Numerical Experiments on Lee Cyclogenesis

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Abstract
Cyclogenesis in the lee of long north-south barriers is studied by use of a numerical weather prediction model. Lee-side development is simulated by the model if an initial situation is chosen, which is favorable for lee cyclogenesis, i.e., a low approaching the barrier from the west in a baroclinic current. Cyclogenesis is investigated in the lee of a barrier of 4000 km length and 2000 m height, which is comparable to the Rocky Mountains, and of another one, which is similar to the massif of Greenland. In both cases, the divergence equation and the vorticity equation are analyzed at the point of greatest pressure fall in the Ice to find out the causes of the lee-side development. Pressure fall begins when the parent low approaches the barrier. It is caused by the warm air that is crossing the barrier in front of the approaching center of low pressure. Vorticity advection aloft is of no importance in the Rocky Mountains case and influences the surface pressure only during the final period of pressure fall in the Greenland case. The development is terminated when the cold front of the parent low passes over the new center of low pressure in the lee. An attempt is made to compare the predictions of the model to observations in the lee of the Rocky Mountains and of Greenland.

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