Publications


Upper atmosphere of Mars up to 120 km: Mars Global Surveyor accelerometer data analysis with the LMD general circulation model

Journal of Geophysical Research E: Planets 109 (2004)

M Angelats i Coll, F Forget, MA López-Valverde, PL Read, SR Lewis

Mars Global Surveyor (MGS) aerobraking accelerometer density measurements are analyzed with the use of the general circulation model (GCM) at the Laboratoire de Météorologie Dynamique (LMD). MGS constant altitude density data are used, obtaining longitudinal wavelike structures at fixed local times which appear to be dominated by low zonal wave number harmonics. Comparisons with simulated data for different seasons and latitudinal bands at constant altitude are performed. Excellent agreement is obtained between the simulated and observational data for low latitudes, with accuracy in both mean and zonal structure. Higher latitudes show a reduction in agreement between GCM results and MGS data. Comparisons that result in good agreement with the observational data allow for the study of wave composition in the MGS data. In particular, the excellent agreement between the simulations and the data obtained at 115 km during areocentric longitude Ls ≈ 65° allows the extraction of the major contributors to the signature, with the eastward propagating diurnal waves of wave numbers one to three being the major players. Significant contributions are also obtained for eastward propagating semidiurnal waves of wave numbers two, three, and five and diurnal wave number five. A sensitivity study is performed to delineate the effects of the near-IR tidal forcing of the upper atmosphere on the wave content at those heights. Simulations without this forcing yield reduced amplitudes for diurnal eastward propagating waves two and three along with a more latitudinally symmetric response for these two components as well as for diurnal eastward propagating wave number one. Copyright 2004 by the American Geophysical Union.


Jupiter's atmospheric composition from the Cassini thermal infrared spectroscopy experiment.

Science 305 (2004) 1582-1586

VG Kunde, FM Flasar, DE Jennings, B Bézard, DF Strobel, BJ Conrath, CA Nixon, GL Bjoraker, PN Romani, RK Achterberg, AA Simon-Miller, P Irwin, JC Brasunas, JC Pearl, MD Smith, GS Orton, PJ Gierasch, LJ Spilker, RC Carlson, AA Mamoutkine, SB Calcutt, PL Read, FW Taylor, T Fouchet, P Parrish, A Barucci, R Courtin, A Coustenis, D Gautier, E Lellouch, A Marten, R Prangé, Y Biraud, C Ferrari, TC Owen, MM Abbas, RE Samuelson, F Raulin, P Ade, CJ Césarsky, KU Grossman, A Coradini

The Composite Infrared Spectrometer observed Jupiter in the thermal infrared during the swing-by of the Cassini spacecraft. Results include the detection of two new stratospheric species, the methyl radical and diacetylene, gaseous species present in the north and south auroral infrared hot spots; determination of the variations with latitude of acetylene and ethane, the latter a tracer of atmospheric motion; observations of unexpected spatial distributions of carbon dioxide and hydrogen cyanide, both considered to be products of comet Shoemaker-Levy 9 impacts; characterization of the morphology of the auroral infrared hot spot acetylene emission; and a new evaluation of the energetics of the northern auroral infrared hot spot.


Jupiter's and Saturn's convectively driven banded jets in the laboratory

Geophysical Research Letters 31 (2004) 1-5

PL Read, YH Yamazaki, SR Lewis, PD Williams, K Miki-Yamazaki, J Sommeria, H Didelle, A Fincham

The banded patterns of cloud and wind are among the most striking features of the atmospheres of Jupiter and Saturn, but their dynamical origin remains poorly understood. Most approaches towards understanding zonation so far (also in the terrestrial oceans) have used highly idealized models to show that it might originate from dynamical anisotropy in a shallow turbulent fluid layer due to the planetary β-effect. Here we report the results of laboratory experiments, conducted on a 14-m diameter turntable, which quantitatively confirm that multiple zonal jets may indeed be generated and maintained by this mechanism in the presence of deep convection and a topographic β-effect. At the very small values of Ekman number (≤2 × 10-5) and large local Reynolds numbers (≥2000, based onjet scales) achieved, the kinetic energy spectra suggest the presence of both energy-cascading and enstrophy-cascading inertial ranges in addition to the zonation near twice the Rhines wave number. Copyright 2004 by the American Geophysical Union.


High levels of atmospheric carbon dioxide necessary for the termination of global glaciation.

