Mid-latitude Cyclones (MLCs)

Science 123		Spring 2026
	MidLatitude Cyclones (MLCs)

Cyclogenesis - the birth of Midlatitude Cyclones (MLCs)
- must have loss of mass (divergence) in the column above the surface
  - divergence = $\frac{\Delta u}{\Delta x} + \frac{\Delta v}{\Delta y}$ units are $\frac{1}{\mbox{seconds}} = \mbox{ sec }^{-1}$
    
    - speed divergence
    
    - path divergence
- preferred regions, along the polar front and :
  - water-land interfaces
  - lee side of mountains (Rockies)
  - jet streak initiation
- typical cyclone paths

Vorticity
- measure of the rotation of a parcel - positive if rotation is "cyclonic" (counter-clockwise in the NH)
- $f$ = planetary vorticity, due to the rotation of the earth
- $\zeta_r$ = relative vorticity = $\frac{\Delta v}{\Delta x} - \frac{\Delta u}{\Delta y}$ units are $\frac{1}{\mbox{seconds}} = \mbox{ sec }^{-1}$
  
  - curvature vorticity : curved flow (gradient winds, for example)
  
  - speed shear vorticity : change of wind speed with a change in distance
- absolute vorticity : $\zeta_a = f + \zeta_r$
  
  - conservation of absolute vorticity - long wave initiation
- potential vorticity : $\displaystyle{\zeta_p = \frac{f + \zeta_r}{\Delta z}}$
  
  - conservation of potential vorticity : $\zeta_r + f = \zeta_p \cdot \Delta z$
  
  explanation of the southward migration of MLCs passing over the Rocky Mountains
- vorticity max
  
  - surface low will likely form when a vorticity maximum moves over a stationary front
  
  - surface low (storm) will likely strengthen when the vorticity maximum moves over the surface low
  
  - in the absence of a stationary front or an organized surface low, an area of organized clouds and precipitation may form in its area
- relationship between vorticity and divergence
  
  - as vorticity increases, divergence decreases (convergence increases)
  
  - as vorticity decreases, divergence increases
Jet streak divergence pattern
- slight increasing and then decreasing pressure gradient force creates a temporary imbalance between the pressure gradient force and the Coriolis effect
- divergence max is associated with heavier precipitation area

Rossby (long) waves
- wavelength $\lambda$ of several thousand kilometers
- 4, 5, or 6 waves encircle the globe
- slow moving waves - speed of a wave is inversely proportional to wavelength
  
  - Rossby wave speed : $c = U_0 + c_0$,
  
  where $U_0$ = speed of the wind
  
  $c_0 = -\beta \left(\frac{\lambda}{2 \pi}\right)^2$, and $\beta = \frac{2\Omega}{R_{earth}}\cos \phi$
  
  - retrograde waves - some move east to west
- wave initiation includes
  
  - barotropic instability
  
  - baroclinic instability - conservation of potential vorticity

Short waves
- shorter wavelength results in greater speed than long waves
- origins include:
  
  - ripples from quasi-stationary low, such as the Gulf of Alaska low
  
  - small, cold pools of air aloft

Barotropic versus baroclinic waves

Baroclinic instability
- 500 mb jet aligns with the polar front at the surface in a barotropic case
- cold advection short wave moves into the trough, causing a local deepening, which leads to a baroclinic case. Now, warm and cold advection work to (1) deepen the low pressure center at the surface, and (2) amplify the wave pattern.
- cold sinking parcels convert PE to KE
- warm, moist parcels release latent energy as moisture condenses to form clouds
- baroclinic instibility explanation

Norwegian Cyclone Model (Bergen School of Meteorology)
- stationary polar front - low pressure trough with inherent cyclonic flow
- frontal wave - initiated by an UA mechanism, closed surface low creates weak rotation, stationary front becomes warm or cold front
- open wave - occurs 12 to 24 hours later, central pressure decreases, cyclonic circulation increases, warm sector is identifiable between the cold and warm front, warm air overruns the warm front creating a large band of cloudiness and light precipitation ahead of the warm front.
- mature (pre-occlusion) - cyclonic circulation increases, cold front catches up to the warm front at the lows center, most intense stage, large areas of cloudiness and precipitation
- advanced occlusion - triple point forms at which a secondary low pressure center often develops
- separation - cold air lies on both sides of occlusion, center is cutoff from an important energy source of rising warm air, the center now begins to fill
Vertical structure
- tilted
- stacked

Conveyor belt model
- useful modification of the Norwegian Cyclone Model for east coast storms
- conveyor belt, or stream of air, corresponding to each of the three air masses in the proximity of the storm
- warm belt - mT
- cold, moist belt - mP
- dry belt from - cP or cT

Decaying cyclone
- occluded surface low is surrounded by cold air
- the surface and upper air lows are cutoff from the main flow
- the surface and upper air lows are stacked
- friction and the lack of upper air support (little divergence aloft) results in the filling of the low.

Science 123

Spring 2026

MidLatitude Cyclones (MLCs)