Publications


Models of venus atmosphere

in Towards Understanding the Climate of Venus: Applications of Terrestrial Models to Our Sister Planet, 11 (2013) 129-156

S Lebonnois, C Lee, M Yamamoto, J Dawson, SR Lewis, J Mendonca, P Read, HF Parish, G Schubert, L Bengtsson, D Grinspoon, SS Limaye, H Schmidt, H Svedhem, DV Titov

© Springer Science+Business Media New York 2013. All rights reserved. In the context of an International Space Science Institute (ISSI) working group, we have conducted a project to compare the most recent General Circulation Models (GCMs) of the Venus atmospheric circulation. A common configuration has been decided, with simple physical parametrization for the solar forcing and the boundary layer scheme. Six models have been used in this intercomparison project. The nominal simulation was run for more than 200 Venus days, and additional sensitivity runs have been done by several models to test the trends visible in these models when parameters are varied: topography, upper and lower boundary conditions, horizontal and vertical resolution, initial conditions. The results show that even in very similar modeling conditions, the wind speeds obtained with the different GCMs are widely different. Super-rotation is obtained, but the shape (with or without marked high-latitude jets) and amplitude of the maximum zonal wind jet is different from one model to the other, from 15 to 50m/s. Minor sensitivity is seen in several models to the upper boundary conditions, the topography or the vertical grid. Horizontal resolution and lower boundary conditions induce variations that are significant, affecting the amplitude and shape of the region of maximum zonal wind. Two models were started from an atmosphere already in super-rotation. The simulations did not converge back to the nominal simulations, maintaining maximum zonal winds over 70m/s (and even 100 m/s) without marked high-latitude jets. This study shows how sensitive GCMs are to the weak forcing of Venus atmosphere, and how difficult it is to draw precise conclusions on the circulation obtained with a single model, as well as on its sensitivity to some parameters.


Simulating the interannual variability of major dust storms on Mars using variable lifting thresholds

Icarus 223 (2013) 344-358

DP Mulholland, PL Read, SR Lewis

The redistribution of a finite amount of martian surface dust during global dust storms and in the intervening periods has been modelled in a dust lifting version of the UK Mars General Circulation Model. When using a constant, uniform threshold in the model's wind stress lifting parameterisation and assuming an unlimited supply of surface dust, multiannual simulations displayed some variability in dust lifting activity from year to year, arising from internal variability manifested in surface wind stress, but dust storms were limited in size and formed within a relatively short seasonal window. Lifting thresholds were then allowed to vary at each model gridpoint, dependent on the rates of emission or deposition of dust. This enhanced interannual variability in dust storm magnitude and timing, such that model storms covered most of the observed ranges in size and initiation date within a single multiannual simulation. Peak storm magnitude in a given year was primarily determined by the availability of surface dust at a number of key sites in the southern hemisphere. The observed global dust storm (GDS) frequency of roughly one in every 3. years was approximately reproduced, but the model failed to generate these GDSs spontaneously in the southern hemisphere, where they have typically been observed to initiate. After several years of simulation, the surface threshold field-a proxy for net change in surface dust density-showed good qualitative agreement with the observed pattern of martian surface dust cover. The model produced a net northward cross-equatorial dust mass flux, which necessitated the addition of an artificial threshold decrease rate in order to allow the continued generation of dust storms over the course of a multiannual simulation. At standard model resolution, for the southward mass flux due to cross-equatorial flushing storms to offset the northward flux due to GDSs on a timescale of ∼3. years would require an increase in the former by a factor of 3-4. Results at higher model resolution and uncertainties in dust vertical profiles mean that quasi-periodic redistribution of dust on such a timescale nevertheless appears to be a plausible explanation for the observed GDS frequency. © 2012 Elsevier Inc.


