Combinations of Straight Jet, Curvature, and Thermal Advection Dynamics


What kind of vertical motion pattern arises near an upper-level jet streak which possesses strong curvature? [Remember that a jet streak is an isolated wind maximum. This is not the same thing as a jet stream, which was treated in the previous section on curvature effects]. Another interesting question concerns what effects cold or warm advection might have upon the resultant vertical motions. These questions have been addressed rigorously by Keyser and Shapiro (1986) in their seminal review paper on upper-level fronts. We will treat each of these questions here in a qualitative manner using simple conceptual models of the vertical motions based upon their semi-geostrophic theoretical arguments.

Mutual consideration of the effects of straight jet streak dynamics and curvature effects is conceptually straightforward. One only needs to simultaneously consider the 4-cell pattern of cross-stream vertical motion for the straight jet streak and the 2-cell along-stream pattern for the curved jet stream, to arrive at the following conclusion: vertical motions will be reinforced on the cyclonic (left) side of the jet streak and suppressed on the anticyclonic (right) side. This is shown below:


Isotachs are shaded in gray for a westerly jet streak in which there is confluence in its entrance region and diffluence in the exit region. Thick red lines denote highly curved geopotential height trough. Plus and minus signs indicate the sense of the omega vertical velocity at jet level (-: ascent, +: descent) resulting from the divergence of the ageostrophic winds, with the size of the signs indicative of the magnitude of the vertical motions.

Now what about the thermal advection effects? Although several different configurations are imaginable, we will only consider here the case of cold advection in the presence of cyclonic shear deformation in combination with the confluence/diffluence pattern representative of the straight jet configuration. The first figure below shows the vertical motion patterns for the simple straight jet situation again (due to confluence in the entrance region and diffluence in the exit region acting upon the cross-front temperature gradient):


Isotachs are shaded in gray for a westerly jet streak in which there is confluence in its entrance region and diffluence in the exit region. Thick red lines are ISOTHERMS (unlike the figure above). Plus and minus signs indicate the sense of the omega vertical velocity at jet level (-: ascent, +: descent) resulting from the divergence of the ageostrophic winds, with the size of the signs indicative of the magnitude of the vertical motions.

The result of the action of cold advection in the presence of cyclonic shear is to create a thermally indirect circulation. The next figure shows that the combined effect of this result with the thermally direct circulation for the straight jet entrance region is to shift the direct circulation laterally toward the anticyclonic side of the jet. The combined actions also result in a shift of the thermally indirect circulation in the exit toward the cyclonic side of the jet.


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