Many mesoscale convective complexes (MCCs) and mesoscale convective systems (MCSs) have been observed to form in environments where the isentropic absolute vorticity may have values that approach zero, resulting in regions with weak inertial stability. Further, if certain vertical stability criteria are met, then symmetric instability may also be present. It has been demonstrated that for a given amount of convective available potential energy (CAPE), deep convective circulations can be modified and enhanced as the symmetric stability is reduced (Emanuel 1982; Xu 1986). Consequently, there has been speculation that the evolution and organization of convection into MCCs (Maddox 1983) and MCSs may be related to the presence of an environment in which the inertial stability is weak or unstable (Emanuel 1979, 1982, 1983; Jascourt et al. 1988; Blanchard 1992). Two typical regions in which these environmental conditions are met are 1) just equatorward of a wind maximum where the anticyclonic shear is large, and 2) in subsynoptic-scale ridges where the anticyclonic curvature is large.In some mesoscale environments, particularly in the springtime over the continental U.S. when CAPE is large and a strong jet stream is still present, the atmosphere is unstable to both upright and slantwise convection. The latter is often referred to as conditional symmetric instability (CSI). Because the time scales of upright and slantwise convection are considerably different, upright convection will typically dominate. It is hypothesized that this upright convection can, over longer time scales, exploit the weak restoring force present in the mesoscale inertial stability. Air parcels move vertically through upright convection and on reaching the equilibrium level expand preferentially in the region that is inertially least stable. Recent modeling work by Seman (1992) suggests that parcel descent occurs on slant trajectories taking the path of least resistance, is directed back toward the generating convection, and occurs preferentially in the inertially unstable region.
In recent papers (Raymond and Jiang 1990; Raymond 1992; Seman 1990, 1992;), the concept of in situ generation of potential vorticity anomalies was developed to explain the upscale growth of convection into mature mesoscale systems. In these studies, there were no regions of inertial instability (i.e., negative potential vorticity) before the onset of convection; the convection itself was responsible for the development of the inertial instability. Although these models are very appealing because they can generate many of the features found in MCCs and MCSs, they fail to address the question of why only some, instead of all, convective systems achieve this condition and grow upscale. A likely explanation may be found in this work.
If the environment is already predisposed to a state of weak inertial stability, as suggested by the typical environments in which MCSs occur, then the divergent outflow at the equilibrium level can exploit the weak restoring force in the inertially unstable (or weakly stable) region. The consequence would be an increase of the upper tropospheric divergence of the cloud and would result in continued stronger updrafts, lower-level perturbation pressure falls and, eventually, the slant downdrafts suggested by CSI theory. Further, the convection is required to do less work to achieve the final state of an upper-level (low-level) negative (positive) potential vorticity anomaly.
To explore the hypothesis that inertial instability plays a role in the development of mesoscale organization of convection, MCSs that occurred in the data-rich PRE-STORM network, and MCS environments sampled with new technology were previously examined (Blanchard 1992). To continue to investigate the role of inertial stability, we used a mesoscale model to simulate these environments using idealized conditions. The model simulations were executed as a series of sensitivity tests with different degrees of inertial [in]stability to develop a comprehensive understanding of the dynamics of this type of atmospheric motion. Results from these numerical simulations are discussed below and are compared with observations.