Session 1: The Solar system A.Adriani
Jupiter's polar regions are not visible from Earth due to Jupiter's low axial tilt and were not seen from any previous space missions prior to Juno [1,2]. The great advantage of Juno relative to the all previous missions to our solar system's largest planet is its mission strategy, namely a polar orbiting spacecraft. Thus, Juno is the first mission ever able to reveal the dynamical structures of Jupiter's polar regions, at latitudes above 80°.
Juno discovered the peculiarities of complex structures of the polar atmospheric circulation and the differences existing between the two poles. Eight circumpolar cyclones have been observed about a single polar cyclone in the North, while in the South, a larger polar cyclone is encircled by another five larger cyclones. The cyclonic structures, organized in quasi-regular polygonal shapes, have been observed with the highest spatial resolution occasionally down to 10 km at the top level of the clouds tops.
Jupiter has recently revealed a new face, with bands/jets at equatorial and mid latitudes and intense vortices at the pole (Adriani et al. 2017). To provide a simplified metaphor for such behavior and explore the interplay of convection and rotation, we considered three-dimensional high-resolution numerical simulations of Rayleigh-Bénard convection, bounded by free-slip horizontal plates and rapidly rotating about an axis tilted with respect to the gravity vector. By varying the angle between the rotation axis and the gravity vector we mimic the different latitudes. We found a sensitive response of the velocity, vorticity and temperature fields to the tilt of the rotation axis, identifying the existence of three different convective regimes depending on the tilt. A weaker heat transport is found at low latitudes along with the formation of organized jet-like structures (intermittent large-scale winds, von Hardenberg et al 2015). At high latitudes, convection is organized in an ensemble of intense vortices. A physical interpretation of the effects of rapid, tilted rotation on convective processes is discussed.
A. Adriani et al. 2018. Clusters of cyclones encircling Jupiter's poles. Nature 555, 216.
J. von Hardenberg, D. Goluskin, A. Provenzale, E.A Spiegel. 2015. Generation of Large-Scale Winds in Horizontally Anisotropic Convection. Phys. Rev. Letters 115, 134501.
VIMS measurements of non-LTE emissions of CH4, HCN and C2H2 represent a dataset with unique coverage to study Titan's upper atmosphere in the altitude region that extends from 500 to 1000 km. This region is the key to a better understanding of the middle atmosphere circulation, and the assessment of the latitudinal and seasonal variations in the distributions of HCN and C2H2 could give important hints, complementary to CIRS observations below 500 km. The inversion of CH4 and HCN at the highest altitudes could also help in the understanding of the complex environment observed by INMS above 1000 km.
An analysis has been carried on a subset of VIMS measurements constituted by 264 limb scanning sequences measured from 2004 to 2012. Non-LTE vibrational temperatures have been calculated for the three molecules in Titan's atmosphere. These temperatures have been used in the modelling of the non-LTE emission for the inversion of the CH4 , HCN and C2H2 VMRs with the GBB code.
The results of the inversion have been averaged in latitudinal bins and in two seasons: winter (2004-2009) and early spring (2010-2012). We will report the results of these analyses.
Session 2: Ice 1
Observational evidence indicates that global glaciation events, termed as “Snowball Earth" events, occurred during 720-635 million years ago. During these events, sea ice of approximately 1000 m thick has probably covered the whole ocean including the equatorial region. The high albedo of sea ice sent the Earth to an extremely cold state, which required a very high atmospheric CO2 level to start the deglaciation, probably with the help of surface dust accumulation in the equatorial region. However, A quantitative estimate of the deglaciation rates is still lacking. Here we evaluate the sea-ice melting rate once the equatorial sea ice has been disintegrated, using a fully coupled atmosphere-ocean general circulation model, CCSM3. We simulate the thick sea ice as flat land with its surface set as glaciers. The open ocean in the equatorial region is assumed to expand gradually and symmetrically relative to the equator as the deglaciation proceeds. A series of simulations are therefore done with the low-latitude ocean having different width, mimicking the state at different stages of the deglaciation process. The melting rate of sea ice is then estimated from these different climate states. The results show that the sea ice could be melted entirely in several hundred years, depending on the parameterization of melt pond on the sea ice surface. The melting rate is low at the beginning, but increases nearly exponentially when the edge of ice reaches the mid-latitude. The results may also help understand the process and timescale of the transformation of an icy planet or satellite into a warm state, under increased stellar radiation or greenhouse forcing.
