Factors Affecting the Intensity and Trajectory of Tropical Cyclone Yasi (February 2011)

Abstract:

Tropical Cyclone (TC) Yasi was a rapidly intensifying, category 5 storm that made landfall in Queensland, Australia on February 3^rd, 2011. An ensemble of physics sensitivity simulations of TC Yasi using WRF v3.4.1 on a 12-km nested domain demonstrates that the choice of cumulus scheme dominates the outcome of the simulated intensity, trajectory and timing of the cyclone. The choice of microphysics parameterization, planetary boundary layer, surface drag coefficient, and ocean mixed layer scheme had smaller effects. The Modified Tiedtke scheme preferentially activates shallow convection, producing an accurate planetary boundary layer and profile of dry and moist air. However, deep convection and storm intensity are hindered. Thus the scheme produces the most accurate trajectory and landfall location but underestimates intensity and depth of the storm. Conversely, the Kain-Fritsch scheme initiates more deep convection and suppresses shallow convection, allowing storm deepening and intensification. However, subsidence from this convection scheme results in dry air entrainment aloft and a more shallow and moist boundary layer than observed. This scheme more accurately simulates depth and intensity but the trajectory and landfall location are too far south. In all ensemble runs, the simulated event reached maximum intensity over the open ocean and dissipated towards landfall, contrary to Yasi’s observed lifecycle. In order to represent potential warming of the East Australian Current (EAC) not resolved in the previous experiments, idealized simulations were performed where a 1°x1° resolution auxiliary sea surface temperature dataset was assimilated into the surface (lower boundary) forcing and 1°K was added to the coastal lagoon. This warming improves the simulated evolution and maintenance of intensity towards landfall and implies that additional warming can fuel further intensification. As the speed, volume transport and temperature of the EAC are predicted to increase with climate change, there are important ramifications for the life cycle of TCs in the area and for coastal settlements in the path of future storms.