Super-rotation is a ubiquitous phenomenon in the four substantial atmospheres possessed by solid bodies in the solar system. This paper reviews recent global model results and reanalyses by data assimilation, in particular for Mars and Venus

Dawson, J.

Lewis, S. R.

Mendonça, J.

Montabone, L.

Read, P. L.

Ruan, T.

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsSuper-rotating jets in the atmospheres of terrestrialplanetsConference or Workshop ItemHow to cite:Lewis, S. R.; Dawson, J.; Read, P. L.; Mendonc¸a, J.; Ruan, T. and Montabone, L. (2012). Super-rotatingjets in the atmospheres of terrestrial planets. In: Comparative Climatology of Terrestrial Planets, 25-28 Jun 2012,Boulder, Co, USA.For guidance on citations see FAQs.c© 2012 The AuthorsVersion: Version of RecordLink(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/climatology2012/pdf/8051.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukSUPER-ROTATING JETS IN THE ATMOSPHERES OF TERRESTRIAL PLANETS.  S. R. Lewis1, J. Daw-son1, P. L. Read2, J. Mendonça2, T. Ruan2, and L. Montabone2,3, 1Department of Physical Sciences, The Open Uni-versity, Walton Hall, Milton Keynes MK7 6AA, UK (s.r.lewis@open.ac.uk), 2Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, OX1 3PU, UK, 3Université Paris VI, Laboratoire de Météorologie Dyna-mique - Tour 45-55, case courrier 99 – 4, place Jussieu, Paris, France.   Introduction:  Super-rotation is a ubiquitous phe-nomenon in the four substantial atmospheres possessed by solid bodies in the solar system. The slowly rotating planet, Venus, and moon, Titan, are both well-known to have atmospheres that rotate substantially more quickly than does the solid surface underneath. The more rapidly rotating planets, Mars and Earth, exhibit less spectacular global super-rotation, but both provide examples of prograde jets near the equator which ro-tate more rapidly than the equatorial surface. It is also worth noting in passing that atmospheric super-rotation is not restricted to planets with solid surfaces and shal-low atmospheres, but that Jupiter and Saturn both pos-sess rapid prograde equatorial jets [e.g. 1] and hence exhibit local super-rotation. In each case the detailed mechanism, or combina-tion of mechanisms, which produces the super-rotating jets might vary, but all require longitudinally asymmet-ric motions, waves or eddies, to transport angular mo-mentum up-gradient into the jets. This paper will re-view recent global circulation model results and, in the case of Mars and Earth, atmospheric reanalyses by data assimilation, in order to diagnose possible mechanisms in each case. Measures of super-rotation:  For comparative purposes it is useful to introduce measures of local and global super-rotation [2, 3]. Local super-rotation, s, is defined as the ratio of the angular momentum of the atmosphere about the rotation axis of the planet to the angular momentum of air at rest relative to the surface at the equator minus one. Global super-rotation, S, is the ratio of the total atmospheric angular momentum of the atmosphere about the rotation axis of the planet to the total angular momentum of an equivalent atmos-phere in solid-body rotation with the planet, again mi-nus one. In both cases, positive values indicate super-rotation compared to that which might be achieved by rearranging an atmosphere initially at rest only by ax-isymmetric processes [4]. Mars and Earth:  The Earth exhibits a small posi-tive global super-rotation with S = 1–2% [2], and rela-tively weak local super-rotation can also be observed, particularly in the westerly phase of the Quasi-Biennial Oscillation (QBO); a prograde wind u = 20 m/s at the equator on Earth is equivalent to a local super-rotation of s = 4%. Modelling studies [3] using the UK (spectral dy-namics with angular momentum conservation en-forced) version of the LMD-UK Mars Global Circula-tion Model (MGCM) [5, 6], as well as results from earlier studies [7], demonstrate that Mars might exhibit stronger super-rotation, with S = 4–6% and s = 8–16% depending on the dust loading and time of year [3]. See also Fig. 1 here.  Figure 1:  Zonal wind (contours) and local super-rotation index (colours) at northern hemisphere autumn equinox (LS = 180°) in the UK MGCM [3]. Only posi-tive values of s are shaded to highlight regions of su-per-rotation. The dust optical depth is varied from moderate to levels approaching planet-encircling dust storms, such as the LS = 185° event in 2001 (τ610Pa = 0.6, 1.2 and 3.0 in the panels from top to bottom).   