Gedday,

Recently our resident Guru Murrumbidgee gave me a paper to internally review whereby he carried out a nice case study within the MesoLAPS environment of a cool change involving coastal frontogenesis and a surging of the front along the coast.  I'll leave the dynamical details for his paper when it appears (Mills:  A case of coastal interaction with a cool change, Australian Met Mag, submitted); but some idea of the his dynamical hypotheses is contained in the Abstract, which is reproduced below:

The cool change we had through Melbourne at the start of this week  (Monday afternoon) is a good one as it showed many of the features pointed out by  the Guru in his paper. In particular, there was an intensification of the surface front as it moved from the ocean into the continental very deep mixed layer; there was also an impressive surge of the front along the coast, apparently associated with a reversal of the pressure gradient.

The sequence is easiest to see in the 5 km LAPS forecast field for the event; and (needless to say), I have put the relevant charts up on my web-page; and for reference, I have also put up the Victorian maptool plot of AWS winds and temps, just to anchor the model to reality.

Beginning with the starting analysis (23 UTC 17 March), over the ocean there is a broad trough with a temperature gradient of the order of 4 degrees per 100 km and a general zone of convergence between northerlies and northwesterlies.  Admittedly this is an "initial model field; so it could be argued  the model hasn't yet had a chance to spin up a good sharp front;  However, the guru's hypothesis (based on observations of many fronts) is that the sharp frontal structure over the sea is masked (or hidden) by the stable maritime boundary layer.  At this stage, (2300 UTC) the sun is already up and you can  see temperatures have started to rise over western Victoria and are already up to 26 C.

Looking at the screen-level temperatures 3, 6 and 9 hours later, we can see an enormous increase in temperature gradient along the coast as the land heats up under diurnal heating, and between 0200 and 0500 UTC a sudden jump of the front to about 100 km inland, followed by a further jump in the next three hours.  The Mills view is that the front moves onto land at about the same time as land surface commences heating.  The heated surface brings about a very deep mixed layer, up to 700 hPa or so (see synoptic discussion of last Monday, where I showed a couple of soundings).  Thus the static stability of the air ahead of the front decreases, the intensity of the cross frontal circulation increases, the isentropes (inferred from the isotherms) become very steeply sloped, the intensity of the vertical circulation is greatly enhanced, and the thermal gradient is enhanced - i.e frontogenesis occurs. Finally, the hitherto extremely regular eastward progression of the frontal ascent maximum accelerates (Mills, 2002 - submitted).

The wind change line closely follows the thermal front (or leading edge of the region of high thermal gradient) so the wind-defined change also jumps inland between 0200 and 0500 UTC, as shown in the actual observations:

                    Obs 02 UTC                                                                                        Obs 04 UTC
 

Another interesting feature Graham points out is the generation of a surge along the coast.  This can be seen clearly in model pressure fields and sfc winds at 0800 UTC, as well as the observations for the same time (all on my web page.)

According to Graham, this surge is generated as a result of a reversed pressure gradient over the western Victorian/Cape Otway region as seen for example on the model pressure fields for 0500 UTC.

The reverse pressure gradient is a hydrostatic response to the surface heating of the land.  This reversal of the preexistent pressure gradient force brings about a sudden change or disruption in the pre-existing "semigeostrophic" balance of forces.  The response of the atmosphere is one of rapid motion due to this imbalance, analogous in some ways to the development of an inertial oscillation when the low levels of the atmosphere become decoupled from the friction layer.

Anyhow, whether or not I/we agree Graham's proposed mechanisms are occurring requires some inspection of the events, and some thought.... But last Monday's frontal passage, and the very clear diagnostics real-time maps that are now available on the internal web, gives us a chance to have a careful look at some of these events.
 

Well... That took up most of my day.... time to go off and check my diary to see what I had actually planned on doing.

cheers

John McB
 

(The Abstract of Graham's paper:  " A surging cool change moved along the Victorian coastline on the afternoon of 21 January 1997.  A mesoscale numerical prediction of this event produced an excellent simulation of the evolution of this front, and is used to diagnose processes leading to the intensification and movement of the change.  It is shown that the development of an internal boundary layer in hot, dry offshore flow can mask the thermal structure of the change at the lowest levels over the ocean, and can also provide a coastal thermal (density) gradient which, with the reversal of the coastal pressure gradient can lead to the surging of a front along the coast.  Differing wind structures before and after the change are seen in the inland and coastal portions of the change, and the implications of these for operational forecasting are discussed."