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  • Journal article
    Lester JG, Lovenduski NS, Graven HD, Long MC, Lindsay Ket al., 2020,

    Internal variability dominates over externally forced ocean circulation changes seen through CFCs

    , Geophysical Research Letters, Vol: 47, Pages: 1-10, ISSN: 0094-8276

    Observations of oceanic transient tracers have indicated that the circulation in the Southern Ocean has changed in recent decades, potentially driven by changes in external climate forcing. Here, we use the CESM Large Ensemble to analyze changes in two oceanic tracers that are affected by ocean circulation: the partial pressure of chlorofluorocarbon‐12 (pCFC12) and the idealized model tracer Ideal Age (IAGE) over the 1991 to 2005 period. The small ensemble mean change in IAGE suggests that there has been very little externally forced change in Southern Ocean circulation over this period, in contrast to strong internal variability. Further, our analysis implies that real‐world observations of changes in pCFC12 may not be a robust way to characterize externally driven changes in Southern Ocean circulation because of the large internal variability in pCFC12 changes exhibited by the individual ensemble members.
  • Journal article
    Shebanits O, Hadid LZ, Cao H, Morooka MW, Hunt G, Dougherty MK, Wahlund J-E, Waite Jr JH, Mueller-Wodarg Iet al., 2020,

    Saturn’s near-equatorial ionospheric conductivities from in situ measurements

    , Scientific Reports, Vol: 10, ISSN: 2045-2322

    Cassini’s Grand Finale orbits provided for the first time in-situ measurements of Saturn’s topside ionosphere. We present the Pedersen and Hall conductivities of the top near-equatorial dayside ionosphere, derived from the in-situ measurements by the Cassini Radio and Wave Plasma Science Langmuir Probe, the Ion and Neutral Mass Spectrometer and the fluxgate magnetometer. The Pedersen and Hall conductivities are constrained to at least 10−5–10−4 S/m at (or close to) the ionospheric peak, a factor 10–100 higher than estimated previously. We show that this is due to the presence of dusty plasma in the near-equatorial ionosphere. We also show the conductive ionospheric region to be extensive, with thickness of 300–800 km. Furthermore, our results suggest a temporal variation (decrease) of the plasma densities, mean ion masses and consequently the conductivities from orbit 288 to 292.
  • Journal article
    Lavraud B, Fargette N, Réville V, Szabo A, Huang J, Rouillard AP, Viall N, Phan TD, Kasper JC, Bale SD, Berthomier M, Bonnell JW, Case AW, Dudok de Wit T, Eastwood JP, Génot V, Goetz K, Griton LS, Halekas JS, Harvey P, Kieokaew R, Klein KG, Korreck KE, Kouloumvakos A, Larson DE, Lavarra M, Livi R, Louarn P, MacDowall RJ, Maksimovic M, Malaspina D, Nieves-Chinchilla T, Pinto RF, Poirier N, Pulupa M, Raouafi NE, Stevens ML, Toledo-Redondo S, Whittlesey PLet al., 2020,

    The heliospheric current sheet and plasma sheet during Parker Solar Probe’s first orbit

    , Letters of the Astrophysical Journal, Vol: 894, Pages: 1-8, ISSN: 2041-8205

    We present heliospheric current sheet (HCS) and plasma sheet (HPS) observations during Parker Solar Probe's (PSP) first orbit around the Sun. We focus on the eight intervals that display a true sector boundary (TSB; based on suprathermal electron pitch angle distributions) with one or several associated current sheets. The analysis shows that (1) the main density enhancements in the vicinity of the TSB and HCS are typically associated with electron strahl dropouts, implying magnetic disconnection from the Sun, (2) the density enhancements are just about twice that in the surrounding regions, suggesting mixing of plasmas from each side of the HCS, (3) the velocity changes at the main boundaries are either correlated or anticorrelated with magnetic field changes, consistent with magnetic reconnection, (4) there often exists a layer of disconnected magnetic field just outside the high-density regions, in agreement with a reconnected topology, (5) while a few cases consist of short-lived density and velocity changes, compatible with short-duration reconnection exhausts, most events are much longer and show the presence of flux ropes interleaved with higher-β regions. These findings are consistent with the transient release of density blobs and flux ropes through sequential magnetic reconnection at the tip of the helmet streamer. The data also demonstrate that, at least during PSP's first orbit, the only structure that may be defined as the HPS is the density structure that results from magnetic reconnection, and its byproducts, likely released near the tip of the helmet streamer.
  • Journal article
    Hunt GJ, Bunce EJ, Cao H, Cowley SWH, Dougherty MK, Provan G, Southwood DJet al., 2020,

