2.2 Internal influences on tropical cyclone formation IWTC-VI Kevin Tory Michael Montgomery 23...

2.2 Internal influences on tropical cyclone formation

IWTC-VIKevin Tory

Michael Montgomery23 November 2006

Introduction

• Genesis requirements• Vorticity tendency fundamentals• MCS role in genesis• Genesis theories of the last decade• Modeling studies• Observations• Recommendations

Genesis Requirements• Necessary environment for genesis:

1. Low- to mid-tropospheric cyclonic absolute vorticity2. Weak vertical wind shear3. Warm ocean4. Moist unstable air mass

• Genesis is the transformation from this environment to a TC scale warm-cored self-sustaining vortex

• Convection drives the transformation

• Proposed 2-step genesis definition.1. Pre-conditioning: the establishment of the necessary environment.2. Vortex construction: the development of a self-sustaining warm-

cored vortex.

Vorticity Tendency Fundamentals

Low- to mid-level vorticity convergence,

balanced by divergence at larger radii.

Vorticity Tendency Fundamentals

Low- to mid-level vorticity convergence,

Vertical advection-like effect

balanced by divergence at larger radii.

balanced by tilting-like effect at updraft edge.

How do we get from this,

…to this?

Somewhere on the web.

MCS role in genesis

Convergence

Divergence

Convergence

Fig. 14 from Houze (2004) Rev. of Geo.

MCS role in genesis

Convergence

Divergence

Convergence

MCS role in genesis

Convergence

Divergence

Convergence

MCS role in genesisDiv

Stratiform Divergence Profile (SDP)

Convective Divergence Profile (CDP)

It is useful to consider genesis (step 2) to involve an atmospheric transition from a mean SDP to a mean CDP (e.g., Mapes and Houze 1995; Raymond et al. 1998).

Genesis TheoriesRitchie, Holland, Simpson: Top-down merger theory.

Bister and Emanuel (1997): Top-down showerhead theory.

a. Mid-level vortex advected down towards the surface, while evaporation of rain moistens the air progressively downward.

b. Moistened cyclonic layer reaches the surface. Surface fluxes increase low-level theta_e.

c. Near downdraft-free convection develops in the moist environment and converges cyclonic vorticity.

Fig. 13 from Bister and Emanuel (1997) Mon. Wea. Rev.

Genesis TheoriesDiv

Bister, and Emanuel (1997): Top-down showerhead theory.

Outstanding Issues:

1. Downward advection of cyclonic vorticity leads to nearby changes in the opposite sense through tilting.

2. Divergence weakens vorticity magnitude near the surface.

3. Dry air inflow would inhibit the moistening process.

4. A moist neutral atmosphere is required for downdraft-free convection. (Zipser, per. com.)

5. But is downdraft-free convection or near downdraft-free convection necessary?

Fig. 13 from Bister and Emanuel (1997) Mon. Wea. Rev.

Genesis Theories

Montgomery,Enagonio: Bottom-up theory.

Consider the interaction of the stratiform and convective vortices.

Genesis Theories

Montgomery,Enagonio: Bottom-up theory.

Assumptions: 1. Convective regions must develop in low-level cyclonic environment.2. Sufficient number and/or size of convective regions are necessaryfor such an interaction to take place.

Genesis Theories

Modeling StudiesChen and Frank (1993), modeling study:

Convective region “takes over”.

Convective region “takes over”. Grid resolution 25 km, hence minimum resolved updraft Scale is about 100 km.

T = 4 h T = 8 h

Stratiform remnant

Issues:1. The convective region quickly dwarfs the stratiform region. What

happened?2. Could this adequately represent the transformation from an SDP dominant

MCS to a CDP dominant MCS in the real world? 3. Or is it just a product of an overly active convective paramaterization

scheme, which forces updrafts on the minimum resolvable scale (~100 km)?

Modeling StudiesMontgomery et al. (2006) modeling study:

Illustrates a possible path from this,

…to this?

Modeling StudiesMontgomery et al. (2006) idealised modeling study:

• RAMS, near cloud resolving, non-hydrostatic, 2 and 3 km grid spacing.

• Intense cyclonic vortices develop on the convective scale (vortical hot towers, VHTs).

• VHTs interact to form larger more intense vortices (upscale vortex cascade).

• Net heating in the VHTs generates inflow above the boundary layer and system-scale convergence of angular momentum (System Scale Intensification, SSI).

Modeling StudiesMontgomery et al. (2006) idealised modeling study:

Vortex Upscale Cascade

System-Scale Intensification

Modeling StudiesDavis and Bosart (2006)coming soon to QJRMS:

15 hours

Squall line

Bore front

Deep convection, reduceddowndrafts

10 m winds, Cloud top temperatures.

