Meteo 422 – Lecture 17 – The Omega Equation Aloft
The derivations below generally follow those in the course text: Holton’s “An Introduction to Dynamic Meteorology”
Goal: Develop an understanding of the role of ageostrophic flow at jet stream level.
· Jet streak example – sketch on blackboard
· Short-wave example – more complex –> discuss in lab
· The right hand term says that when the geostrophic wind advects in slower wind, the ageostrophic wind goes left of the jet in the N.H., i.e. roughly poleward.
· Likewise, when the geostrophic wind advects in faster wind, the ageostrophic wind goes left of the jet in the N.H., i.e. roughly equatorward.
· Sketch streamlines on the jet-level map.
· Air flowing into a region of tight pressure gradient finds itself going too slowly for Coriolis to balance the PGF, so the air accelerates towards lower pressure.
· The resulting cross-contour ageostrophic flow experiences Coriolis forcing too, acting to accelerate the along-contour flow back towards geostrophy.
· The reverse happens when air exits a region of tight pressure gradient.
· Entry region circulation
· Exit region circulation
· Where would you expect clouds and precipitation?
· Consider the stability of the upper troposphere
· Adiabatic heating and cooling
· Destruction of the upwind end of a jet streak
· Extension of the downwind end of a jet streak
· Ageostrophy holds the height and wind fields together to maintain some semblance of geostrophy
· Vertical advection there causes much more adiabatic heating and cooling than does similar motion in the troposphere
· So the column-average temperature and thus the surface pressure change more when the tropopause is advected up or down
· Enhanced cyclogenesis – think about why
· Upper tropospheric fronts
· Formed by intrusions of stratospheric air into the troposphere in the entrance region of jet streaks and short-waves
· Eliminated both by mixing with tropospheric air and by retraction into the stratosphere in the exit region of jet streaks and short-waves
· Wind shear
· Strong stability and sloping inversion means strong horizontal temperature gradient
· Hence strong thermal wind
· Hence strong vertical wind shear
· If the shear becomes strong enough to overcome the stability Kelvin-Helmholtz waves form, creating dangerous turbulence
· Nuclear fallout
· Radioactive particles get rained out of the troposphere within a week
· So if someone fights a nuclear war on the other side of the world, little of that fallout reaches you.
· In contrast, radioactive particles that rise with the mushroom cloud into the stratosphere can stay there for months.
· These particles travel around the globe until they are injected into the troposphere by tropopause folds.
· The resulting fronts can carry the radiation to the surface or into storms, which rain it out.
· Creating a global fallout hazard from even local nuclear wars.