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.