Kinematic and Moisture Characteristics of a Nonprecipitating Cold Front Observed during IHOP. Part II: Alongfront Structures

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.

Section: ) Stably Stratified Casessupporting

confidence: 80%

Section: ) Stably Stratified Casesmentioning

confidence: 99%

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

Kawashima

2011

“…Initially studied and documented by fluid dynamicists Helmholtz (1868) and Lord Kelvin (1871), later studies were done using both visual (Ludlam 1967;Reiss and Corona 1977) and laboratory observations (Drazin 1958;Thorpe 1968Thorpe , 1971Thorpe , 1973Simpson 1969Simpson , 1982Scotti and Corcos 1972;Britter and Simpson 1978;Koop and Browand 1979). As technology advanced, K-H waves were also observed by an assortment of instrumentation including ground-based radars (Hicks and Angell 1968;Browning and Watkins 1970;Mueller and Carbone 1987;Weckwerth and Wakimoto 1992;Chapman and Browning 1997;Petre and Verlinde 2004;Luce et al 2008) and airborne radars equipped with additional instrumentation (Browning et al 1973, Hardy et al 1973Wakimoto et al 1992;Wakimoto and Bosart 2001;Friedrich et al 2008;Geerts and Miao 2010), and were numerically simulated (Fritts 1979;Sykes and Lewellen 1982;Droegemeier and Wilhelmson 1986;Fritts et al 1996).…”

Section: Introductionmentioning

confidence: 99%

Polarimetric Doppler Radar Observations of Kelvin–Helmholtz Waves in a Winter Storm

Houser

Bluestein

2011

Kelvin-Helmholtz waves were observed by the Twin Lakes, Oklahoma (KTLX), Weather Surveillance Radar-1988 Doppler (WSR-88D); the Norman, Oklahoma (KOUN), polarimetric WSR-88D; and the polarimetric Collaborative Adaptive Sensing of the Atmosphere (CASA) radars on 30 November 2006 during a winter storm in central Oklahoma. The life cycle and structure of the waves are analyzed from the radar data, and the nearby atmospheric conditions are examined. The initial perturbations associated with the waves are first evident only in the radars' radial velocity fields. As the waves mature, perturbations become discernable in the reflectivity factor Z and spectrum width (SW) fields of both radars, and in the differential reflectivity Z dr and, to a lesser extent, the cross-correlation coefficient r hv fields of KOUN. As the waves break and begin to dissipate, the perturbations subside.A dual-Doppler analysis is synthesized to examine the kinematic structure of the waves and to relate the polarimetric observations to the kinematics. It is determined that Z and Z dr are enhanced in regions of upward motion (wave crests), and r hv is reduced in the same vicinity and near the base of the wave circulations. Vertical velocity perturbations transport horizontal momentum upward and downward, inducing horizontal wind perturbations that are approximately 908 out of phase and downstream from their corresponding vertical velocity perturbations. Perturbations in Z, Z dr , and r hv are observed in the vicinity of wave crests while SW perturbations occur predominately in and just upstream from wave troughs. It is determined that perturbations in the polarimetric variables are a result of the waves modifying local precipitation microphysics. Perturbations in Z and Z dr are hypothesized to be the result of columnar ice crystal generation whereas those in r hv likely result from the mixing of ice crystals of various shapes and sizes. Perturbations in SW are a result of turbulent motions likely associated with wave breaking and downward advection of a strong shear layer.

“…Individual case studies in the literature suggest numerous environments in which KH instability can be released: KH waves (or ''billows'' as they are commonly referred to) have been observed in or near cumulonimbus anvils (Petre and Verlinde 2004;Trier 2016), near the tropopause (Mahalov et al 2011;Trier et al 2012), at the base of low-level jets (e.g., Nakanishi et al 2014;Lin et al 2019), near the top of density currents associated with a sea breeze (Sha et al 1991), behind a cold front (Geerts et al 2006;Friedrich et al 2008;Samelson and Skyllingstad 2016), at the upper boundaries of other cold-air intrusions (Zhou and Chow 2013), in midlatitude cyclones (Wakimoto et al 1992), and above mountainous terrain (Geerts and Miao 2010;Medina and Houze 2016).…”

Section: Introductionmentioning

confidence: 99%

Detailed Dual-Doppler Structure of Kelvin–Helmholtz Waves from an Airborne Profiling Radar over Complex Terrain. Part I: Dynamic Structure

Grasmick

Geerts

2020

Kelvin–Helmholtz (KH) waves are remarkably common in deep stratiform precipitation systems associated with frontal disturbances, at least in the vicinity of complex terrain, as is evident from transects of vertical velocity and 2D circulation, obtained from a 3-mm airborne Doppler radar, the Wyoming Cloud Radar. The high range resolution of this radar (~40 m) allows detection and depiction of KH waves in fine detail. These waves are observed in a variety of wavelengths, depths, amplitudes, and turbulence intensities. Proximity rawinsonde data confirm that they are triggered in layers where the Richardson number is very small. Complex terrain may locally enhance wind shear, leading to KH instability. In some KH waves, the flow remains mostly laminar, while in other cases it breaks down into turbulence. KH waves are frequently locked to the terrain, and occur at various heights, including within the free troposphere, at the boundary layer top, and close to the surface. They are observed not only upwind of terrain barriers, as has been documented before, but also in the wake of steep terrain, where the waves can be highly turbulent. Vertical-plane dual-Doppler analyses of KH waves reveal the mixing of layers of differential momentum across the high-shear zone. Doppler radar data are used to explore the dynamics of KH waves, including the response of thermodynamic and kinematic variables above, below, and within the instability layer.