    Saturn's auroral field-aligned currents: observations from the Northern Hemisphere dawn sector during cassini's proximal orbits

    , Journal of Geophysical Research: Space Physics, Vol: 125, ISSN: 2169-9380

    We examine the azimuthal magnetic field signatures associated with Saturn's northern hemisphere auroral field‐aligned currents observed in the dawn sector during Cassini's Proximal orbits (April 2017 and September 2017). We compare these currents with observations of the auroral currents from near noon taken during the F‐ring orbits prior to the Proximal orbits. First, we show that the position of the main auroral upward current is displaced poleward between the two local times (LT). This is consistent with the statistical position of the ultraviolet auroral oval for the same time interval. Second, we show the overall average ionospheric meridional current profile differs significantly on the equatorward boundary of the upward current with a swept‐forward configuration with respect to planetary rotation present at dawn. We separate the planetary period oscillation (PPO) currents from the PPO‐independent currents and show their positional relationship is maintained as the latitude of the current shifts in LT implying an intrinsic link between the two systems. Focusing on the individual upward current sheets pass‐by‐pass we find that the main upward current at dawn is stronger compared to near‐noon. This results in the current density been ~1.4 times higher in the dawn sector. We determine a proxy for the precipitating electron power and show that the dawn PPO‐independent upward current electron power ~1.9 times higher than at noon. These new observations of the dawn auroral region from the Proximal orbits may show evidence of an additional upward current at dawn likely associated with strong flows in the outer magnetosphere.
  • Journal article
    Verniero JL, Larson DE, Livi R, Rahmati A, McManus MD, Pyakurel PS, Klein KG, Bowen TA, Bonnell JW, Alterman BL, Whittlesey PL, Malaspina DM, Bale SD, Kasper JC, Case AW, Goetz K, Harvey PR, Korreck KE, MacDowall RJ, Pulupa M, Stevens ML, de Wit TDet al., 2020,

    Parker Solar Probe Observations of Proton Beams Simultaneous with Ion-scale Waves

    , ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 248, ISSN: 0067-0049
  • Journal article
    Bowen TA, Bale SD, Bonnell JW, de Wit TD, Goetz K, Goodrich K, Gruesbeck J, Harvey PR, Jannet G, Koval A, MacDowall RJ, Malaspina DM, Pulupa M, Revillet C, Sheppard D, Szabo Aet al., 2020,

    A Merged Search-Coil and Fluxgate Magnetometer Data Product for Parker Solar Probe FIELDS

    , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380
  • Journal article
    Staniland N, Dougherty M, Masters A, Bunce Eet al., 2020,

    Determining the nominal thickness and variability of the magnetodisc current sheet at saturn

    , Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-15, ISSN: 2169-9380

    The thickness and variability of the Saturnian magnetodisc current sheet is investigated using the Cassini magnetometer data set. Cassini performed 66 fast, steep crossings of the equatorial current sheet where a clear signature in the magnetic field data allowed for a direct determination of its thickness and the offset of its center. The average, or nominal, current sheet half‐thickness is 1.3 R S , where R S is the equatorial radius of Saturn, equal to 60,268 km. This is thinner than previously calculated, but both spatial and temporal dependencies are identified. The current sheet is thicker and more variable by a factor ∼2 on the nightside compared to the dayside, ranging from 0.5–3 R S . The current sheet is on average 50% thicker in the nightside quasi‐dipolar region (≤15 R S ) compared to the dayside. These results are consistent with the presence of a noon‐midnight electric field at Saturn that produces a hotter plasma population on the nightside compared to the dayside. It is also shown that the current sheet becomes significantly thinner in the outer region of the nightside, while staying approximately constant with radial distance on the dayside, reflecting the dayside compression of the magnetosphere by the solar wind. Some of the variability is well characterized by the planetary period oscillations (PPOs). However, we also find evidence for non‐PPO drivers of variability.
  • Journal article
    Fuselier, Petrinec, Sawyer, Mukherjee, Masters Aet al., 2020,