Modeling Studies

36 hours

Relatively large cyclonic vortex results.

Davis and Bosart (2006)coming soon to QJRMS:

Modeling Studies

48 hours

Deep convection preferred to the east of the circulation centre.

“The small-scale vorticity anomalies were the result of relatively intense convection and heating on the scale of individual cumulonimbi or convective systems less than ~50 km in length.”

Modeling Studies

60 hours

Modeling Studies

72 hours

Modeling Studies

84 hours

TC develops on the poleward side (cyclonic shear) of the monsoon westerly surge.

A number of low and mid-level PV anomalies merge/ are axisymmetrized into a central PV monolith.

Shear tilts developing PV cores, whereas convection on the down-tilt side serves to realign them.

850 PV anomaly at the initial circulation centre is axisymmetrized into the PV monolith by 14 hours.

Significant tilt on the developing PV monolith at 8,10 hours with strongest convection on the downtilt side.

Continued convection on the downtilt side aligns the developing PV monolith.

Tory et al. (2006): Elcho Island storm

Modeling Studies

Elcho Island Storm

125 126 127 128 129 130 131 132 133

850 hPa PV, wind500 hPa PV, wind

250 hPa up motion

1—3 Pa/s 3—5 Pa/s

117 118 119 120 121 122 123 124 125

R eference Vectors

Elcho Island Storm

125 126 127 128 129 130 131 132 133

250 hPa up motion

1—3 Pa/s 3—5 Pa/s

117 118 119 120 121 122 123 124 125

R eference Vectors

Elcho Island Storm

125 126 127 128 129 130 131 132 133

250 hPa up motion

1—3 Pa/s 3—5 Pa/s

117 118 119 120 121 122 123 124 125

R eference Vectors

Elcho Island Storm

125 126 127 128 129 130 131 132 133

250 hPa up motion

1—3 Pa/s 3—5 Pa/s

117 118 119 120 121 122 123 124 125

R eference Vectors

125 126 127 128 129 130 131 132 133

Elcho Island Storm

125 126 127 128 129 130 131 132 133

250 hPa up motion

1—3 Pa/s 3—5 Pa/s

117 118 119 120 121 122 123 124 125

R eference Vectors

Elcho Island Storm

125 126 127 128 129 130 131 132 133

250 hPa up motion

1—3 Pa/s 3—5 Pa/s

117 118 119 120 121 122 123 124 125

R eference Vectors

Elcho Island Storm

125 126 127 128 129 130 131 132 133

250 hPa up motion

1—3 Pa/s 3—5 Pa/s

117 118 119 120 121 122 123 124 125

R eference Vectors

125 126 127 128 129 130 131 132 133

Elcho Island Storm

125 126 127 128 129 130 131 132 133

250 hPa up motion

1—3 Pa/s 3—5 Pa/s

117 118 119 120 121 122 123 124 125

R eference Vectors

Elcho Island Storm

125 126 127 128 129 130 131 132 133

250 hPa up motion

1—3 Pa/s 3—5 Pa/s

117 118 119 120 121 122 123 124 125

R eference Vectors

Elcho Island Storm

125 126 127 128 129 130 131 132 133

250 hPa up motion

1—3 Pa/s 3—5 Pa/s

117 118 119 120 121 122 123 124 125

R eference Vectors

Elcho Island Storm

125 126 127 128 129 130 131 132 133

250 hPa up motion

1—3 Pa/s 3—5 Pa/s

117 118 119 120 121 122 123 124 125

R eference Vectors

Elcho Island Storm

125 126 127 128 129 130 131 132 133

250 hPa up motion

1—3 Pa/s 3—5 Pa/s

117 118 119 120 121 122 123 124 125

R eference Vectors

125 126 127 128 129 130 131 132 133

Elcho Island Storm

125 126 127 128 129 130 131 132 133

250 hPa up motion

1—3 Pa/s 3—5 Pa/s

117 118 119 120 121 122 123 124 125

R eference Vectors

Modeling Studies

Davis and Bosart (2006): Similar development to Montgomery, Hendricks. VHTs play an important role.

Tory et al. (2006): Again similar development, except coarser resolution (~15 km grid spacing) leads to larger-scale features.

Note: while genesis was underway downdrafts were present in all three higher resolution studies.

Modeling Studies• Modeling studies show a transition from mean SDP to mean CDP without the need for downdraft free convection.

• Downdraft free convection (or near downdraft free convection) may result in time.

• Good results using 0.15 grid spacing in TC-LAPS suggests genesis may be driven largely by scales resolvable by this model.

Observations• Until recently observations have been too sparse to assess the behavior predicted in the higher resolution modeling studies.

• Instead only profiles of MCS divergence, vorticity and vertical velocity have been determined from most field experiments.

