Mean radiosonde soundings for the Australian monsoon/cyclone season

Abstract

Motivated in part by a potential application to modelling tropical cyclones in the Australian region, mean radiosonde soundings are determined for the three northern Australian stations, Willis Island, Darwin and Weipa, during the core months of the cyclone season (December-February). More than 8500 individual soundings are examined in 30-year datasets for Willis Island and Darwin (1980-2010) and a 15-year dataset for Weipa (1998-2013). These soundings are stratified into three groups according to the low-level wind direction (monsoon regime, easterly flow regime and the rest). The mean soundings for the monsoon regime (low-level winds in the sector west to north) are compared at the three stations and diurnal differences are investigated at stations with two soundings per day. The mean monsoon Willis Island sounding is compared also with the Dunion moist tropical (MT) sounding, which is frequently used as an environmental sounding in the numerical modelling of tropical cyclones. The Willis Island sounding is 1-3 degrees C warmer and somewhat drier than the Dunion MT sounding through the entire troposphere, although the relative humidity differences are relatively small (less than 5% at most observed levels). Idealized numerical simulations of tropical cyclone evolution are performed to assess the implications of using one thermodynamic sounding or another for tropical cyclones in the Australian region. The simulations highlight the importance of not only the environmental sounding for the intensification of model storms, but also the sea surface temperature combined with the sounding.