Low-Level Mesocyclonic Concentration by Nonaxisymmetric Transport. Part I: Supercell and Mesocyclone Evolution

Abstract

An idealized simulation of a supercell using the Regional Atmospheric Modeling System (RAMS) was able to produce a low-level mesocyclone near the intersection of the forward-and rear-flank downdrafts. The creation of the low-level mesocyclone is similar to previous studies. After 3600 s, the low-level mesocyclone underwent a period of rapid intensification, during which its form changed from an elongated patch to a compact center. This transition was also accompanied by a sudden decrease in pressure (to 12 mb below that of the neighboring flow), and was found to occur even in the absence of nested grids.It is shown that the stage of strong intensification does not begin aloft, as in the dynamic pipe effect, and then descend to the surface. Rather, the vortex is initiated near the surface, and then builds upward. The process is completed in 5 min, and the final vortex can be clearly distinguished from the larger-scale mesocyclone at the cloud base. The reduction of pressure can be explained as a consequence of the evacuation of mass in the horizontal convergence equation. This is in contrast to axisymmetric models of vortex intensification, which generally rely on the evacuation of mass in the vertical divergence equation. In the latter cases a positive horizontal convergence tendency is what initiates the concentrated vortex. However, nondivergent models prove that vorticity concentration can occur in the absence of any horizontal convergence. Here the concentration is associated with a negative horizontal convergence tendency.

Characteristics of Submeso Winds in the Stable Boundary Layer

“…The unavailability of network observations at multiple levels and the uncertainty of the measured vertical motion at larger scales precludes meaningful analysis of the three-dimensional dynamics of the submeso-scale structures and also precludes more rigorous analysis of the kinematics, such as presented by Chong et al (1990) and applied to model output by Gaudet and Cotton (2006). The network data do provide for simple analysis of the kinematics of the horizontal flow.…”

Relative sensitivities of simulated rainfall to fixed shape parameters and collection efficiencies

“…RAMS is a nonhydrostatic mesoscale model that is frequently used to simulate deep convective storms (e.g. Grasso, ; Gaudet and Cotton, ; Barth et al , 2007; Lerach et al, ; Nair et al, ; Federico et al, ; Grant and van den Heever, ). The model was initialized using a horizontally homogeneous thermodynamic vertical profile (Figure ) adapted from Gaudet et al ().…”

Climatology of Long-Track Tornadoes

“…Precise time observations from initial supercell development to tornado formation are not as well documented as times obtained from modeling results. Typically, 50-100 min (with longer times possible) elapse in idealized threedimensional numerical simulations of supercells before they are mature enough, in tornadosupporting environments, to permit the development and support the maintenance of tornado-like vortices (e.g., Wicker and Wilhelmson 1995;Gaudet and Cotton 2006;Coffer and Parker 2017;Markowski 2020).…”

“…We have added supporting information from observations that include varying cycle times from Darkow and Roos (1970), Burgess et al (1982), and from VORTEX (Dowell and Bluestein 2002). Citations to model results include cyclic times from Alderman et al (1999), Adlerman and Droegemeier (2002); Wicker and Wilhelmson (2006); Gaudet and Cotton (2006); Coffer and Parker 2017;and Markowski (2020). Markowski (2020) who examined the effects of varying the initial PBL perturbations in supercell simulations and found "very limited intrinsic predictability".…”