Turbulence is of tremendous importance in a wide range of astrophysical and geophysical flows. Unfortunately, the equations of motion are notoriously difficult to solve. I will introduce an approach to low-dimensional modeling of turbulent flows that focuses on the the large, coherent flow structures which often occur, such as convection rolls in the atmosphere or ocean currents. These structures and their dynamics can be described with relatively few variables using a model consisting of stochastic ordinary differential equations. As a model system to test this approach, we use Rayleigh-Benard convection experiments, in which a container is filled with water and heated from below. Buoyancy drives a flow which organizes into a roll-shaped circulation. This convection roll exhibits a wide range of dynamics including erratic meandering, spontaneous flow reversals, and several oscillation modes, all of which are reminiscent of phenomena observed in astro/geophysical flows. A simple model of stochastic motion in a potential quantitatively reproduces all of these observed flow dynamics. The potential term is a direct function of boundary geometry (i.e. topography), and is found to accurately predict the different flow dynamics observed in experiments with different boundary geometries. This approach may lead to more general and relatively easy to solve models for turbulent flows with potential applications to climate, weather, and even the turbulent dynamo that generates Earth’s magnetic field.
Horizontal shear in the rotating, stratified ocean : Linear theory and nonlinear evolution
Submesoscale instabilities and mixing are poorly understood. We focus our work on barotropic shear with Rossby and Froude numbers of O (1). Instabilities and nonlinear cascades are possible in this regime even though stable stratification is significant. We have demonstrated previously (Arobone and Sarkar, JFM 2012) that the linear stability of the shear layer shows new aspects for strong stratification and moderate rotation rates. In this regime stratification acts to stabilize the inertial instability but greatly increase the range of vertical wavenumbers unstable to barotropic instability when Ro ∼ −1.
Nonlinear simulations are used to explore the shear layer with Ro(t). Coherent structure evolution varies greatly between cases with different moderate anticyclonic values of Ro0, but cyclonic rotation and strong anticyclonic rotation modify the flow in a more straightforward manner. Possible instability mechanisms, e.g. elliptic, zigzag, inertial, and barotropic instabilities are related to the simulation results. Enstrophy budgets from the simulations show a marked transition corresponding to sign reversal of centerline absolute vorticity, consistent with the linear modification of barotropic instability when Ro ∼ −1. New results analyzing saturation of inertial instability in the presence of strong stratification will be presented, noting strong differences from unstratified flows.
Why did the 2010 Eyjafjallajökull volcanic eruption cloud last so long?
The global economic consequences of the relatively small Eyjafjallajok̈ ull eruption in the spring of 2010 caught the world off guard. That the eruption cloud lasted for several months rather than weeks, efficiently disrupting air travel and the holiday plans of thousands of Northern Europeans, drew arguably more attention and a certainly garnered a highly emotional response. The unexpected longevity of this eruption cloud was touted to be the consequence of unusual ”perfect-storm-like” weather patterns that also conspired to produce the very dry conditions leading to the massive Russian fires later that summer. It was called ”an anomaly”. However, this anomaly nearly repeated itself the following year in the form of the 2011 Grimsvoẗ n eruption cloud. Indeed, in the geological record, possibly 45% of explosive eruptions produced long-lasting clouds similar to the 2010 Eyjafjallajok̈ ull event, which is clearly not so unusual.
A major reason that the behavior of the 2010 Eyjafjallajok̈ ull eruption cloud was surprising is that ”standard” models for how ash sedimentation works (i.e., heavy particles fall out of the cloud faster than light particles) are incomplete with significant consequences not just for assessing hazards to air traffic, but also for understanding, for example, the effect of volcanism on climate. Observations of the 2010 Eyjafjallajok̈ ull and 2011 Grimsvoẗ n umbrella clouds, as well as the structure of atmospheric aerosol clouds from the 1991 Mt Pinatubo event, suggest that an additional key process in addition to particle settling is the production of internal layering. I will use analog experiments on turbulent particle-laden umbrella clouds understood with simple models to show that this layering occurs where natural convection driven by particle sedimentation and the differential diffusion of primarily heat and fine particles give rise to a large scale instability leading to this layering. This “particle diffusive convection” strongly influences cloud longevity where volcanic umbrella clouds are enriched in fine ash. More generally, however, volcanic cloud residence times will depend on ash fluxes related to both individual particle settling and diffusive convection. I will discuss a new sedimentation model that includes both sedimentation processes captures real-time measurements of the rate of change of particle concentration in the 1982 El Chichon, 1991 Mt Pinatubo and 1992 Mt Spurr ash-clouds. Finally, although we have made progress to understanding how volcanic ash clouds ultimately work, there remain some fundamental problems that I will discuss, depending on time.
