tropical cyclogenesis in a tropical wave critical layer

Tropical Cyclogenesis in a Tropical Wave Critical Layer:

Easterly Waves

Chengfeng FENG

Dunkerton T J, Montgomery M T, Wang Z. Tropical cyclogenesis in a tropical wave critical layer: Easterly waves[J]. Atmospheric Chemistry and Physics, 2009, 9(15): 5587-5646.

North West Research AssociatesRedmond, WAVice President, Senior Research Scientist, [email protected]

Introduction

• The genesis of tropical cyclones is unsolved.

Difficulty: observation, model, theory

Author: Lagrangian nature

• Montgomery et al. (2006b)

• The TC genesis problem was posed as the metamorphosis of a mid-level mesoscale convective vortex (MCV) with weak cyclonic circulation at the ocean surface into a self sustaining warm-core tropical depression vortex.

• Yet to be answered, is how this embryo and its surrounding circulation are created in the first place and whether the initial proto-vortex can survive long enough in realistic environments to enable the transition to a self-sustaining warm-core vortex.

Introduction

• Tropical waves

Sweet spot: low pressure in the lower troposphere

• The easterly waves of the Atlantic and Pacific basins

• The Rossby-gravity waves of the central and far eastern Pacific Oceans

• Tropical depression disturbance of the western tropical Pacific

• ……

• Then, extend this line of research to other types of tropical waves.

Objective

• Marsupial paradigm.

• To explain the generation and maintenance of the hurricane embryo within its parent synoptic-scale wave or monsoon trough environment.

• To advance beyond M06 and understand more completely the role of moist convective turbulence that operates within the embryo.

• Theory -> Observation

Critical layer

• Hydrodynamically neutral waves emerge from an unstable source region upstream, propogatewestward into the domain of interest.

• Africa -> Atlantic Sector

• Three components

• Lagrangian flow

• Stable/unstable

An idealized easterly jet

• Barotropic

• Shear

• Absolute vorticity

• Meridional gradient

• Where is the critical level?

Interested in cyclonic rather that anticyclonic vorticity.

Thorncroft and Hoskins (1994a)

The order of enumeration

• The order of enumeration:

The cat’s eye is a result of wave propagation into the region of interest, while convective amplification and aggregation is aided subsequently by formation of the cat’s eye.

(Killworth and McIntyre, 1985)

• Important to this sequence:

The easterly jet is located in the lower, not middle, troposphere

First hypothesis

• Proto-vortex cyclonic eddies instrumental in TC formation are intimately associated with the parent wave’s critical latitude in the lower troposphere. The critical layer and Kelvin cat’s eye within, formed as a result of the wave’s finite-amplitude interaction with its own critical latitude, contain a region of cyclonic rotation and weak straining/shearing deformation in which synoptic waves and mesoscale vorticity anomalies, moving westward together, amplify and aggregate on a nearly zero relative mean flow. This multi-scale interaction provides a dynamical pathway to “bottom-up” development of the proto-vortex from below.

Second hypothesis

• The critical-layer cat’s eye of the parent wave provides a set of quasi-closed material contours inside of which air is repeatedly moistened by convection, protected to some degree from lateral intrusion of dry air and impinging vertical shear, and (thanks to its location near the critical latitude) able to keep pace with the parent wave until the proto-vortex has strengthened into a self-maintaining entity.

Third hypothesis

• The parent wave is maintained and possibly enhanced by diabatically amplified eddies within the wave (proto-vortices on the mesoscale), a process favored in regions of small intrinsic phase speed.

• This hypothesis, incidentally, agrees with the common observation that a tropical wave is weakened or eliminated when the diabatic vortex leaves the pouch, having acquired its own identify and propagation characteristics independent of the parent wave.

Datasets and Time Period

• Datasets

• ERA-40:

~125km or ~1.125 degrees, 6hourly, 60 vertical levels

• TRMM:

satellite-derived precipitation, 0.25 degrees

• NHC best-track data

• The time period of interest:

The peak hurricane seasons of 1998-2001

Sample: 55 storms

• 1998-2001 August and September

54 0f the 61 named storms (one half of the 136 TD) :

“Monochromic easterly wave”

• Among the 54 storms

Many formed from African easterly waves

Eastern Pacific: ITCZ

• A single case from the central Pacific

Common characteristics of 55 storms

• Persistent convective precipitation occurs in a quasi-closed recirculating gyre in the co-moving frame.

• The tropical storm develops near the intersection of the critical latitude and the trough axis.

• Five representative storms during August 2000

The evolution of easterly waves and convection

before TC genesis.

2.5-9 day bandpass meridional velocity

• A region of anomalous cyclonic vorticity

• First two: uniform patterns

• Common characteristics of 55 storms:

• Present prior to genesis time.

• Reasonably monochromatic for at least the two preceding days.

2.5-9 day bandpass meridional velocity

• First four examples: aligned almost perfectly in the vertical

• Shanshan: the tile diminishes approaching genesis time

• Common characteristics of 55 storms:

• Become well aligned vertically in the lower troposphere below 500hPa before genesis occurs.

Characteristics: one ore more

• A group of wave phases originating to the west (east) of the genesis location, followed by eastward (westward) apparent group propagation across the diagram

• Abrupt amplification of the wave packet near the genesis point

• Diminution of wave amplitude towards the end of the storm track

• A pronounced wave packet in the upper right quadrant of the Hovmoller diagram indicating either that the incident wave is maintained for a time by diabatic processes operating within the proto-votex or that new waves are excited by the vortex as it develops into a full-fledged tropical storm, and these waves subsequently propagate as a group to the east.

Central Atlantic: hurricane Debby

• Outside the closed gyre: shear vorticity

850hPa Relative vorticity

10−5𝑠−1


• Saddle point; Critical layer; Critical latitude; H1


10−5𝑠−1

A resting frame

• Saddle point misplaced.

• Vorticity: little relation.

• No closed gyre at 600hPa.

• The co-moving frame is better than the resting frame.

10−5𝑠−1


• Horizontal advection; precipitation cells

850hPa Saturation fraction

percent


• Closed circulation; H2

600hPa 3-h precipitation

mm/day

East Pacific: tropical storm Fabio

• Closed streamlines; strong cyclonic vorticity; genesis


10−5𝑠−1


• 𝑂𝑊 = 𝜍2 − 𝑈𝑥 − 𝑉𝑦2− 𝑉𝑥 + 𝑈𝑦

2

600hPa OW parameter

10−10𝑠−2


• High saturation fraction; a strong gradient

850hPa Saturation fraction

percent


• Near CL: sustained and focused; Diffluent region

600hPa 3-h precipitation

mm/day


• Weak vertical shear; not the deciding factor

𝑚𝑠−1

850hPa 850-500hPa


• Influenced by: convection, and synoptic activity

𝑚𝑠−1

600hPa 850-200hPa

Survey of events 1998-2001

• Red: the translating gyre

• Green: a 3*3 matrix of ERA-40 grid points

• Black circles: “climatological” values for August-September 1998-2001 at the various gyreor box locations

• Fill circles: the genesis time

• Open circles: the 36-h interval preceding and slightly overlapping genesis time


• Moist

• Precipitation

• Vorticity

• Okubo-Weiss

• Exceptionally large values: Near the genesis location

• Vertical shear

• Local shear is often smaller than the gyre-wide shear, favorable

• 3-12m/s, not large


• Wave amplitude declines, which may support H3

Thank You!

tropical cyclogenesis in a tropical wave critical layer

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