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Lecture 20: Air masses

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a look at a complex case where pressure field shows ... storm and a lee trough - difficult or impossible to detect surface fronts. any spare time - questions ... – PowerPoint PPT presentation

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Title: Lecture 20: Air masses


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

2
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 QH0
700-500 mb layer neutral w.r.t. saturated
adiabatic motion (well-mixed)
3
Fig. 9-4
Fig. 9-10
Fig. 9-4
warm air cut off from the surface by the meeting
of two cold fronts
4
Can we diagnose fronts associated with this
Manitoba storm?
The red dots are points of reference
00Z, 23 April 2006
5
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
6
Would/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.
7
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

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.
9
break in the high cloud in lee of Alberta Rockies
is sign of adiabatic compression of descending
current
17Z Nov 28, 2003.
10
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11
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
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