Transcript Atmospheric Circulation: Thermal Structure and the

Atmospheric Circulation: Thermal
Structure and the Mesospheric
Refrigerator
How do Atmospheric Gravity Waves couple to the
mean circulation to produce the coldest place on
earth?
Jonathan S. Friedman
with particular thanks to C. Y. She and S.
Harrell
(using lots of “borrowed” materials)
6 June 2013
Mesospheric Refrigerator
NAIC Arecibo Observatory
Atmospheric
Structure
Relatively inert, dynamically
unstable
Dynamically Stable, rising air does not penetrate
easily, but waves can propagate.
Weather & orography
Mesospheric Refrigerator
NAIC Arecibo Observatory
At the mesopause level (~80 km) the summer
polar region is on the average 70-80°C colder
than the winter polar region. Such a temperature
distribution is far from radiative equilibrium and
clearly must be dynamically maintained by
adiabatic cooling (heating) due to ascent
(subsidence) in the summer (winter) hemisphere.
(Holten, J. Atmos. Sci., 1982)
Mesospheric Refrigerator
NAIC Arecibo Observatory
Winter
Mesospheric Refrigerator
NAIC Arecibo Observatory
What’s the problem?
•
•
•
Problem:
The thermal structure of the
stratosphere/mesosphere is far from radiative
equilibrium ⇒ mechanical forcing is required.
Complication:
Simple upwelling in the sunlit summer polar
region provides insufficient forcing to induce the
non-intuitive structure.
Which raises the question:
What drives the refrigeration process and how?
Mesospheric Refrigerator
NAIC Arecibo Observatory
Definitions
• Radiative Heating/Cooling
Temperature change by absorption/emission of radiation
• Atmospheric Gravity Waves
Fluctuations where gravity is the restoring force
• Geostrophic Winds
Winds generated and controlled by pressure gradients and the
coriolis force
• Coriolis “Force”
Deflection in the motion of an object moving in a rotating frame of
reference. Actually a statement of conservation of angular
momentum.
Refrigerator
A system of dynamical cooling through expansion and
compression.
Mesospheric Refrigerator
NAIC Arecibo Observatory
What Contributes to Circulation?
• Solar Tides
» Surface heating
» Ozone absorption of Solar UV
• Rotation of the Earth — Coriolis Force
• Geostrophic winds (pressure gradients)
• Atmospheric Gravity Waves
• Planetary Waves
Mesospheric Refrigerator
NAIC Arecibo Observatory
Starting with tropospheric circulation
http://www.ux1.eiu.edu/~cfjps/1400/circulation.html
Mesospheric Refrigerator
NAIC Arecibo Observatory
Stratospheric Circulation
• Hadley Cell
• Ferrel Cell
• Polar Cell
Mesospheric Refrigerator
NAIC Arecibo Observatory
Gravity Waves
• Atmospheric pressure waves produced by
processes such as convection (thunder storms),
winds passing over mountains, etc.
• Amplitude increases as it propagates upwards
to thinner atmosphere.
• Can carry large amounts of energy, which is
deposited where the wave breaks, affecting
wind flow.
Video, courtesy
Dave Fritts
Courtesy:
Paul Castleberg
Mesospheric Refrigerator
NAIC Arecibo Observatory
Mesospheric Refrigerator
NAIC Arecibo Observatory
Mesospheric Refrigerator
NAIC Arecibo Observatory
If the relative velocity between the wave source and
wind is fast enough ...
Mesospheric Refrigerator
NAIC Arecibo Observatory
Mesospheric Refrigerator
NAIC Arecibo Observatory
In the summer mesosphere, gravity waves break
down the zonal wind. The resulting momentum is
poleward towards the winter pole. The result is high
winter pole winds and warmer temperatures at 87 km.
Mesospheric Refrigerator
NAIC Arecibo Observatory
Conclusions
•
•
•
The summer mesopause, at between 85 & 90 km, is
the coldest place on earth.
The solstice temperature structure in the upper
mesosphere is inverted from the intuitive (radiatively
controlled) form. This implies dynamical heat transfer »
a mesospheric refrigerator.
The evaporator/compressor cycle is driven by gravity
waves, which break down the geostropic summer zonal
wind flow and enhance the winter zonal flow. The result
is pole-to-pole airflow that produces the observed
thermal structure.
Mesospheric Refrigerator
NAIC Arecibo Observatory
some References
•
•
•
•
•
Andrews, D. G., J. R. Holton, and C. B. Leovy, Middle Atmospheric
Dynamics, Academic Press, San Diego, CA, 1987.
Fritts, D. C. and R. A. Vincent (1987), Mesospheric momentum flux studies
at Adelaide, Australia: Observations and a gravity-wave–tidal interaction
model, J. Atmos. Sci, 44 (3), 605–619.
Gardner, C. S. and W. Yang (1998), Measurements of the dynamical cooling
rate associated with the vertical transport of heat by dissipating gravity
waves in the mesopause region at the Starfire Optical Range, New Mexico,
J. Geophys. Res., 103, 16,909–16,926.
Holton, J. R. and M. J. Alexander (2000), The role of waves in the transport
circulation of the middle atmosphere, in Atmospheric Science Across the
Stratopause, edited by D. E. Siskind, S. L. Ekermann, and M. E. Summers,
Geophys. Monogr. Ser., pp. 21–35, American Geophysical Union.
Concept of a two-level mesopause: Support through new lidar observations,
J. Geophys. Res., 103, 5855–5864.
Mesospheric Refrigerator
NAIC Arecibo Observatory