To incoming students of meteorology_ the notion of “balanced flow

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To incoming students of meteorology_ the notion of “balanced flow Powered By Docstoc

   To incoming students of meteorology, the notion of “balanced flow” conveys the idea of an approx-
imate balance of Coriolis, centripetal, and pressure-gradient forces in the large-scale flow of the atmos-
phere. It is also a state of minimum energy to which the flow might relax through “geostrophic adjust-
ment” and the radiation of gravity waves. But in the last thirty years the idea has undergone considerable
development and refinement to include related concepts such as (i) the existence or otherwise of a “slow
manifold” on which solutions to the primitive equations might remain if properly initialized, (ii) the use
of potential vorticity (PV) and PV inversion to diagnose and predict a balanced flow, (iii) more accurate
theoretical models of balance or “hyperbalance,” and (iv) how such models might be used to minimize
imbalances in the initial conditions of numerical models of the primitive equations.

    Consequently there is a lot more for a student to learn about the subject than was the case thirty years
ago. Alas, there is still more to be discovered. We are beginning to understand that geostrophic adjust-
ment is more than a process brought about by an unbalanced initial condition introduced by a professor
teaching a class. Imbalance can arise spontaneously as a result of balanced motions. The spontaneity of
imbalance, absence of external forcing, and development at subcritical Richardson number distinguishes
this process from the traditional idea of geostrophic adjustment and from other mechanisms of gravity
wave excitation (e.g., when balanced flow encounters topography and is forced to flow over rather than
around the ridge, when penetrative convection hits a stable layer, or when flows from opposing directions
overlap in the vertical, generating shear instabilities that radiate gravity waves into the far field). It is now
appreciated that balanced flows spontaneously generate local regions of imbalance without any of the
above mechanisms acting, simply as a result of horizontal advection and deformation. This process of
“spontaneous imbalance” and the subsequent radiation of gravity waves has an important, though incom-
plete, parallel in the spontaneous radiation of acoustic waves by fluid flows, a subject pioneered by the
late Sir James Lighthill. Instability remains an efficient way to produce imbalance, but might include (in
addition to parcel instabilities) the familiar baroclinic instability of meteorology, or jet streaks moving
through baroclinic wave troughs. Certain types of hybrid instability also exist in which balanced and un-
balanced motions in different regions are fused together.

    Outside of hurricanes, nature does not make spontaneous imbalance easy. There is normally a wide
separation of scales (temporal and spatial) between balanced and unbalanced motions. The wider the gap
(as in the ocean) the more difficult it is for gravity waves to be spontaneously excited. Despite valiant ef-
forts in recent years, oceanographers have yet to find a “smoking gun” pointing to an unambiguous case
of spontaneous imbalance. (Even gravity waves observed under sea ice might be forced from the surface,
as so often is the case elsewhere in the ocean.) For atmospheric scientists the gap is less severe and gravi-
ty wave excitation by this mechanism may be easier to detect. Regardless of amplitude and scale, geo-
physical flows are usually turbulent (in a quasi-2D sense) somewhere and will therefore tend to close the
spectral gap as a result of the forward enstrophy cascade, thereby creating local regions of imbalance.
Nonlocal mechanisms of spectral energy transfer (such as instability) that accelerate this cascade are like-
ly to be more efficient in creating imbalance than geophysical turbulence per se. Unstable currents exist in
atmosphere and ocean alike. So there is hope that instances of spontaneous imbalance might be discov-
ered in the ocean, and on Mars, despite the prevalence of surface forcings.

   This special collection of the Journal of the Atmospheric Sciences is composed of papers developed
from the First Spontaneous Imbalance Workshop: Perspectives from Atmospheric and Oceanic Dynamics
in August 2006 at the University of Washington in Seattle, Washington. The workshop was sponsored by
the National Science Foundation and Northwest Research Associates.

                                                                            Tim Dunkerton & Pascale Lelong
                                                                                             Guest Editors
                                                                              Chris Snyder

  List of Articles

2265    Ngan et al.        Dissipation of Synoptic-Scale Flow by Small-Scale Turbulence
2336    Afanasyev et al.   Emission of Inertial Waves by Baroclinically Unstable Flows:
                           Laboratory Experiments with Altimetric Imaging Velocimetry
2351    Snyder et al.      Inertia-gravity waves generated within a dipole vortex
2404    Sugimoto et al.    Parameter Sweep Experiments on Spontaneous Gravity Wave
                           Radiation from Unsteady Rotational Flow in an f-Plane Shallow
                           Water System
2455    Riley and Lindborg Stratified Turbulence: A Possible Interpretation of Some
                           Geophysical Turbulence Measurements
2477    Knox et al.        Application of the Lighthill–Ford Theory of Spontaneous
                           Imbalance to Clear-Air Turbulence Forecasting
2480    Williams et al.    Inertia–Gravity Waves Emitted from Balanced Flow: Observa
                           tions, Properties, and Consequences
2481    Zeitlin            Decoupling of Balanced and Unbalanced Motions and Inertia–
                           Gravity Wave Emission: Small versus Large Rossby Numbers
2482    Zhang and Zhang Tracking Gravity Waves in Baroclinic Jet-Front Systems
2489    Wu and Eckermann Global Gravity Wave Variances from Aura MLS: Characteristics
                           and Interpretation
2490    Schecter           The Spontaneous Imbalance of an Atmospheric Vortex at High
                           Rossby Number
2494    Vanneste           Exponential Smallness of Inertia--Gravity--Wave Generation at
                           Small Rossby Number
2499    Hakim              A Probabilistic Theory for Balance Dynamics
2538    McIntyre           Spontaneous Imbalance and Hybrid Vortex--Gravity Structures
2539    Sato and Yoshiki    Gravity Wave Generation around the Polar Vortex in the
                           Stratosphere Revealed by Three Hourly Radiosonde Observations
                           at Syowa Station

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