Publications


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.


Erratum: 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 (Physical Review E (2002) 66 (056302))

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 68 (2003) 199031-

J Sukhatme, RT Pierrehumbert


Glacial flow of floating marine ice in ''Snowball Earth''

JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 108 (2003) ARTN 3308

JC Goodman, RT Pierrehumbert


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.


A combined laboratory and numerical study of heat transport by baroclinic eddies and axisymmetric flows

JOURNAL OF FLUID MECHANICS 489 (2003) 301-323

PL Read


Reply to "Modern precipitation stable isotope vs. elevation gradients in the High Himalaya" by Hou Shugui et al.

EARTH AND PLANETARY SCIENCE LETTERS 209 (2003) 401-403

DB Rowley, BS Currie, RT Pierrehumbert


Modeling the Martian dust cycle 1. Representations of dust transport processes

Journal of Geophysical Research 107 (2002) 5123 18pp-

PL Read, C. E. Newman, S. R. Lewis, F. Forget


Bifurcations and instabilities in rotating, two-layer fluids: II. beta-plane

NONLINEAR PROC GEOPH 9 (2002) 289-309

AF Lovegrove, IM Moroz, PL Read

In this paper, we show that the behavior of weakly nonlinear waves in a 2-layer model of baroclinic instability on a P-plane with varying viscosity is determined by a single degenerate codimension three bifurcation. In the process, we show how previous studies, using the method of multiple scales to derive evolution equations for the slowly varying amplitude of the growing wave, arise as special limits of the general evolution description.


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.

Physical review. E, Statistical, nonlinear, and soft matter physics 66 (2002) 056302-

J Sukhatme, RT Pierrehumbert

We examine the decay of passive scalars with small, but nonzero, diffusivity in bounded two-dimensional (2D) domains. The velocity fields responsible for advection are smooth (i.e., they have bounded gradients) and of a single large scale. Moreover, the scale of the velocity field is taken to be similar to the size of the entire domain. The importance of the initial scale of variation of the scalar field with respect to that of the velocity field is strongly emphasized. If these scales are comparable and the velocity field is time periodic, we see the formation of a periodic scalar eigenmode. The eigenmode is numerically realized by means of a deterministic 2D map on a lattice. Analytical justification for the eigenmode is available from theorems in the dynamo literature. Weakening the notion of an eigenmode to mean statistical stationarity, we provide numerical evidence that the eigenmode solution also holds for aperiodic flows (represented by random maps). Turning to the evolution of an initially small scale scalar field, we demonstrate the transition from an evolving (i.e., non-self-similar) probability distribution function (pdf) to a stationary (self-similar) pdf as the scale of variation of the scalar field progresses from being small to being comparable to that of the velocity field (and of the domain). Furthermore, the non-self-similar regime itself consists of two stages. Both stages are examined and the coupling between diffusion and the distribution of the finite time Lyapunov exponents is shown to be responsible for the pdf evolution.


Testing paleogeographic controls on a Neoproterozoic snowball Earth

GEOPHYSICAL RESEARCH LETTERS 29 (2002) ARTN 1515

CJ Poulsen, RL Jacob, RT Pierrehumbert, TT Huynh


The advection-diffusion problem for stratospheric flow. Part II: Probability distribution function of tracer gradients

JOURNAL OF THE ATMOSPHERIC SCIENCES 59 (2002) 2830-2845

YY Hu, RT Pierrehumbert


The hydrologic cycle in deep-time climate problems.

Nature 419 (2002) 191-198

RT Pierrehumbert

Hydrology refers to the whole panoply of effects the water molecule has on climate and on the land surface during its journey there and back again between ocean and atmosphere. On its way, it is cycled through vapour, cloud water, snow, sea ice and glacier ice, as well as acting as a catalyst for silicate-carbonate weathering reactions governing atmospheric carbon dioxide. Because carbon dioxide affects the hydrologic cycle through temperature, climate is a pas des deux between carbon dioxide and water, with important guest appearances by surface ice cover.


Surface quasigeostrophic turbulence: The study of an active scalar.