Experimental signatures of critically balanced turbulence in MAST

Physical Review Letters 110 (2013)

YC Ghim, A Schekochihin, AR Field, IG Abel, M Barnes, G Colyer, SC Cowley, FI Parra, D Dunai, S Zoletnik

Beam emission spectroscopy (BES) measurements of ion-scale density fluctuations in the MAST tokamak are used to show that the turbulence correlation time, the drift time associated with ion temperature or density gradients, the particle (ion) streaming time along the magnetic field, and the magnetic drift time are consistently comparable, suggesting a "critically balanced" turbulence determined by the local equilibrium. The resulting scalings of the poloidal and radial correlation lengths are derived and tested. The nonlinear time inferred from the density fluctuations is longer than the other times; its ratio to the correlation time scales as ν*i-0. 8±0.1, where ν*i=ion  ⠀Šcollision   rate/streaming   rate. This is consistent with turbulent decorrelation being controlled by a zonal component, invisible to the BES, with an amplitude exceeding those of the drift waves by ∼ν*i-0.8. Published by the American Physical Society.


The dynamics and circulation of venus atmosphere

in Towards Understanding the Climate of Venus: Applications of Terrestrial Models to Our Sister Planet, 11 (2013) 73-110

PL Read

© Springer Science+Business Media New York 2013. All rights reserved. In this chapter we introduce a number of basic dynamical ideas and concepts that are useful in understanding the large-scale circulation of Venus's atmosphere. Some of these are of interest from an historical viewpoint, having influenced thinking on this subject at an earlier time. But most are still very relevant for interpreting modern observations, measurements and for formulating and interpreting models. We begin by considering basic conservation principles that provide key constraints on the circulation and go on to investigate the main dynamical balances prevalent in the atmosphere. The chapter goes on to discuss the main eddy processes that are likely to play a significant role in maintaining Venus's atmospheric super-rotation, including planetary waves, gravity waves and thermal tides, their likely origins and how they interact with the zonal flow. The chapter concludes with a brief discussion of how the atmosphere interacts with the underlying surface.


Robust elements of Snowball Earth atmospheric circulation and oases for life

JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 118 (2013) 6017-6027

DS Abbot, A Voigt, D Li, G Le Hir, RT Pierrehumbert, M Branson, D Pollard, DDB Koll


Strange news from other stars

NATURE GEOSCIENCE 6 (2013) 81-83

RT Pierrehumbert


Data assimilation in the laboratory using a rotating annulus experiment

Quarterly Journal of the Royal Meteorological Society (2012)

RMB Young, PL Read

The thermally driven rotating annulus is a laboratory experiment important for the study of the dynamics of planetary atmospheres under controllable and reproducible conditions. We use the analysis correction method to assimilate laboratory data into an annulus model. We analyze the 2S and 3AV regular flow regimes between rotation rates of 0.75 and 0.875 rad s and the 3SV chaotic flow regime between rotation rates of 2.2 and 3.1 rad s. Our assimilated observations are irregularly distributed, which is more meteorologically realistic than gridded observations as used in recent applications of data assimilation to laboratory measurements. We demonstrate that data assimilation can be used successfully and accurately in this context. We examine a number of specific assimilation scenarios: a wave-number transition between two regimes, information propagation from data-rich to data-poor regions, the response of the assimilation to a strong disturbance to the flow, and a vortex-shedding instability phenomenon at high rotation rate. At the highest rotation rates we calculated the barotropic E-vectors using unobserved variables such as temperature and the vertical structure of the velocity field that are only available via the assimilation. These showed that the mean flow is weakened by the action of eddies, going some way towards explaining why vortices are shed at the very highest rotation rates but not at lower rotation. Rossby-wave stability theory suggests that the underlying instability leading to vortex shedding may be baroclinic in character. © 2012 Royal Meteorological Society.


Hydrogen-nitrogen greenhouse warming in Earth's early atmosphere.

Science (New York, N.Y.) 339 (2013) 64-67

R Wordsworth, R Pierrehumbert

Understanding how Earth has sustained surface liquid water throughout its history remains a key challenge, given that the Sun's luminosity was much lower in the past. Here we show that with an atmospheric composition consistent with the most recent constraints, the early Earth would have been significantly warmed by H(2)-N(2) collision-induced absorption. With two to three times the present-day atmospheric mass of N(2) and a H(2) mixing ratio of 0.1, H(2)-N(2) warming would be sufficient to raise global mean surface temperatures above 0°C under 75% of present-day solar flux, with CO(2) levels only 2 to 25 times the present-day values. Depending on their time of emergence and diversification, early methanogens may have caused global cooling via the conversion of H(2) and CO(2) to CH(4), with potentially observable consequences in the geological record.


