The Australian Summertime Cool Change. Part II: Mesoscale Aspects

Abstract

Observations of four cold-frontal systems traversing the coastal region of southeast Australia in late spring and early summer are described in terms of process occurring on the mesoscale. A conceptual model is presented which summarizes the main results of the data analysis. Features found in common with other studies of cold fronts include: (i) the multiple-line nature of the frontal transition zone (FTZ); (ii) concentration of cyclonic relative vorticity at a height z≈1 to 1.5 km in the rear of the FTZ; and (iii) the existence of a prefrontal jet at z≈1.5 km, northerly in our case, southerly in the Northern Hemisphere. The change lines within the FTZ (and at the leading edge if there is no sea breeze) are most probably convective instability lines whose alignment and movement depend on the large-scale, cloud-layer winds. The lines are evident as mesoscale cloud bands from satellite imagery and as rainbands from radar. At least one of these develops into a vigorous squall line whose cold outflow produces a pressure jump, and related wind-shift line. Movement of the pressure-jump line depends both on the gravity-current nature of the cold outflow and the environmental wind field. The squall line and pressure-jump line are associated with mesoscale high and low pressure features to which the boundary-layer wind field responds. The structure of the FTZ up to z=2 km appears to be dominated by the presence of the squall line, with upwards motion ahead and downwards behind. On a horizontal scale of 100 km, cyclonic vorticity reaches twice the Coriolis parameter f in the vicinity of the squall line. Frontogenesis occurs largely within the FTZ with horizontal convergence and deformation processes being of comparable importance. The prefrontal jet is broadly in thermal wind balance with the horizontal temperature gradient which is, itself, determined by the fact that prefrontal air closest to the FTZ originates farther to the north and is therefore hotter than prefrontal air more distant from the zone.