Publications


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.


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.


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.


Future Mars geophysical observatories for understanding its internal structure, rotation, and evolution

Planetary and Space Science 68 (2012) 123-145

V Dehant, B Banerdt, P Lognonné, M Grott, S Asmar, J Biele, D Breuer, F Forget, R Jaumann, C Johnson, M Knapmeyer, B Langlais, M Le Feuvre, D Mimoun, A Mocquet, P Read, A Rivoldini, O Romberg, G Schubert, S Smrekar, T Spohn, P Tortora, S Ulamec, S Vennerstrom

Our fundamental understanding of the interior of the Earth comes from seismology, geodesy, geochemistry, geomagnetism, geothermal studies, and petrology. For the Earth, measurements in those disciplines of geophysics have revealed the basic internal layering of the Earth, its dynamical regime, its thermal structure, its gross compositional stratification, as well as significant lateral variations in these quantities. Planetary interiors not only record evidence of conditions of planetary accretion and differentiation, they exert significant control on surface environments. We present recent advances in possible in-situ investigations of the interior of Mars, experiments and strategies that can provide unique and critical information about the fundamental processes of terrestrial planet formation and evolution. Such investigations applied on Mars have been ranked as a high priority in virtually every set of European, US and international high-level planetary science recommendations for the past 30 years. New seismological methods and approaches based on the cross-correlation of seismic noise by two seismic stations/landers on the surface of Mars and on joint seismic/orbiter detection of meteorite impacts, as well as the improvement of the performance of Very Broad-Band (VBB) seismometers have made it possible to secure a rich scientific return with only two simultaneously recording stations. In parallel, use of interferometric methods based on two Earth-Mars radio links simultaneously from landers tracked from Earth has increased the precision of radio science experiments by one order of magnitude. Magnetometer and heat flow measurements will complement seismic and geodetic data in order to obtain the best information on the interior of Mars. In addition to studying the present structure and dynamics of Mars, these measurements will provide important constraints for the astrobiology of Mars by helping to understand why Mars failed to sustain a magnetic field, by helping to understand the planet's climate evolution, and by providing a limit for the energy available to the chemoautotrophic biosphere through a measurement of the surface heat flow. The landers of the mission will also provide meteorological stations to monitor the climate and obtain new measurements in the atmospheric boundary layer. © 2011 Elsevier Ltd.


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


Dynamics and circulation regimes of terrestrial planets

PLANETARY AND SPACE SCIENCE 59 (2011) 900-914

PL Read


Ion heat transport studies in JET

Plasma Physics and Controlled Fusion 53 (2011)

P Mantica, C Angioni, B Baiocchi, M Baruzzo, MNA Beurskens, JPS Bizarro, RV Budny, P Buratti, A Casati, C Challis, J Citrin, G Colyer, F Crisanti, ACA Figueiredo, L Frassinetti, C Giroud, N Hawkes, J Hobirk, E Joffrin, T Johnson, E Lerche, P Migliano, V Naulin, AG Peeters, G Rewoldt, F Ryter, A Salmi, R Sartori, C Sozzi, G Staebler, D Strintzi, T Tala, M Tsalas, D Van Eester, T Versloot, PC DeVries, J Weiland

Detailed experimental studies of ion heat transport have been carried out in JET exploiting the upgrade of active charge exchange spectroscopy and the availability of multi-frequency ion cyclotron resonance heating with 3He minority. The determination of ion temperature gradient (ITG) threshold and ion stiffness offers unique opportunities for validation of the well-established theory of ITG driven modes. Ion stiffness is observed to decrease strongly in the presence of toroidal rotation when the magnetic shear is sufficiently low. This effect is dominant with respect to the well-known ω E×B threshold up-shift and plays a major role in enhancing core confinement in hybrid regimes and ion internal transport barriers. The effects of T e/T i and s/q on ion threshold are found rather weak in the domain explored. Quasi-linear fluid/gyro-fluid and linear/non-linear gyro-kinetic simulations have been carried out. Whilst threshold predictions show good match with experimental observations, some significant discrepancies are found on the stiffness behaviour. © 2011 IOP Publishing Ltd.


