Simulating deep convection initiation by misocyclones

Understanding the mechanisms controlling the location and timing of deep moist convection is of paramount importance for improved nowcasts of developing severe thunderstorms. The research to be presented represents a numerical counterpart to observational projects like STEPS and MOCISE whose objectives include investigating convection initiation along convergence lines like the dry line. The study of moist convective forcing from boundary layer misocyclone circulations is a natural outgrowth of previous work involving misocyclones along boundary layer convergence/shear boundaries by Lee and Wilhelmson.

A three-dimensional, non-hydrostatic, quasi-compressible convective cloud model (MSTFLOW) is employed to simulate a weak outflow that possesses marked horizontal shear (a vertical vortex sheet) at its leading edge. Thermodynamic conditions typical of an eastern Colorado spring/summer convective environment are utilized. Given the characteristic observed dimensions of misocyclones present along boundaries (~1-3 km), a domain grid with Dx=Dy=120 m, Dz [level 1]=40 m is used to provide the high resolution necessary to simulate these circulations.

Prior related non-supercell tornado studies indicated that the misocyclones had the potential to markedly influence storm initiation and structure by controlling the low-level updraft distribution. More recent model simulations designed to look directly at this relationship have revealed interesting results. These experiments have shown that for certain vertical shear profiles, the deep convection that initiates above the boundary is located directly above the misocyclones. In these cases, the misocyclones are controlling the distribution of updraft maxima along the boundary and thus, the low-level moist convective forcing that is influencing the development of deep convection. Moisture plumes associated with each misocyclone are a prominent feature of the results and consistent with Sub-VORTEX observations along the Denver Convergence and Vorticity Zone. In other cases, we have found that the misocyclones alter the horizontal and vertical structure of the outflow boundary leading edge which also highly influences updraft magnitude and distribution. The results to be presented represent an important finding directly applicable to nowcasting deep convection.