JP6J.4 THE STRUCTURE AND DYNAMICS OF ATMOSPHERIC BORES AND SOLITONS AS DETERMINED FROM REMOTE SENSING AND MODELING EXPERIMENTS DURING IHOP Steven E. Koch, Mariusz Pagowski1,2, James W. Wilson3, Frederic Fabry4, Cyrille Flamant5, Wayne Feltz6, Geary Schwemmer7, and Bart Geerts8 1 NOAA Research – Forecast Systems Laboratory, Boulder, Colorado 2 Cooperative Institute for Research in the Atmosphere, Colorado State University 3 National Center for Atmospheric Research 4 Radar Observatory, McGill University 5 Institut Pierre-Simon Laplace, Paris, France 6 Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin 7 NASA Goddard Space Flight Center 8 Department of Atmospheric Sciences, University of Wyoming 1. Introduction 2. Background material on bores Solitary waves are a class of gravity waves The presence of low-level stratification favors consisting of a single elevation of finite amplitude the ordered evolution of gravity currents into that, owing to a balance between nonlinearity and turbulent bores, bores, solitons, and finally solitary dispersion, propagates without change of form. A waves (Christie et al. 1979; Simpson 1987). One family of solitary waves, which is termed a characteristic of gravity currents is a “feeder flow” “soliton”, forms as the natural consequence of the of air directed from behind the head of the gravity evolution of a bore – a type of gravity wave current toward its leading edge in a current-relative (hydraulic jump) generated as a density current framework. Passage of gravity currents at a (such as cold air from a thunderstorm) intrudes ground site is usually identifiable by an abrupt into a fluid of lesser density, which in the case of pressure jump hydrostatically related to the mean the atmosphere, occurs beneath a low-level cooling throughout the depth of the current, a inversion. These phenomena were observed sharp wind shift caused by the horizontal gradient repeatedly by a multitude of ground-based and of the pressure perturbation field, and increased airborne remote sensing systems during the six- gustiness due to strong vertical mixing in the head. week field phase of the International H20 Project (IHOP), even though bores were not one of the An internal bore in the atmosphere is a type of primary IHOP objectives (Weckwerth et al. 2004). gravity wave disturbance that propagates on a These observing systems together produced the low-level inversion ahead of gravity currents. most extensive set of observations ever collected Bores typically are generated as a gravity current of the evolving structure of bores, solitons, and intrudes into a stably-stratified boundary layer solitary waves. In spite of this, a more complete (SBL) of sufficient depth near the ground, resulting understanding of the dynamics of these in a sustained elevation of the inversion layer. phenomena and their interactions required use of Amplitude-ordered solitary waves (a train of which very high-resolution numerical weather prediction is referred to as a soliton) can evolve from bores in models initialized with IHOP data. These some instances. Because the speed of a solitary observations and the numerical modeling together wave is proportional to the wave amplitude, a offer the opportunity for an unprecedented study of dispersive family of waves evolves from the initial the evolving structure of bores, solitons, and bore with amplitudes inversely related to their solitary waves. widths. The number of waves increases continuously with time to a finite value, though the Corresponding author address: Steven E. Koch, NOAA/OAR/FSL, R/FS1, 325 Broadway, Boulder, CO number of waves is highly dependent upon the 80305-3328; e-mail < [email protected]> turbulent dissipation. According to the hydraulic theory of Rottman defined solitary waves may propagate ahead of and Simpson (1989), hereafter RS89, bore the gravity current. propagation speed depends upon its depth (db), the SBL inversion depth (h0), and the speed of a This prediction for the strength of a bore may be long gravity wave (Cgw) as follows: compared to another one based upon hydraulic theory and the laboratory work of RS89, which 1 2 does not require knowledge of the inflow velocity, C C = !0.5 d h 1+ d h # . (1) bore gw " ( b 0 )( b 0 )$ but instead depends upon: Koch and Clark (1999) have applied other Cgc Cgc Cgc predictive equations for bore speed to account for Fr = = 1 = C 2 N h such factors as finite fluid depth, vanishingly small gw (g !" h0 "vw ) 0 depth of the stable layer, and energy loss and (3) restricted to the neutral layer above the SBL d waveguide (Klemp et al. 1997). In general, the H = gc SBL must be sufficiently deep and intense to h0 support a bore of a given strength. The modeling work of Haase and Smith (1989b), hereafter HS89, where Fr is the Froude number, H is a measure of and Jin et al. (1996) sheds light on this issue. the depth of the gravity current to the inversion HS89 suggest that two parameters govern depth, and the potential temperature change whether an undular bore will be generated from an across the inversion is given by !" . It is of some intrusive gravity current: interest to note that the Froude number used in RS89 is proportional to the inverse of the µ (U ! Cgc ) (U ! Cgc ) F = = parameter used in HS89, i.e., the critical value C* g "# dgc #vw µ >0.7 equates in the RS89 nomenclature to: and (2) Cgc 2 C0 2N h0 $ 0.91 . (4) µ = = < = N h0 !