Tasmanian springtime frontal events
Origins of dry air
Background
• Increase in severity of springtime events
in the last several decades
• Objectively verified
• Why?
(Recent) Approaches to frontal case
studies
Mills 2008 (a, b)
(and 2002, 2003, 2005a,b, 2006,
2007, 2010)
Dry slots, and short-term sfc dryings
assw frontal circulations
Parallel strand of research in US:
Zimet et al 2007
Kaplan et al 2008
Huang et al 2009
Events
• 2 high-end spring fire weather days:
– 7 November 2002 (40+ scrub fires Hobart
area)
– 12 October 2006 (800 Ha burnt Hobart
eastern shore. FFDI 100+ for several hours.)
7 November 2002: Broad environment
MSLP: 0000 UTC 5, 6 and 7 November
FFDI at Hobart Airport
300 hPa
Trough increasingly tilted as it approaches Tasmania
IR Imagery
Development evident in period from 06/2030 to 07/0530
WV @ 0430 UTC 7 November
Meanwhile…Hobart Ap 06/2300 UTC
Dry aloft, unstable, 50 kt at 850 hPa
Airmass Origins
Courtesy of
Alan Wain
1500m less
“cluttered” than
surface
What’s going on in Eucla?
Very dry
around 2000m
Moisture from Bight…?
Model vertical sounding
near 38S 136E ,
from 24 hour forecast of
mesoLAPS, initialised at
0000 UTC 05 November
• Maybe not!
• Strong MBL
inversion
(for model)
Cross-section through line at left
Hobart at arrow location above
Cross-section displays RH (black, %)
and PVOR (red, PVU)
And windspeed/isentropes…
MBL north coast
Foehn effect
Decreased, but not unusual,
RH
12 October 2006 – antecedent conditions
MSLP
Hobart Airport
Hobart Airport Weather and FFDI 12 October 2006
140
40
windspeed
FFDI
Windspeed (km/h) and FFDI
dewpoint
30
100
20
80
10
60
0
40
-10
20
Time (UTC)
:0
0
12
9:
00
6:
00
3:
00
0:
00
:0
0
-20
21
18
:0
0
0
Temperature and Dewpoint Temperature
(Celcius)
air temp
120
300 hPa + cross-sections
Hobart Airport 11/2300 UTC
Airmass origin…
Along the way…
Again, N coast MBL
Lee downslide winds
Low (but not exceptional)
RH
Satellite image support/verification
Compare events…
Both reveal effects of Tasmanian orography
– foehn effect/MBL north coasts
12 October 2006 – no evidence of
intrusion of drier air from aloft. Airmass
is very dry anyway
But… what’s different? Both have
approaching change, jet streak nearby
So…
• Isentropes essentially horizontal
• While tropopause depression is evident,
intrusion doesn’t extend to low levels
• Axis of trough well west of Tasmania
Summary
MesoLAPS resolved dynamics well in
both cases, it seems - scope for using
xn to diagnose possible dry air descent
But low level moisture fields not wellresolved (take note GFE regions!)
Variety (>=2!) of mechanisms
possible for dry air to occur ahead of
Tas fronts
Some common features
Finally - Possible alert trigger (?)
Wind speed vs Dewpoint depression Hobart Airport
1992-2010
45
40
Wind speed (m/s)
35
30
25
20
15
10
5
0
-10
0
10
20
30
40
Dewpoint depression
50
60
70