Homework 4: Due at Beginning of Class, September 30, 2011).
Explain the following from Chapter 2 of Doswell and Bosart:
"...rising motion favors a decrease in static stability below the level of peak ascent, usually somwhere in the mid troposphere....therefore, ETC's should exhibit some asymmetry in their vertical motion patterns beyond that derived from QG theory; ascent should be more intense than descent...."
You need to make an attempt to explain both halves of the statement. Why is the first part true? And, given that, why is the second half true as well. Specifically refer to the QG omega equation in your explanations for a textbook type wave cyclone.
For a textbook, or typical, wave cyclone, cold advection characterizes the region northwest of the cold front and warm advection characterizes the region northeast of the warm front. Further, cyclonic vorticity advection, increasing with height, is usually centered over or slightly east of the surface low pressure area. If one assumes that the absolute magnitude of the advections are equal, then the western half of ETCs should have descent (or the first two terms will return an ambiguous result) and the eastern half of ETCs should have ascent (because both terms return forcing for upward motion there).
However, this assumes that the static stability parameter is the same everywhere in the domain of the ETC. But the net ascent in the eastern half of the ETCs should destablize the atmosphere (for the usual thermodynamic and moisture stratification) there, and stabilize it in the western half. This would make the effectiveness of the forcing functions greater in the eastern half than in the western half, producing a stronger upward motion center over the eastern half, than the downward motion center over the western half.
(Not part of the question explicitly, so not graded for, is this: in addition, as ETCs exit the Rockies a plume of conditionally unstable air with high moisture content moves northward in the eastern half of ETCs. This creates an even lower static stability parameter in a narrowly focussed zone, creating a nearly mesoscale pocket of extremely high (synoptic-scale) upward motion centers there).