A key to improved ion core confinement in the JET tokamak: Ion stiffness mitigation due to combined plasma rotation and low magnetic shear

Physical Review Letters 107 (2011)

P Mantica, C Angioni, C Challis, G Colyer, L Frassinetti, N Hawkes, T Johnson, M Tsalas, PC Devries, J Weiland, B Baiocchi, MNA Beurskens, ACA Figueiredo, C Giroud, J Hobirk, E Joffrin, E Lerche, V Naulin, AG Peeters, A Salmi, C Sozzi, D Strintzi, G Staebler, T Tala, D Van Eester, T Versloot

New transport experiments on JET indicate that ion stiffness mitigation in the core of a rotating plasma, as described by Mantica et al. [Phys. Rev. Lett. 102, 175002 (2009)PRLTAO0031-900710.1103/PhysRevLett.102.175002] results from the combined effect of high rotational shear and low magnetic shear. The observations have important implications for the understanding of improved ion core confinement in advanced tokamak scenarios. Simulations using quasilinear fluid and gyrofluid models show features of stiffness mitigation, while nonlinear gyrokinetic simulations do not. The JET experiments indicate that advanced tokamak scenarios in future devices will require sufficient rotational shear and the capability of q profile manipulation. © 2011 American Physical Society.


Bifurcations leading to summer Arctic sea ice loss

JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 116 (2011) ARTN D19120

DS Abbot, M Silber, RT Pierrehumbert


HYDROGEN GREENHOUSE PLANETS BEYOND THE HABITABLE ZONE

ASTROPHYSICAL JOURNAL LETTERS 734 (2011) ARTN L13

R Pierrehumbert, E Gaidos


Thermal structure and dynamics of Saturn's northern springtime disturbance.

Science 332 (2011) 1413-1417

LN Fletcher, BE Hesman, PGJ Irwin, KH Baines, TW Momary, A Sanchez-Lavega, FM Flasar, PL Read, GS Orton, A Simon-Miller, R Hueso, GL Bjoraker, A Mamoutkine, T del Rio-Gaztelurrutia, JM Gomez, B Buratti, RN Clark, PD Nicholson, C Sotin

Saturn's slow seasonal evolution was disrupted in 2010-2011 by the eruption of a bright storm in its northern spring hemisphere. Thermal infrared spectroscopy showed that within a month, the resulting planetary-scale disturbance had generated intense perturbations of atmospheric temperatures, winds, and composition between 20° and 50°N over an entire hemisphere (140,000 kilometers). The tropospheric storm cell produced effects that penetrated hundreds of kilometers into Saturn's stratosphere (to the 1-millibar region). Stratospheric subsidence at the edges of the disturbance produced "beacons" of infrared emission and longitudinal temperature contrasts of 16 kelvin. The disturbance substantially altered atmospheric circulation, transporting material vertically over great distances, modifying stratospheric zonal jets, exciting wave activity and turbulence, and generating a new cold anticyclonic oval in the center of the disturbance at 41°N.


Plasma rotation and transport in MAST spherical tokamak

Nuclear Fusion 51 (2011)

AR Field, C Michael, RJ Akers, J Candy, G Colyer, W Guttenfelder, YC Ghim, CM Roach, S Saarelma

The formation of internal transport barriers (ITBs) is investigated in MAST spherical tokamak plasmas. The relative importance of equilibrium flow shear and magnetic shear in their formation and evolution is investigated using data from high-resolution kinetic- and q-profile diagnostics. In L-mode plasmas, with co-current directed NBI heating, ITBs in the momentum and ion thermal channels form in the negative shear region just inside qmin. In the ITB region the anomalous ion thermal transport is suppressed, with ion thermal transport close to the neo-classical level, although the electron transport remains anomalous. Linear stability analysis with the gyro-kinetic code GS2 shows that all electrostatic micro-instabilities are stable in the negative magnetic shear region in the core, both with and without flow shear. Outside the ITB, in the region of positive magnetic shear and relatively weak flow shear, electrostatic micro-instabilities become unstable over a wide range of wave numbers. Flow shear reduces the linear growth rates of low-k modes but suppression of ITG modes is incomplete, which is consistent with the observed anomalous ion transport in this region; however, flow shear has little impact on growth rates of high-k, electron-scale modes. With counter-NBI ITBs of greater radial extent form outside qmin due to the broader profile of E × B flow shear produced by the greater prompt fast-ion loss torque. © 2011 IAEA, Vienna.


