Numerical experiments on the genesis of Sharav cyclones: idealized simulations

Abstract

A three-level channel flow model is used to investigate the generation of Sharav cyclones in the lee of the Atlas mountains through “idealized” numerical experimentation. A typical initial state consists of a barotropic high—low system, embedded in baroclinic shear flow and centered to the north of the Atlas mountains. If this “perturbation” is of sufficient strength, a cold-air outbreak occurs in the rear of the low as well as a southward excursion of the trough at upper levels. The cold air is blocked by the Atlas mountains, if the initial location of the low is chosen properly. The upper-level trough intensifies slightly when crossing the mountain range and induces then a low-level cyclone in the lee. The trough turns eastward later on and the new cyclone is staying close to this upper-level centre. A simple two-dimensional three-layer model is used to explain salient features of this type of lee cyclogenesis. Moreover, additional experiments are conducted in order to elucidate details of the cyclogenetic process.