Solar Semidiurnal Tide in the

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Solar Semidiurnal Tide in the Atmosphere
Jeff Forbes Department of Aerospace Engineering
Sciences University of Colorado, Boulder, CO
80309-0429

• Forcing of the Semidiurnal Tide
• Vertical Propagation of the Semidiurnal Tide and
  its Interactions with the Overlying Atmosphere
• Distortion by Zonal Mean Winds
• Modulation by Longitude Variations in Mean Winds
• (Stationary Planetary Waves)
• Modulation by Traveling Planetary Waves (e.g.,
  2-day wave)
• Solar Semidiurnal Tide in Mars Dusty Atmosphere

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The ITM System
400 km
ITM System
60 km
0 km
Pole
Equator
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The semidiurnal tide is just one example from a
whole spectrum of waves that couple different
atmospheric regions and produce observable
phenomena.
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Thermal Excitation of the Semidiurnal Tide
150
100
Height (km)
50
0
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In the local time frame, the heating may be
represented as
Q
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Solar Heating Distribution from a Space-Based
Perspective
To an observer in space, it looks like the
heating bulge (and the tides it generates) are
fixed with respect to the Sun, and the planet is
rotating beneath. To an observer on the
ground, the heating bulge, and the tides it
generates, are moving westward or migrating at
the apparent motion of the Sun.
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Meridional wind field at 103 km (April)
associated with the semidiurnal tide propagating
upward from the lower atmosphere, mainly excited
by UV absorption by O3 in the stratosphere-mesosph
ere
Courtesy M. Hagan
The tide propagates westward with respect to the
surface once per day, and is locally seen as the
same semidiurnal tide at all longitudes.
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UV Absorption by O3
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Tidal Variability
Eastward Winds over Saskatoon, Canada, 65-100 km
Note the transition from easterlies (westerlies)
below 80-85 km to westerlies (easterlies) above
during summer (winter), due to GW filtering and
momentum deposition.
Note the predominance of the semidiurnal tide
at upper levels, with downward phase progression.
Courtesy of C. Meek and A. Manson
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Longitude variations are taken into account with
zonal wavenumbers s ? n.
A spectrum of tides thus exists, to first order
representable as a linear superposition of waves
of various frequencies (n) and zonal wavenumbers
(s)
The waves with s ? n are referred to as
non-migrating tides because they do not migrate
with respect to the Sun to a planetary-fixed
observer.
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200
150
100
Height (km)
50
UV Absorption by O3
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Zonal Mean Winds due to Dissipation of
Semidiurnal Tides
SW2 SW1 SW3
SW2 only
SW1 SW3
Angelats i Coll and Forbes, 2002
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The total atmospheric response to solar forcing
is the result of constructive and destructive
interference between migrating and nonmigrating
tidal components, giving rise to a different
tidal response at each longitude.
TIMED/SABER Semidiurnal Temperatures 110 km
April 2004
Zhang et al., 2006
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400
Penetration into upper thermosphere ionosphere
350
300
Height (km)
50
UV Absorption by O3
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Solar Semidiurnal Tide in the Dusty Mars
Atmosphere
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The Model

• Time-dependent global model of the
  mutually-interactive semidiurnal tide and zonal
  mean circulation
• Parameterization employed to handle convective
  instability -- eddy diffusivity introduced to
  keep wave amplitude at stable limit.
• Heating rates used based on
• observed dust distributions
• validated against surface pressure
• perturbations measured by
• Viking-1and Viking-2 landers.

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Solar Semidiurnal Tide in Mars Atmosphere, Ls
270, High Dust (t 2.3)
Solar Semidiurnal Tide in Earths Atmosphere,
Ozone Heating
Solar Semidiurnal Tide in Mars Atmosphere, Dust
Heating
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Semidiurnal Temperature Perturbation
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Eddy diffusion Coefficient due to Breaking
Semidiurnal Tide
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Zonal Mean Acceleration of the Atmosphere due to
the Dissipating Semidurnal Tide
Zonal Mean Zonal Wind, Low Dust
Zonal Mean Zonal Wind Difference, ? 2.3
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Solar Semidiurnal Tide in the Atmosphere
CONCLUDING REMARKS

• The semidiurnal tide and its effects are
  pervasive and ubiquitous in Earths atmosphere
• There are new things to be learned, and
  probably, to be discovered
• The semidiurnal tide is just one example from a
  whole spectrum of waves that couple different
  atmospheric regions and produce observable
  phenomena.
• The solar semidiurnal tide is important in
  vertically-coupling Mars atmosphere, with
  potential importance to aerobraking.