The Multi-Scale Structure of the Australian Cool Changes |
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| Authors: | H Kraus C M Ewenz M Kremer J M Hacker |
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| Institution: | Meteorologisches Institut der Universit?t Bonn, Auf dem Hügel 20, D-53121 Bonn,
Flinders Institute for Atmospheric and Marine Sciences, Adelaide, Australia, AU
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| Abstract: | Summary In Southern Australia summertime deep cold fronts are frequently preceded by a shallow cold frontal line connected to a prefrontal
lower tropospheric trough. The advance of this line defines a “cool change” which in many cases causes severe weather events.
The goal of this paper is to analyze the multi-scale structure of these cool changes using aircraft observations and synoptic-scale
analyses. The aircraft measurements on cross-frontal tracks of horizontal lengths of up to 300 km are performed with an average
resolution of 3 to 4 m along the track. Thus a multi-scale analysis from micro-scale events up to the synoptic-scale phenomena
can be presented. All flights and thus all meso- and micro-scale analyses are performed over water only. The obviously very
different characteristics of the cool change structure elements over land are not investigated.
The synoptic analyses for one very typical case show a prefrontal trough as characterized by its position in relation to the
main deep cold front, its source region in Western Australia and its extent to the southeast. Fields of strong wind shear,
temperature gradients, vertical wind and Q-vectors are displayed.
The meso-β-scale x, z-cross-sections derived from two aircraft missions (data of the second one in brackets) show: a shallow cold front with a
160 (60) km wide transition zone in which the near surface potential temperature drops rather steadily by 9 °C (20 °C); a
shallow feeder flow topped by a strong inversion with a vertical gradient of potential temperature up to 5 °C/100 m between
the top of the feeder flow at 400 (200) m and 1500 (700) m; a cross-frontal circulation expressed by the ageostrophic wind
components u
ϕ,subscale and w with a center at 1200 m over the frontal edge of the feeder flow (for one mission only); a strong shear of the along-frontal
wind component v
ϕ with a large increase of the negative v
ϕ-values with height, which very well fits to the synoptic-scale view of the wave structure of the geostrophic wind (well-known
from the upper level synoptic charts) at different heights; a jet core of this along-frontal wind in the center of the cross-frontal
circulation, again for one mission only.
A very striking example of a micro-scale event is an approximately 1 km wide head of a frontal squall line. It shows dramatic
changes of all meteorological parameters. The event is displayed in a horizontal domain of 4 km with full resolution (∼ 4 m).
Derivatives of the measured parameters in the cross-frontal direction add information to the space series of the parameters
themselves. Deformation frontogenesis of potential temperature and specific humidity show very large values on the scale resolved
here. Fortunately the squall line could be sampled again at the same height, but in a somewhat degenerated state 1? h later.
Received September 3, 1999 Revised December 14, 1999 |
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