Thermodynamic Structure of the Maritime Troposphere Around the Australian Continent

A set of 1672 radiosonde profiles taken during a 26-year period in the maritime region around Australia have been analysed. The data were stratified in six subregions of which three can be regarded as tropical, two as subtropical and one as representing mid-latitude climatic conditions. Skew-T diagrams together with Rossby diagrams were used to diagnose the atmospheric structure. A Rossby diagram uses potential temperature and water mixing ratio as independent coordinates. This set of coordinates facilitates the comparison between different atmospheric states and climatological regimes. Characteristic curves on the Rossby diagram are explained in terms of relevant transport and air mass transformation process. The results presented in this paper indicate that in order to compare different climatological regimes it is important to distinguish between levels above and below the average top of the boundary layer. The most robust method to identify the mean top of the boundary layer is to designate it as the level of minimum equivalent potential temperature. Thermodynamic profiles in the northern tropical region shift from a deep convective equilibrium structure in the (monsoon) summer to a stable regime in winter with temperature profiles following the wet-adiabat. The deep convective equilibrium structure is neutral to parcels ascending from within the boundary layer to the mid-troposphere without shedding their liquid water, i.e. the reversible wet-adiabatic ascent. Furthermore, there appears to exist a large separation in thermodynamic structure between the group of three tropical regions and the group of two subtropical regions and one mid-latitude region. The analysis was supplemented by plotting relationships between the precipitable water and surface air temperature. The precipitable water is a well described function of the surface air temperature, in agreement with reports in the literature.