Tropical Cyclone Formation and Extratropical Transition

IWTC – V Recommendations• There is a strong need for a consistent definition of tropical

cyclone formation such that operational priorities may be satisfied, and high-quality data sets will be defined based on physically relevant formation characteristics – region-independent – benefit the research community for studies that address synoptic-

scale and mesoscale environmental influences on tropical cyclone formation plus allow testing of theories of tropical cyclone formation in observation and modeling frameworks

• Tropical cyclone formation is dependent on interactions among a variety of space and time scales. There is a strong need to define factors that would provide diagnostic evaluation of potential tropical cyclone formation.– examination of tropical cyclone formation in numerical prediction

models – measure of uncertainty in the ability for tropical cyclone formation

to be related to large-scale factors in numerical model simulation

Tropical Cyclone Formation and Extratropical Transition

IWTC – VI Summary

• Two-stage process (Zehr 1993, Gray 1998, Karyampudi and Pierce

2000)

– (I) Preconditioning via sub-synoptic (synoptic-) scale organization of an environment favorable for genesis [EXTERNAL]

– (II) Concentration of environment vorticity by the mesoscale [INTERNAL]

• Kinematic – establish low-level cyclonic vorticity• Thermodynamic – establish a moist-neutral

environment, downdraft-free convection

Synoptic-scale [External] Influences

Stage I• Tropical waves that propagate zonally provide local flow

perturbations that contribute to a favorable environment for tropical cyclone formation via enhancement of

– Vertical motion– Low-level vorticity– Changing vertical wind

shear

• Most tropical cyclones form under the influence of wave circulations.

• If the waves could be forecast (with demonstrable skill), genesis could be forecast.

1200 UTC 07 Oct 2002

Synoptic-scale [External] Influences

Stage I• Such predictions are very difficult in the tropics using

current deterministic models.• Tropical waves are predictable using statistical

techniques. Other statistical genesis prediction techniques are also showing promise.

• Therefore, combined statistical/deterministic forecasts offer promise forforecasts of genesis. Possible lead times of up to the time-scale of the MJO.

1200 UTC 07 Oct 2002

Mesoscale Organization [Internal] Influences

Stage II

Bottom UpTop Down

BU

Top-Down vs. Bottom-Up

TD

BU

Top-Down vs. Bottom-Up

TD

BU

Top-Down vs. Bottom-Up

TD

BU

Top-Down vs. Bottom-Up

TD

BU

Top-Down vs. Bottom-Up

TD

Mesoscale Organization [Internal] Influences

Stage II• Issues associated with the relative roles of

stratiform and convective-type vortex enhancement– Establishment of downdraft-free convection– Vortex hot tower contribution to the establishment of a

moist-neutral environment– Necessity of these conditions?– Factors that impact generation of sufficient vortex hot

tower activity– Scales: pre-genesis clusters are observed on the scale of

100 km. • Are they comprised of downdraft-free convection?• What mechanisms are responsible for these

conditions?• Why do most fail to warm core, surface-concentrated

vortices?

Stages I and II

• Genesis process is driven by large-scale dynamics

• Given proper initialization and representation of the large-scale environment (Gray’s necessary genesis conditions) models should be able to simulate the genesis process. – Contains the dynamical forcing that

initiates convection

Forecast Issues• Diagnosis of tropical cyclone formation based on

the 850 to 500 hPa wind profiles.

• Cases of operational global model success– Favorable large-scale environment

• Tropical waves• MJO

• Cases of operational global model failures– Smaller scale, external influences– Trough intrusions– Low shear– Differential steering

• Major factor is to identify role of large scale versus mesoscale processes

Images from: http://www.nrlmry.navy.mil/sat_products.html

Formation Alert:0600 UTC 4 July 2006

First Forecast: 0800 UTC 8 July 2006

All 2005 Atlantic Tropical Cyclones through Alpha (excluding Vince)

Too weak

Too strong

Pasch et al. 2006, AMS Conference on Hurricanes and Tropical Meteorology

2005 Atlantic Tropical CyclonesUKM: Probability of Detection

0

0.1

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12 24 36 48 60 72 84 96 108 120

Forecast Interval (h)

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abili

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2005 Atlantic Tropical CyclonesNOGAPS: Probability of Detection

0

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12 24 36 48 60 72 84 96 108 120

Forecast Interval (h)

Pro

babi

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All Forecasts Forecasts > Vort. Thresh.