Extended spherical shells of ice, supported by an underlying ocean are common to multiple planetary objects including Earth. Such shells covered Earth partially or completely during snowball epochs, and challenged the sustainability of life. Similar shells on icy moons are scarred with terrain whose origin is still under debate. Evidence of water-vapour plums over icy moons imply the possible formation of cracks and rifts in their ice cover, that reach the subglacial ocean. Similar rifts form in present terrestrial ice sheets and are believed to be an important stage in calving.
Here we combine theoretical modeling together with laboratory-scale experiments of ice sheets, to investigate the dynamical stability of icy shells that propagate over oceans. Theoretically, we model the deformation of ice as a thin film of power-law fluid under free- slip conditions at its ocean base and surface. The fluid is discharged at constant flux and axisymmetrically with respect to the pole and propagates towards the equator. Our model demonstrates that the propagating front in such a situation may become unstable due to its failure to sustain large extensional forces, resulting in the formation of rifts when the fluid is sufficiently strain-rate softening. It also suggests that open rifts can get closed at times that we can predict. We confirmed this instability in the limit of infinite planetary curvature through laboratory-scale experiments using shear-thinning fluids representing ice that were driven by buoyancy into a bath of an inviscid fluid that represented an ocean.
Climate change, amplified in the far north, has led to rapid sea ice decline in recent years. In the summer, melt ponds form on the surface of Arctic sea ice, significantly lowering the ice reflectivity (albedo) and thereby accelerating ice melt. Pond geometry controls the details of this crucial feedback; however, a reliable model of pond geometry does not currently exist. Here we show that a simple model of voids surrounding randomly sized and placed overlapping circles reproduces the essential features of pond patterns. The only two model parameters, the characteristic circle radius and coverage fraction, are chosen by comparing, between the model and the aerial photographs of the ponds, two correlation functions which determine the typical pond size and their connectedness. Using these parameters, the void model robustly reproduces the ponds' area-perimeter and area-abundance relationships over more than 6 orders of magnitude. By analyzing the correlation functions of ponds on several dates, we also find that the pond scale and the connectedness are surprisingly constant across different years and ice types. Moreover, we find that ponds resemble percolation clusters near the percolation threshold. These results demonstrate that the geometry and abundance of Arctic melt ponds can be simply described, which can be exploited in future models of Arctic melt ponds that would improve predictions of the response of sea ice to Arctic warming.
Session 3: Ice 2Tilte2
1Istituto per le Applicazioni del Calcolo “M. Picone", Rome (It);
2Université des Sciences et technologies de Lille 1 UL1 · Laboratoire de Mécanique de Lille (Fr);
3Department of Mathematics and Computer Science, Eindhoven University of Technology, Eindhoven (Nl)
The drastic decrease of the sea ice cover is one of the most critical effects of global warming and climate change, and it is responsible for their amplification, due to the consequently diminished albedo [1]. An important role in such feedback mechanism is played by the presence, on the sea ice surface, of ponds of melt water [2, 3, 4]. Nevertheless, processes involving sea ice melt ponds are still, somehow, underrepresented in Global Climate Models and often poorly understood. We will present a new continuum model of sea ice, which includes an explicit description of melt pond evolution [5]. Unlike models à la Thorndike [6], where an evolution equation for the Ice Thickness Distribution (ITD), g(h; x, t), is integrated, reflecting the obvious lack of detailed information below the scale of the mesh size of climatological interest, we study the coupled dynamics of the sea ice topography, h(x,t), and of the melt water depth field, w(x, t). This approach can be seen, then, as a kind of subgrid modelling for the ITD.
The sea ice thickness evolves under the action of a thermodynamic driving, re presenting the melting rate for a given radiation power [7], and a source-sink term, as in Thorndikeʼs equation, modelling possible transitions among thickness categories due to mechanical processes (ridging, rafting, etc). The melting rate includes, in an effective way, dynamical processes occurring within ponds, such as turbulent thermal convection [8], which is known to enhance the melting. The model we propose is similar in principle to the one put forward in [9], but it takes into account explicitly the horizontal transport of melt water down slopes of sea ice topography h; moreover it features and a non-uniform seepage rate of melt water through the porous ice medium and a minimal coupling with the atmosphere via a surface wind shear term, τs.
After providing a derivation of the model, based on phenomenological arguments, we will present results obtained from its numerical integration. We simulate the summertime evolution of sea ice, i.e. only melting is considered and water refreezing is disregarded. The impact of different conditions on the final ITD is considered, namely: i) the initial sea ice topography; ii) the presence/absence of melt ponds, through the dependence of the melting rate on w; iii) the coupling with the atmosphere (through the wind term). In particular, we found that, when the coupling with melt ponds is present, the ITD is peaked at a lower value of h and its form cannot be fitted with the analytical form predicted in [10]. We will also show that the wind affects the morphology of the melt pond configurations, quantified in terms of the fractal dimension of the set of points where w(x, t) is non-zero [11].
