.5oN)
An upright vortex of Typhoon Fengshen was formed from the superposition of the two disturbances.
-1 day -3 day+1 day
TROUGH
TROUGH
VORTEX
Hovmöller Diagrams of Relative Vorticity
850hPa, 5-10N 500hPa, 7.5-12.5N
The vortex superposition took place due to the slowdown of the westward propagation in the lower troposphere.
Slowdown Leading to Vortex Superposition
U (10day, 850hPa, 2.5S-2.5N) V (3-10day, 850hPa, 5-10N)
The propagation speed of disturbances was decreased within a zonal confluent region of MJO.
(i.e., wave accumulation, Aiyyer and Molinari 2003, JAS)
Easterly EasterlyConfluence Confluence
Slowdown Leading to Vortex Superposition
The slowdown is not significant in the middle troposphere.
V (3-10day, 500hPa, 10-15N) V (3-10day, 850hPa, 5-10N)
Easterly EasterlyConfluence Confluence
PALAU observation
Merger of Cloud Systems
Merger of clouds system before the Fengshen’s upgrade into TS/TY
•Cloud systems F1-F3:
successive development near the slow-moving surface vortex
•Cloud system R:
constant propagation speed (7 m/s) before and after the merger
F1
F2
F3
R
Rotated Cartesian
Coordinate
Vortex center(at surface)
Merger
Cloud Distribution and Vertical Wind Profile
VORTEX CENTER
R R
Dopper radar observation revealed Cloud system R involving a mid-level vortex, corresponding to the mid-level disturbance
Cloud Distribution and Vertical Wind Profile
18 JUN
19 JUN
17 JUN
16 JUN
15 JUN
20 JUN
14 JUN
21 JUN
R
Woleai
OLR (MJO)
Cloud system R corresponds to a westward-propagating disturbance, developing in the convectively-active area of MJO.
R
Mesoscale processes simulated by a global cloud-resolving
model
JAMSTEC’s Research on Typhoon Formation
Observation in East Philippine SeaObservation in East Philippine Sea
• PALAU field experiment in early summer season (June-July) of 2005, 2008, and 2010
• Using ground-based and ship-borne Doppler radars, upper-air sounding arrays, oceanic buoys
• To capture the structure and evolution of mesoscale convective systems embedded in a pre-typhoon vortex
Global cloud-resolving simulationGlobal cloud-resolving simulation
• Using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM), developed at JAMSTEC
• Explicit cloud physics, no cumulus parameterization, with horizontal resolution of 3.5 km
• To understand the key process of typhoon formation, under influences of synoptic- and large-scale waves and disturbances (e.g., MJO)
JAMSTECEarth Simulater 2
IcosahedralIcosahedralCoordinatesCoordinates
Simulated Overall Evolution (R<100km)
Formation of a deep symmetric vortex with warm core,in concurrence with pressure drop, after 03UTC 18 June
Simulated Vortex Superposition
Vortex Center(at surface)
Pressurefall
Genesis of Fengshen from the vortex superposition was represented well by the NICAM simulation with 3 days of lead time.
Mid-level vortex
Simulated Vortex Superposition
Pressure fallstarted
Symmetric Component of Tangential Wind
The deep upright vortex was in gradient-wind balance from the lower to upper troposphere
Before superposition After superposition
Grids not in gradient-wind balance are indicated by “X”
Vorticity budget analysis
HADV(10-8 s-2)
VADV(10-8 s-2)
STR(10-8 s-2)
TILT(10-8 s-2)
TMCH(10-8 s-2)
Relative vorticity (10-4 s-1)
• Negative effect of horizontal advection
[HADV] before superposition.
• Elimination of negative HADV and
increase in stretching [STR] after
superposition (bottom-up building).
before after
Change in the Convective Structure
Azimuthal distribution of surface rain(R<100km)
Transformation of a mesoscale convective system (MCS) into a partial eyewall after the
vortex superposition
MCS
eyewall
Rain, Wind at surface
Pressure and Rain Rate at Surface
The pressure drop initially took place near the MCS, and its location moved to the vortex center as the MCS transformed into a partial eyewall.
(in a resting frame)
Track ofPressure minimum
Inertial Stability(Schubert and Hack 1982, JAS)
• A measure of the resistance to the movement of air parcels in the radial-vertical (r, z) plane
• Increased inertial stability means reduced adiabatic cooling (N2 w) and more efficient diabatic warming of the air due to convection.
• Large value in tropical cyclone, more than [100 * f2] in the inner core(Holland and Merrill 1984)
f: Coriolis parameterr: radial distancev: tangential wind component
3 Hour Before Superposition (18/00)
A remaining unidirectional flow
Tilted vortex axis
Weak inertial stability
Outward movement of MCS due to vertical wind shear
Displacement of an MCS-induced warm core (MCV)
3 Hour After Superposition (18/06)
Upright axis of vortex
Increased inertial stability (> 100 * f2)
Formation of a protected area
Evolution of a partial eyewall with warm core and low pressure near the surface vortex center
P’ < -2 hPa
Role of Vertical Shear on Convective Structure
Location of MCS relative the to vortex center
Mean wind (R<100km)
before after Before superposition:
Large vertical shear due to the unidirectional flow in the middle to upper troposphere.
MCS leaving behind the vortex center, according to the large shear.
After superposition:
Reduced vertical shear caused the inward movement of MCS and structural change into the partial eyewall
Schematic View of The Fengshen’s Formation
Separated vortex centers in the lower and middle troposphere, causing vertical wind shear over the surface vortex center
MCS leaving behind the surface vortex due to the vertical shear
Formation of an upright vortex in gradient wind balance
Transformation of MCS into a partial eyewall under the environmentwith reduced vertical shear and
increased inertial stability
The vortex superposition was the principal factor in the genesis of Fengshen
Summary
• Synoptic and mesoscale processes leading to the genesis of Typhoon Fengshen (2008) were investigated based on observations and numerical simulations.
• This typhoon was formed when a mid-tropospheric trough was superposed upon a lower-tropospheric vortex (TD-type disturbance). The presence of two separated vortices before the genesis was supported by the observations.
• The simulation represented the transformation of an MCS into eyewall clouds under a condition with the increased inertial stability and reduced vertical shear (due to vortex superposition).
• These results suggests an importance of vortex superposition for tropical cyclogenesis in the tropical western Pacific.
• The results also suggest the importance of correctly reproducing the vertical structure of incipient disturbances for simulating typhoon formation.