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of Ro as well as Fr. We use a barotropic unstable jet flow as a basic state and add forcing to maintain this flow. By choosing a forcing parameter, we maintain an unsteady rotational flow that changes periodically with time. Spontaneous gravity wave radiation from an unsteady rotational flow is then generated continuously. We have investigated continuous gravity wave radiation from an unsteady rotational flow using a similar system with a fixed parameter value ( Sugimoto et al. 2007b ). In the
of Ro as well as Fr. We use a barotropic unstable jet flow as a basic state and add forcing to maintain this flow. By choosing a forcing parameter, we maintain an unsteady rotational flow that changes periodically with time. Spontaneous gravity wave radiation from an unsteady rotational flow is then generated continuously. We have investigated continuous gravity wave radiation from an unsteady rotational flow using a similar system with a fixed parameter value ( Sugimoto et al. 2007b ). In the
1. Introduction Gravity waves (GWs) play a key role in the global meteorology, climate, chemistry, and microphysics of the stratosphere and mesosphere ( Fritts and Alexander 2003 ). Because finite computational resources force global climate–chemistry and weather prediction models to run at spatial resolutions that do not adequately resolve GW dynamics, these important GW-induced effects must be parameterized (e.g., McLandress 1998 ; Kim et al. 2003 ). Arguably the greatest weakness in
1. Introduction Gravity waves (GWs) play a key role in the global meteorology, climate, chemistry, and microphysics of the stratosphere and mesosphere ( Fritts and Alexander 2003 ). Because finite computational resources force global climate–chemistry and weather prediction models to run at spatial resolutions that do not adequately resolve GW dynamics, these important GW-induced effects must be parameterized (e.g., McLandress 1998 ; Kim et al. 2003 ). Arguably the greatest weakness in
-friendly version of the Lighthill–Ford theory, and discuss each forcing term and the relationship of the leading-order terms to established CAT forecasting techniques. In section 3 the method used to apply this theory as a CAT forecasting tool is explained. In section 4 a case study is presented showing the utility of Lighthill–Ford theory in CAT forecasting. Section 5 examines a season-long database that demonstrates the superiority of applying this theory versus other CAT forecasting methods, and
-friendly version of the Lighthill–Ford theory, and discuss each forcing term and the relationship of the leading-order terms to established CAT forecasting techniques. In section 3 the method used to apply this theory as a CAT forecasting tool is explained. In section 4 a case study is presented showing the utility of Lighthill–Ford theory in CAT forecasting. Section 5 examines a season-long database that demonstrates the superiority of applying this theory versus other CAT forecasting methods, and
1. Introduction Gravity waves are atmospheric waves with a restoring force of buoyancy, which are characterized by their small spatial scales and short periods. Gravity waves have the ability to transport momentum, mostly in the vertical, over a long distance and deposit it in the mean field through dissipation and breaking processes. Since the importance of this ability of gravity waves in the middle atmosphere was recognized in early 1980s, many observational, numerical, and theoretical
1. Introduction Gravity waves are atmospheric waves with a restoring force of buoyancy, which are characterized by their small spatial scales and short periods. Gravity waves have the ability to transport momentum, mostly in the vertical, over a long distance and deposit it in the mean field through dissipation and breaking processes. Since the importance of this ability of gravity waves in the middle atmosphere was recognized in early 1980s, many observational, numerical, and theoretical
the spontaneous generation of exponentially small fast oscillations analogous to IGWs. In this model, the slow part of the motion is represented by the forcing term, assumed to be determined a priori and independent of the evolution of the fast variables. This misses a central aspect of the dynamics of two-time-scale systems, namely that there is in general no exact split between slow and fast variables. Nevertheless, the forced harmonic oscillator provides an instructive example that serves to
the spontaneous generation of exponentially small fast oscillations analogous to IGWs. In this model, the slow part of the motion is represented by the forcing term, assumed to be determined a priori and independent of the evolution of the fast variables. This misses a central aspect of the dynamics of two-time-scale systems, namely that there is in general no exact split between slow and fast variables. Nevertheless, the forced harmonic oscillator provides an instructive example that serves to