June 14, 2013: Giant Planet Formation and Internal Structure
The two main models for giant planet formation are known as core accretion, the standard model, and disk instability. There are substantial differences between these formation models, including formation timescale, favorable formation location, ideal disk properties for planetary formation, early evolution, planetary composition, etc. First, I will summarize current knowledge of the internal structures of solar- and extrasolar- giant planets, and the two formation models including their substantial differences, advantages, and disadvantages. I will then present the predicted planetary composition in each model, and discuss how theoretical models should be connected to available (and future) data.
June 21, 2013: Thermophysical Modeling and Measurements of Martian-Like Particulated Materials: Effect of Temperature and Cementing Phases
Remote temperature measurements have increased our understanding of the physical properties of the Martian surface layer. Typical grain sizes, rock abundances, subsurface layering, soil cementation, bedrock exposures, and ice presence/compositions have been derived and mapped using temperature data in conjunction with subsurface models of heat conduction, and have helped to constrain numerous global-scale processes. However, the simplicity of these models precludes more significant advances in the characterization of the physical nature of the Martian surface. For this seminar, I will present a new model of heat conduction for planetary soils derived from a combination of finite element modeling and laboratory measurements for homogeneous particulated media accounting for the grain size, porosity, gas pressure and composition, temperature, and the effect of any cementing phase. I will show that incorporating the temperature dependence of bulk conductivity alters the predicted diurnal and seasonal temperatures as compared to temperatures predicted with a temperature-independent conductivity model. Inconsistencies between observed temperatures and those predicted using temperature-independent conductivity models have been interpreted to result from subsurface heterogeneities, but they may partially be explained by a temperature-dependency of the thermal inertia, with additional implications on the derived grain sizes. Cements are shown to significantly increase the bulk conductivity of a particulated medium, and bond fractions <5% per volume are consistent with Martian thermal inertia observations previously hypothesized to correspond to a global duricrust. I will conclude with general thoughts on the predicted thermophysical properties of particulated materials on other planetary bodies with atmospheres.
Postdoc Hilke Schlichting simulates planet formation
Hubble postdoctoral fellow and soon-to-be Assistant Professor at Massachusetts Institute of Technology, Hilke Schlichting is no stranger to traveling long distances. On a daily basis, Schlichting ventures three-to-five billion miles from Earth to the Kuiper Belt, a primordial ring of icy bodies in the outer reaches of our solar system. “I’m interested in all aspects of planet formation,” said Schlichting. “Our solar system provides an opportunity to study it in a way that we cannot study elsewhere.”
Of course, Schlichting does all her space traveling from the comfort of her office at UCLA. Using data from the nearly 1500 Kuiper Belt Objects (KBOs) scientists have identified since 1992, Schlichting studies size distribution, especially for objects larger than 100 km across. Objects of this size, including the well-known dwarf planet, Pluto, are important because “planet formation never proceeded to completion for these bodies,” said Schlichting. “It’s an ideal laboratory for testing planet formation theories.”
No one knows exactly how planets form. “We know there is a gaseous disk that surrounds a star or stars in the beginning,” said Schlichting, “but we can’t observe the early stages of formation because the gas blocks our view.” To predict exactly what happens in the early stages of planet formation, Schlichting creates models that try to reproduce the distribution of objects observed in the Kuiper Belt. From her models she has learned that planet formation goes through a phase called “runaway growth,” a time when a relatively small fraction of the total mass coalesces very quickly into large objects. “The model matches the large KBOs, which are frozen in the runaway growth phase,” she said.
Schlichting has also conducted research to identify very small objects in the Kuiper Belt. “The objects are too small to reflect much sunlight back to Earth,” said Schlichting. Instead, she observes a large number of background stars in hopes that a small object will pass in front of one of the stars, thereby blocking out some of the starlight. Seeing these ephemeral objects is rare, and Schlichting has detected only two from an immense data set that was collected by Hubble Space Telescope over a period of more than 16 years.
In July 2013, Schlichting, her husband, and her Alaskan malamute, Amir, will leave sunny California to brave the shores of the Atlantic in Boston, Massachusetts. “Amir is such a California dog,” Schlichting said. “He won’t even go outside if it’s raining.” Regardless of 100-pound Amir’s reluctance to brave the elements, Schlichting looks forward to continuing to solve the mysteries of planetary formation at her new institute. “I’ve learned many things from UCLA’s Department of Earth & Space Sciences that I never would have learned in an astrophysics department. It has been a very stimulating place,” she said. “Moving to Boston will be quite a challenge, hopefully in a good way.”
Watch a video profile of Hilke here. Learn more about her research here.