[1] D.K. Perovich and J.A. Richter-Menge, Annu. Rev. Mar. Sci. 1, 417-441 (2009).
[2] H. Eicken, T.C. Grenfell, D.K. Perovich, J.A. Richter-Menge and K. Frey, J. Geophys. Res. 109, C08007 (2004).
[3] D.K. Perovich, J. Geophys. Res. 110, C03002 (2005).
[4] D. Flocco, D. Schroeder, D.L. Feltham and E.C. Hunke, J. Geophys. Res. 117, C09032 (2012).
[5] D. Flocco and D.L. Feltham, J. Geophys. Res. 112, C08016 (2007).
[6] A.S. Thorndike et al, J. Geophys. Res. 80(33), 4501-4513 (1975).
[7] I. Eisenman and J.S. Wettlaufer, Proc. Natl. Acad. Sci. USA 106, 28-32 (2009).
[8] B. Rabbanipour Esfahani, S.C. Hirata, S. Berti and E. Calzavarini, arXiv:1801.03694 (2018).
[9] M. Lüthje, D.L. Feltham, P.D. Taylor and M.G. Worster, J. Geophys. Res. 111, C02001 (2006).
[10] S. Toppaladoddi and J.S. Wettlaufer, Phys. Rev. Lett. 115, 148501 (2015).
[11] C. Hohenegger, B. Alali, K.R. Steffen, D.K. Perovich and K.M. Golden, The Cryosphere 6, 1157-1162 (2012).
Ongoing and future space missions aim to identify potentially habitable planets in our Solar System and beyond. Planetary habitability is determined not only by a planet's current stellar insolation and atmospheric properties, but also by the evolutionary history of its climate. It has been suggested that icy planets and moons become habitable after their initial ice shield melts as their host stars brighten. Here we show from global climate model simulations that a habitable state is not achieved in the climatic evolution of those icy planets and moons that possess an inactive carbonate–silicate cycle and low concentrations of greenhouse gases. Examples for such planetary bodies are the icy moons Europa and Enceladus, and certain icy exoplanets orbiting G and F stars. We find that the stellar fluxes that are required to overcome a planet's initial snowball state are so large that they lead to significant water loss and preclude a habitable planet. Specifically, they exceed the moist greenhouse limit, at which water vapour accumulates at high altitudes where it can readily escape, or the runaway greenhouse limit, at which the strength of the greenhouse increases until the oceans boil away. We suggest that some icy planetary bodies may transition directly to a moist or runaway greenhouse without passing through a habitable Earth-like state.
Europa is one of the most probable places in the solar system to find extra-terrestrial life motivating the study of its deep (~100 km) ocean and thick icy shell. The chaotic terrain patterns on Europa's surface have been associated with vertical convective motions within the ice. Horizontal gradients of ice thickness are expected due to the large equator-to-pole gradient of surface temperature and can drive a global horizontal ice flow, yet such a flow and its observable implications have not been studied. We present a global ice flow model for Europa composed of warm, soft ice flowing beneath a cold brittle rigid ice crust. The model is coupled to an underlying (diffusive) ocean and includes the effect of tidal heating and convection within the ice. We show that Europa's ice can flow meridionally due to pressure gradients associated with equator-to-pole ice thickness differences, which can be up to a few km and can be reduced both by ice flow and due to ocean heat transport. The ice thickness and meridional flow direction depend on whether the ice convects or not; multiple (convecting and non-convecting) equilibria are found. Measurements of the ice thickness and surface temperature from future Europa missions18,19 can be used with our model to deduce whether Europa's icy shell convects and to constrain the effectiveness of ocean heat transport.
Session 4: Exoplanets 1
The ice-albedo feedback on rapidly-rotating terrestrial planets in the habitable zone can lead to abrupt transitions (bifurcations) between a warm and a snowball (ice-covered) state, bistability between these states, and hysteresis in planetary climate. This is important for planetary habitability because snowball events may trigger rises in the complexity of life, but could also endanger complex life that already exists. Recent work has shown that planets tidally locked in synchronous rotation states will transition smoothly into the snowball state rather than experiencing bifurcations. Here we investigate the structure of snowball bifurcations on planets that are tidally influenced, but not synchronously rotating, so that they experience long solar days. We use PlaSIM, an intermediate-complexity global climate model, run with a thermodynamic mixed layer ocean. We find that the amount of hysteresis (range in stellar flux for which there is bistability in climate) is significantly reduced for solar days with lengths of tens of Earth days, and disappears for solar days of hundreds of Earth days. These results indicate that tidally influenced planets orbiting M and K-stars that are not synchronously rotating will have much less hysteresis associated with the snowball bifurcations than they would if they were rapidly rotating. This implies that the amount of time it takes them to escape a snowball state via CO2 outgassing will be greatly reduced, as will the period of cycling between the warm and snowball state if they have a low CO2 outgassing rate.
