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Analysis of a Lower-Tropospheric Gravity Wave Train Using Direct and Remote Sensing Measurement Systems

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Analysis of a Lower-Tropospheric Gravity Wave Train Using Direct and Remote Sensing Measurement Systems JULY 2017 T O M S E T A L . 2791 Analysis of a Lower-Tropospheric Gravity Wave Train Using Direct and Remote Sensing Measurement Systems a BENJAMIN A. TOMS School of Meteorology, University of Oklahoma, Norman, Oklahoma JESSICA M. TOMASZEWSKI Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, Colorado b DAVID D. TURNER AND STEVEN E. KOCH National Severe Storms Laboratory, Norman, Oklahoma (Manuscript received 8 June 2016, in final form 31 January 2017) ABSTRACT On 10 August 2014, a gravity wave complex generated by convective outflow propagated across much of Oklahoma. The four-dimensional evolution of the wave complex was analyzed using a synthesis of near- surface and vertical observations from the Oklahoma Mesonet and Atmospheric Radiation Measurement (ARM) Southern Great Plains networks. Two Atmospheric Emitted Radiance Interferometers (AERI)— one located at the ARM SGP central facility in Lamont, Oklahoma, and the other in Norman, Oklahoma— were used in concert with a Doppler wind lidar (DWL) in Norman to determine the vertical characteristics of the wave complex. Hydraulic theory was applied to the AERI-derived observations to corroborate the ob- servationally derived wave characteristics. It was determined that a bore-soliton wave packet initially formed when a density current interacted with a nocturnal surface-based inversion and eventually propagated independently as the density current became diffuse. The soliton propagated within an elevated inversion, which was likely induced by ascending air at the leading edge of the bore head. Bore and density current characteristics derived from the observations agreed with hydraulic theory estimates to within a relative difference of 15%. While the AERI did not accurately resolve the postbore elevated inversion, an error propagation analysis suggested that uncertainties in the AERI and DWL observations had a negligible influence on the findings of this study. 1. Introduction Karan and Knupp 2009), sea-breeze fronts (Clarke et al. 1981; Sheng et al. 2009), or even by the interaction be- Lower-tropospheric gravity waves are ubiquitous and tween convective outflow and sea-breeze fronts often take the form of either internal bores or internal (Wakimoto and Kingsmill 1995; Kingsmill and Crook solitary waves. Such gravity waves may be generated by 2003). The vertical perturbation of the stably stratified the disturbance of a surface-based inversion by density layer generates an oscillatory motion that may propa- currents in the form of intense cold fronts (Smith et al. gate ahead of the wave source and be maintained if at- 1982; Karyampudi et al. 1995; Hartung et al. 2010), moist mospheric wave ducting properties are present. This convective outflow (Knupp 2006; Coleman et al. 2009; wave may take the form of either a bore, a solitary wave, or a packet of waves (Christie et al. 1979; Crook 1986; a Current affiliation: Department of Atmospheric Science, Colorado Rottman and Simpson 1989). State University, Fort Collins, Colorado. The passage of an atmospheric bore commonly in- b Current affiliation: NOAA/Earth System Research Laboratory, stigates sustained modifications to the prewave envi- Boulder, Colorado. ronment (Clarke et al. 1981; Rottman and Simpson 1989; Koch et al. 1991). Initially, air near the surface is Corresponding author: Benjamin A. Toms, ben.toms@colostate. vertically displaced in advance of the bore head. The edu vertically displaced air cools adiabatically, creating a DOI: 10.1175/MWR-D-16-0216.1 Ó 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). 2792 MONTHLY WEATHER REVIEW VOLUME 145 compact column of relatively dense air aloft, allowing therefore essential to understanding the evolution of for an addition of mass in the total atmospheric column such lower-tropospheric gravity waves. in the evacuated space. Additional mass transported by The undular nature of a bore is dependent upon the the bore contributes to the total increase of mass in the ratio of the depth of the bore to the depth of the surface- 21 atmospheric column. Thus, an abrupt increase in surface based inversion (i.e., bore strength; dbh0 )(Benjamin pressure occurs as a result of bore passage, which is and Lighthill 1954; Rottman and Simpson 1989). For 1 , 21 , hydrostatically consistent with the magnitude of adia- dbh0 2, undulations are formed following the leading batic cooling induced by the rising motion. The in- bore and the bore is undular in form, with energy dis- creased surface pressure gradient causes surface winds sipation dependent upon vertical wave propagation. to increase in magnitude and rotate toward the direction These undulations are often observed following the of bore movement. On the back side of the bore head, passage of undular bores as an amplitude-ordered downward mixing of horizontal momentum further in- packet of solitary waves, known as a soliton. The creases the near-surface