The lee cyclone

Westerly airflow that is forced over a north-south mountain barrier undergoes vertical contraction over the ridge and expansion on the lee side. This vertical movement creates compensating lateral expansion and contraction, respectively. Hence there is a tendency for divergence and anticyclonic curvature over the crest, and convergence and cyclonic curvature in the lee of the barrier. Wave troughs may be set up in this way on the lee side of low hills (see Figure 6.13) as well as major mountain chains such as the Rocky Mountains. The airflow characteristics and the size of the barrier determine whether or not a closed low-pressure system actually develops. Such depressions, which at least initially tend to remain 'anchored' by the barrier, are frequent in winter to the south of the Alps, when the mountains block the low-level flow of northwesterly airstreams. Fronts often develop in these depressions, but the low does not form as a wave along a frontal zone. Lee cyclogenesis is common in Alberta and Colorado, in the lee of the Rocky Mountains, and in northern Argentina in the lee of the Andes.

Continue reading here: The thermal low

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