Observations• Kingsmill and Houze (1999)

Updraft-downdraft interface

Deep layer inflow

Saturated theta_e decreasing with height in updraft inflow.

High boundary layer theta_e

The higher the BL theta_e is the longer lived and larger the MCSs tend to be.

Very low theta_e mid-level inflow.

Uncertain connection to the convective scale downdrafts.

Shallow patches of low theta_e air in the precipitating regions of the BL.

VHTs ?

Observations• VHT’s? Montgomery et al. (2006) show low-level theta_e distribution consistent with Kingsmill and Houze.

Shallow patches of low theta_e air in the precipitating regions of the BL.

Observations•Reasor et al. (2005) and Sippel et al. (2006) used ground based and airborne Doppler radar respectively to investigate some of the vorticity structures present during the formation of Hurricane Dolly and TS Allison.

•The development process was consistent with the modeling study of Hendricks et al. (2004).

•At some point in the development all three showed an area of enhanced low-level cyclonic vorticity, with a feeder band of discrete vortices spiralling toward the enhanced area.

ObservationsHendricks et al. (2004). Reasor et al. (2005)

Feeder band

Enhanced vorticity

ObservationsHendricks et al. (2004). Sippel et al. (2006)

Feeder band

Enhanced vorticity

Where to from here?

• Genesis theories were influenced by a number of assumptions.

• Modeling studies prompt us to reconsider these assumptions.

• High-resolution observations (so far) seem to support the modeling studies.

• The assumptions:

1. Existance of the surface anticyclone below an MCS.

- Is it an obstacle to be overcome?

2. The need for (near) downdraft-free convection.

- Is it necessary for the transition from mean SDP to CDP?

3. The importance of the MCV.

- Does the MCV need to extend to the surface before mean CDP?

- Are MCVs always required to reduce LR and facilitate efficient transfer of heat to rotational momentum, or can the environment (e.g., monsoon low) be sufficiently cyclonic to do this?

Recommendations

• Establish a widely accepted genesis definition. Stage 1: the establishment of synoptic, sub-synoptic environment

favorable for genesis. Stage 2: the mesoscale organisation of this environment into a warm-cored TC-like vortex.

• Continue investigating the thermodynamic evolution of MCSs in the tropical oceanic environment.

• Continue investigating the nature of vorticity enhancement in MCSs and their immediate environment.

• Determine why most tropical disturbances fail to become warm-cored, surface-concentrated vortices.

Vorticity Tendency Fundamentals (cont)This horizontal redistribution of PV (a ) to build an upright vortex core,

is achieved largely through horizontal convergence and a vertical advection-like effect. In isentropic coordinates,

na JJJ~~~ QvuJa )0,,(

~ )0,,(~

Note, the vertical (isentropic-normal) components of the two vectors Ja and Jn are zero. This is the basis of Haynes and McIntyre’s two statements on the previous slide.

=density, Q = PV, Q =PV substance (PVS), Ja and Jn are the advective and non-advective fluxes respectively. (Isentropic coordinates)

Vorticity Tendency Fundamentals (cont)

The tracer concentration will change when fluid leaves or enters the layer vertically.

Convergent flow concentrates PVS = local PV enhancement.

Isentropic surface

Convergence “concentrates” the tracer

Vorticity tendency Fundamentals (cont)

Blue arrows = updraft. Thin lines = vortex lines. Dashed lines = isentropic surfaces at t=0. Red arrows = vorticity vectors.

Vorticity magnitude is inversely proportional to vortex line spacing.

PV magnitude= the component of absolute vorticity normal to the isentropes.

Isentropic dipping leads to an increase in vorticity magnitude (red arrows) due to the movement of the isentropes into an area of higher vorticity magnitude.

Vertical advection-like effect.

Tilting of isentropes weakens the isentropic normal component of vorticity (i.e., PV) by increasing the angle between isentropic normal and the vortex lines.Tilting-like effect

In a deep convective updraft the isentropes will dip substantially relative to the fluid flow. In our TC genesis simulations this can lead to anticyclonic PV generation on the updraft edges, which suggests the isentropic surface has been tilted beyond parallel to the vortex line (relative to the fluid flow).

Cyclonic PVAnticyclonic PV

Hypothesis: SDP to CDP with downdrafts

Rainfall ensures less evaporative cooling can take place in downdrafts than condensational heating in updrafts. Result: net heating.

Heat energy is projected onto gravity waves and rotational modes.

With an increased cyclonic environment (reduced Rossby deformation radius):

(i) more energy will be projected onto the rotational mode, (ii) lower tropospheric convergence will be enhanced, and(iii) cyclonic vorticity will increase.

Hypothesis: SDP to CDP with downdrafts

2.2 Internal influences on tropical cyclone formation IWTC-VI Kevin Tory Michael Montgomery 23...

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