Jet Stream Circulations

1. Jet Stream Circulations April 16, 2004

6. Tools • Vorticity advection • Thermal advection • Jet stream ageostrophic circulation • Isentropic lift To begin to discuss cyclogenesis, we have to define divergence and convergence

7. Divergence Divergence is the horizontal spreading out of air. Convergence is the horizontal shrinking of air. By conservation of mass, we know that vertical stretching (shrinking) occurs with horizontal convergence (divergence)

8. Vertical Velocity & Divergence If we consider the atmosphere to have fixed boundaries at the bottom (ground) and top (tropopause), we can associate vertical motions with divergence and convergence as shown on the next slide.

9. Tropopause Convergence Divergence Ground Divergence Convergence From the above we can also say that mid-tropospheric downward motion has divergence below and convergence aloft and vice-versa

10. Divergence and Vertical motion From everything we have just discussed, you could be justified in saying that as long as we can measure divergence, we should be able to measure vertical motion and therefore regions of cyclone development (Cyclogenesis). Unfortunately this is not straightforward, but we can find a connection between divergence and something that is relatively easy to measure………………….VORTICITY

11. Vorticity & Vertical Motion • It has been stated that PVA increasing with height promotes rising motion

12. Jestreams and Cyclogenesis Putting this all together gives us the following Upper Convergent/Divergent pattern. A: Convergent area  Sfc Divergence B: Divergent area  Sfc Convergence Favourable for weather development C: Divergent area  Sfc Convergence Favourable for weather development D: Convergent area  Sfc Divergence

14. Jet-Strm Divergence

15. Jet-Strm Divergence PVA increasing w/ height