Nature 429 (2004) 646-649

RT Pierrehumbert

The possibility that the Earth suffered episodes of global glaciation as recently as the Neoproterozoic period, between about 900 and 543 million years ago, has been widely discussed. Termination of such 'hard snowball Earth' climate states has been proposed to proceed from accumulation of carbon dioxide in the atmosphere. Many salient aspects of the snowball scenario depend critically on the threshold of atmospheric carbon dioxide concentrations needed to trigger deglaciation. Here I present simulations with a general circulation model, using elevated carbon dioxide levels to estimate this deglaciation threshold. The model simulates several phenomena that are expected to be significant in a 'snowball Earth' scenario, but which have not been considered in previous studies with less sophisticated models, such as a reduction of vertical temperature gradients in winter, a reduction in summer tropopause height, the effect of snow cover and a reduction in cloud greenhouse effects. In my simulations, the system remains far short of deglaciation even at atmospheric carbon dioxide concentrations of 550 times the present levels (0.2 bar of CO2). I find that at much higher carbon dioxide levels, deglaciation is unlikely unless unknown feedback cycles that are not captured in the model come into effect.


Nonconservation of Ertel potential vorticity in hydrogen atmospheres

JOURNAL OF THE ATMOSPHERIC SCIENCES 61 (2004) 1953-1965

PJ Gierasch, BJ Conrath, PL Read


Predicting weather conditions and climate for Mars expeditions

JBIS-J BRIT INTERPLA 57 (2004) 75-86

PL Read, SR Lewis, SJ Bingham, CE Newman

Weather and climatic conditions are among the most important factors to be taken into account when planning expeditions to remote and challenging locations on Earth. This is likely to be equally the case for expedition planners on Mars, where conditions (in terms of extremes of temperature, etc.) can be at least as daunting as back on Earth. With the success of recent unmanned missions to Mars, such as NASA's Mars Pathfinder, Mars Global Surveyor and Mars Odyssey, there is now a great deal of information available on the range of environmental conditions on Mars, from the tropics to the CO2 ice-covered polar caps. This has been further supplemented by the development of advanced numerical models of the Martian atmosphere, allowing detailed and accurate simulations and predictions of the weather and climate across the planet. This report discusses the main weather and climate variables which future Martian human expedition planners will need to take into account. The range of conditions likely to be encountered at a variety of typical locations on Mars is then considered, with reference to predictions from the ESA Mars Climate Database.


Warming the world.

Nature 432 (2004) 677-

RT Pierrehumbert


Jupiter's and Saturn's convectively driven banded jets in the laboratory

Geophysical Research Letters 31 (2004) L22701 5pp-

Y Yamazaki, P.L. Read, J. Sommeria, P.D. Williams


An intense stratospheric jet on Jupiter

Nature 427 (2004) 132-135

SB Calcutt, Achtergerg, Flasar, Kunde


Hydrothermal plume dynamics on Europa: Implications for chaos formation

JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS 109 (2004) ARTN E03008

JC Goodman, GC Collins, J Marshall, RT Pierrehumbert


Investigating atmospheric predictability on Mars using breeding vectors in a general-circulation model

Quarterly Journal of the Royal Meteorological Society 130 (2004) 2971-2989

CE Newman, PL Read, SR Lewis

A breeding vectors approach is used to investigate the hypothesis that the Martian atmosphere is predictable at certain times of year, by identifying the fastest-growing modes of instability ar different times in a Mars general-circulation model. Results indicate that the period from northern mid-spring until mid-autumn is remarkably predictable, with negative global growth rates for a range of conditions, in contrast to the situation on the earth. From northern late autumn to early spring growing modes do occur, peaking in northern high latitudes and near winter solstice. Reducing the size of the initial perturbations increases global growth rates in most cases, supporting the idea that instabilities which saturate nonlinearly at lower amplitudes have generally faster growth rates. In late autumn/early winter the fastest-growing modes ('bred vectors') are around the north pole, increase with dust loading, and probably grow via barotropic as well as baroclinic energy conversion. In northern late winter/early spring the bred vectors are around the north pole and are strongly baroclinic in nature. As dust loading (and with it the global circulation strength) is increased their growth rates first decrease, as the baroclinic mode is suppressed, then increase again as the fastest-growing instabilities switch to being those which dominated earlier in the year. If dust levels are very low during late northern autumn (late southern spring) then baroclinic modes are also found around the spring pole in the south, though for a slight increase in dust loading the dominant modes shift back to northern high latitudes. The bred vectors are also used as perturbations to the initial conditions for ensemble simulations. One possible application within the Mars model is as a means of identifying regions and times when dust-lifting activity (related to surface wind stress) might show significant interannual variability for a given model configuration, without the need to perform long, computationally expensive multi-year model runs with each new set-up. This is tested for a time of year when previous multi-year experiments showed significant variability in dust storm onset in the region north of Chryse. Despite the model having no feedbacks between dust lifting and atmospheric state (unlike the original multi-year mn), the ensemble members still show maximum divergence in this region in terms of near-surface wind stress, suggesting both that this application deserves further testing, and that the intrinsic atmospheric variability alone may be important in producing interannual variability in this storm type. © Royal Meteorological Society, 2004.