Sea glacier flow and dust transport on Snowball Earth

GEOPHYSICAL RESEARCH LETTERS 38 (2011) ARTN L17501

D Li, RT Pierrehumbert


THE MARTIAN ATMOSPHERIC BOUNDARY LAYER

REVIEWS OF GEOPHYSICS 49 (2011) ARTN RG3005

A Petrosyan, B Galperin, SE Larsen, SR Lewis, A Maeaettaenen, PL Read, N Renno, LPHT Rogberg, H Savijarvi, T Siili, A Spiga, A Toigo, L Vazquez


Some fine points on radiative forcing Reply

PHYSICS TODAY 64 (2011) 12-12

RT Pierrehumbert


Overview of physics results from MAST

Nuclear Fusion 51 (2011)

B Lloyd, RJ Akers, F Alladio, S Allan, LC Appel, M Barnes, NC Barratt, N Ben Ayed, BN Breizman, M Cecconello, CD Challis, IT Chapman, D Ciric, G Colyer, JW Connor, NJ Conway, M Cox, SC Cowley, G Cunningham, A Darke, M De Bock, E Delchambre, G De Temmerman, RO Dendy, P Denner, MD Driscoll, B Dudson, D Dunai, M Dunstan, S Elmore, AR Field, G Fishpool, S Freethy, L Garzotti, KJ Gibson, MP Gryaznevich, W Guttenfelder, J Harrison, RJ Hastie, NC Hawkes, TC Hender, B Hnat, DF Howell, MD Hua, A Hubbard, G Huysmans, D Keeling, YC Kim, A Kirk, Y Liang, MK Lilley, M Lisak, S Lisgo, YQ Liu, GP Maddison, R Maingi, SJ Manhood, R Martin, GJ McArdle, J McCone, H Meyer, C Michael, S Mordijck, T Morgan, AW Morris, DG Muir, E Nardon, G Naylor, MR O'Brien, T O'Gorman, J Pláeník, A Patel, SD Pinches, MN Price, CM Roach, V Rozhansky, S Saarelma, SA Sabbagh, A Saveliev, R Scannell, SE Sharapov, V Shevchenko, S Shibaev, D Stork, J Storrs, W Suttrop, A Sykes, P Tamain, D Taylor, D Temple, N Thomas-Davies, A Thornton, MR Turnyanskiy, M Valovic, RGL Vann, G Voss, MJ Walsh, SEV Warder, HR Wilson, M Windridge

Major developments on the Mega Amp Spherical Tokamak (MAST) have enabled important advances in support of ITER and the physics basis of a spherical tokamak (ST) based component test facility (CTF), as well as providing new insight into underlying tokamak physics. For example, L-H transition studies benefit from high spatial and temporal resolution measurements of pedestal profile evolution (temperature, density and radial electric field) and in support of pedestal stability studies the edge current density profile has been inferred from motional Stark effect measurements. The influence of the q-profile and E × B flow shear on transport has been studied in MAST and equilibrium flow shear has been included in gyro-kinetic codes, improving comparisons with the experimental data. H-modes exhibit a weaker q and stronger collisionality dependence of heat diffusivity than implied by IPB98(y,2) scaling, which may have important implications for the design of an ST-based CTF. ELM mitigation, an important issue for ITER, has been demonstrated by applying resonant magnetic perturbations (RMPs) using both internal and external coils, but full stabilization of type-I ELMs has not been observed. Modelling shows the importance of including the plasma response to the RMP fields. MAST plasmas with q > 1 and weak central magnetic shear regularly exhibit a long-lived saturated ideal internal mode. Measured plasma braking in the presence of this mode compares well with neo-classical toroidal viscosity theory. In support of basic physics understanding, high resolution Thomson scattering measurements are providing new insight into sawtooth crash dynamics and neo-classical tearing mode critical island widths. Retarding field analyser measurements show elevated ion temperatures in the scrape-off layer of L-mode plasmas and, in the presence of type-I ELMs, ions with energy greater than 500 eV are detected 20 cm outside the separatrix. Disruption mitigation by massive gas injection has reduced divertor heat loads by up to 70%. © 2011 IAEA, Vienna.


Saturn: Storm-clouds brooding on towering heights

Nature 475 (2011) 44-45

P Read

Pages