Thunderstorms above Frontal Surfaces in Environments without Positive CAPE. Part II: Organization and Instability Mechanisms

Abstract

The second of two papers describing thunderstorms that occur above frontal surfaces, frequently in environments without positive convective available potential energy (CAPE), focuses on an impressive outbreak of elevated thunderstorms during AVE-SESAME I. It is shown that the thunderstorms occurred in three convective impulses, each of which developed in the warm sector before propagating onto the frontal surface; subsequent thunderstorms developed over the frontal surface. While in the warm sector, the convection was supported by an extremely unstable boundary layer. However, this convective energy quickly diminished above the frontal surface and thunderstorms continued and developed for many hours in an essentially stable hydrostatic environment. During the lifetime of these impulses, mesoscale updrafts developed and moved with the convective areas, maintaining nearly steady-state systems with strong low-level inflow. The environment was found to be symmetrically neutral in the region of the inflow. Numerous pressure waves were observed in association with the elevated thunderstorms, yet thew features were evidently not important in triggering of the storms. An investigation of a convective band that formed above the frontal surface revealed that the development probably took place in two steps. Initially, high ?e air overlying the frontal inversion was stable to vertical displacements, but inertially unstable. Then, along the instantaneous path of the unstable parcel, the thermodynamic structure changed, the parcel became gravitationally unstable, and upright convection resulted.