    Suppression of magnetic reconnection at Saturn’s low-latitude magnetopause

    , Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-16, ISSN: 2169-9380

    Observations from the Cassini Plasma Spectrometer/Electron Spectrometer (CAPS/ELS) are used in an in‐depth investigation of the occurrence and location of reconnection at Saturn's magnetopause. Heated, streaming electrons parallel and/or antiparallel to the magnetic field in the magnetosheath adjacent to the magnetopause indicate that reconnection is occurring somewhere on the boundary. In these instances, the Cassini spacecraft is connected to open magnetic field lines that thread the magnetopause boundary. A survey of 99 crossings with sufficient pitch angle coverage from CAPS/ELS indicates that 65% of the crossings had this evidence of reconnection. Specific crossings from this survey are used to demonstrate that there are times when reconnection at Saturn's low‐latitude magnetopause may be suppressed.
  • Journal article
    Mackie A, Wild M, Brindley H, Folini D, Palmer Pet al., 2020,

    Observed and CMIP5-Simulated Radiative Flux Variability Over West Africa

    , EARTH AND SPACE SCIENCE, Vol: 7
  • Conference paper
    Quaas J, Gryspeerdt E, Vautard R, Boucher Oet al., 2020,

    Climate impact of aircraft-induced cirrus assessed from satellite observations before and during COVID-19

    , EGU General Assembly, Publisher: Copernicus GmbH, Pages: 1-1

    Aircraft produce contrail in suitable atmospheric conditions, and these may spread out into cirrus. However, it is unclear how large this effect and its implied radiative forcing is. Here we use the opportunity of the COVID-19 related aircraft traffic reduction in boreal spring 2020 in comparison to the traffic in 2019 to assess satellite data. MODIS retrievals are examined for 2020 vs. the climatology 2011 to 2019. In order to account for weather variability, circulation analogues are defined for each region and day of the Spring 2020 period, and the cirrus coverage and emissivity in springtimes 2011 - 2019 is assessed for comparison to 2020. In conclusion, we find that cirrus are reduced by 9±1.5% in absolute terms. This is consistent with a trend analysis. The implied radiative forcing by aviation-induced cirrus is assessed at 49±28 Wm-2.
  • Journal article
    Nordheim TA, Wellbrock A, Jones GH, Desai RT, Coates AJ, Teolis BD, Jasinski JMet al., 2020,

    Detection of Negative Pickup Ions at Saturn's Moon Dione

    , GEOPHYSICAL RESEARCH LETTERS, Vol: 47, ISSN: 0094-8276
  • Journal article
    Wilson LB, Chen L-J, Wang S, Schwartz SJ, Turner DL, Stevens ML, Kasper JC, Osmane A, Caprioli D, Bale SD, Pulupa MP, Salem CS, Goodrich KAet al., 2020,

    Electron Energy Partition across Interplanetary Shocks. III. Analysis

    , ASTROPHYSICAL JOURNAL, Vol: 893, ISSN: 0004-637X
  • Journal article
    Wild O, Voulgarakis A, O'Connor F, Lamarque J-F, Ryan EM, Lee Let al., 2020,

    Global sensitivity analysis of chemistry-climate model budgets of tropospheric ozone and OH: exploring model diversity

    , Atmospheric Chemistry and Physics, Vol: 20, Pages: 4047-4058, ISSN: 1680-7316

    Projections of future atmospheric composition change and its impacts on air quality and climate depend heavily on chemistry–climate models that allow us to investigate the effects of changing emissions and meteorology. These models are imperfect as they rely on our understanding of the chemical, physical and dynamical processes governing atmospheric composition, on the approximations needed to represent these numerically, and on the limitations of the observations required to constrain them. Model intercomparison studies show substantial diversity in results that reflect underlying uncertainties, but little progress has been made in explaining the causes of this or in identifying the weaknesses in process understanding or representation that could lead to improved models and to better scientific understanding. Global sensitivity analysis provides a valuable method of identifying and quantifying the main causes of diversity in current models. For the first time, we apply Gaussian process emulation with three independent global chemistry-transport models to quantify the sensitivity of ozone and hydroxyl radicals (OH) to important climate-relevant variables, poorly characterised processes and uncertain emissions. We show a clear sensitivity of tropospheric ozone to atmospheric humidity and precursor emissions which is similar for the models, but find large differences between models for methane lifetime, highlighting substantial differences in the sensitivity of OH to primary and secondary production. This approach allows us to identify key areas where model improvements are required while providing valuable new insight into the processes driving tropospheric composition change.
  • Journal article
    Weiss Z, Pickering JC, 2020,

    Charge transfer from doubly charged ions of transition elements in a neon glow discharge: evidence based on emission spectra

