Seasonal Tropical Cyclone Prediction with POAMA

Kay Shelton1, Andrew Charles2, Harry Hendon1 and

Yuriy Kuleshov2

1CAWCR, BoM and 2NCC, BoM

What is a tropical cyclone?

• A tropical cyclone (TC) is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation (Holland 1993).

• The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone".

Schematic structure

of mature TC in SH(Adapted from NOAA NH image)

http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html

http://www.bom.gov.au/weather/cyclone/faq

How do they form?

• Ingredients for TC formation:

– sufficiently large background vorticity (generally 3° from

the Equator is minimum requirement),

– small vertical wind shear over the disturbance,

– a warm ocean to a sufficient depth (SST>26.5°C),

– conditionally unstable atmospheric column,

– moist mid-levels,

– a pre-existing disturbance.

How do they form?

• Ingredients for TC formation:

– sufficiently large background vorticity (generally 3° from

the Equator is minimum requirement),

– small vertical wind shear over the disturbance,

– a warm ocean to a sufficient depth (SST>26.5°C),

– conditionally unstable atmospheric column,

– moist mid-levels,

– a pre-existing disturbance.Large-scale environment,

strongly modulated by ENSO

How do they form?

• Ingredients for TC formation:

– sufficiently large background vorticity (generally 3° from

the Equator is minimum requirement),

– small vertical wind shear over the disturbance,

– a warm ocean to a sufficient depth (SST>26.5°C),

– conditionally unstable atmospheric column,

– moist mid-levels,

– a pre-existing disturbance.Large-scale environment,

strongly modulated by ENSO

Stochastic component

How do we make a TC prediction?

Use objective TC tracking scheme (Tory et al. 2012):

• Identify disturbances in the model at each time,

• Join these disturbances together to form a track,

• When certain threshold criteria have been met for 3

successive times, the disturbance is declared a “TC”.

t0 t1 t2 t3

How do we make a TC prediction?

Use objective TC tracking scheme (Tory et al. 2012):

• Identify disturbances in the model at each time,

• Join these disturbances together to form a track,

• When certain threshold criteria have been met for 3

successive times, the disturbance is declared a “TC”.

t0 t1 t2 t3

How do we make a TC prediction?

Use objective TC tracking scheme (Tory et al. 2012),:

• Identify disturbances in the model at each time,

• Join these disturbances together to form a track,

• When certain threshold criteria have been met for 3

successive times, the disturbance is declared a “TC”.

t0 t1 t2 t3

How do we make a TC prediction?

• TCs are tracked every day (in the 1 Oct run) for the entire SH

TC season (Nov-Apr) in each member of the POAMA

ensemble.

How do we make a TC prediction?

• TCs are tracked every day (in the 1 Oct run) for the entire SH

TC season (Nov-Apr) in each member of the POAMA

ensemble.

Pre-TC track TC track

How do we make a TC prediction?

• The number of TCs in each region in each member is

determined.

• The proportion of the ensemble above/below the long-term

hindcast average provides the probabilistic prediction.

Aus regions map

Obs. climo. avg.: 7

POAMA climo. avg.: 4

Prob. above: 40%

Obs. climo. avg.: 11

POAMA climo. avg.: 9

Prob. above: 56%

How well does POAMA do?

• The confidence in the TC prediction for each region is

determined by comparing the ensemble hindcast to

observations.

• Interannual variability in POAMA TCs compares well

with observations.

Grey bars are 1 standard deviation from ensemble mean, based on 30 members

How well does POAMA do?

Observations POAMA

ENSO years defined member Niño 3.4 SST anomalyENSO years defined following Kuleshov et al. (2008).

TC prediction products

What can we provide?

• Regional probabilistic TC predictions.

• Spatial distribution of anomalous TC activity.

EXPERIMENTAL

Future directions

• TC intensity predictions – the likelihood of more intense systems; accumulated cyclone energy (ACE) – a measure of the number of cyclone days and aggregated TC intensity.

• Intraseasonal TC predictions – predictions of TC activity for the next one, two or 3 months released more frequently throughout the TC season (POAMA multi-week system).

• Improved TC simulation – with POAMA 3 based on the ACCESS model, expect differences in model physics, dynamics and higher resolution to provide improvements in TC representation, seasonal and interannual variability.

Thank you

For further information

Kay Shelton

Research Scientist

Email: K.Shelton@bom.gov.au

Phone: (03) 8638 8288

Tory, K. J., Dare, R. A., Davidson, N. E., McBride, J. L., and Chand, S. S., 2012a: The importance of low-deformation vorticity in tropical cyclone formation, Atmos. Chem. Phys. Discuss., 12, 17539-17581, doi:10.5194/acpd-12-17539-2012.

Tory, K.J., S. S. Chand, R. A. Dare, and J. L. McBride, 2012b: The development and assessment of a model-, grid- and basin-independent tropical cyclone detection scheme. J Climate, accepted.

Tory, K.J., S. S. Chand, R. A. Dare, and J. L. McBride, 2012c: An assessment of a model-, grid- and basin-independent tropical cyclone detection scheme in selected CMIP3 global climate models. J Climate, accepted.