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 Conditions under which actual lapse rate (ELR) approximates DALR or SALRDALR or SALR

925-720 mb layer “neutral w.r.t. unsaturated adiabatic motion” (well-mixed)

700-500 mb layer “neutral w.r.t. saturated adiabatic motion” (well-mixed)

“well-mixed” implies QH=0

warm air cut off from the surface by the meeting of two cold fronts

Fig. 9-10Fig. 9-4

Fig. 9-4

Hudson’s Bay(data sparse) (ice?)

??12 hr motion12 hr motion

??

The red dots are points of reference

00Z, 23 April 2006

Can we diagnose fronts associated with this Manitoba storm?

Can we diagnose fronts associated with this Manitoba storm? The red dots are points of reference

00Z, 23 April 2006

warm frontal surface slopes up to N?cold frontal surface slopes up to W?

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.

Would/could you diagnose a front (or fronts) associated with this N. Alberta storm?

very coldvery cold

coldcold

mildmild

plenty of contrast in:• T• wind dir’n• p trend+ well defined troughs

Tight gradient –Chinook winds

• 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

Alberta Lee TroughAlberta Lee Trough

CMC 700 mb analysis, 12Z Nov 28, 2003.

• 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

17Z Nov 28, 2003.

break in the high cloud in lee of Alberta Rockies is sign of adiabatic compression of descending current

CMC 850 mb analysis, 12Z Nov 28, 2003

• trough of warm air aloft (trowal)• isotherms helpful in thinking about fronts ??

CMC surface 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 coldvery cold

coldcold

mildmild