June 4th, 2013: Giant impacts, magma oceans and the origin of the early atmosphere
Earth’s violent accretion likely generated multiple magma oceans. In particular, the Moon-forming giant impact is often thought to have produced a whole mantle magma ocean, which would have homogenized any pre-existing chemical heterogeneity within the mantle. The ratio of primordial 3He to primordial 22Ne in the mantle preserves a record of magma oceans on the early Earth. Importantly, the 3He/22Ne ratio of the Earth’s shallow depleted mantle is significantly higher than the deep mantle. To explain this observation, I propose that at least two giant impact-induced atmospheric blow-off and magma ocean degassing episodes are required and that the last giant impact did not generate a whole mantle magma ocean. New Xe isotopic data indicate that the catastrophic mantle outgassing and atmospheric blow-off events inferred from3He/22Ne ratios were accomplished between ~30 to 55 Myrs after the start of the Solar System. Therefore, outgassing associated with giant impacts, including the Moon-forming impact, must have occurred within this time window. Previous calculations of impact-induced atmospheric erosion have, however, found that it is difficult to completely remove the atmosphere from a body as large as Earth by a giant impact. The need for atmospheric loss inferred from the noble gas data could be reconciled with the dynamics of giant impacts by considering the new high-spin Moon formation hypothesis. I will further show that the current inventory of primordial noble gases in the atmosphere must largely be derived from late accreting planetesimals, a conclusion that has implications for the composition of the early atmosphere.
June 5th, 2013: Protoplanetary Disks and Exoplanetary Atmospheres: Hubble and Beyond
The composition and spatial distribution of molecular gas in the inner few AU of young (< 10 Myr) cirmcumstellar disks are important components to our understanding of the formation and evolution of planetary systems. Following the growth of planetary systems, the energetic radiation environment plays a critical role in atmospheric heating and chemistry in these worlds. In the first part of the talk, I will present recent results on protoplanetary disks and the radiation environment in exoplanetary systems, using data from the new and refurbished spectrographs aboard the Hubble Space Telescope. We have completed the first ultraviolet spectroscopic survey of the inner molecular disks around these stars, characterizing the spatial distribution of H2 and CO at planet-forming radii (a < 10 AU), the inner molecular regions of transitional disks, and the Lyman-alpha radiation environment. I will also describe first results from a spectrocopic survey of M-dwarf exoplanet host stars. This work highlights the ubiquity of chromospheric time-variability and the importance of strong Lyman-alpha radiation to planets in the habitable zones of these systems. In the second part of the talk, I will describe current and future UV/visible space instrumentation being developed by the ultraviolet astrophysics group at the University of Colorado. We are actively involved in the development and flight-testing of next-generation optical coatings, detector systems, and grating in support of future NASA Explorer-class and flagship missions. Many of the graduate students are involved in all phases of a space-flight mission: from instrument development and mission planning to launch and data acquisition to analysis and publication of the results.
June 7th, 2013: Groundwater sapping, watefall erosion, and formation of bedrock canyons on Earth and Mars
Networks of valleys with steep, amphitheater-shaped headwalls are prominent features on the surfaces of Earth and Mars. These landforms are commonly used as diagnostic indicators of undermining and headwall retreat by groundwater-seepage erosion. In this presentation, I question the link between seepage erosion and canyon form, and present an alternate hypothesis for canyon formation: waterfall retreat during large-scale flooding. To support this hypothesis I will discuss three interrelated studies and some ongoing work. First I investigated several canyons in Idaho, which have long been thought to be formed by groundwater sapping because they contain some of the largest springs in the United States. 4He and 14C dates, plunge pools, and sediment transport modeling indicate, however, that these canyons most likely formed during a catastrophic flood about 48,000 years ago. Second, Canyon Lake Gorge, Texas, was examined as a rare modern example of bedrock canyon formation during a single flood event. Results show that the combination of well jointed rock and high flow discharges caused block plucking, waterfall formation, and a rate of erosion that was limited only by the ability of the flow to transport sediment. Finally, theoretical modeling and flume experiments are used to show that steep headwalls can persist during canyon formation due to waterfall-induced toppling in fractured rock.
June 6th, 2013: Consequences of Dipolarization Front Braking in the Tail – Dipole Transition Regions
It is well established that high-speed flows in the magnetotail plasma sheet, separated from the ambient plasma by dipolarization fronts, are braked in the tail-dipole transition region of the near-Earth magnetotail. Kinetic and electromagnetic energy of the flow burst and dipolarization front is therefore converted to thermal energy of plasma and radiated by electromagnetic and plasma waves. Details of the energy conversion as yet remain unclear, largely due to the lack of multi-point observations in the transition region.
Taking advantage of THEMIS probes and geosynchronous (GEO) satellite conjunctions repeated in two events, we can study physical connections of the dipolarization front braking between X=-11 and -9 RE and magnetic and plasma oscillations observed at X=-8 RE and at GEO. It has been found that, despite different background plasma conditions in both events, slow-mode oscillations were excited in the dipole-dominated magnetotail region in response to the front braking in the transition region. No signatures of front rebounding were found. The slow-mode wave, observed at X=-8 RE, was not directly driven by dipolarized flux tube oscillations. The data analysis has shown that the slow-mode oscillations observed were triggered by the plasma pressure enhancements ahead of the front.