Atmospheric composition, irreversible climate change, and mitigation policy.

in Climate Science for Serving Society Research, Modeling and Prediction Priorities, Springer Science & Business Media (2013) 15

S Solomon, RT Pierrehumbert, HD matthews, JS Daniel, P Friedlingstein

This volume offers a comprehensive survey and a close analysis of efforts to develop actionable climate information in support of vital decisions for climate adaptation, risk management and policy.


Planetary science: Plumbing the depths of Uranus and Neptune

Nature 497 (2013) 323-324

P Read


Global gyrokinetic turbulence simulations of MAST plasmas

Plasma Physics and Controlled Fusion 54 (2012)

S Saarelma, P Hill, A Bottino, G Colyer, AR Field, B McMillan, A Peeters, CM Roach

Electrostatic gyrokinetic analyses are presented for an L-mode discharge with an internal transport barrier, from the spherical tokamak, MAST. Local and global microstability analysis finds similar linear growth rates for ion temperature gradient (ITG) driven modes. When the electron response is assumed to be adiabatic, growth rates are found to be lower than the experimental E×B flow shearing rate. Including kinetic electrons, without collisions, increases the ITG growth rates above the flow shearing rate, and these modes are found to be linearly unstable in the outer part of the plasma only. In global simulations the flow shear stabilization is found to be asymmetric with respect to the direction of the flow: there is a small destabilizing effect at low flow shear when the flow is in the co-direction. Global non-linear simulations with kinetic electrons and including the flow shear effects predict turbulent ion heat transport that is well above the neoclassical level in the region outside the internal transport barrier in this MAST plasma. In non-linear simulations we also find turbulence extending from the outer part of the plasma into the linearly stable core region. © 2012 IOP Publishing Ltd.


Phase synchronization between stratospheric and tropospheric quasi-biennial and semi-annual oscillations

Quarterly Journal of the Royal Meteorological Society 138 (2012) 1338-1349

PL Read, AA Castrejón-Pita

A combination of singular systems analysis and analytic phase techniques are used to investigate the possible occurrence in observations of coherent synchronization between quasi-biennial and semi-annual oscillations (QBOs; SAOs) in the stratosphere and troposphere. Time series of zonal mean zonal winds near the Equator are analysed from the ERA-40 and ERA-interim reanalysis datasets over a ∼ 50-year period. In the stratosphere, the QBO is found to synchronize with the SAO almost all the time, but with a frequency ratio that changes erratically between 4:1, 5:1 and 6:1. A similar variable synchronization is also evident in the tropical troposphere between semi-annual and quasi-biennial cycles (known as TBOs). Mean zonal winds from ERA-40 and ERA-interim, and also time series of indices for the Indian and West Pacific monsoons, are commonly found to exhibit synchronization, with SAO/TBO ratios that vary between 4:1 and 7:1. Coherent synchronization between the QBO and tropical TBO does not appear to persist for long intervals, however. This suggests that both the QBO and tropical TBOs may be separately synchronized to SAOs that are themselves enslaved to the seasonal cycle, or to the annual cycle itself. However, the QBO and TBOs are evidently only weakly coupled between themselves and are frequently found to lose mutual coherence when each changes its frequency ratio to its respective SAO. This suggests a need to revise a commonly cited paradigm that advocates the use of stratospheric QBO indices as a predictor for tropospheric phenomena such as monsoons and hurricanes. © 2012 Royal Meteorological Society.


Cumulative carbon as a policy framework for achieving climate stabilization.