Data assimilation for Mars [8, 9] now means that the atmospheric super-rotation can be diagnosed from a re-analysis spanning almost six complete martian 8051.pdfComparative Climatology of Terrestrial Planets (2012)years covering the period for which thermal and dust observations are available from the Thermal Emission Spectrometer on Mars Global Surveyor and the Mars Climate Sounder on Mars Reconnaissance Orbiter.  Venus and Titan:  The slowly-rotating bodies are the most remarkable cases of super-rotation in the solar system, with s > 60, S ~ 10 for Venus. Such large val-ues of S are consistent with a scaling analysis using a global Rossby number [2] derived from the broadly similar wind speeds to those seen on Earth and Mars, the similar planetary  radii and the much slower plane-tary rotation rate. Recent global circulation models have produced super-rotation for both Venus [10, 11, 12, 13, 14 and references therein] and Titan [15, 16], but results are highly dependent on the model resolu-tion, initial state, parameterizations used and even the dynamical core. Results will be shown from both a finite difference model primarily developed in Oxford [11, 17], which features more complete physical parameterizations, and a model run that shares the spectral dynamical core of the UK MGCM, run at The Open University [14], and see Fig. 2. A more complete international inter-comparison will be presented in [18].    Figure 2:  Upper panel: longitudinal-mean wind in the OU VGCM after 250 Venus sidereal days (about 60,750 Earth days) of spin-up from rest. Positive val-ues indicate prograde motion. Lower panel: local su-per-rotation index, s, derived from the winds shown in the upper panel.  Conclusions:  Local and global super-rotation re-quire the transfer of angular momentum up-gradient with respect to the longitudinal-mean gradient of angu-lar momentum. This transfer must be accomplished by eddy processes, which might include thermal tides, large-scale Kelvin or Rossby waves or small-scale inertio-gravity waves or even turbulent diffusive pro-cesses. On Earth especially, strong tropical convection will tend to mix angular momentum and to suppress the development of clear, local super-rotating jets in the troposphere; without the latent heat resulting from the condensation of large quantities of water, this pro-cess might be less important on other planets. Models permit the investigation and quantification of the mechanisms behind atmospheric super-rotation, for example [3] demonstrates that the thermal tides are the most important mechanism behind the formation of the low-level equatorial jets seen in Fig. 1, although an almost exactly equal global super-rotation will occur, with jets peaking elsewhere in the model if the thermal tides are artificially suppressed. The development of data assimilation techniques for other planets now permits the analysis of super-rotation in less highly idealized cases using an atmospheric reanalysis as would be done for the Earth.  References:  [1] Irwin P. G. J. (2009) Giant Plan-ets of Our Solar System: Atmospheres, Composition and Structure, Springer, 436pp. [2] Read P. L. (1986) Q. J. R. Meteorol. Soc., 112, 231–251. [3] Lewis S. R. and Read P. L. (2003) JGR, 108, E4, 5034, 1–15. [4] Hide R. (1969) JAS, 26, 841–853. [5] Forget F. et al. (1999) JGR, 104, 24155–24176. [6] Lewis S. R. et al. (1999) JGR, 104, 24177–24194. [7] Wilson R. J. and Hamilton K. (1996) JAS, 53, 1290–1326. [8] Lewis S. R. et al. (2007) Icarus, 192, 327–347. [9] Montabone L. et al. (2006) Icarus, 185, 113–132. [10] Yamamoto M. and Takahashi M. (2003) GRL, 30(9), 090,000-1. [11] Lee C. et al. (2007) JGR, 112, E04S11.1–10. [12] Lee C. and Richardson M. I. (2010) JGR, 115, 4002. [13] Lebonnois S. et al. (2010) JGR, 115, 6006. [14] Lewis S. R. et al. (2012) in Bengtsson et al. (eds.) To-wards Understanding the Climate of Venus: Applica-tions of Terrestrial Models to Our Sister Planet, Springer, Chapter 6. [15] Hourdin F. et al. (1995) Ica-rus, 117, 358–374. [16] Rannou P. et al. (2004) Icarus, 170, 443–462. [17] Mendonça J. et al. (2012) this meeting. [18] Lebonnois S. et al. (2012) in Bengtsson et al. (eds.) Towards Understanding the Climate of Venus: Applications of Terrestrial Models to Our Sis-ter Planet, Springer, Chapter 7. 8051.pdfComparative Climatology of Terrestrial Planets (2012)

Super-rotating jets in the atmospheres of terrestrial planets

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