Mesoscale Observations of a Prefrontal Squall Line

Abstract

Phase I of the Cold Fronts Research Programme was carried out during November/December 1980 in south-eastern Australia. Data from a frontal event on 27 November are analyzed, with particular emphasis on a squall line that formed over the Southern Ocean and moved across the predominantly land-based network. Potential instability between 750 and 550 mb, arising from the subsynoptic scale circulation associated with the frontal system, existed ahead of the squall fine. Boundary layer forcing resulting from convergence of the cold outflow and presquall air was responsible for release of this instability as the line passed through the observing network. The lowest 200 m of the nocturnal boundary layer were undisturbed by the cold outflow, as this radiatively cooled layer was potentially colder than downdraft air. A surface pressure jump of 3 mb was recorded and the low-level wind rotated through 360° in two hours in response to the associated mesohigh. Two-dimensional (x?z) cross sections of wind and thermodynamic variables composited from aircraft, radiosonde and pibal data reveal centre of upward motion at 800 mb near the leading edge of the outflow and 50 km behind at 500 mb, with the latter being the stronger of the two. Cooling extends to 620 mb and the important role played by relatively dry air entering the system from the rear at middle levels is clearly shown.