Philosophical transactions. Series A, Mathematical, physical, and engineering sciences 370 (2012) 4365-4379

HD Matthews, S Solomon, R Pierrehumbert

The primary objective of the United Nations Framework Convention on Climate Change is to stabilize greenhouse gas concentrations at a level that will avoid dangerous climate impacts. However, greenhouse gas concentration stabilization is an awkward framework within which to assess dangerous climate change on account of the significant lag between a given concentration level and the eventual equilibrium temperature change. By contrast, recent research has shown that global temperature change can be well described by a given cumulative carbon emissions budget. Here, we propose that cumulative carbon emissions represent an alternative framework that is applicable both as a tool for climate mitigation as well as for the assessment of potential climate impacts. We show first that both atmospheric CO(2) concentration at a given year and the associated temperature change are generally associated with a unique cumulative carbon emissions budget that is largely independent of the emissions scenario. The rate of global temperature change can therefore be related to first order to the rate of increase of cumulative carbon emissions. However, transient warming over the next century will also be strongly affected by emissions of shorter lived forcing agents such as aerosols and methane. Non-CO(2) emissions therefore contribute to uncertainty in the cumulative carbon budget associated with near-term temperature targets, and may suggest the need for a mitigation approach that considers separately short- and long-lived gas emissions. By contrast, long-term temperature change remains primarily associated with total cumulative carbon emissions owing to the much longer atmospheric residence time of CO(2) relative to other major climate forcing agents.


The origin and evolution of saturn's 2011-2012 stratospheric vortex

Icarus 221 (2012) 560-586

LN Fletcher, BE Hesman, RK Achterberg, PGJ Irwin, G Bjoraker, N Gorius, J Hurley, J Sinclair, GS Orton, J Legarreta, E García-Melendo, A Sánchez-Lavega, PL Read, AA Simon-Miller, FM Flasar

The planet-encircling springtime storm in Saturn's troposphere (December 2010-July 2011, Fletcher, L.N. et al. [2011]. Science 332, 1413-1414; Sánchez-Lavega, A. et al. [2011]. Nature 475, 71-74; Fischer, G. et al. [2011]. Nature 475, 75-77) produced dramatic perturbations to stratospheric temperatures, winds and composition at mbar pressures that persisted long after the tropospheric disturbance had abated. Thermal infrared (IR) spectroscopy from the Cassini Composite Infrared Spectrometer (CIRS), supported by ground-based IR imaging from the VISIR instrument on the Very Large Telescope and the MIRSI instrument on NASA's IRTF, is used to track the evolution of a large, hot stratospheric anticyclone between January 2011 and March 2012. The evolutionary sequence can be divided into three phases: (I) the formation and intensification of two distinct warm airmasses near 0.5. mbar between 25 and 35°N (B1 and B2) between January-April 2011, moving westward with different zonal velocities, B1 residing directly above the convective tropospheric storm head; (II) the merging of the warm airmasses to form the large single 'stratospheric beacon' near 40°N (B0) between April and June 2011, disassociated from the storm head and at a higher pressure (2 mbar) than the original beacons, a downward shift of 1.4 scale heights (approximately 85. km) post-merger; and (III) the mature phase characterised by slow cooling (0.11. ±. 0.01. K/day) and longitudinal shrinkage of the anticyclone since July 2011. Peak temperatures of 221.6. ±. 1.4. K at 2. mbar were measured on May 5th 2011 immediately after the merger, some 80. K warmer than the quiescent surroundings. From July 2011 to the time of writing, B0 remained as a long-lived stable stratospheric phenomenon at 2. mbar, moving west with a near-constant velocity of 2.70. ±. 0.04. deg/day (-24.5. ±. 0.4. m/s at 40°N relative to System III longitudes). No perturbations to visible clouds and hazes were detected during this period.With no direct tracers of motion in the stratosphere, we use thermal windshear calculations to estimate clockwise peripheral velocities of 200-400m/s at 2mbar around B0. The peripheral velocities of the two original airmasses were smaller (70-140m/s). In August 2011, the size of the vortex as defined by the peripheral collar was 65° longitude (50,000km in diameter) and 25° latitude. Stratospheric acetylene (C 2H 2) was uniformly enhanced by a factor of three within the vortex, whereas ethane (C 2H 6) remained unaffected. The passage of B0 generated a new band of warm stratospheric emission at 0.5mbar at its northern edge, and there are hints of warm stratospheric structures associated with the beacons at higher altitudes (p<0.1mbar) than can be reliably observed by CIRS nadir spectroscopy. Analysis of the zonal windshear suggests that Rossby wave perturbations from the convective storm could have propagated vertically into the stratosphere at this point in Saturn's seasonal cycle, one possible source of energy for the formation of these stratospheric anticyclones. © 2012 Elsevier Inc.