Forecasts > Warm Core Thresh. Forecasts > both Thresh.

2005 Atlantic Tropical CyclonesGFS: Probability of Detection

0

0.1

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12 24 36 48 60 72 84 96 108 120

Forecast Interval (h)

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NOGAPS: rarely exceeds vorticity threshold, but does exceed thermodynamic threshold

UKM: slightly better detection relative to vorticity threshold

GFS: equal detection relative to dynamic and thermodynamic parameters

Pasch et al. 2006 AMS

Extratropical TransitionIWTC-V Summary

• IWTC-V recognises that a consistent definition of extratropical transition (ET) of a tropical cyclone does not exist and recommends that WMO support the development of an operationally and physically consistent definition for ET for use by the operational and research communities, and that this ET definition be presented at IWTC-VI. This should include conceptual models of wind, precipitation and ocean surface wave distributions.

• To advance the understanding of the process of extratropical transition, it is recommended that a comprehensive research program be developed including the use of existing data sets, and field experiments in co-operation with existing programs such as THORPEX– Precipitation – Expansion of wind field– Ocean wave fields– Role of the ocean in ET

Extratropical TransitionIWTC-V Summary

Observations and Forecasts• Timing of extratropical transition:• Phase with the midlatitude trough• Structural changes• Physical processes• Important observations• Impacts on forecasts of downstream

weather– Re-intensification?– Dissipation?

Timeline of events during Extratropical Transition over the western North Pacific

SL

P (

mb

)

Time (h) 0 30 46 81

Step 1 Step 2 Step 3 End ET

Little or no re-intensification (SLP > 1000 mb)

Moderate re-intensification (980 mb < SLP < 999 mb

Deep re-intensification, including rapid deepening (SLP < 980 mb)

957 mb +/- 25 mb

976 mb +/- 18 mb

993 mb +/- 7 mb

Transformation Re-Intensification

Adapted from Klein et al. (2000) Weather and Forecasting

10-10 K m-1 s-110-5 s-1

Hurricane Floyd 0000 UTC 16 Sep 1999

Deep Re-intensification

10-10 K m-1 s-1

10-10 K m-1 s-110-5 s-1

Hurricane Fran 0000 UTC 06 Sep 1996

Dissipation

10-10 K m-1 s-110-5 s-1

Hurricane Bonnie 1200 UTC 26 Aug 1998

Moderate Re-intensification

Extratropical TransitionPhysical Processes and Downstream

Impacts• Scale of physical processes involved in

the ET process ranges from microscale to planetary scale – most are associated with phase changes and are difficult to observe/model/diagnose

• There are variety of physical processes that contribute to the sensitivity of the downstream response to the TC and the upstream state during ET?

GFS 500 hPa ensemble +108 h VT 1200 UTC 20 Sep 03

GFS ensemble members +00 0000 UTC 16 Sep 2003500 hPa height (m) at a 240 m interval

Hurricane Isabel

Extratropical Transition as Tropical/Extratropical Interaction

• Role of the tropical cyclone structure – prior to recurvature– at recurvature– following recurvature

• What must be observed (space, time, parameter) to increase predictability of– important weather parameters?– downstream impacts?

• Role in seasonal variability (Hart 2006 AMS Conference)

Summary

• Progress– Observational capabilities– Operational Model capabilities– Recognition of the importance of scale

interactions

• Requirements– Explaining variability– Continued analysis of scale interactions – Definitions