1. Cause of vertical motions
Orography:
Rising: upslope flow – forced (KE to PE)
Surface heating (valley wind)
Sinking: downslope flow – forced (PE to KE)
Surface cooling (mountain wind)
Daytime – valley breeze
Nighttime – mountain breeze
Drainage flow

2. Cause of vertical motions
Surface thermal contrasts:
Land – sea breeze
Nighttime – land breeze
Daytime – sea breeze

3. Cause of vertical motions
Land sea + mountain valley breeze
Stronger effect
Daytime
Valley breeze
Sea breeze
ocean
California!
ocean

4. Cause of vertical motions
Land sea + mountain valley breeze
Enhanced winds
Night time
Mountain breeze
Land breeze
ocean
California!
ocean

5. Cause of vertical motions
Buoyancy (in clouds):
Rising – Condensation and latent heat release
Sinking – Evaporation and latent heat consumption
Kinermatically or thermally forced divergence zone:
e.g., central Florida
Sea breeze
Large scale (synoptic scale) dynamical/thermal forcing (i.e., PVA):
Rising: PVA aloft, positive thickness advection below,
Right of the entrance/left of the exit of Jet streak (next class)
Sinking: NVA aloft, negative thickness advection below,
Left of the entrance/right of the exit of Jet streak (nest class)

6. Vertical motions and Static Stability
Synoptic scale rising motion:
500 mb
Surface
Tropopause
None-divergent level T
initial
Final
Divergence
Stabilization
Destabilization

7. Vertical motions and Static Stability
Synoptic scale sinking motion:
500 mb
Surface
Tropopause
None-divergent level T
initial
Final
Stabilization
Destabilization

8. Middle latitude upper level waves
Rossby waves:
restoring force – Coriolis force
Absolute vorticity: N
At the beginning:
Anticyclonic
circulation
Cyclonic
circulation

9. Middle latitude upper level waves
wind speed
Trough
Ridge

10. Middle latitude upper level waves
Assuming that the shear effect is not important
For short waves,
For long waves

11. Long Rossby Waves (try nature coordinates)
wind speed
Trough
Ridge
V > 0
V > 0
f max
f min I II
f min
Rossby waves, I:
II:

12. Long Rossby Waves Long wave propagating direction.
wind speed
Trough
Ridge
Long wave propagating direction.
Wave propagate upwind for long Rossby waves (westward).
This is opposite to what we learned before for short (Rossby) waves.

13. Rossby Waves
Waves on a uniform current in a two-dimensional nondivergent fluid system, rotating with varying angular speed about the local vertical (beta plane). It takes into account the variability of the Coriolis parameter. These waves actually propagate upstream, i.e., from east to west against the westerly winds. Their speed of propagation depends on the latitude, their wave length, and the speed of the westerly wind. In the late 1930’s, G Rossby derived a formula for estimating these speeds on the assumptions that
The wind is exact geostrophic balance
The height contours vary sinusoidally about a latitude line in wavelength L,
There is no shear in the y direction (all vorticity is from curvature), and
The mean zonal wind speed is constant in time and space

14. Rossby Waves
With these assumptions, using sine wave to represent the height field and the barotropic vorticity equation, the Rossby wave phase speed (C, m/s):

15. Rossby Waves
Be careful about what information is provided on a weather map.
(Z geopotential height or gz geopotential )

16. Rossby Waves
C is the zonal speed of a Rossby wave with respect to the ground. For short waves (the “beta” term is small because of a small wavelengh), C is about 4 m/s in mid latitudes for L ~ 1000 km. For such a wave, C is a little smaller than U and the wave would travel from west to east at speeds about 4 m/s, slower than the mean westerly wind of the current in which it is embedded.
As the wavelenght increases, the “beta” term gets larger and C decreases. For very long waves, C becomes negative, meaning that the ultra-long waves actually move from east to west, or retrograde.
Assume that U = 20 m/s, L=4000 km,
What is the value of C?

17. Rossby Waves