The influence of topography on midlatitude cyclones on Australia's east coast

Abstract

The east coast of Australia has a relatively high frequency of midlatitude cyclones, locally known as East Coast Lows (ECLs), which can cause severe weather including widespread flooding and coastal erosion. The elevated topography close to the east coast has been hypothesized to play a role in both the genesis and impacts of cyclones in this region, but existing studies have been limited to case studies of individual events. In this paper we present the results from two 20 year simulations over the Australian region using the Weather Research and Forecasting Model and assess the results from removing all topography in the region on both mean atmospheric circulation and ECL frequency. Removing topography results in an increase in sea level pressure to the south of Australia and an increase in moisture flux convergence and rainfall near the east coast, as well as a decrease in potential vorticity to the north of the ECL region. This results in a change in the spatial distribution of cyclones, with a 37% decrease in the frequency of cyclones that develop to the south of the ECL region but a 20% increase in cyclones near the east coast. This results in little overall change in the frequency of ECLs and suggests that coarse topography is unlikely to be responsible for the difficulties in simulating coastal cyclones in global climate models.

Orography Drives the Semistationary West Australian Summer Trough

“…Similar declines in moisture flux and rainfall are also observed in the other three seasons, resulting in a greater than 40% decline in total annual rainfall (Pepler et al, ). There is, however, a wet bias in the control WRF simulation in inland Australia during the rest of the year, associated with anomalous northeasterlies in inland Australia, which may exaggerate the role of orography in rainfall during these seasons.…”

“…The model simulations used for this paper were described in detail in Pepler et al (2017). In brief, version 3.6 of the Weather Research and Forecasting model (WRF; Skamarock et al, 2008) was used to downscale the ERA-Interim reanalysis (Dee et al, 2011) over the CORDEX-Australasia region (Figure 1).…”

“…This research was supported by the Earth Systems and Climate Change (ESCC) Hub of the Australian Government's National Environmental Science Programme (NESP). This paper uses model simulations generated by Pepler et al (), which are archived through the UNSW Research Data Archive, with postprocessed datasets available through the National Computational Infrastructure. Lisa Alexander, Jason Evans, and Steven Sherwood played important roles in the original experiment design, with Alejandro Di Luca providing valuable assistance in running the WRF simulations.…”

“…More complex models, such as 3D general circulation models, have not been used to analyze the influence of orography on the WCT, although the most recent generation of global climate models simulate the mean summer circulation over Australia well, including the WCT (Moise et al, 2015). However, a recent study assessing the influence of removing Australian orography on east Australian rainfall also identified a substantial change in the mean circulation over WA (Pepler et al, 2017). The 21-year regional climate model simulations performed in that study offer a useful data set to reassess the role of Australian orography in the generation of the WCT.…”