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Understanding Cold Front Dynamics and Cyclogenesis in the Rockies

This text delves into the complex interactions of cold fronts and cyclogenesis as they traverse the Rocky Mountains. It highlights the destruction of lower portions of Pacific weather systems, while the upper short-wave and its thermal structure manage to cross the mountain barrier. Key concepts include mesoscale troughing, QG leeside cyclogenesis, and the dynamics of upper and lower atmospheric interactions. The evolution of trough structures and the impact of prevailing wind shear are also discussed, with a focus on notable regions like Alberta and Colorado.

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Understanding Cold Front Dynamics and Cyclogenesis in the Rockies

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  1. Cold Front Aloft Model Comments • The lower portions of Pacific weather systems are destroyed as they cross the Rockies. • The upper short-wave and its associated thermal structure can pass across the barrier. • Troughing on the eastern side can occur on two scales: • small mesoscale locked to the terrain associated with downslope warming. • QG leeside cyclogenesis

  2. CFA • The low level leeside troughing cannot move eastward as fast as the upper trough and a natural forward tilt develops. • With upward motion leading the upper trough, precipitation natural leads the slow-moving lower trough. Often a potentially unstable environment exists ahead of the trough, leading to convective precipitation. • Eventually cold air is able to move into the eastward moving trough, producing a more classical cold frontal structure.

  3. CFA • Another way to look at this. There is a natural tendency for normal wind shear with height to shear apart systems, changing the phasing between lower and upper levels. • Naturally occurs even without mountains: decrease in slope with time. • The mutual support of upper and lower structures and interacting vertical motions tends to delay this decoupling. (Palmen figure) • Shearing off the lower portion of a system, and establishing a new trough on the eastern side that is not dynamically coupled to the system aloft facilitates the rapid development of a forward tilting system. (e.g., no low level cold air advection that supports structure aloft)

  4. Locations of Lee Trough/Cyclogenesis • Two major foci: Alberta (Alberta Clippers) and Colorado/New Mexico (Colorado Lows). • A map reveals why…

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