1. 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 NAIC Arecibo ObservatoryMesospheric Refrigerator

2. NAIC Arecibo ObservatoryMesospheric Refrigerator Relatively inert, dynamically unstable Weather & orography Dynamically Stable, rising air does not penetrate easily, but waves can propagate. Atmospheric Structure

3. 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) NAIC Arecibo ObservatoryMesospheric Refrigerator

4. NAIC Arecibo Observatory Winter Mesospheric Refrigerator

5. What’s the problem? NAIC Arecibo Observatory Problem: The thermal structure of the stratosphere/mesosphere is far from radiative equilibrium ⇒ mechanical forcing is required. 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. 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? Which raises the question: What drives the refrigeration process and how? Mesospheric Refrigerator

6. 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

7. NAIC Arecibo ObservatoryMesospheric Refrigerator 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

8. NAIC Arecibo Observatory http://www.ux1.eiu.edu/~cfjps/1400/circulation.html Mesospheric Refrigerator Starting with tropospheric circulation

9. NAIC Arecibo Observatory Hadley CellHadley Cell Ferrel CellFerrel Cell Polar CellPolar Cell Mesospheric Refrigerator Stratospheric Circulation

10. 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 NAIC Arecibo ObservatoryMesospheric Refrigerator Video, courtesy Dave Fritts

11. Mesospheric RefrigeratorNAIC Arecibo Observatory

12. Mesospheric RefrigeratorNAIC Arecibo Observatory

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

14. NAIC Arecibo ObservatoryMesospheric Refrigerator

15. 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. NAIC Arecibo ObservatoryMesospheric Refrigerator

16. NAIC Arecibo Observatory Conclusions The summer mesopause, at between 85 & 90 km, is the coldest place on earth. 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 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. 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

17. 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. NAIC Arecibo ObservatoryMesospheric Refrigerator