Atmospheric Circulation: Thermal Structure and the Mesospheric Refrigerator How do Atmospheric Gravity Waves couple to the mean circulation to produce.

Atmospheric Circulation: Atmospheric Circulation: Thermal Structure and the Thermal Structure and the Mesospheric RefrigeratorMesospheric Refrigerator

How do Atmospheric Gravity Waves couple to the How do Atmospheric Gravity Waves couple to the mean circulation to produce the coldest place on mean circulation to produce the coldest place on

earthearth??

Jonathan S. FriedmanJonathan S. Friedmanwith particular thanks to C. Y. She and S. with particular thanks to C. Y. She and S.

HarrellHarrell(using lots of (using lots of ““borrowedborrowed”” materials) materials)

6 June 20136 June 2013NAIC Arecibo ObservatoryMesospheric Refrigerator

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Relatively inert, Relatively inert, dynamically unstabledynamically unstable

Weather & orographyWeather & orography

Dynamically Stable, rising air does not Dynamically Stable, rising air does not penetrate easily, but waves can propagate.penetrate easily, but waves can propagate.

Atmospheric Atmospheric StructureStructure

At the mesopause level (~80 km) the summer At the mesopause level (~80 km) the summer polar region is on the average 70-80°C colder than polar region is on the average 70-80°C colder than

the winter polar region. Such a temperature the winter polar region. Such a temperature distribution is far from radiative equilibrium and distribution is far from radiative equilibrium and

clearly must be dynamically maintained by clearly must be dynamically maintained by adiabatic cooling (heating) due to ascent adiabatic cooling (heating) due to ascent

(subsidence) in the summer (winter) hemisphere.(subsidence) in the summer (winter) hemisphere. (Holten, J. Atmos. Sci., 1982)(Holten, J. Atmos. Sci., 1982)

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Winter

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WhatWhat’’s the problem?s the problem?

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• Problem:Problem:The thermal structure of the The thermal structure of the stratosphere/mesosphere is far from stratosphere/mesosphere is far from radiative equilibrium radiative equilibrium ⇒⇒ mechanical forcing mechanical forcing is required.is required.

• Complication:Complication:Simple upwelling in the sunlit summer Simple upwelling in the sunlit summer polar region provides insufficient forcing to polar region provides insufficient forcing to induce the non-intuitive structure.induce the non-intuitive structure.

• Which raises the question:Which raises the question:What drives the refrigeration process and What drives the refrigeration process and how?how?

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Definitions• Radiative Heating/Cooling

Temperature change by absorption/emission of radiation

• Atmospheric Gravity WavesFluctuations where gravity is the restoring force

• Geostrophic WindsWinds 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.

RefrigeratorA system of dynamical cooling through expansion and compression.

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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

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http://www.ux1.eiu.edu/~cfjps/1400/circulation.html

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Starting with tropospheric circulation

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••Hadley CellHadley Cell••Ferrel CellFerrel Cell••Polar CellPolar Cell

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Stratospheric Circulation

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.

Courtesy: Paul Castleberg

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Video, courtesy Dave Fritts

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Mesospheric Refrigerator NAIC Arecibo Observatory

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If the relative velocity between the wave source and wind is fast enough ...

Mesospheric Refrigerator

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In the summer mesosphere, gravity waves breakdown 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.

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ConclusionsConclusions

• The summer mesopause, at between 85 & 90 The summer mesopause, at between 85 & 90 km, is the coldest place on earth.km, is the coldest place on earth.

• The solstice temperature structure in the upper The solstice temperature structure in the upper mesosphere is inverted from the intuitive mesosphere is inverted from the intuitive (radiatively controlled) form. This implies (radiatively controlled) form. This implies dynamical heat transfer » a mesospheric dynamical heat transfer » a mesospheric refrigerator.refrigerator.

• The evaporator/compressor cycle is driven by The evaporator/compressor cycle is driven by gravity waves, which break down the geostropic gravity waves, which break down the geostropic summer zonal wind flow and enhance the winter summer zonal wind flow and enhance the winter zonal flow. The result is pole-to-pole airflow that zonal flow. The result is pole-to-pole airflow that produces the observed thermal structure.produces the observed thermal structure.

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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.

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