- Title:
-
On Easterly Changes Over Elevated Terrain in Australia's Southeast
- Date:
- April 2007
- Organisations
- Bureau of Meteorology
- Authors:
- Graham A. Mills
- Location:
- Australia, Australia
Overview
This investigation of the structure and evolution of easterly changes through the southern highlands of New South Wales, Australia, was initially prompted by the apparent association of a band of extremely low humidity that was observed just before an easterly change in Canberra on the afternoon of 18 January 2003 (the day of the catastrophic bushfires) with the passage of that cool change. A broader investigation of similar extreme drying events revealed ten easterly change events over the highlands of southeastern Australia that were preceded by similar marked reductions in near-surface humidity. An interesting feature of several of these events is the presence of a long, narrow, dark (dry) band in the water vapour channel (6.7 μ) geostationary satellite imagery parallel to the easterly cool change.
These ten events show a remarkably similar structural evolution, with frontogenesis commencing during the morning near the top of the coastal escarpment, but inland penetration of the cool change not commencing until a deep mixed layer has formed over the plateau on the warm side of the developing front and the frontal temperature gradient has increased to near the maximum value it attains on a given day. It is shown that while these ‘easterly cool changes’ have gravity-current like characteristics, which has led to them being considered sea-breeze fronts, their origin appears to be on the escarpment rather than the coastline.
A strong, thermally direct cross-frontal circulation is a common feature of
these fronts, and the strong pre-frontal ascent generates a gravity wave in the
more stable westerly flow above the deep mixed layer on the warm (western)
side of the front. It is hypothesised that the narrow, dark (dry) bands seen in
the water vapour channel (6.7μ) geostationary satellite imagery mark the downward motion on the eastern (downstream) side of this gravity wave. It is also hypothesised that the strong vertical motions associated with the crossfrontal circulation enhance the entrainment of dry mid-tropospheric air from just above the mixed layer, thus leading to the near-surface humidity reductions observed just before frontal passage.
