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) http://virtualskies.arc.nasa.gov/weather/tutorial/tutorial3.html Daytime – valley breeze Nighttime – mountain breeze Drainage flow

Cause of vertical motions Surface thermal contrasts: Land – sea breeze http://virtualskies.arc.nasa.gov/weather/tutorial/tutorial3.html Nighttime – land breeze Daytime – sea breeze

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

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

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)

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

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

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

Middle latitude upper level waves wind speed Trough Ridge

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

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:

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.

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

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):

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

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?

Rossby Waves