Model consistent schemes to force gravity waves from tropospheric sources

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Model consistent schemes to force gravity waves
from tropospheric sources

• Fabrizio Sassi
• NCAR/CGD

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TROPOSPHERIC SOURCES OF GRAVITY WAVES

• Gravity waves are produced as a result of
  adjustment processes in a stratified fluid. Flow
  over obstacles, convection and frontal zones are
  typical sources of gravity waves.
• Two examples are shown next waves generation
  from a mid-latitude frontal zone (numerical
  simulation) and from a tropical convective storm
  (observation numerical simulation).

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FRONTAL ZONE

• The early stages of a frontal development show a
  moderate tightening of the gradients of potential
  temperature and pressure while zonal winds are
  moderately strong.
• In the mature stage, gradients are substantially
  tighter, and zonal winds are stronger.
• Accompanying those tight gradients, at higher
  levels gravity waves are visible in vertical
  velocity and temperature perturbations.
• These waves are fairly slow (6 m/s) with a
  horizontal scale 150 km and vertical scale
  2.5 km.

Zhang, 2004 JAS
W
W, ? at 13 km
T,P,U at 8 km
early stage
mature stage
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Radar reflectivity
CONVECTION

• Radar reflectivity shows two storms near Darwin
  (north/west of Australia).
• Radar reflectivity is converted to
  precipitation ? then latent heat ? and vertical
  velocity regions of stronger activity clearly
  show vertically propagating gravity waves.
• Momentum fluxes for the upper troposphere/lower
  stratosphere region are then inferred, showing
  an eastward (westerly) preference with horizontal
  scales less than 100 km.

Alexander et al, 2004 JGR
Vertical velocity
Mom. flux
Phase velocity
Horiz. scale
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January U
January T

• THE ROLE OF GRAVITY WAVES IN M.A. CLIMATE
• Zonal wind reversal in mesosphere
• Winter polar mesopause warmer than summer polar
  mesopause
• The stratopause is at higher elevation in winter
  than in summer.

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Why bother?

• We are concerned with unresolved (horizontally
  and also vertically) gravity waves.
• A correct simulation of the middle atmosphere
  relies critically on detailed aspects of gravity
  wave properties both at their source and sink.
• A model-consistent parameterization is intended
  to capture those aspects, and represent them as
  realistically as possible (consistently with the
  underlying model physics).
• This effort is important for simulations of
  climate change scenarios.

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WACCM3

• Based on CAM3
• Horizontal resolution 2x2.5 (latxlon). 66
  vertical levels from the ground to about 150 km.
  Dynamics is finite-volume (Lin, 2004).
• Same lower atmospheric physics of CAM3 with the
  addition of
• Lindzen-type gravity wave scheme
• Molecular diffusion
• Above 70 km, non-LTE scheme for the 15 ?m band of
  CO2
• Fully interactive chemistry
• Includes thermospheric processes (ion drag, Joule
  heating, etc..)

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Gravity Waves Schemes

• McFarlane (1987) parameterization is used to
  account for the momentum deposition due to
  orographic gravity waves (c0) .
• Spectrum-Base case gravity wave source is
  zonally uniform, operates continuously with an
  intermittency factor of 1/32.
• Spectrum range is from -80 m/s to 80 m/s by 2.5
  m/s (65 waves in total)
• The spectral shape is gaussian, centered about
  the wind at the source level.
• Source level is the first model interface above
  500 hPa.
• In the tropics (between 30S and 30N), the gravity
  wave's spectrum is launched with 1/5 of the
  amplitude of the extra-tropics (to avoid
  generation of tropical wind oscillations).

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Gravity Waves Schemes

• Spectrum-Charon and Manzini (2002 JAS) scheme
• The Frontogenesis function (F) identifies
  locations where the circulation is such that it
  tightens the magnitude of the temperature
  gradient (Hoskins, 1982),
• As the temperature gradients are tightening, an
  ageostrofic circulation follows relieving those
  gradients (Hoskins, 1982). It is assumed that it
  is through the ageostrofic circulation that
  gravity waves are radiated away.
• Subjectively, a critical value of Frontogenesis
  function is chosen When and where the critical
  values is exceeded, gravity waves are launched
  like in the standard scheme.

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Mom. Flux of Wly waves at 500 hPa
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• Conclusions
• Model consistent gravity wave schemes are in
  general preferable to ad hoc solutions. While
  gravity waves properties at source level are the
  greatest uncertainty, typically they are
  specified so that a reasonable middle atmosphere
  results from numerical simulations. This feature
  does not allow for changes of source properties
  in different climate change scenarios. A correct
  representation of gravity wave processes in the
  middle atmosphere is critical toward a proper
  representation of the climate thereof.
• Whereas both source and sink processes are
  important, the greatest challenge upon us is a
  better understanding of the source
  characteristics (spatially and temporally).

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• Can observations help? Yes, but
• Satellite measurements will suffer from asynoptic
  sampling problems at least until more orbiting
  platforms are launched simultaneously.
• Ground observations are limited in spatial
  coverage.
• A deterministic view of gravity wave sources is
  probably beyond our attainable goals but a
  statistical approach may yield valuable results.