    , Plasma Sources Science and Technology, Vol: 29, Pages: 1-12, ISSN: 0963-0252

    An extensive study of Mn II, Fe II, Ti II, Cr II and Cu II emission spectra from a Grimm-type glow discharge in neon was performed, using the formalism of transition rate (TR) diagrams. In this method, radiative depopulation rates of individual excited levels of a species under study are established based on the emission spectrum, prospective contributions from radiative decay of higher excited levels (cascade excitation) are subtracted and the resulting net depopulation rates are plotted as function of energy of the levels involved. A peak at a particular energy in such a diagram reflects a collisional process in operation, selectively populating levels in a narrow interval around that energy. By comparing net TR diagrams of ionic spectra of the elements listed above, a common pattern was found indicating that singly charged ions of these elements are created, in addition to other mechanisms, by charge transfer between doubly charged ions of the element under study and metastable neutral neon atoms. This mechanism appears to be significant and needs to be taken into account in collisional–radiative models describing excitation and ionization of some elements in neon glow discharges.
  • Journal article
    Bercic L, Larson D, Whittlesey P, Maksimovic M, Badman ST, Landi S, Matteini L, Bale SD, Bonnell JW, Case AW, de Wit TD, Goetz K, Harvey PR, Kasper JC, Korreck KE, Livi R, MacDowall RJ, Malaspina DM, Pulupa M, Stevens MLet al., 2020,

    Coronal electron temperature inferred from the strahl electrons in the inner heliosphere: parker solar probe and helios observations

    , The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 892, Pages: 1-14, ISSN: 0004-637X

    The shape of the electron velocity distribution function plays an important role in the dynamics of the solar wind acceleration. Electrons are normally modeled with three components, the core, the halo, and the strahl. We investigate how well the fast strahl electrons in the inner heliosphere preserve the information about the coronal electron temperature at their origin. We analyzed the data obtained by two missions, Helios, spanning the distances between 65 and 215 R S, and Parker Solar Probe (PSP), reaching down to 35 R S during its first two orbits around the Sun. The electron strahl was characterized with two parameters: pitch-angle width (PAW) and the strahl parallel temperature (T s∥). PSP observations confirm the already reported dependence of strahl PAW on core parallel plasma beta (${\beta }_{\mathrm{ec}\parallel }$). Most of the strahl measured by PSP appear narrow with PAW reaching down to 30°. The portion of the strahl velocity distribution function aligned with the magnetic field is for the measured energy range well described by a Maxwellian distribution function. T s∥ was found to be anticorrelated with the solar wind velocity and independent of radial distance. These observations imply that T s∥ carries the information about the coronal electron temperature. The obtained values are in agreement with coronal temperatures measured using spectroscopy, and the inferred solar wind source regions during the first orbit of PSP agree with the predictions using a PFSS model.
  • Journal article
    Nair V, Heus T, Van Reeuwijk M, 2020,

    Dynamics of subsiding shells in actively growing clouds with vertical updrafts

    , Journal of the Atmospheric Sciences, Vol: 77, Pages: 1353-1369, ISSN: 0022-4928

    The dynamics of a subsiding shell at the edges of actively growing shallow cumulus clouds with updrafts is analyzed using direct numerical simulation. The actively growing clouds have a fixed in-cloud buoyancy and velocity. Turbulent mixing and evaporative cooling at the cloud edges generate a subsiding shell which grows with time. A self-similar regime is observed for first and second order moments when normalized with respective maximum values. Internal scales derived from integral properties of the flow problem are identified. Self-similarity analysis conducted by normalizing using these scales reveal that contrary to classical self similar flows, the turbulent kinetic energy budget terms and velocity moments scale according to the buoyancy and not with the mean velocity. The shell thickness is observed to increase linearly with time. The buoyancy scale remains time-invariant and is set by the initial cloud-environment thermodynamics. The shell accelerates ballistically with a magnitude set by the saturation value of the buoyancy of the cloud-environment mixture. In this regime, the shell is buoyancy driven and independent of the in-cloud velocity. Relations are obtained for predicting the shell thickness and minimum velocities by linking the internal scales with external flow parameters. The values thus calculated are consistent with the thickness and velocities observed in typical shallow cumulus clouds. The entrainment coefficient is a function of the initial state of the cloud and the environment, and is shown to be of the same order of magnitude as fractional entrainment rates calculated for large scale models.
  • Journal article
    Adhikari S, Shay MA, Parashar TN, Pyakurel PS, Matthaeus WH, Godzieba D, Stawarz JE, Eastwood JP, Dahlin JTet al., 2020,

