The effect of mesoscale heterogeneity on the initiation and subsequent evolution of simulated convective systems

This two-part study investigates the initiation and evolution of quasi-linear convective systems (QLCSs) within complex mesoscale environments. Convective outflows and other mesoscale features appear to affect the simulated system attributes of timing and location, as well as the rotational characteristics and associated dynamics of these systems. Thus, in Part I, real-data numerical simulations of the 6 July 2003 and 24 October 2001 QLCS events have been performed to: (i) identify and characterize the various ambient mesoscale features that modify the structure and evolution of simulated QLCSs; and then (ii) determine the nature of interaction of such features with the systems, emphasizing the dynamics of genesis and evolution of low-level mesovortices. Significant low-level mesovortices develop in both simulated QLCSs regardless of the degree of heterogeneity present in their mesoscale environments. While the details of their genesis are case-dependent, this result underscores the fact that such variability is not a necessary condition for rotation (and ultimately severe weather production) within these systems. While not necessary, meso-γ-scale (order 10 km) and meso-β-scale (order 100 km) heterogeneities in the form of convective outflow and due to differing air masses, respectively, are sufficient to affect mesovortex strength within the simulated QLCS of 6 July 2003. In Part II, ensemble Kalman filter (EnKF) simulations of the 4 July 2003 QLCS event have been performed to: (i) assess the sensitivity of the QLCS forecast to improved model representation of mesoscale environmental structures through the assimilation of surface observations; and (ii) investigate the relative response of QLCS dynamics to these mesoscale structures. Improvements in the representation of the QLCS environment, as well as simulated system attributes such as timing and location, are achieved without consideration of radar observations. Assimilation of the observed response to a mesoscale gravity-wave event—the mechanism for observed convective initiation—was not, in turn, followed by the model generation of the phenomenon, but provides the forced lifting necessary for simulated convection initiation. The most observationally consistent solutions are produced when vertical localization length scales extend at least to the boundary layer top and for longer assimilation periods