Australian region tropical cyclones: Influence of environment at different scales.

摘要

This dissertation explores the influence of environmental factors on a variety of spatial and temporal scales on tropical cyclones (TCs) in the Australian region. Chapter 1 provides the motivation for the work presented, and leads into a discussion on the current state of knowledge of large-scale factors affecting the interannual variability of TCs in each of the seven global TC basins (Chapter 2). Chapter 3 is an investigation of the role of large-scale environmental factors, notably sea surface temperature (SST), low-level relative vorticity, and deep tropospheric vertical wind shear, for the interannual variability of November-April tropical TC activity in the Australian region. Extensive correlation analyses were carried out between TC frequency and intensity and the above-mentioned large-scale parameters, using TC data for 1970-2006 from the official Australian TC data set. Large correlations were found between the seasonal number of TCs and SST in the Nino 3.4 and Nino 4 regions. These correlations were greatest (-0.73) during the August-October period, immediately preceding the Australian TC season. The correlations remain almost unchanged for the July-September period and therefore can be viewed as potential seasonal predictors of the forthcoming TC season. In contrast, only weak correlations (+0.37) were found with the local SST in the region north of Australia where many TCs originate these were reduced almost to zero when the ENSO component of the SST was removed by partial correlation analysis. The annual frequency of TCs was strongly correlated with 850-hPa relative vorticity and vertical shear of the zonal wind over the main TC genesis areas of the Australian region. A Principal Component Analysis of the SST data set revealed two main modes of Pacific Ocean SST variability that match very closely with the basin-wide patterns of correlations between SST and TC frequencies. It was also found that the above-mentioned large correlations could be increased markedly (e.g. from -0.73 to -0.80 for the August-October period) by a weighted combination of SST time series from weakly correlated regions.When only the eastern region subset of the Australian TC data set was considered (Chapter 4), including the annual number of landfalling TCs in the northeastern state of Queensland, the correlations between TC number and ENSO decreased substantially. These correlations were reduced to less than +0.1 during the warm phase of Interdecadal Pacific Oscillation from 1979-1998, suggesting that the relationship between TC activity and ENSO fluctuates on interdecadal time scales. The number of landfalling TCs was highly correlated (+0.68) with total number of TCs forming in the eastern region each year.The interaction between complex terrain and a landfalling TC over northeastern Australia is investigated in Chapter 5 using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) Mesoscale Model (MM5). Severe TC Larry (March 2006) made landfall over an area of steep coastal orography and caused extensive damage. The damage pattern suggested that the mountainous terrain had a large influence on the TC wind field, with highly variable damage across relatively small distances. The major aims in this study were to reproduce the observed features of TC Larry, including track, intensity, speed of movement, size, decay rate, and the three-dimensional wind field, using realistic high-resolution terrain data and a nested grid with a horizontal spacing of 1 km for the finest domain (referred to as CTRL), and to assess how the above parameters change when the terrain height is set to zero (NOTOPOG). The TC track for CTRL, including the timing and location of landfall, was in close agreement with observation, with the model eye overlapping the location of the observed eye at landfall. Setting the terrain height to zero resulted in a more southerly track and a more intense storm at landfall. The orography in CTRL had a large impact on the TC's 3-D wind field, particularly in the boundary layer where locally very high wind speeds, up to 68 m s -1, coincided with topographic slopes and ridges. The orography also affected precipitation, with localized maxima in elevated regions matching observed rainfall rates. In contrast, the precipitation pattern for the NOTOPOG TC was more symmetric and rainfall totals decreased rapidly with distance from the storm's center. Parameterized maximum surface wind gusts were located beneath strong boundary-layer jets. Finally, small-scale banding features were evident in the surface wind field over land for the NOTOPOG TC, owing to the interaction between the TC boundary layer flow and land surface characteristics.