The Australian Summertime Cool Change. Part II: Mesoscale Aspects

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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 outflo... 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 outflo...

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