Multicell stage of the Munich storm of 12 July 1984: a numerical study

The multicell stage of the Munich hailstorm is investigated using a three-dimensional, non-hydrostatic mesoscale model. During this stage, the storm developed in a wind profile that had little directional shear yet an extensive series of two-dimensional numerical experiments could not produce a self-sustaining and realistic convective system. The modelled storm is a multicellular, three-dimensional system featuring right-flank development of new cells. It is constituted of two principal flow branches, namely, an intense updraught that travels at the speed of a mid-level steering current and a weak downdraught. A mid-level relative inflow overtakes the storm from the rear but undergoes unsaturated descent of only about 100 mb, fails to establish the classical downdraught associated with mid-latitude storms, and exits ahead of the storm. Weak, shallow downdraughts originate beneath an inversion and from ahead of the storm. Viewed in a cross section parallel to the direction of storm motion, trajectory analyses show that the cold pool is fed by two types of shallow downdraught, namely a gravity or solitary wave undergoes minimal vertical displacement and an overturning jump that causes substantial evaporative cooling despite its small vertical displacement. A density current circulation fails to form in this plane but one does develop in the perpendicalur section and its interaction with the convergence field and vertical shear causes secondary cell initiation at right angles to that found in multicell squall lines.