The Interactions of Gravity Waves with Mesocyclones: Preliminary Observations and Theory

Abstract

Apparent interactions between ducted gravity waves and preexisting mesocyclones are investigated. Preliminary analyses of Weather Surveillance Radar-1988 Doppler (WSR-88D) observations from several cases reveal that the intersection of fine lines, whose propagation speed is consistent with that of gravity waves, and existing mesocyclones leads to an increase in the rotational velocity of the mesocyclone. Utilizing simplified ducted wave kinematics and the vorticity equation, changes in vorticity associated with convergence-divergence and perturbation wind shear within the gravity wave are examined. Convergence ahead of wave ridges may be significant, causing mesocyclone intensification through vorticity stretching. It will also be shown that a wave may significantly change the vertical wind shear and streamwise vorticity in storm inflow. A simple one-dimensional model is presented, which shows that vorticity decreases temporarily ahead of the wave ridge, then increases rapidly behind the ridge as positive tilting and stretching act together. The mesocyclone vorticity reaches a peak just ahead of the wave ridge, then begins to rapidly decrease behind the ridge. Model results compared very well to actual measurements in a sample case in which a mesocyclone interacted with two gravity waves of different amplitudes.

Meteorological Overview of the Devastating 27 April 2011 Tornado Outbreak

“…During the merger of the second RS at 2148 UTC, the rotational velocity within the nascent supercell increased from about 7 m s -1 at 2134 to near 18 m s -1 at 2148 UTC, 17 min before tornadogenesis occurred. This pattern is consistent with that reported in Coleman and Knupp (2008). Because the merger event occurred within the dual-Doppler lobe formed by the MAX and KHTX radars (34-km baseline), the detailed kinematics of the RS and the response of the storm intersected by RS features can be examined.…”

“…Such a behavior has been recently simulated by Lewellen (2012). We have also noted (e.g., Coleman et al 2010) that tornadogenesis may rapidly occur on the downtrack side of ridges, as illustrated in Fig. 15 for one of the midday QLCS tornadoes.…”

“…9), where surface temperatures of ~17°C were maintained after the midday QLCS by persistent rain showers and cloud cover. Between 1855 and 1955 UTC, a packet of ducted gravity waves propagated over the MIPS site, producing significant surface pressure oscillations of about ±1 hPa that were correlated with wind fluctuations [consistent with the impedance relation as defined in Gossard and Hooke (1975) and refined by Coleman and Knupp (2010)] and laminar wave cloud formations (Murphy et al 2012). Since the waves moved directly over the MIPS site, the wave properties and boundary layer changes associated with the wave passages were documented.…”

Factors Affecting Surface Wind Speeds in Gravity Waves and Wake Lows

“…Although prediction of the amplitude of a gravity wave or wake low is difficult, Coleman (2008), combining equations from Holton (1992) and Lighthill (1978), showed that the surface pressure perturbation due to the vertical motion in a disturbance is proportional to the static stability of the atmosphere and to the vertical displacement of air parcels. The wind perturbation is directly proportional to the amplitude, so the pressure perturbations associated with a disturbance must be ascertained.…”

Review and case studies of non-traditional severe local windstorms

“…Pressure drops in a wake low are sometimes directly associated with the downward depression of a surfacebased cold pool (e.g., Stumpf et al 1991;Johnson 2001). Downward motion in stable air produces a larger warm perturbation and a more intense low pressure area at the surface (e.g., Bosart and Seimon 1988;Coleman 2008). Utilizing the hydrostatic equation and results from Lighthill (1978), Coleman (2008) showed the following for pressure perturbations associated with vertical displacements of air:…”