µc Cgc Cgc The presence of the gravity current is no The first parameter F is the ratio of the front- longer critical to the evolution of the undular bore relative “inflow velocity” of the cold air (U - C ) to gc and solitary waves once the bore has been the densimetric speed of a gravity current (C*), the generated (Christie 1989). In the simplest case, latter being a function of the gravity current depth the bore is generated on top of a surface-based, d and temperature contrast ( ). The second gc µ pre-frontal stable layer of sufficient depth as the parameter is the ratio of the long gravity wave gravity current intrudes into the SBL. In the real phase speed C0 to the gravity current speed (Cgc). atmosphere, complex stratification, vertical wind Note that C0 depends upon the inversion layer shear, elevated inversions, and unsteady and/or strength (N) and depth (h0). The parameter µ multiple gravity currents may complicate matters. determines whether an undular bore (or solitary In addition, the PBL must be sufficiently deep and wave) will form and propagate ahead of the gravity intense to support a bore of a given strength current as required. When µ <0.7 (the (Rottman and Simpson 1989). If a mechanism is “supercritical regime”), the gravity current not present to trap the vertical propagation of wave propagates faster than any gravity waves and a energy, the bore will quickly diminish (Crook 1988; well-defined feeder flow is present. When the Koch et al. 1991; Koch and Clark 1999). Wave value of µ increases to near 0.7, but still within trapping ability is typically measured by the Scorer parameter defined as: this supercritical regime, undular bores may form. Bores and solitary waves are spawned when 2 2 2 >0.7 (the “subcritical regime”) as the gravity 2 Nm " U "z 2 µ m = ! ! k , (5) 2 U C wave propagates considerably faster than the (U ! Cbore ) ( ! bore ) gravity current. For large values of F, several well- where k and m are the horizontal and vertical windshift into the direction of movement of the wavenumbers, Nm is the moist Brunt-Väisälä disturbance associated with a sudden pressure frequency, U is the background wind in the jump, and a net cooling in the lower troposphere. direction of bore propagation, and Cbore is the bore The degree of cooling should be hydrostatically propagation speed. A rapid decrease of m with consistent with the sustained surface pressure height supports wave trapping. Of particular increase that follows an initial abrupt pressure relevance to bores is the low-level jet mechanism, jump, as recorded in sensitive surface which seems to be quite prevalent in bore case microbarograph data. Cooling associated with studies (Koch et al.1991), since it gives rise to gravity currents is due primarily to horizontal strong trapping related to curvature in the velocity advection of the denser air, whereas adiabatic profile (the second term in the Scorer equation). ascent is the cause of cooling in the case of a bore. Thus, pronounced surface cooling is not Koch and Clark (1999) provide a methodology characteristic of bores; in fact, weak warming (and for determining whether observed phenomena are drying) may occur as the result of turbulent gravity currents, bores, or solitary waves. The downward mixing of warmer (and typically drier) minimum requirements that must be satisfied to air from above the inversion into the SBL. support the hypothesis that a phenomenon is actually a gravity current are that: Gravity currents may generate other kinds of gravity waves besides bores and solitary waves. • Its depth as predicted hydrostatically using Model simulations show that a broad spectrum of gravity current theory agrees with that gravity waves may be produced by gravity measured by the remotely sensed data. currents, including trapped lee-type waves created • Its speed of propagation observed prior to by strong flow over the head of the gravity current deceleration is consistent with that predicted (Jin et al. 1996). Trapped lee waves display no from gravity current theory. vertical tilt and are motionless relative to the • A region of pronounced positive front-relative gravity current head. Lee-wave trapping occurs “feeder flow” is observed behind the leading only under a very special condition in which the edge of the gravity current above the surface Scorer parameter decreases sufficiently rapidly layer (so as to minimize frictional effects).