Numerical Weather Modelling of the Factors affecting the Rapid Intensification of Tropical Cyclone Yasi

Tropical Cyclone (TC) Yasi was a rapidly intensifying storm that began cyclogenesis just northeast of Fiji on January 29th 2011. It tracked southwest, continually gaining intensity until reaching category 5 by the time it made landfall at Mission Beach on the Australian Eastern coastline in the early hours of February 3rd 2011. Yasi was measured at 600km wide and produced a storm surge of up to 6m in height, wind speeds of up to 300km/h, and a minimum central pressure of 929hPa recorded at landfall. The storm had a large impact on the coastline and particularly the Great Barrier Reef ecosystem, causing mechanical damage to the reef structures, reducing salinity and temperature, and bringing anomalously high levels of nutrient and sediment loading into the reef lagoons as flood waters receded from land.

A full suite of numerical weather modeling simulations has been carried out with the National Center for Atmospheric Research (NCAR) Weather Research and Forecasting (WRF) Model v.3.4.1 to simulate this TC event. The simulations with WRF were utilized to determine an appropriate model set up and the boundary and initial condition data required to accurately simulate the track, timing and intensity of this extreme event. Simulations reveal the difficulties in capturing the intensity of an event of this size and magnitude but also important details about the characteristics of the storm environment that supported such an extreme event. Sensitivity simulations demonstrate how an intense TC can be sensitive to environmental factors in the region such as sea surface temperature (SST) values and gradients.

This modeling work is combined with analysis and quantification of changes to SST; nutrient and sediment loading; and chlorophyll a concentrations in surface waters along the coastline from before to after TC Yasi with remotely sensed data sets such as from MODIS. The data validates the wide spread interaction that TC Yasi and events of this size and intensity have with the ocean surface and the negative and long term impacts they can have on coastal and marine ecosystems.