Clouds and Snowball Earth deglaciation

GEOPHYSICAL RESEARCH LETTERS 39 (2012) ARTN L20711

DS Abbot, A Voigt, M Branson, RT Pierrehumbert, D Pollard, G Le Hir, DDB Koll


Assimilating and modeling dust transport in the martian climate system

Proceedings of the International Astronomical Union 8 (2012) 326-328

T Ruan, L Montabone, PL Read, SR Lewis

A meteorological data assimilation system has been developed recently for analyzing measurements of temperature and dust opacity on Mars and has been successfully applied in several studies (e.g. Montabone et al. 2005, Lewis et al. 2007) to study various atmospheric phenomena. A more sophisticated data assimilation system, now with full dust transport incorporated, is becoming available to represent more accurately and realistically the physical transport of dust. Copyright © International Astronomical Union 2014.


Diversity of planetary atmospheric circulations and climates in a simplified general circulation model

Proceedings of the International Astronomical Union 8 (2012) 297-302

Y Wang, P Read

The parametric dependence of terrestrial planetary atmospheric circulations and climates on characteristic parameters is studied. A simplified general circulation model - PUMA is employed to investigate the dynamic effects of planetary rotation rate and equator-to-pole temperature difference on the circulation and climate of terrestrial planetary atmospheres. Five different types of circulation regime are identified by mapping the experimental results in a 2-D parameter space defined by thermal Rossby number and frictional Taylor number. The effect of the transfer and redistribution of radiative energy is studied by building up a new two-band semi-gray radiative-convective scheme, which is capable of modelling greenhouse and anti-greenhouse effects while keeping the tunable parameters as few as possible. The results will provide insights into predicting the habitability of terrestrial exoplanets. Copyright © International Astronomical Union 2014.


TRANSP modelling of neutron emissivity on MAST

39th EPS Conference on Plasma Physics 2012, EPS 2012 and the 16th International Congress on Plasma Physics 3 (2012) 1786-1789

I Wodniak, M Gorelenkova, S Conroy, M Cecconello, R Akers, M Turnyanskiy, S Sangaroon, D Keeling, C Michael, G Colyer, G Ericsson

The aim of this paper is to validate a set of methods to model the measured neutron emission on Mega Amp Spherical Tokamak (MAST) using TRANSP. Neutrons are measured along a set of collimated lines of sight (LoS) at MAST by a Neutron Camera (NC). The neutron emissivity is modeled with TRANSP [1] for different plasma discharges using a selfconsistent data set produced by a data preparation and analysis tool. The modeled neutron emissivity is then compared with NC experimental data using a full 3D solid angle calculation to model the transport of neutrons from plasma to detector. A good agreement between the modelled and experimental neutron rate was found.


Zonal winds at high latitudes on Venus: An improved application of cyclostrophic balance to Venus Express observations

Icarus 217 (2012) 629-639

JM Mendonça, PL Read, CF Wilson, SR Lewis

Recent retrievals of zonal thermal winds obtained in a cyclostrophic regime on Venus are generally consistent with cloud tracking measurements at mid-latitudes, but become unphysical in polar regions where the values obtained above the clouds are often less than or close to zero. Using a global atmospheric model, we show that the main source of errors that appear in the polar regions when retrieving the zonal thermal winds is most likely due to uncertainties in the zonal wind intensity in the choice of the lower boundary condition.Here we suggest a new and robust method to better estimate the lower boundary condition for high latitudes, thereby improving the retrieved zonal thermal winds throughout the high latitudes middle atmosphere. This new method is applied to temperature fields derived from Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) data on board the Venus Express spacecraft. We obtain a zonal thermal wind field that is in better agreement with other, more direct methods based on either retrieving the zonal winds from cloud tracking or from direct measurements of the meridional slope of pressure surfaces. © 2011 Elsevier Inc.


Phase synchronization between stratospheric and tropospheric quasi-biennial and semi-annual oscillations

Quarterly Journal of the Royal Meteorological Society (2012)

PL Read, AA Castrejón-Pita

Pages