High-Resolution Numerical Simulation of Tropical Cyclone Larry (2006): Sensitivity to Orography

Severe tropical cyclone Larry made landfall near the town of Innisfail, Queensland, on the morning of 20 March 2006, causing approximately $360 million in damages to buildings and crops in the region. This region is important from an Australian tropical cyclone (TC) perspective owing to the complex nature of the terrain in close proximity to the coastline, with peaks reaching 1600 m. Many previous studies on the influence of orography on TCs have focused on regions such as Taiwan, Mexico, and the Caribbean Islands, but so far the little work has been done for the Australian region.

Analysis of damage following the passage of TC Larry suggested that the steep terrain played a significant role in determining wind speed over land, as evidenced by varying degrees of damage over relatively small distances. Engineers from James Cook University found that wind speed up over topographic ridges did indeed lead to an increase in damage to buildings, while structures sheltered by topography suffered much less damage. In addition, some towns suffered a disproportionate amount of damage relative to their distance from the core winds at landfall.

The current study uses the fifth-generation Pennsylvania State Univeristy-National Center for Atmospheric Research (PSU-NCAR) Mesoscale Model (MM5) to simulate TC Larry. Two simulations are carried out: (i) a control simulation (CTRL) with very high-resolution terrain data (~ 900 m) used to investigate the influence of complex terrain, and (ii) a sensitivity simulation (NOTOPO) in which the terrain is totally removed. The land surface characteristics for both simulations are the same. All simulations are performed on a quadruply nested two-way interactive mesh with dimensions and grid spacings of 93x100, 27 km (D1), 220x210, 9 km (D2), 445x286, 3 km (D3), and 385x268, 1km (D4), respectively. Model physics include a Mellor-Yamada type 2.5 order planetary boundary layer (PBL) scheme, Betts-Miller cumulus parameterization and Reisner mixed-phase cloud microphysics.

Results show that the terrain plays a significant role in the track, maximum intensity, and distribution of wind and precipitation of the TC. The CTRL TC makes landfall with a central pressure of 927 hPa. The southern part of the eye passes directly over Innisfail in excellent agreement with observation. The NOTOPO TC makes landfall 80 km further south with a central pressure of 911 hPa. An examination of simulated radar reflectivity and boundary layer winds reveals a highly symmetric structure relative to the CTRL TC for which the maximum winds occur in the western half of the circulation over land as well as the coastal fringe exposed to the southern part of the eye wall. The CTRL simulation also shows several local orographically-induced wind maxima well removed from the core eye-wall winds. In addition, a significant westerly downslope windstorm develops over the mountain range west of Port Douglas, about 100 km north of the eye. Finally, it was found that the NOTOPO TC decays at a faster rate than the CTRL TC, suggesting that the overall decay time is relatively insensitive to the underlying terrain.