-scale imbalance in generating mesoscale gravity waves were further examined most recently in Plougonven and Zhang (2007) . However, as noted in Lane et al. (2004) , without a sophisticated wave source analysis it is often difficult to determine unambiguously whether mesoscale structures, such as jets and upper-level fronts, are the source of the gravity waves or a response to some other forcing that also generates the waves. The ray-tracing technique has been widely used to investigate gravity wave sources
-scale imbalance in generating mesoscale gravity waves were further examined most recently in Plougonven and Zhang (2007) . However, as noted in Lane et al. (2004) , without a sophisticated wave source analysis it is often difficult to determine unambiguously whether mesoscale structures, such as jets and upper-level fronts, are the source of the gravity waves or a response to some other forcing that also generates the waves. The ray-tracing technique has been widely used to investigate gravity wave sources
forcing and shown reasonable agreement with full simulations. Our approach is to simulate numerically an idealized vortex dipole. The numerical solutions begin from the surface-trapped QG dipole for a uniform potential vorticity fluid of Muraki and Snyder (2007) . This dipole is associated with a potential temperature anomaly on a flat horizontal boundary. In terms of atmospheric jet streaks, the rigid boundary may be thought of as a simple model for the tropopause and the computational domain can
forcing and shown reasonable agreement with full simulations. Our approach is to simulate numerically an idealized vortex dipole. The numerical solutions begin from the surface-trapped QG dipole for a uniform potential vorticity fluid of Muraki and Snyder (2007) . This dipole is associated with a potential temperature anomaly on a flat horizontal boundary. In terms of atmospheric jet streaks, the rigid boundary may be thought of as a simple model for the tropopause and the computational domain can
studied the simplest thought experiment in which the phenomenon arises. For unstratified, nonrotating, compressible flow in an unbounded domain with no gravity or other external force, he asked how a freely evolving vortical flow occupying some finite region might emit sound waves, even when the Mach number M = U / c s ≪ 1. Here, U is a typical flow speed and c s is the sound speed. By combining simple mathematics with a careful and powerful heuristic argument based on physical insight
studied the simplest thought experiment in which the phenomenon arises. For unstratified, nonrotating, compressible flow in an unbounded domain with no gravity or other external force, he asked how a freely evolving vortical flow occupying some finite region might emit sound waves, even when the Mach number M = U / c s ≪ 1. Here, U is a typical flow speed and c s is the sound speed. By combining simple mathematics with a careful and powerful heuristic argument based on physical insight
interaction between the flow and a physical obstruction (e.g., generation in the wake of a ship; Lighthill 1978 ), which is the mechanism by which mountains generate atmospheric gravity waves ( Hines 1989 ). Direct forcing of the ocean by the atmosphere is a known source of oceanic gravity waves ( Wunsch and Ferrari 2004 ). Finally, inertia–gravity waves are also radiated during the geostrophic adjustment of a hypothetical fluid ( Rossby 1938 ), in which geostrophic balance is approached from an
interaction between the flow and a physical obstruction (e.g., generation in the wake of a ship; Lighthill 1978 ), which is the mechanism by which mountains generate atmospheric gravity waves ( Hines 1989 ). Direct forcing of the ocean by the atmosphere is a known source of oceanic gravity waves ( Wunsch and Ferrari 2004 ). Finally, inertia–gravity waves are also radiated during the geostrophic adjustment of a hypothetical fluid ( Rossby 1938 ), in which geostrophic balance is approached from an
Coriolis force is perpendicular to the wall. The nose of the jet propagates in the cyclonic direction around the tank and eventually a closed cyclonic circulation is established in the tank. The baroclinic jet leaning on the wall becomes unstable and small-scale meanders form along the jet. These small-scale initial instabilities can be clearly seen in Fig. 2a between 12 and 6 o’clock positions. Larger meanders grow near the source and then propagate with the current. It is interesting to note that
Coriolis force is perpendicular to the wall. The nose of the jet propagates in the cyclonic direction around the tank and eventually a closed cyclonic circulation is established in the tank. The baroclinic jet leaning on the wall becomes unstable and small-scale meanders form along the jet. These small-scale initial instabilities can be clearly seen in Fig. 2a between 12 and 6 o’clock positions. Larger meanders grow near the source and then propagate with the current. It is interesting to note that