Upper atmosphere of Mars up to 120 km: Mars Global Surveyor accelerometer data analysis with the LMD general circulation model

JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS 109 (2004) ARTN E01011

MAI Coll, F Forget, MA Lopez-Valverde, PL Read, SR Lewis


Hydrothermal plume dynamics on Europa: Implications for chaos formation

Journal of Geophysical Research E: Planets 109 (2004)

JC Goodman, GC Collins, J Marshall, RT Pierrehumbert

Hydrothermal plumes may be responsible for transmitting radiogenic or tidally generated heat from Europa's rocky interior through a liquid ocean to the base of its ice shell. This process has been implicated in the formation of chaos regions and lenticulae by melting or exciting convection in the ice layer. In contrast to earlier work, we argue that Europa's ocean should be treated as an unstratified fluid. We have adapted and expanded upon existing work describing buoyant plumes in a rotating, unstratified environment. We discuss the scaling laws governing the flow and geometry of plumes on Europa and perform a laboratory experiment to obtain scaling constants and to visualize plume behavior in a Europa-like parameter regime. We predict that hydrothermal plumes on Europa are of a lateral scale (at least 25-50 km) comparable to large chaos regions; they are too broad to be responsible for the formation of individual lenticulae. Plume heat fluxes (0.1-10 W/m2) are too weak to allow complete melt-through of the ice layer. Current speeds in the plume (3-8 mm/s) are much slower than indicated by previous studies. The observed movement of ice blocks in the Conamara Chaos region is unlikely to be driven by such weak flow. Copyright 2004 by the American Geophysical Union.


A new general circulation model of Jupiter's atmosphere based on the UKMO Unified Model: three-dimentional evolution of isolated vortices and zonal jets in mid-latitudes

Planetary and Space Science 52 (2004) 423-445

Y Yamazaki, D.R. Skeet, P.L. Read


The effect of a global dust storm on simulations of the Martian water cycle

GEOPHYSICAL RESEARCH LETTERS 31 (2004) ARTN L22702

HM Bottger, SR Lewis, PL Read, F Forget


Equatorial jets in the dusty Martian atmosphere

JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS 108 (2003) ARTN 5034

SR Lewis, PL Read


Abrupt climate change.

Science (New York, N.Y.) 299 (2003) 2005-2010

RB Alley, J Marotzke, WD Nordhaus, JT Overpeck, DM Peteet, RA Pielke, RT Pierrehumbert, PB Rhines, TF Stocker, LD Talley, JM Wallace

Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past, when the Earth system was forced across thresholds. Although abrupt climate changes can occur for many reasons, it is conceivable that human forcing of climate change is increasing the probability of large, abrupt events. Were such an event to recur, the economic and ecological impacts could be large and potentially serious. Unpredictability exhibited near climate thresholds in simple models shows that some uncertainty will always be associated with projections. In light of these uncertainties, policy-makers should consider expanding research into abrupt climate change, improving monitoring systems, and taking actions designed to enhance the adaptability and resilience of ecosystems and economies.


Risk and reason: Safety, law, and the environment

NATURE 422 (2003) 263-263

RT Pierrehumbert


Decay of passive scalars under the action of single scale smooth velocity fields in bounded two-dimensional domains: From non-self-similar probability distribution functions to self-similar eigenmodes (vol E 66, art no 056302, 2002)

PHYSICAL REVIEW E 68 (2003) ARTN 019903

J Sukhatme, RT Pierrehumbert


Equatorial jets in the dusty Martian atmosphere

Journal of Geophysical Research E: Planets 108 (2003) 15-11

SR Lewis, PL Read

We investigate the production of equatorial jets which demostrate strong local superrotation in an atmospheric general circulation model of Mars. These westerly jets are driven by diurnal thermal tides, and their strength is shown to be closely related to the amount of dust in the atmosphere. The superrotating jets are strongest near to equinox and under conditions of high atmospheric dust loading. If there is sufficient dust, in amounts corresponding to dust storm conditions, the westerly equatorial jets can occur at any time of year and reach speeds of over 40 m/s, peaking between 10 and 20 km altitude. For more moderate dust amounts, typical of background levels on Mars, the jets are still strong when the subsolar point is close to the equator and latitudinally symmetric tidal modes are forced. Strong easterly retrograde winds are also found high above the equator, and it is shown that the thermal tides play a major role in their formation. This process is especially relevant close to equinox when the cross-equatorial meridional circulation is weak.

Pages