Previous studies have shown that atmospheric dynamics plays an important role in the climate, habitable zone and characterization for synchronously rotating exoplanets around low-mass stars. Whether ocean dynamics could play a similar role is unknown and represents a major challenge to our understanding of the planets. Here we show that day-to-night ocean heat transport decreases with increasing stellar flux, and near the inner edge of the habitable zone, atmospheric dynamics dominates the energy transport. This results from the interactions between stellar energy, clouds and water vapor and between surface temperature gradients, surface winds and ocean currents. We further show that ocean dynamics moves the hottest spot to the east of the substellar point for planets in the habitable zone’s middle range, which could be observed by future space telescope(s), but it has no significant effect on the thermal phase curve of planets near the inner edge. These results indicate that ocean dynamics does not shift the location of the inner edge, future studies on the inner edge can focus on the atmosphere only, but fully coupled ocean-atmosphere is necessary in the middle range of the habitable zone.
Understanding how Snowball episodes function on Earth-like exoplanets is critical for planetary habitability. Previous work has shown that glaciated planets in the habitable zone with low outgassing rates could be perpetually cycling through warm and Snowball states. On Earth, the Neoproterozoic Snowball events were followed by increases in the complexity of life and rises in atmospheric oxygen.
Gaps in thick, semi-global ice coverage (sea glaciers) could be maintained at the equator by ocean-ice-atmosphere dynamics. We investigate this idea by modifying a global ocean- thick-marine-ice model developed for modeling Neoproterozoic Snowball Events to account for gaps in thick ice and interactions with atmospheric dynamics. Asynchronous coupling of the marine-ice model and a 2D seasonal energy balance model will be used to simulate the climate of tidally-locked, glaciated exoplanets around M-stars. Our hypothesis is that in the parameter regime that allows for sea glacier flow, ice flow will make gaps in the thick ice, and therefore an open ocean solution, less likely. This would suggest that oases in thick ice are a more viable survival mechanism for photosynthetic life during a Snowball event.
Session 5: Exoplanets 2
Earth-sized exoplanets are extremely common in the galaxy and many of them are likely tidally locked, such that they have permanent day- and nightsides. Astronomers have started to probe the atmospheres of such planets, which raises the question: can tidally locked planets support habitable climates and life?
Several studies have explored this question using global circulation models (GCMs). Not only did these studies find that tidally locked Earth analogs can indeed sustain habitable climates, their large day-night contrast should also create a distinct cloud structure that could help astronomers identify such planets. These studies, however, relied on GCMs which do not explicitly resolve convection, raising the question of how robust their results are.
Here we consider the dynamics of clouds and convection on a tidally locked planet using the System for Atmospheric Modeling (SAM) cloud-resolving model. We simulate a 3d 'channel', representing an equatorial strip that covers both day- and nightside of a tidally locked planet. We use interactive radiation and an interactive slab ocean surface and investigate the response to changes in the stellar constant. We find mean climates that are broadly comparable to those produced by a GCM. However, when the slab ocean is shallow, we also find internal variability that is far bigger than in a GCM or similar variability under Earth-like conditions. Convection in a tidally locked domain self-organizes in a dramatic fashion, with large outbursts of convection followed by periods of relative calm. We show that one of the timescales for this behavior is set by the time it takes for a dry gravity wave to travel between day- and nightside. The quasi-periodic self-organization of clouds can vary the planetary albedo by up to ~50%. Changes this large are potentially detectable with future space telescopes, which raises the prospect of using convectively driven variability to identify high priority targets in the search for life around other stars.
Traditional habitable zone theory assumes that the silicate-weathering feedback regulates the atmospheric CO2 of planets within the habitable zone to maintain surface temperatures that allow for liquid water. There is non-definitive evidence that it has been working on Earth, but it is uncertain whether it operates on other planets within the habitable zone. The concept of the habitable zone relies on it to maintain habitable conditions, making it crucial to determine its functioning on other planets. We can test the silicate-weathering feedback directly by using a statistical approach with future instruments such as JWST, LUVOIR, or HabEx. Those will be used to make low-precision CO2 measurements of planets at different radii within the habitable zone.