    Reconnection from a turbulence perspective

    , Physics of Plasmas, Vol: 27, Pages: 1-10, ISSN: 1070-664X

    The spectral properties associated with laminar, anti-parallel reconnection are examined using a 2.5D kinetic particle in cell simulation. Both the reconnection rate and the energy spectrum exhibit three distinct phases: an initiation phase where the reconnection rate grows, a quasi-steady phase, and a declining phase where both the reconnection rate and the energy spectrum decrease. During the steady phase, the energy spectrum exhibits approximately a double power law behavior, with a slope near −5/3 at wave numbers smaller than the inverse ion inertial length and a slope steeper than −8/3 for larger wave numbers up to the inverse electron inertial length. This behavior is consistent with a Kolmogorov energy cascade and implies that laminar reconnection may fundamentally be an energy cascade process. Consistent with this idea is the fact that the reconnection rate exhibits a rough correlation with the energy spectrum at wave numbers near the inverse ion inertial length. The 2D spectrum is strongly anisotropic with most energy associated with the wave vector direction normal to the current sheet. Reconnection acts to isotropize the energy spectrum, reducing the Shebalin angle from an initial value of 70° to about 48° (nearly isotropic) by the end of the simulation. The distribution of energy over length scales is further analyzed by dividing the domain into spatial subregions and employing structure functions.
  • Journal article
    Malaspina DM, Szalay JR, Pokorny P, Page B, Bale SD, Bonnell JW, de Wit TD, Goetz K, Goodrich K, Harvey PR, MacDowall RJ, Pulupa Met al., 2020,

    In Situ Observations of Interplanetary Dust Variability in the Inner Heliosphere

    , ASTROPHYSICAL JOURNAL, Vol: 892, ISSN: 0004-637X
  • Journal article
    Krasnoselskikh V, Larosa A, Agapitov O, de Wit TD, Moncuquet M, Mozer FS, Stevens M, Bale SD, Bonnell J, Froment C, Goetz K, Goodrich K, Harvey P, Kasper J, MacDowall R, Malaspina D, Pulupa M, Raouafi N, Revillet C, Velli M, Wygant Jet al., 2020,

    Localized Magnetic-field Structures and Their Boundaries in the Near-Sun Solar Wind from Parker Solar Probe Measurements

    , ASTROPHYSICAL JOURNAL, Vol: 893, ISSN: 0004-637X
  • Journal article
    Johnston CD, Cargill PJ, Hood AW, De Moortel I, Bradshaw SJ, Vaseekar ACet al., 2020,

    Modelling the solar transition region using an adaptive conduction method

    , Astronomy & Astrophysics, Vol: 635, Pages: A168-A168, ISSN: 0004-6361

    Modelling the solar Transition Region with the use of an Adaptive Conduction (TRAC) method permits fast and accurate numerical solutions of the field-aligned hydrodynamic equations, capturing the enthalpy exchange between the corona and transition region, when the corona undergoes impulsive heating. The TRAC method eliminates the need for highly resolved numerical grids in the transition region and the commensurate very short time steps that are required for numerical stability. When employed with coarse spatial resolutions, typically achieved in multi-dimensional magnetohydrodynamic codes, the errors at peak density are less than 5% and the computation time is three orders of magnitude faster than fully resolved field-aligned models. This paper presents further examples that demonstrate the versatility and robustness of the method over a range of heating events, including impulsive and quasi-steady footpoint heating. A detailed analytical assessment of the TRAC method is also presented, showing that the approach works through all phases of an impulsive heating event because (i) the total radiative losses and (ii) the total heating when integrated over the transition region are both preserved at all temperatures under the broadening modifications of the method. The results from the numerical simulations complement this conclusion.
  • Journal article
    Bruneau N, Wang S, Toumi R, 2020,

    Long memory impact of ocean mesoscale temperature anomalies on tropical cyclone size

    , Geophysical Research Letters, Vol: 47, ISSN: 0094-8276

    Mesoscale ocean temperature anomalies modify a tropical cyclone (TC). Through a modeling study we show that, while the maximum wind speed is rapidly restored after the TC passes a warm‐ or cold‐ (eddy size) sea surface temperature (SST) anomaly, the storm size changes are more significant and persistent. The radius of gale force winds and integrated kinetic energy (IKE) can change by more than 10% per degree and this endures several days after crossing an SST anomaly. These properties have a long memory of the impact from the ocean fluxes and depend on the integrated history of SST exposure. They are found to be directly proportional to the storm total precipitation. Accurate continuous forecast of the SST along the track may therefore be of central importance to improving predictions of size and IKE, while instantaneous local SST near the TC core is more important for the forecast of maximum wind speed.
  • Journal article
    Kaweeyanun N, Masters A, Jia X, 2020,

