The Influence of Horizontal Environmental Variability on Numerically Simulated Convective Storms. Part I: Variations in Vertical Shear

Abstract

Severe convective storms are typically simulated using either an idealized, horizontally homogeneous environment (i.e., single sounding) or an inhomogeneous environment constructed using numerous types of observations. Representing opposite ends of the spectrum, the former allows for the study of storm dynamics without the complicating effects of either land surface or atmospheric variability, though arguably at the expense of physical realism, while the latter is especially useful for prediction and data sensitivity studies, though because of its physical completeness, determination of cause can be extremely difficult. In this study, the gap between these two extremes is bridged by specifying horizontal variations in environmental vertical shear in an idealized, controlled manner so that their influence on storm morphology can be readily diagnosed. Simulations are performed using the Advanced Regional Prediction System (ARPS), though with significant modification to accommodate the analytically specified environmental fields. Several steady-state environments are constructed herein that retain a good degree of physical realism while permitting clear interpretation of cause and effect. These experiments are compared to counterpart control simulations in homogeneous environments constructed using single wind profiles from selected locations within the inhomogeneous environment domain. Simulations in which steady-state vertical shear varies spatially are presented for different shear regimes (storm types). A gradient of weak shear across the storm system leads to preferred cell development on the flank with greater shear. In a stronger shear regime (i.e., in the borderline multicell/supercell regime), however, cell development is enhanced on the weaker shear flank while cell organization is enhanced on the strong shear side. When an entire storm system moves from weak to strong shear, changes in cell structure are influenced by local mesoscale forcing associated with the cold pool. In this particular experiment, cells near the leading edge of the cold pool, where gust front convergence occurs along a continuous line, evolve into a bow-echo structure as the shear increases. In contrast, simulated cells that remain relatively isolated on the flank of the cold pool tend to develop supercellular characteristics.

The Role of Momentum Transport in the Motion of a Quasi-Idealized Mesoscale Convective System

“…The simulation also includes a background baroclinic environment (an element largely neglected by many purely idealized modeling studies of the past). This approach allows a more realistic treatment of the larger-scale environment by including an upper-level jet stream that is in thermal wind balance, and accounting for the influence of the Coriolis force on MCS evolution (important to MCS motion over extended time intervals); Skamarock et al (1994), Jewett and Wilhelmson (2006), and Richardson et al (2007) also discuss the advantages of such a framework.…”

The Initiation and Evolution of Multiple Modes of Convection within a Meso-Alpha-Scale Region

“…While the majority of the findings discussed thus far deal with how a storm initially organizes within a particular shear environment, Richardson et al (2007) showed that variations in environmental shear during the course of a storm's lifetime may have an impact on its organization as well. Using idealized numerical simulations with horizontally varying vertical wind shear, the authors found that storms became increasingly organized as they moved into a region with stronger vertical shear.…”

Vorticity Evolution Leading to Tornadogenesis and Tornadogenesis Failure in Simulated Supercells

“…This follows many prior supercell modeling studies that have studied the NSE in relation to the resulting storm type and behavior (e.g., Klemp and Wilhelmson 1978;Weisman and Klemp 1982). Previous observational and modeling studies have shown or suggested that nonhomogeneous features (i.e., preexisting baroclinic regions and vertical vorticity) can also influence low-level rotation and tornado potential in supercells (e.g., Maddox et al 1980;Markowski et al 1998;Atkins et al 1999;Fierro et al 2006;Richardson et al 2007). By excluding these nonhomogeneous features, the ability (or inability) of a modeled storm to produce a tornado can be limited to the NSE.…”