Jet Stream Circulations

April 16, 2004

• Tools

– Vorticity advection

– Thermal advection

– Jet stream ageostrophic circulation

– Isentropic lift

To begin to discuss cyclogenesis, we have to define divergence and convergence

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)

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.

Convergence

From the above we can also say that mid-tropospheric downward motion has divergence below and convergence aloft and vice-versa

Divergence

ConvergenceDivergence

Ground

Tropopause

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

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

motion

Jestreams and Cyclogenesis

Putting this all together gives us the following Upper Convergent/Divergent pattern.

A: Convergent area Sfc DivergenceB: Divergent area Sfc Convergence

Favourable for weather development

C: Divergent area Sfc Convergence Favourable for weather development

D: Convergent area Sfc Divergence

Jet-Strm Divergence

Jet-Strm Divergence

PVA increasing w/ height