Chaos (Woodbury, N.Y.) 12 (2002) 439-450

J Sukhatme, RT Pierrehumbert

We study the statistical and geometrical properties of the potential temperature (PT) field in the surface quasigeostrophic (SQG) system of equations. In addition to extracting information in a global sense via tools such as the power spectrum, the g-beta spectrum, and the structure functions we explore the local nature of the PT field by means of the wavelet transform method. The primary indication is that an initially smooth PT field becomes rough (within specified scales), though in a qualitatively sparse fashion. Similarly, initially one-dimensional iso-PT contours (i.e., PT level sets) are seen to acquire a fractal nature. Moreover, the dimensions of the iso-PT contours satisfy existing analytical bounds. The expectation that the roughness will manifest itself in the singular nature of the gradient fields is confirmed via the multifractal nature of the dissipation field. Following earlier work on the subject, the singular and oscillatory nature of the gradient field is investigated by examining the scaling of a probability measure and a sign singular measure, respectively. A physically motivated derivation of the relations between the variety of scaling exponents is presented, the aim being to bring out some of the underlying assumptions which seem to have gone unnoticed in previous presentations. Apart from concentrating on specific properties of the SQG system, a broader theme of the paper is a comparison of the diagnostic inertial range properties of the SQG system with both the two- and three-dimensional Euler equations. (c) 2002 American Institute of Physics.


Abrupt Climate Change: Inevitable Surprises

National Academies Press, 2002

COAC Change, NR Council, BOASA Climate, DOEAL Studies, PR Board, OS Board

Based on the best and most current research available, this book surveys the history of climate change and makes a series of specific recommendations for the future.


Modeling the Martian dust cycle - 2. Multiannual radiatively active dust transport simulations

JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS 107 (2002) ARTN 5124

CE Newman, SR Lewis, PL Read, F Forget


Modeling the Martian dust cycle 1. Representations of dust transport processes

Journal of Geophysical Research E: Planets 107 (2002) 6-1

CE Newman, SR Lewis, PL Read, F Forget

A dust transport scheme has been developed for a general circulation model of the Martian atmosphere. This enables radiatively active dust transport, with the atmospheric state responding to changes in the dust distribution via atmospheric heating, as well as dust transport being determined by atmospheric conditions. The scheme includes dust lifting, advection by model winds, atmospheric mixing, and gravitational sedimentation. Parameterizations of lifting initiated by (1) near-surface wind stress and (2) convective vortices known as dust devils are considered. Two parameterizations are defined for each mechanism and are first investigated offline using data previously output from the non-dust- transporting model. The threshold-insensitive parameterizations predict some lifting over most regions, varying smoothly in space and time. The threshold-sensitive parameterizations predict lifting only during extreme atmospheric conditions (such as exceptionally strong winds), so lifting is rarer and more confined to specific regions and times. Wind stress lifting is predicted to peak during southern summer, largely between latitudes 15° and 35°S, with maxima also in regions of strong slope winds or thermal contrast flows. These areas are consistent with observed storm onset regions and dark streak surface features. Dust devil lifting is also predicted to peak during southern summer, with a moderate peak during northern summer. The greatest dust devil lifting occurs in early afternoon, particularly in the Noachis, Arcadia/Amazonis, Sirenum, and Thaumasia regions. Radiatively active dust transport experiments reveal strong positive feedbacks on lifting by near-surface wind stress and negative feedbacks on lifting by dust devils.


Modeling the Martian dust cycle 2. Multiannual radiatively active dust transport simulations

Journal of Geophysical Research E: Planets 107 (2002) 7-1

CE Newman, SR Lewis, PL Read, F Forget

Multiannual dust transport simulations have been performed using a Mars general circulation model containing a dust transport scheme which responds to changes in the atmospheric state. If the dust transport is "radiatively active," the atmospheric state also responds to changes in the dust distribution. This paper examines the suspended dust distribution obtained using different lifting parameterizations, including an analysis of dust storms produced spontaneously during these simulations. The lifting mechanisms selected are lifting by (1) near-surface wind stress and (2) convective vortices known as dust devils. Each mechanism is separated into two types of parameterization: threshold-sensitive and -insensitive. The latter produce largely unrealistic annual dust cycles and storms, and no significant interannual variability. The threshold-sensitive parameterizations produce more realistic annual and interannual behavior, as well as storms with similarities to observed events, thus providing insight into how real Martian dust storms may develop. Simulations for which dust devil lifting dominates are too dusty during northern summer. This suggests either that a removal mechanism (such as dust scavenging by water ice) reduces opacities at this time or that dust devils are not the primary mechanism for storm production. Simulations for which near-surface wind stress lifting dominates produce the observed low opacities during northern spring/ summer, yet appear unable to produce realistic global storms without storm decay being prevented by the occurrence of large-scale positive feedbacks on further lifting. Simulated dust levels are generally linked closely to the seasonal state of the atmosphere, and no simulation produces the observed amount of interannual variability.

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