Transcript Slide 1

Lecture 20: Air masses & fronts (Ch 9)
• conditions under which ELR approximates DALR or SALR
• trying to locate surface fronts – example of guidance from 850 mb map
• a look at a complex case where pressure field shows distinct influences
of a mid-lat. storm and a lee trough - difficult or impossible to detect
surface fronts
• any spare time - questions
Conditions under which actual lapse rate (ELR) approximates
DALR or SALR
925-720 mb layer “neutral
w.r.t. unsaturated adiabatic
motion” (well-mixed)
“well-mixed”
implies
QH=0
700-500 mb layer “neutral w.r.t. saturated
adiabatic motion” (well-mixed)
Fig. 9-4
Fig. 9-4
warm air cut off from the
surface by the meeting of two
cold fronts
Fig. 9-10
Can we diagnose fronts associated
with this Manitoba storm?
12 hr motion
The red dots are points of reference
?
Hudson’s Bay
(data sparse)
(ice?)
?
00Z, 23 April 2006
Can we diagnose fronts associated
with this Manitoba storm?
The red dots are points of reference
warm frontal surface slopes up to N?
cold frontal surface slopes up to W?
00Z, 23 April 2006
Would/could you diagnose a front (or fronts) associated with this
N. Alberta storm?
very cold
cold
plenty of contrast in:
•T
• wind dir’n
• p trend
+ well defined troughs
mild
Tight gradient –
Chinook winds
CMC surface analysis, 12Z Nov 28, 2003. Storm trough through C. and NE. Ab, plus
wind induced lee trough in the SW complicating pattern; wind warning for SW Ab.
Alberta Lee Trough
• when wind impinges on an obstacle, no matter what the scale of the obstacle, one
usually sees a pressure drop from upwind side to downwind side, ie. relatively low
pressure in the wake (or lee) of the obstacle.
• thus on the synoptic scale when a strong wind impinges on a mountain barrier,
the consequence may be the development of a trough of low pressure in the lee
(the strong pressure gradient associated with this trough goes hand in hand with the
strong Chinook type wind)
• sometimes a closed surface low forms in/from the lee trough… name for this
event is "Lee Cyclogenesis''
• the fact that the lee trough phenomenon is occurring complicates the
interpretation of this low
• SW current aloft across Rockies
(associated with coastal trough)
• trough of warm air aloft (trowal)
• drier in the trowal as well
• trowal is signature of adiabatic
compression of descending
current
CMC 700 mb analysis, 12Z Nov 28, 2003.
break in the high cloud in lee of
Alberta Rockies is sign of adiabatic
compression of descending current
17Z Nov 28, 2003.
• trough of warm air aloft (trowal)
• isotherms helpful in thinking about
fronts
??
CMC 850 mb analysis, 12Z Nov 28, 2003
Impossible to place surface fronts on basis of classic signs… in
case of doubt, pointless to insist they exist…
very cold
cold
mild
CMC surface analysis, 12Z Nov 28, 2003