Title: Lecture 20: Air masses
1Lecture 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
2Conditions 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 QH0
700-500 mb layer neutral w.r.t. saturated
adiabatic motion (well-mixed)
3Fig. 9-4
Fig. 9-10
Fig. 9-4
warm air cut off from the surface by the meeting
of two cold fronts
4Can we diagnose fronts associated with this
Manitoba storm?
The red dots are points of reference
00Z, 23 April 2006
5Can 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
6Would/could you diagnose a front (or fronts)
associated with this N. Alberta storm?
very cold
cold
- plenty of contrast in
- T
- wind dirn
- 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.
7Alberta 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
8- 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.
9break in the high cloud in lee of Alberta Rockies
is sign of adiabatic compression of descending
current
17Z Nov 28, 2003.
10(No Transcript)
11Impossible 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