To do this we combine two approaches. First, a radiative-transfer model is used to compute the amount of CO2 necessary to maintain surface liquid water on planets for different values of insolation. We run a large ensemble of Earth-like planets with different masses, atmospheric masses, inert atmospheric composition, cloud composition and level, and other greenhouse gases. Second, we determine on how many planets measurements would need to be made to effectively marginalize over factors like surface temperature and pressure, and other uncertainties like clouds and greenhouse gases. With a sufficient number of planets, we will be able to detect a trend in the amount of atmospheric CO2 as a function of insolation and a robust detection of functioning silicate-weathering feedback. We determine the instrumental limits under which this is possible for future instruments such as JWST, LUVOIR, HabEx. When data from those missions becomes available, we will compare low-precision measurements of CO2 to statistical estimates made using the radiative transfer model. The results of this work may influence the usage of JWST and will enhance mission planning of LUVOIR and HabEx.
Recent observations have provided evidences of subsurface oceans covered by icy crusts on several objects in the Solar System, called ocean worlds, as the icy moons of Jupiter (Europa, Ganymede and Callisto) and Saturn (Titan and Enceladus), and dwarf planets (Ceres and Pluto). Titan and Ceres represent two end-members of this class of objects both due to the diversity in their geological activity and internal structure. The static gravity field data are available up for Titan to degree 3 measured during Cassini mission and for Ceres up to degree 18 measured during the Dawn mission. In addition, we have the estimation of the tidal Love number of Titan. We used gravity field measurements to infer the interior structure both of Titan and Ceres.
1 Institute of Atmospheric Sciences and Climate, Consiglio Nazionale delle Ricerche (ISAC-CNR); 2 University of Turin, Physics Department; 3 INAF – Trieste Astronomical Observatory; 4 Institute of Geosciences and Earth Resources, Consiglio Nazionale delle Ricerche (IGG-CNR)
The Planet Simulator (PlaSIM) is an Earth system Model of Intermediate Complexity (EMIC) which combines good skill in reproducing the observed earth's climate, also at low resolutions, with flexibility and speed of execution provided by a compact code and simple parameterizations. We present recent work devoted to tuning the model in a coupled configuration, both with a mixed-layer ocean (ML) and with a full 3D ocean (LSG), for achieving a realistic reproduction of present-day climate. In particular, the energy balance of the model is tuned and appropriate choices for meridional diffusivity in the ML case and for vertical oceanic diffusivity in LSG are determined. Thanks to its flexibility in the configurations (aquaplanets or different land-sea masks can be easily implemented, together with changes in solar and orbital parameters) PlaSIM is particularly suited for paleoclimatic and exoplanetary studies. In particular, we have recently adapted the model to allow in a flexible manner the exploration of tidally locked configurations and first results are presented.
Session 6: Earth
According to the snowball Earth hypothesis, a large quantity of CO2 must build up during an event in order to cause eventual deglaciation. One prediction of this model is a depletion in atmospheric oxygen-17 as a result of stratospheric chemical reactions, which has been observed in preserved barite minerals. This represents one of the most dramatic and compelling pieces of evidence in support of the snowball Earth hypothesis. The inference of high CO2 from anomalous oxygen-17 measurements of barite minerals, however, was made by assuming that the stratosphere-troposphere mass exchange rate and mixing within the stratosphere were the same as the present. In this contribution we test these assumptions with simulations of modern and hard snowball atmospheric conditions using the global climate model ECHAM5. We report that the mass exchange rate is comparable in these two cases, meaning the paleoCO2 estimates from barite studies don't need to be altered. Further work is necessary to see how robust this result is, and then to quantify the mixing efficiency within the stratosphere.
Major Sudden Stratospheric Warming events (SSWs) occur in the Arctic stratosphere during winter at a frequency of about six events per decade. An SSW features a distorted or completely reversed stratospheric polar vortex, as well as tens of degrees of warming within several days. SSW events affect the Arctic Oscillation (AO) / Northern Annular Mode (NAM), and are related to extreme weather events. We consider the effects of the Madden-Julian Oscillation (MJO) on the SSW frequency, and in particular the response of SSWs to a strengthening of the MJO which is expected in a warmer climate. We show that Rossby wave trains excited by a stronger MJO can nearly double the frequency of SSWs. Two specific teleconnection mechanisms were identified: a direct propagation of MJO-forced transient waves to the Arctic stratosphere, and a nonlinear enhancement of stationary waves emanating from the mid-latitudes by the MJO-forced transient waves. The stationary wave enhancement is shown to be a result of a nonlinear interaction between the mid-latitude jet and the MJO-forced waves that strengthen the zonal asymmetry of the mid-latitude jet. Decrease in hysteresis of planetary climate for planets with long solar days
