From: John McBride <jmb@bom.gov.au>
To: synoptic_discussion@bom.gov.au
Cc: Dale Hess <dzh@bom.gov.au>, k.tory@bom.gov.au
Subject: Fires: dry adiabat to 600hPa; lower tropospheric drying
 

Gedday,

A) DRY ADIABAT UP TO 600 hPa

Last night I looked over the Bureau 1995 report and a couple of other
papers that have been written on the '94 fires.  As several people have
pointed out, it is a remarkably similar situation to the current one.

As with the current system, there was a very deep mixed layer at Sydney on
the major fire-danger days, with a dry adiabat up to about 600hPa.

Does anyone have any background knowledge on the cause of these incredibly
deep mixed layers?  How common are they in summer?  What is their cause?
Are they related to the very strong winds at 80hPa associated with the low
south of Tassie?

Are Dale Hess and Kevin Tory on this discussion group?   I know they have
worked on mixed-layer depth and boundary layer height.  I have seen maps of
boundary-layer height from meso-Laps on the web somewhere, but can't find
it today.  Anyone know where this is kept?  Also I know Kevin Tory does
diagnostics comparing the boundary layer heights from meso-Laps with the
soundings. Do you put them on the web anywhere Kevin?
 

B) HOW DID IT GET SO DRY?
The other day we (I?) were discussing the large very dry area over NSW
stretching across to northern WA, with dewpoint temperatures down to -3C.
We were speculating on how it got so dry and we proposed that it could have
simply been a dilution effect of the surface moisture, with it being mixed
up through such a deep layer.  Looking again at the sounding, that doesn't
explain it... the sounding has dried out at all levels up to 600hPa, so
there has been a net large scale loss of moisture....any further ideas on
this?  Do the LAPS people do maps anywhere of 850 hpa mixing ratio?  Are
there archives, anomoly maps etc of parameters like this?  Maybe I'll have
to stroll down the corridor to the Guru-Murrumbidgee and get some scripts
to (heaven forbid) run these jobs myself?

Cheers

JMcB

From Kevin Tory

Hi John,

I have been producing maps of the boundary layer height at
Melbourne and Sydney airports on a daily basis, and they have
been posted on the AAQFS web site.  This site has access
restrictions.  With the demise of Ansett, we lost 80-90% of the
data and the plots have been mostly meaningless.  In addition to
this I have been producing equivalent pt050 LAPS plots at
Melbourne and Sydney Airports and at Shane Park and
Broadmeadows. The pt050 LAPS plots at Sydney airport rarely
show deep mixed layers due to the influence of the sea nearby.
However, the Shane Park pt050 LAPS plots (~ 50 km inland, I
think) regularly show mixing depths reaching 700 hPa.  I would
expect the mixing depths to continue to increase further inland.
(These latter plots are not posted on the web.  I have hard copies in
my office, and electronic copies on sam that can be retrieved with
a bit of effort.)

Cheers,

Kevin.

From Harald Richter

I have seen many well-mixed dry PBLs west of a late spring dryline in
Texas and Oklahoma.  PBL heights up to ~500 hPa were not uncommon,
while tens of kilometres east (i.e. E of the dryline) a rather humid
PBL topped out around 850 hPa.

A deep dry well-mixed layer makes a lot of sense to me as being conducive
to fire conditions.  Near-surface moisture (from ET and half-empty beer cans)
can quickly be mixed out and removed from the fuel on the ground.
Deep mixed layers can also transport higher mid-level (600 hPa)
momentum to the surface, creating and adding to the woes of gusty surface
winds.
Deep dry layers offer little in the way of absoption for incoming solar
radiation.

> B) HOW DID IT GET SO DRY?

For me this is the old question of partitioning a summary effect
into (i) vertical mixing through a deep layer (driven by the annual
maximum in incoming solar radiation) + (ii) horizontal advection
(parcels on slowly descending trajectories? / low-level trajectories that
haven't seen a body of water in ages?).  Once the soil is dry and
the plants are stressed and the clouds are gone,  all in situ sources
of moisture are absent.

I feel like a drink now,  cheers,
Harald