Modelling the Ocean Response to a Tropical Cyclone on Australia's Northwest Shelf

Madeleine Cahill, CMAR

Peter Craig, CMAR

Mike Herzfeld - CMAR

Lou Mason - UTas

Can we model the ocean response to tropical cyclones ?

• Tropical cyclones: infrequent, small, energetic

• Dynamic atmospheric models have little skill in generating TCs

• Observations of cyclones in Australia are generally land based

• In situ observations of ocean response are rare.

• Effects are devastating (oil rigs, coastal regions…)

Ocean observations NWS – 1995 current meter deployment

ADCP

x current meter mooring

Path of TC Bobby

SHOC - CSIRO’s Coastal Ocean Model

3-D primitive equation hydrodynamic model Arakawa C-grid free surface vertical z coordinates a choice of turbulence closure schemes

Our application 4km & 20km horizontal grid resolution 46 z-levels k- turbulence closure scheme Horizontally uniform vertical temperature profile

with a 20m mixed layer

Tropical Cyclone Model

Double vortex Holland model

- track prescribed

- radius to maximum winds determined by fitting to

available wind and pressure observations

Available marine wind observing sites

Cyclone model fit to wind observations: Site 1

Model fit to wind observations: site 2

Tropical Cyclone Model Parameters

Radius to maximum winds Central pressure deficit

Two Holland wind fields: Bobby1 (blue), Bobby2 (pink)

Holland wind fields - wind stress magnitude ocean current vectors (pink)

Bobby1 Bobby2

Filtered current vector timeseriesADCP mooring in 125m (central site)

Modelled ocean response at ADCP mooring

Wind forcing: Bobby1 Bobby2

Comparing response to two wind fields: Bobby1 (m) and Bobby2 (black) at the ADCP (125m)

Alongshore component Cross-Shelf component

Comparing response to Bobby1(m) and Bobby2(black) at offshore mooring (M2 in 300m)

Alongshore component Cross-shelf component

Filtered current vector timeseries Offshore mooring in 300m

Modeled response at offshore mooring:

windfield1 windfield2

Velocities at ADCP location Observations (blue) v SHOC (mag)

Conclusions

• Model-data fit is starting to look pretty good • Model response is very sensitive to TC extent and location

Further work• compare SHOC results with ROMS• explore the effect of different vertical mixing schemes (using

temperature as well velocity for verification)• There are a further 11 TCs for which we have some current and

temperature observations from the NWS.

Thank you

Email: madeleine.cahill@csiro.au

Web: www.cmar.csiro.au