    Favorable conditions for magnetic reconnection at ganymede’s upstream magnetopause

    , Geophysical Research Letters, Vol: 47, Pages: 1-10, ISSN: 0094-8276

    Ganymede is the only Solar System moon known to generate a permanent magnetic field. Jovian plasma motions around Ganymede create an upstream magnetopause, where energy flows are thought to be driven by magnetic reconnection. Simulations indicate Ganymedean reconnection events may be transient, but the nature of magnetopause reconnection at Ganymede remains poorly understood, requiring an assessment of reconnection onset theory. We present an analytical model of steady‐state conditions at Ganymede's magnetopause, from which the first Ganymedean reconnection onset assessment is conducted. We find that reconnection may occur wherever Ganymede's closed magnetic field encounters Jupiter's ambient magnetic field, regardless of variations in magnetopause conditions. Unrestricted reconnection onset highlights possibilities for multiple X lines or widespread transient reconnection at Ganymede. The reconnection rate is controlled by the ambient Jovian field orientation and hence driven by Jupiter's rotation. Future progress on this topic is highly relevant for the JUpiter ICy moon Explorer mission.
  • Journal article
    Fargette N, Lavraud B, Øieroset M, Phan TD, ToledoRedondo S, Kieokaew R, Jacquey C, Fuselier SA, Trattner KJ, Petrinec S, Hasegawa H, Garnier P, Génot V, Lenouvel Q, Fadanelli S, Penou E, Sauvaud J, Avanov DLA, Burch J, Chandler MO, Coffey VN, Dorelli J, Eastwood JP, Farrugia CJ, Gershman DJ, Giles BL, Grigorenko E, Moore TE, Paterson WR, Pollock C, Saito Y, Schiff C, Smith SEet al., 2020,

    On the ubiquity of magnetic reconnection inside flux transfer event‐like structures at the earth's magnetopause

    , Geophysical Research Letters, Vol: 47, Pages: 1-9, ISSN: 0094-8276

    Flux transfer events (FTEs) are transient phenomena frequently observed at the Earth's magnetopause. Their usual interpretation is a flux rope moving away from the reconnection region. However, the Magnetospheric Multiscale Mission revealed that magnetic reconnection sometimes occurs inside these structures, questioning their flux rope configuration. Here we investigate 229 FTE‐type structures and find reconnection signatures inside 19% of them. We analyze their large‐scale magnetic topology using electron heat flux and find that it is significantly different across the FTE reconnecting current sheets, demonstrating that they are constituted of two magnetically disconnected structures. We also find that the interplanetary magnetic field (IMF) associated with reconnecting FTEs presents a strong By component. We discuss several formation mechanisms to explain these observations. In particular, the maximum magnetic shear model predicts that for large IMF By, two spatially distinct X lines coexist at the magnetopause. They can generate separate magnetic flux tubes that may become interlaced.
  • Conference paper
    Gryspeerdt E, Smith T, O'Keefe E, Christensen M, Goldsworth Fet al., 2020,

    Impact of ship emission controls recorded by cloud properties

    <jats:p> &amp;lt;p&amp;gt;The impact of aerosols on cloud properties is one of the largest uncertainties in the anthropogenic forcing of the climate system. As large, isolated sources of aerosol, ships provide the ideal opportunity to investigate aerosol-cloud interactions. However, their use for quantifying the aerosol impact on clouds has been limited by a lack on information on the aerosol perturbation generated by the ship.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;In this work, satellite cloud observations are combined with ship emissions estimated from transponder data. Using over 17,000 shiptracks during the implementation of emission controls, the central role of sulphate aerosol in controlling shiptrack properties is demonstrated. Meteorological factors are shown to have a significant impact on shiptrack formation, particularly cloud-top relative humidity. Accounting for this meteorological variation, this work also demonstrates the potential for satellite retrievals of ship sulphate emissions, providing a pathway to the use of cloud observations for monitoring air pollution.&amp;lt;/p&amp;gt; </jats:p>
  • Journal article
    Fujita R, Graven H, 2020,

    Impact of atmospheric radiocarbon and stable isotope measurements on understanding the global CH4 budget over 1850&amp;#8211;2015

