Balanced and Unbalanced Circulations in a Primitive Equation Simulation of a Midlatitude MCC. Part II: Analysis of Balance

Abstract

A nonlinear balance condition, which permits the diagnosis of both balanced divergent and nondivergent flows, is presented. This analysis approach is applied to the results of a numerical simulation of a midlatitude mesoscale convective complex (MCC) to assess the degree of balance of these and similar convective weather systems. It is found that, to a large extent, the simulated MCC represents a highly balanced fluid system. The nondivergent component of the MCC wind field was found to be largely balanced from the time of initial convection to dissipation. Perhaps more surprisingly, the storm-induced divergent model winds are also balanced to a fair degree, though certainly less so than the nondivergent flow. Further, the balanced divergent flow makes up a significant portion of the total balanced flow in some regions of the MCC. System-scale divergence profiles of the model and balanced winds are compared and found to agree reasonably well, especially in the growth and mature stages of the MCC. Within a stationary averaging volume enclosing the MCC, the greatest disparity between the model and balanced circulations is found in the downward vertical motion. The model downward mass flux significantly exceeds the balanced downward flux at most times during the simulation, suggesting that the process of mass adjustment due to convective heating is largely dominated by unbalanced fast-manifold processes, such as inertia?gravity waves. The unbalanced flow is found to be composed largely of divergent circulations of periodic nature (i.e., gravity waves). The appearance and characteristics of these features are found to be in good agreement with current theoretical predictions regarding the atmospheric response to convective heating and associated compensating subsidence. The (modified) Rossby radii (?R) for two lowest-order gravity wave modes are calculated. The mesoscale convective vortex (MCV) within the storm is larger than ?R for all but the gravest mode. The ?Rn for the mature storm as an ensemble also indicates a good degree of balance with ?Rn=1 scaling similar to the MCC and larger values of n scaling smaller than the system as a whole. These ?R values strongly suggest that this simulated MCC represents an inertially stable balanced mesoscale convective system.