Date of this Version
Monthly Weather Review 139(10) October 2011 pp. 3125-3138; DOI: 10.1175/MWR-D-10-05036.1
The sensitivity of storm longevity to the pattern of deep convection initiation (e.g., multiple, quasi-linearly arranged initial deep convective cells versus an isolated deep convective cell) is examined using idealized cloud-resolving simulations conducted with a low-shear initial environment. When multiple deep convective cells are initialized in close proximity to one another using either a line of thermals or a shallow airmass boundary, long-lived storms are produced. However, when isolated deep convection is initiated, the resultant storm steadily decays following initiation. These results illustrate that a quasi-linear mechanism, such as a preexisting airmass boundary, that initiates multiple deep convective cells in close proximity can lead to longer-lived storms than a mechanism that initiates isolated deep convection.
The essential difference between the experiments conducted is that an isolated initial storm produces a shallower cold pool than when a quasi-linear initiation is used. It is argued that the deep cold pools promote deep forced ascent, systematic convective cell redevelopment, and thus long-lived storms, even in environments with small values of vertical shear. The difference in cold pool depth between the simulations is attributed to differences in the horizontal flux of cold air to the gust front. With a single initial storm, the few convective cells that subsequently form provide only a limited source of cold air, leading to a cold pool that is shallow and incapable of fostering continued updraft redevelopment.