    <jats:p> &amp;lt;p&amp;gt;Measurements of stable isotope ratios of atmospheric CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;&amp;amp;#160;(&amp;amp;#948;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C-CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;, &amp;amp;#948;D-CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;) have been utilized to evaluate contributions of individual CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;&amp;amp;#160;sources and sinks to global atmospheric CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;&amp;amp;#160;budget. However, given the uncertainty of both the source isotope signatures and kinetic isotope effects, recent estimates of the global atmospheric CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;&amp;amp;#160;budget using stable isotope observations are still inconclusive&amp;lt;span&amp;gt;.&amp;amp;#160;&amp;lt;/span&amp;gt;Radiocarbon measurements (&amp;amp;#916;&amp;lt;sup&amp;gt;14&amp;lt;/sup&amp;gt;C-CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;) could provide &amp;lt;span&amp;gt;stronger&amp;amp;#160;&amp;lt;/span&amp;gt;additional&amp;amp;#160;&amp;lt;span&amp;gt;constraint&amp;amp;#160;&amp;lt;/span&amp;gt;&amp;lt;span&amp;gt;on&amp;amp;#160;&amp;lt;/span&amp;gt;&amp;lt;span&amp;gt;the&amp;amp;#160;&amp;lt;/span&amp;gt;&amp;lt;span&amp;gt;fossil-fuel&amp;amp;#160;&amp;lt;/span&amp;gt;CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;&amp;amp;#160;&amp;lt;span&amp;gt;sources &amp;lt;/span&amp;gt;(i.e.,&amp;lt;sup&amp;gt;14&amp;lt;/sup&amp;gt;C-free)&amp;lt;span&amp;gt;, but the uncertainty of &amp;lt;sup&amp;gt;14&amp;lt;/sup&amp;gt;CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;g
  • Conference paper
    Sourdeval O, Gryspeerdt E, Mülmenstädt J, Krämer M, Quaas Jet al., 2020,

    Satellite-based estimate of the climate forcing due to aerosol - ice cloud interactions

    <jats:p> &amp;lt;p&amp;gt;Substantial efforts have been led over the last decades to improve our understanding of the interactions between clouds and anthropogenic aerosols (aci). The effective radiative forcing associated with these interactions (ERFaci), which combines the radiative forcing (i.e. Twomey effect) and cloud adjustments, still constitutes a large part of our current uncertainties on climate predictions.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Important progress has been made in the assessment of ERFaci for liquid clouds, partly due to advances in the joint use of satellite and modelling data to tackle this problem. More particularly, the retrieval of the droplet number concentration from satellite remote sensing - a property closely related to droplet nucleation processes - has been extremely helpful to better quantify ERFaci. However, similar estimations for ice clouds have for long suffered from a lack of observational constraint on the ice crystal number concentration (N&amp;lt;sub&amp;gt;i&amp;lt;/sub&amp;gt;), a challenging task due to the high complexity of the physical processes associated with the nucleation and growth of ice crystals. However, a novel long-term global dataset of N&amp;lt;sub&amp;gt;i&amp;lt;/sub&amp;gt; from active satellite measurements has recently (DARDAR-Nice) opened the door to new observation-based estimates of RFaci for ice clouds.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;This study investigates aerosol - ice clouds interactions using N&amp;lt;sub&amp;gt;i&amp;lt;/sub&amp;gt; profiles from the DARDAR-Nice product together with collocated aerosol information from the Copernicus Atmospheric Monitoring Service (CAMS) reanalyses. A multitude of cloud regimes, subdivided into seasonal and regional bins, are considered in order to disentangle meteorological effects from the aci signature. First results of joint-histograms between N&am
  • Journal article
    Nowack P, Runge J, Eyring V, Haigh Jet al., 2020,

    Causal networks for climate model evaluation and constrained projections

    , Nature Communications, Vol: 11, ISSN: 2041-1723

    Global climate models are central tools for understanding past and future climate change. The assessment of model skill, in turn, can benefit from modern data science approaches. Here we apply causal discovery algorithms to sea level pressure data from a large set of climate model simulations and, as a proxy for observations, meteorological reanalyses. We demonstrate how the resulting causal networks (fingerprints) offer an objective pathway for process-oriented model evaluation. Models with fingerprints closer to observations better reproduce important precipitation patterns over highly populated areas such as the Indian subcontinent, Africa, East Asia, Europe and North America. We further identify expected model interdependencies due to shared development backgrounds. Finally, our network metrics provide stronger relationships for constraining precipitation projections under climate change as compared to traditional evaluation metrics for storm tracks or precipitation itself. Such emergent relationships highlight the potential of causal networks to constrain longstanding uncertainties in climate change projections.
  • Journal article
    Hajra R, Henri P, Vallières X, Galand M, Rubin M, Tsurutani BT, Gilet N, Bucciantini L, Nemeth Zet al., 2020,

