Jeff Snyder
EF5
This post is in reference to some discussion in the 6/24/10 FCST thread.
The ageostrophic response in the left-exit region of a low-level jet streak would result in divergence (as I'm sure you know). For upper-level jet streaks, as a result of the very high static stability that resides at and above the tropopause, most of the vertical motion that results from the divergence/convergence within upper-level jet streak quadrants (i.e. the transverse circulation) occurs below the jet streak where static stability is much lower. In the low-levels, since the ground is impermeable, there is a hard boundary that may result in most of the vertical motion associated with ageostrophic winds in the quads of a low-level jet streak being above the jet streak. In other words, while divergence would still occur in the left exit region of a low-level jet, this may actually result in SUBSIDENCE as air "fills the void" left by the quasihorizontal low-level divergence. Of course, you'd really need to examine the vertical variation in static stability to judge the amount of air that will rise from below vs. sink from above. In addition, one can't really look at a single level to determine vertical motion; one really needs to examine the change in divergence/convergence with height to determine mesoscale/synoptic-scale vertical motions associated with jet streaks.
To make matters more complicated, as a result of the intensification of the low-level jet after peak heating, there may be substantial low-level convergence at the nose of the LLJ. In addition, if you add in a front or thermal boundary, there may well be enhanced isentropic lift / warm-air advection near the nose of the LLF, which can be destabilizing and serve as a focus for (often elevated) thunderstorm initiation.
For those who are interested in learning more about divergence patterns and the transverse circulation associated with upper-level jet streaks, I wholeheartedly and strongly recommend the excellent COMET MetEd module titled Jet Streak Circulations.
The ageostrophic response in the left-exit region of a low-level jet streak would result in divergence (as I'm sure you know). For upper-level jet streaks, as a result of the very high static stability that resides at and above the tropopause, most of the vertical motion that results from the divergence/convergence within upper-level jet streak quadrants (i.e. the transverse circulation) occurs below the jet streak where static stability is much lower. In the low-levels, since the ground is impermeable, there is a hard boundary that may result in most of the vertical motion associated with ageostrophic winds in the quads of a low-level jet streak being above the jet streak. In other words, while divergence would still occur in the left exit region of a low-level jet, this may actually result in SUBSIDENCE as air "fills the void" left by the quasihorizontal low-level divergence. Of course, you'd really need to examine the vertical variation in static stability to judge the amount of air that will rise from below vs. sink from above. In addition, one can't really look at a single level to determine vertical motion; one really needs to examine the change in divergence/convergence with height to determine mesoscale/synoptic-scale vertical motions associated with jet streaks.
To make matters more complicated, as a result of the intensification of the low-level jet after peak heating, there may be substantial low-level convergence at the nose of the LLJ. In addition, if you add in a front or thermal boundary, there may well be enhanced isentropic lift / warm-air advection near the nose of the LLF, which can be destabilizing and serve as a focus for (often elevated) thunderstorm initiation.
For those who are interested in learning more about divergence patterns and the transverse circulation associated with upper-level jet streaks, I wholeheartedly and strongly recommend the excellent COMET MetEd module titled Jet Streak Circulations.
Last edited by a moderator:
