Examining Storm Evolution Within Idealized Heterogeneous Environments

Many of our conceptual models for severe storm development and lifecycle are derived from theories and numerical simulations that assume a horizontally homogeneous environment in which CAPE and vertical shear have constant values over 100s of kilometers.  While these studies have proven to be very valuable, they do not address many real-life situations in which these quantities vary greatly over these distances.  Richardson (1999) was the first to incorporate idealized mesoscale heterogeneity into storm simulations.  However, computing resources at that time did not allow for fine-resolution simulations with multi-moment microphysics schemes.  Therefore, though these simulations were able to address storm-scale changes in morphology, they could not address changes in low-level mesocyclones adequately.  This is possible with today's computing power, and the changes in storm dynamics as a storm encounters changes in its environment remain a relevant and interesting problem, particularly with regard to increased or diminished chances for tornadogenesis.  

To study the influence of heterogeneous moisture and vertical shear on low-level rotation, we employ the method of Richardson (1999) and Richardson et al. (2007) within CM1, using double-moment microphysics and grid spacing of 250 m in the horizontal, with a stretched grid with 50-m resolution at low levels in the vertical.  The moisture and vertical shear variations will be idealized in order to capture realistic changes in storm behavior while allowing the clear assignment of cause and effect that is not possible in a full-blown observational study.  The heterogeneous simulations will be compared with homogeneous control runs using soundings taken from various locations within the heterogeneous environment.  Circuit analyses are employed to deduce the change in low-level circulation as the storm moves through a changing environment.  We also examine changes in the low-level perturbation pressure-gradient force as increasing low-level vertical shear is encountered, to determine if increases in this quantity enhance the low-level vertical vorticity.