Response of Simulated Squall Lines to Low-Level Cooling

Abstract

Organized convection has long been recognized to have a nocturnal maximum over the central United States. The present study uses idealized numerical simulations to investigate the mechanisms for the maintenance, propagation, and evolution of nocturnal-like convective systems. As a litmus test for the basic governing dynamics, the experiments use horizontally homogeneous initial conditions (i.e., they include neither fronts nor low-level jet streams).The simulated storms are allowed to mature as surface-based convective systems before the boundary layer is cooled. In this case it is then surprisingly difficult to cut the mature convective systems off from their source of near-surface inflow parcels. Even when 10 K of the low-level cooling has been applied, the preexisting system cold pool is sufficient to lift boundary layer parcels to their levels of free convection. The present results suggest that many of the nocturnal convective systems that were previously thought to be elevated may actually be surface based. With additional cooling, the simulated systems do, indeed, become elevated. First, the CAPE of the near-surface air goes to zero: second, as the cold pool's temperature deficit vanishes, the lifting mechanism evolves toward a bore atop the nocturnal inversion. Provided that air above the inversion has CAPE, the system then survives and begins to move at the characteristic speed of the bore. Interestingly, as the preconvective environment is cooled and approaches the temperature of the convective outflow, but before the system becomes elevated, yet another distinct behavior emerges. The comparatively weaker cold pool entails slower system motion but also more intense lifting, apparently because it is more nearly balanced by the lower-tropospheric shear. This could explain the frequent observation of intensifying convective systems in the evening hours without the need for a nocturnal low-level jet. The governing dynamics of the simulated systems, as well as the behavior of low-level tracers and parcel trajectories, are addressed for a variety of environments and degrees of stabilization.

Dependence of the precipitation intensity in mesoscale convective systems to temperature lapse rate

“…The importance of the CAPE distribution is manifested evidently in lower CAPE environments than in higher CAPE cases, supporting the idea of Takemi and Satomura (2000). The vertical distribution of CAPE is also recognized to play an important role in the mechanisms for the maintenance and evolution of nocturnal convective systems (Parker, 2008).…”

The 2015 Plains Elevated Convection at Night Field Project

“…Exactly how these structures interact with their environment to favor or deter the upscale growth and organization of convection is not well understood. Schmidt and Cotton (1990), Parker (2008), andSchumacher (2009) used numerical models to demonstrate scenarios in which convectively generated gravity waves near the surface supported the organization and maintenance of MCSs. However, prior to PECAN, scant observational evidence existed with which to analyze these structures.…”

“…This work suggests that an elevated bore can be an integral component of a mature MCS moving through a region with elevated CAPE. In these simulations, bore ascent dominates when the surface-based CAPE vanishes, yet gust-front-driven systems may persist at the surface despite significant boundary layer stabilization (Parker 2008). Several case studies of nocturnal bores show dramatic net displacement of air in the lower troposphere (e.g., Weckwerth et al 2004;Whiteman et al 2006;Knupp 2006;Koch et al 2008;Marsham et al 2011).…”

“…The presence of an SBL may result in undular bores propagating ahead of the parent density current (Crook et al 1990;Carbone et al 2002;Wilson and Roberts 2006;Coleman and Knupp 2011). Idealized simulations utilizing CPMs have been used to explore this last feedback mechanism, in particular the transition to elevated convection and the formation of a bore (e.g., Parker 2008;French and Parker 2010). This work suggests that an elevated bore can be an integral component of a mature MCS moving through a region with elevated CAPE.…”

Observations of dual slantwise circulations above a cool undercurrent in a mesoscale convective system

“…We shall refer to this as an undercurrent. Parker (2008), in his modelling study of an MCS with elevated convection, identified a similar undercurrent which he referred to as an 'underflow regime'.…”