q 2001 American Meteorological Society Some Simple Simulations of Thunderstorm Outflows

Three idealized simulations of thunderstorm outflows are presented. Each outflow is a response to an instan-taneous low-level cooling. The vertical structures of the coolings differ as do the environments in which the outflows form, and consequently the dynamics of the outflows differ. One outflow is a gravity current, another is a gravity wave, and the third comprises both a gravity current and a gravity wave. The horizontal transport of mass is important for the advance of the gravity-current outflow, but not for the gravity wave outflow, and it is suggested that this is the defining dynamical distinction between the two outflows. The simulations are compared to observations and it is suggested that some outflows previously characterized as gravity currents may better fit the gravity wave or gravity current/wave archetypes. It is also noted that the gravity wave component of an outflow may be generated directly by low-level cooling in addition to the commonly suggested mechanism of the interaction of a gravity current with a stable layer. 1