Gedday all... A good squall line (a beauty) on the Broome radar.

Radar Image:   256 Km:  20.40   20.50   2100   2110   2120   2130   2140
                        512km:     20.40   20.50   2100   2110   2120   2130   2140

Sat pics:  00z, 03z  05z 06  09 11  12 15  17  18  21  (Oz-scale:  00  09 21)

I'm really busy today... so don't intend saying much.  To the WA RFC:
How common is this sort of squall line?  Have they been studied?  Is much
known about them?

Talk to you later.

JMcB
 

Later:

Gedday,

I said I was too busy to talk today, and I am... However, I couldn't resist
having a look at the Broome sounding for this morning, measured an hour or so
after the large tropical squall line I pointed out earlier.

(If you go there this afternoon, it will be on
http://serva.sa.bom.gov.au/data/skewt/map.html..... I'll put it up on my web-
page when I have a chance.

Soundings 23 Z 23rd Jan

Anyway.. This sounding has a nice bulgey onion shape at low-levels, underneath
a layer of very high humidity, which presumably is in the trailing stratiform.

I have never studied tropical squall lines in any depth; but this sounding is
familiar to me... I'm sure I have heard Bob Houze or Ed Zipser or someone talk
about the "onion sounding" over the years.  Looking at it, it is clearly
modified air brought down from upper levels: high theta (hence warm) low
thetae (hence dry).

I'll cc this to Peter May and Tom Keenan.. They may have a reference.

Cheers

JMcB

Barry Hanstrum

John

Baz here from the wild west.  I've had some interest in these squall lines
at Broome for a number of years now.  They typically form late
afternoon/early evening over the inland Kimberley and blast out to sea
through Broome overnight, typically 10pm -midnight.  They occasionally
produce severe winds.  There have been a couple I recall with gusts up to
70 knots or so that have done some damage in Broome and been a problem for
ships in the port.  They can often be tracked over large distances to the
west and occasionally cause problems with unexpected gusty wind shifts at
the gas platforms offshore from Karratha.

The environment in which they form is normally the high shear/large CAPE
environment that supports a long lived organized squall line, in which some
sort of balance exists between the circulation driven by the cold pool
associated with the storm downdraft and the circulation in the opposite
sense implied by the background shear.  The typical set up for the shear is
low level westerlies associated with a trough inland overlain by easterlies
from say 750 hPa upwards.  Typical shear values between the surface and 700
hPa have a magnitude of around 30 knots or so...

This particular case doesn't fit the normal shear criteria, sfc to 700 Hpa
shear at Broome at 17z is weak, of the order of 15 knots or less, not
usually enough to organize a line in that fashion.  The 22z winds at Broome
following the passage of the line are interesting, there seems to be
evidence for a rear inflow jet with winds up to 40 knots from the east at
700 hPa.  By 05z today the winds had dropped down to below 20knots again..

Aany thoughts on how a line like this becomes organized in a relatively low
shear environment??

Baz

John McBride

> Any thoughts on how a line like this becomes organized in a relatively low
> shear environment??
>

And another question is "Why Broome?"; why not Learmonth or Wyndham?"

JMcb

Tom Keenan

Barry: The high shear condition is optimum for long lived squalls but
not necessary-see Rotunno's1988 (JAS , 45, 463-485) paper.  With an out
of balance situation e.g.  a dominant cold pool forcing can lead to
intense convective situations that will propagate.  Tom