Numerical Study of Horizontal Shear Instability Waves along Narrow Cold Frontal Rainbands

Abstract

The effects of variations in low-level ambient vertical shear and horizontal shear on the alongfront variability of narrow cold frontal rainbands (NCFRs) that propagate into neutral and slightly unstable environments are investigated through a series of idealized cloud-resolving simulations.In cases initialized with slightly unstable sounding and weak ambient cross-frontal vertical shears, core-gap structures of precipitation along NCFRs occur that are associated with wavelike disturbances that derive their kinetic energy mainly from the mean local vertical shear and buoyancy. However, over a wide range of environmental conditions, core-gap structures of precipitation occur because of the development of a horizontal shear instability (HSI) wave along the NCFRs.The growth rate and amplitude of the HSI wave decrease significantly as the vertical shear of the ambient cross-front wind is reduced. These decreases are a consequence of the enhancement of the low-level local vertical shear immediately behind the leading edge. The strong local vertical shear acts to damp the vorticity edge wave on the cold air side of the shear zone, thereby suppressing the growth of the HSI wave through the interaction of the two vorticity edge waves. It is also noted that the initial wavelength of the HSI wave increases markedly with increasing horizontal shear. The local vertical shear around the leading edge is shown to damp long HSI waves more strongly than short waves, and the horizontal shear dependency of the wavelength is explained by the decrease in the magnitude of the vertical shear relative to that of the horizontal shear.

The formation of a large summertime Saharan dust plume: Convective and synoptic-scale analysis

“…As mentioned in section 4.3 the simulated cold pool flow veers from a radial flow until it is almost parallel to the edge of the cold pool. This behavior is also suggested in the observed dust plume by the formation of structures consistent with horizontal shear instability (Figure 1 ) [ Kawashima , 2011 ]. This shows that the cold pool is being strongly influenced by rotational effects.…”

Reexamining the United Kingdom’s Greatest Tornado Outbreak: Forecasting the Limited Extent of Tornadoes along a Cold Front

“…Misovortices have diameters of 1−4 km (Fujita 1981) and have been suggested to be the parent circulation from which the tornadoes form. Different explanations have been offered to explain misovortex formation, including the release of horizontal shearing instability (e.g., Carbone 1982;Hobbs and Persson 1982;Lee and Wilhelmson 1997b;Jorgensen et al 2003;Wheatley and Trapp 2008;Kawashima 2011), advection of hydrometeors (Locatelli et al 1995), trapped gravity waves (Brown et al 1999), tilting of vorticity along the cold front (Carbone 1983), or combinations of the above.…”

Doppler Radar Observations of Horizontal Shearing Instability in Quasi-Linear Convective Systems

“…Shear instability within the vertical plane (Kelvin-Helmholtz) and horizontal plane (HSI) extracts energy from the mean flow, converting it into discrete maxima of vorticity (Haurwitz 1949;Markowski and Richardson 2010). HSI has been hypothesized to be responsible for the formation of small-scale vortices within vigorous narrow cold frontal rainbands (NCFR; Carbone 1982Carbone , 1983Kawashima 2011), outflow boundaries (Mueller and Carbone 1987;Wakimoto and Wilson 1989;Lee and Wilhelmson 1997a,b, hereafter LW97), and idealized nearsurface boundaries (Buban and Ziegler 2016a,b, hereafter BZ16). LW97 found that lobe and cleft instabilities, which develop near the outflow nose, can trigger HSI and the formation of misovortices which undergo a life cycle of vortex sheet roll up, subharmonic interaction, consolidation, and dissipation.…”