Midlatitude Cyclones - Upper Level Patterns
- Vorticity
- Measure of speed of rotation (angular velocity) of air columns
- Focus on rotation around local vertical - in horizontal wind fields
- Measured in units of angular velocity - revolutions (radians) per second
- Cyclonic spin defined as positive
- Plays key role in dynamics of Rossby waves
- Changes in vorticity closely related to convergence and divergence
- Can be used to diagnose vertical motions associated with horizontal convergence and divergence
- Absolute vorticity = planetary + relative vorticity
- Planetary vorticity is spin produced by earth's rotation
- Component of earth's rotation oriented around local vertical
- Always positive in Northern Hemisphere
- 0 at equator, increases northward
- Relative vorticity is spin produced by winds moving relative to surface
- Curvature + lateral shear (change in speed moving across flow perpendicular to wind direction
- Relative vorticity is positive in cyclonic vorticies and troughs, negative in anticyclonic vorticies and ridges
- In most large-scale midlatitude flows
so
absolute vorticity usually positive - Vorticity tends to be high in troughs and on north flank of jetstream
- Vorticity Changes and Divergence
- Convergence/divergence associated with changes in absolute vorticity
- Conservation of angular momentum
- Increasing vorticity - horizontal flow converges toward axis of rotation
- Decreasing vorticity - horizontal flow diverges away from axis of rotation
- Upper level maximum vorticity occurs in troughs, minimum in ridges
- Upper level convergence to west as flow spins up entering deepening trough
- Upper level divergence to east as flow spins down approaching rising ridge
- Dynamic highs and lows created by convergence and divergence in upper level flows
- Pressure features tilt westward with height in developing midlatitude cyclone
- Rossby Wave Dynamics
- Maintenance of Rossby waves
- Conservation of absolute vorticity
- Start with flow moving southeasterward in straight line
- Absolute vorticity = planetary vorticity
- Relative vorticity = 0
- Planetary vorticity decreases at lower latitude
- Air columns now spinning faster than surface, so relative vorticity increases
- Produces cyclonic curvature of flow in trough, accelerates back northward
- Absolute vorticity = planetary vorticity + relative vorticity
- Relative vorticity > 0
- Flow heads north, becomes straight again
- Absolute vorticity = planetary vorticity
- Relative vorticity = 0
- Planetary vorticity increases at higher latitudes
- Air columns now spinning slower than surface, so relative vorticity decreases
- Produces anticyclonic curvature of flow in ridge, accelerates back southward
- Absolute vorticity = planetary vorticity + relative vorticity
- Relative vorticity < 0
- Change in f with latitude is restoring force
- Amplifying Rossby waves
- Absolute vorticity changes associated with convergence and divergence
- Absolute vorticity increases as flow converges into deepening trough
- Absolute vorticity decreases as flow diverges heading into amplifying ridge
- Ridge building due to warm air advection in warm sector east of surface low
- Amplifying upper level wave produces stronger convergence entering trough
- Above surface high
- Stronger divergence leaving trough
- Above surface low
- Surface pressure features intensify, increasing warm and cold advection
- Intensification only occurs when surface and upper level features offset
- Upper level trough west of surface low
- 300mb height and velocity fields for 4:00am PST on Feb. 6
- 500mb height and vorticity fields for 4:00am PST on Feb. 6
- 700mb height and vertical velocity fields for 4:00am PST on Feb. 6
- 850mb height and temperature fields for 4:00am PST on Feb. 6
- Sea level pressure field for 4:00am PST on Feb. 6
- Propagation of Rossby waves
- Waves propagate out of source region conserving absolute vorticity
- Wave speed proportional to wavelength
- U is zonal wind speed, c is wave speed, L is wavelength
- Wave speed always negative (westward) relative to wind speed
- Wind faster, so air moves through waves
- For wavelengths of waves associated with midlatitude cyclones:
- So waves propagate eastward relative to surface
- Animation of 500mb forecast maps, 2/29/08-3/5/08