    Ionospheric total electron content of comet 67P/Churyumov-Gerasimenko

    , Astronomy & Astrophysics, Vol: 635, Pages: A51-A51, ISSN: 0004-6361

    We study the evolution of a cometary ionosphere, using approximately two years of plasma measurements by the Mutual Impedance Probe on board the Rosetta spacecraft monitoring comet 67P/Churyumov-Gerasimenko (67P) during August 2014–September 2016. The in situ plasma density measurements are utilized to estimate the altitude-integrated electron number density or cometary ionospheric total electron content (TEC) of 67P based on the assumption of radially expanding plasma. The TEC is shown to increase with decreasing heliocentric distance (rh) of the comet, reaching a peak value of ~(133 ± 84) × 109 cm−2 averaged around perihelion (rh < 1.5 au). At large heliocentric distances (rh > 2.5 au), the TEC decreases by ~2 orders of magnitude. For the same heliocentric distance, TEC values are found to be significantly larger during the post-perihelion periods compared to the pre-perihelion TEC values. This “ionospheric hysteresis effect” is more prominent in the southern hemisphere of the comet and at large heliocentric distances. A significant hemispheric asymmetry is observed during perihelion with approximately two times larger TEC values in the northern hemisphere compared to the southern hemisphere. The asymmetry is reversed and stronger during post-perihelion (rh > 1.5 au) periods with approximately three times larger TEC values in the southern hemisphere compared to the northern hemisphere. Hemispheric asymmetry was less prominent during the pre-perihelion intervals. The correlation of the cometary TEC with the incident solar ionizing fluxes is maximum around and slightly after perihelion (1.5 au < rh < 2 au), while it significantly decreases at larger heliocentric distances (rh > 2.5 au) where the photo-ionization contribution to the TEC variability decreases. The results are discussed based on cometary ionospheric production and loss processes.
  • Journal article
    Zappa G, Ceppi P, Shepherd TG, 2020,

    Time-evolving sea-surface warming patterns modulate the climate change response of subtropical precipitation over land

    , Proceedings of the National Academy of Sciences of the United States of America, Vol: 117, Pages: 4539-4545, ISSN: 0027-8424

    Greenhouse gas (GHG) emissions affect precipitation worldwide. The response is commonly described by two timescales linked to different processes: a rapid adjustment to radiative forcing, followed by a slower response to surface warming. However, additional timescales exist in the surface-warming response, tied to the time evolution of the sea-surface-temperature (SST) response. Here, we show that in climate model projections, the rapid adjustment and surface mean warming are insufficient to explain the time evolution of the hydro-climate response in three key Mediterranean-like areas—namely, California, Chile, and the Mediterranean. The time evolution of those responses critically depends on distinct shifts in the regional atmospheric circulation associated with the existence of distinct fast and slow SST warming patterns. As a result, Mediterranean and Chilean drying are in quasiequilibrium with GHG concentrations, meaning that the drying will not continue after GHG concentrations are stabilized, whereas California wetting will largely emerge only after GHG concentrations are stabilized. The rapid adjustment contributes to a reduction in precipitation, but has a limited impact on the balance between precipitation and evaporation. In these Mediterranean-like regions, future hydro-climate–related impacts will be substantially modulated by the time evolution of the pattern of SST warming that is realized in the real world.
  • Journal article
    Agapitov OV, de Wit TD, Mozer FS, Bonnell JW, Drake JF, Malaspina D, Krasnoselskikh V, Bale S, Whittlesey PL, Case AW, Chaston C, Froment C, Goetz K, Goodrich KA, Harvey PR, Kasper JC, Korreck KE, Larson DE, Livi R, MacDowall RJ, Pulupa M, Revillet C, Stevens M, Wygant JRet al., 2020,

    Sunward-propagating Whistler Waves Collocated with Localized Magnetic Field Holes in the Solar Wind: <i>Parker Solar Probe</i> Observations at 35.7 <i>R<sub>⊙</sub></i> Radii

    , ASTROPHYSICAL JOURNAL LETTERS, Vol: 891, ISSN: 2041-8205

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