winds, while the downward Morning Glory waves in northeastern Australia are mixing of potentially warmer air leads to a positive historically the most commonly cited form of such un- perturbation in surface temperature. Aside from pres- dular bores (Clarke et al. 1981; Smith et al. 1982; Goler sure perturbations, which may be observed at the sur- and Reeder 2004). For increasing bore strength, how- face no matter the vertical placement of the wave, the ever, turbulent motions on the rear side of the bore head influences of a solitary wave are similar to those of a become increasingly important for wave-energy dissi- bore, although they are transient and confined to the pation and inhibit the development of solitary waves vertical region within which the solitary wave resides that may form behind the bore (Rottman and Simpson (Rottman and Einaudi 1993; Knupp 2006; Koch 1989). Of similar concern is the potential for solitary et al. 2008a). waves to assist in vertical mixing if the solitary waves The evolution of gravity wave complexes has been propagate behind a turbulent bore (Koch et al. 2008a). shown to be dependent upon the prewave kinematic and Knowledge of each wave packet’s evolution is therefore thermodynamic environment (Knupp 2006; Koch et al. critical to the assessment of both the characteristics of 2008a,b). Atmospheric bores and solitons generated by the waves and their potential impacts on the surround- density currents propagate within stable layers near the ing environment. ground and are commonly confined to the lowest kilo- Depending on the wave characteristics, bores and meters of the atmosphere (Rottman and Simpson 1989; solitary waves may generate enough vertical motion to Rottman and Grimshaw 2002). The vertical wind shear overcome near-surface static stability and generate between two stably stratified layers may limit the verti- moist convection. Multiple case studies have shown cal propagation of gravity waves, with winds orthogonal bores to decrease the stability of the boundary layer and to the direction of wave propagation being the most generate vertical motion and turbulent mixing associ- conducive to wave ducting (Crook 1986, 1988; Rottman ated with mass convergence on the leading edge of the and Simpson 1989; Rottman and Einaudi 1993; Koch wave (Karyampudi et al. 1995; Koch and Clark 1999; and Clark 1999). These physical relationships are Coleman and Knupp 2011). The cooling generated by mathematically explained via the Scorer parameter, the vertical motion at the leading edge of the bore can defined here: lead to cloud formation if the atmospheric moisture content is sufficient. This condensation has been docu- 2 N(z) [›2U(z)/›z2] mented to result in the formation of laminar, roll-shaped l2(z) 5 2 (1) [U(z)2 C]2 U(z)2 C clouds, termed Morning Glory waves in Australia [e.g., Haase and Smith (1984) and Goler and Reeder (2004)], where l2 is the Scorer parameter, N is the Brunt–Väisälä or even strong to severe convection if the free atmo- frequency, U is the magnitude of the wind in the di- sphere is conditionally unstable (e.g., Karyampudi et al. rection of bore propagation, and C is the propagation 1995; Koch and Clark 1999; Locatelli et al. 2002; Watson speed of the bore (Scorer 1949). The first term repre- and Lane 2016). sents the contribution of stably stratified layers to wave- Because of the complexity of their evolution, bores energy ducting, while the second represents the ducting and solitons pose significant diagnostic and prognostic induced by the curvature of the wind. Accordingly, the difficulty. The modifications to the atmospheric profile spatial characteristics of bores often vary substantially by bores and solitons and the related potential for moist due to mesoscale variability in near-surface kinematic convection are difficult to quantify unless specific char- and thermodynamic characteristics (Hartung et al. acteristics of the waves are known (Koch and Clark 2010). The study of low-level atmospheric structure is 1999). High-resolution observations of gravity waves are JULY 2017 T O M S E T A L . 2793 TABLE 1. Properties of observation systems utilized. Observing system Extracted variables Sampling resolution (min) Location Oklahoma Mesonet T, Td, w, p, wind speed, wind direction 5 Oklahoma; statewide ARM surface stations T, Td, w, p, wind speed, wind direction 1 North-central OK Doppler wind lidar Wind speed, wind direction 2 Norman, OK AERI T, w, LWP 2 Norman, OK, and Lamont, OK NEXRAD radar Base reflectivity 5 TLX, DDC, VNX, INX, ICT therefore crucial for accurate analyses of their evolu- Prior to discussing the wave complex, we provide tion, influences on their surrounding environment, and details of the utilized instruments in section 2. The their potential to generate moist convection. Recently, prewave environment is discussed using both atmo- the Plains Elevated Convection at Night (PECAN) field spheric profiles and synoptic weather analyses in section campaign utilized a high concentration of mesonet sta- 3, and the wave complex is analyzed using observational tions and atmospheric profilers to better understand the data and hydraulic theory in section 4.
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