A numerical study of storm splitting that leads to long lived storms.

A three-dimensional cloud model was used to investigate the splitting of an initially isolated storm in a one-directional east/west shear. The simulated evolution of storm splitting in some cases follows all four stages suggested by Achtemeier after analysis of radar data, including the development of two self-sustaining storms. One of these storms moves to the right of the mean wind vector and the other to the left. In the right moving storm the updraft rotates cyclonically and the downdraft anticyclonically, forming a vortex pair, as depicted in the schematic model of Fankhauser. The vortex pair structure is also similar to that observed with Doppler radar and analysed by Ray. The downdraft induced gust front interacts with the low level environmental wind to produce the convergence necessary to sustain the storm. This convergence extends to the south and west of the storm, and if enough low level moisture is avilable a flanking line develops. The distribution of rainwater within the updraft suggests the existenc of an over-hang and hook typically observed in severe storms. To understand when splitting might occur the strength and distribution of the vertical wind shear were varied. The various simulations suggest that strong shear at and just above cloud base is important for th splitting process to be successful. For splitting to occur the low level inflow from the east in our simulations must be sufficiently stron to inhibit the propagation of the gust front toward the east. If the gust front (or wind shift line) can propagate away from the storm toward the east, the region of low level convergence moves away from the storm and initial splitting in the lower updraft cannot be sustained. Further, without the precipitation induced downdraft and associated low level outflow splitting does not occur. (A)