Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F12: Natural Pattern Formation and Earth's Climate SystemFocus
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Sponsoring Units: GPC GSNP Chair: Mary Silber, University of Chicago Room: 271 |
Tuesday, March 14, 2017 11:15AM - 11:51AM |
F12.00001: The Volume of Earth's Lakes Invited Speaker: B. B. Cael How much water do lakes on Earth hold? Global lake volume estimates are scarce, highly variable, and poorly documented. We develop a mechanistic null model for estimating global lake mean depth and volume based on a statistical topographic approach to Earth's surface. The volume-area scaling prediction is accurate and consistent within and across lake datasets spanning diverse regions. We applied these relationships to a global lake area census to estimate global lake volume and depth. The volume of Earth's lakes is 199,000 km$^{\mathrm{3}}$ (95{\%} confidence interval 196,000-202,000 km$^{\mathrm{3}})$. This volume is in the range of historical estimates (166,000-280,000 km$^{\mathrm{3}})$, but the overall mean depth of 41.8 m (95{\%} CI 41.2-42.4 m) is significantly lower than previous estimates (62 - 151 m). These results highlight and constrain the relative scarcity of lake waters in the hydrosphere and have implications for the role of lakes in global biogeochemical cycles. We also evaluate the size (area) distribution of lakes on Earth compared to expectations from percolation theory. [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:27PM |
F12.00002: Arctic sea ice melt pond fractal dimension - explained Invited Speaker: Predrag Popovic As Arctic sea ice starts to melt in the summer, pools of melt water quickly form on its surface, significantly changing its albedo, and impacting its subsequent evolution. These melt ponds often form complex geometric shapes. One characteristic of their shape, the fractal dimension of the pond boundaries, D, when plotted as a function of pond size, has been shown to transition between the two fundamental limits of D $=$ 1 and D $=$ 2 at some critical pond size. Here, we provide an explanation for this behavior. First, using aerial photographs, we show how this fractal transition curve changes with time, and show that there is a qualitative difference in the pond shape as ice transitions from impermeable to permeable. Namely, while ice is impermeable, maximum fractal dimension is less than 2, whereas after it becomes permeable, maximum fractal dimension becomes very close to 2. We then show how the fractal dimension of a collection of overlapping circles placed randomly on a plane also transitions from D $=$ 1 to D $=$ 2 at a size equal to the average size of a single circle. We, therefore, conclude that this transition is a simple geometric consequence of regular shapes connecting. The one physical parameter that can be extracted from the fractal transition curve is the length scale at which transition occurs. We provide a possible explanation for this length scale by noting that the flexural wavelength of the ice poses a fundamental limit on the size of melt ponds on permeable ice. If this is true, melt ponds could be used as a proxy for ice thickness. [Preview Abstract] |
Tuesday, March 14, 2017 12:27PM - 12:39PM |
F12.00003: Spatial distirbution of Antarctic mass flux due to iceberg transport Darin Comeau, Elizabeth Hunke, Adrian Turner Under a changing climate that sees amplified warming in the polar regions, the stability of the West Antarctic ice sheet and its impact on sea level rise is of great importance. Icebergs are at the interface of the land-ice, ocean, and sea ice systems, and represent approximately half of the mass flux from the Antarctic ice sheet to the ocean. Calved icebergs transport freshwater away from the coast and exchange heat with the ocean, thereby affecting stratification and circulation, with subsequent indirect thermodynamic effects to the sea ice system. Icebergs also dynamically interact with surrounding sea ice pack, as well as serving as nutrient sources for biogeochemical activity. The spatial pattern of these fluxes transported from the continent to the ocean is generally poorly represented in current global climate models. We are implementing an iceberg model into the new Accelerated Climate Model for Energy (ACME) within the MPAS-Seaice model, which uses a variable resolution, unstructured grid framework. This capability will allow for full coupling with the land ice model to inform calving fluxes, and the ocean model for freshwater and heat exchange, giving a complete representation of the iceberg lifecycle and increasing the fidelity of ACME southern cryosphere simulations. [Preview Abstract] |
Tuesday, March 14, 2017 12:39PM - 12:51PM |
F12.00004: Statistical Mechanics and the Climatology of the Arctic Sea Ice Thickness Distribution John Wettlaufer, Srikanth Toppaladoddi We study the seasonal changes in the thickness distribution of Arctic sea ice, $g(h)$, under climate forcing. Our analytical and numerical approach is based on a Fokker-Planck equation for $g(h)$, in which the thermodynamic growth growth rates are determined using observed climatology. In particular, the Fokker-Planck equation is coupled to an observationally consistent thermodynamic model. We find that due to the combined effects of thermodynamics and mechanics, $g(h)$ spreads during winter and contracts during summer. This behavior is in agreement with recent satellite observations from CryoSat-2. Because $g(h)$ is a probability density function, we quantify all of the key moments (e.g., mean thickness, fraction of thin/thick ice, mean albedo, relaxation time scales) as greenhouse-gas radiative forcing, $\Delta F_0$, increases. The mean ice thickness decays exponentially with $\Delta F_0$, but {\em much slower} than do solely thermodynamic models. This exhibits the crucial role that ice mechanics plays in maintaining the ice cover, by redistributing thin ice to thick ice--far more rapidly than can thermal growth alone. [Preview Abstract] |
Tuesday, March 14, 2017 12:51PM - 1:03PM |
F12.00005: Patterned surfaces pattern convection Srikanth Toppaladoddi, John Wettlaufer Turbulent convection over rough surfaces is ubiquitous in many engineering, geophysical and astrophysical settings. However, the effects of a rough surface on the turbulent transport of mass, momentum and heat are not well understood. To this end, we use highly resolved numerical simulations to study turbulent Rayleigh-Benard convection in a cell with sinusoidally rough upper and lower surfaces in two dimensions for $Pr = 1$ and $Ra = \left[4 \times 10^6, 3 \times 10^9\right]$. By varying the wavelength $\lambda$ at a fixed amplitude, we find an optimal wavelength $\lambda_{\text{opt}}$ for which the Nusselt-Rayleigh scaling relation is $\left(Nu-1 \propto Ra^{0.483}\right)$ maximizing the heat flux. This is consistent with the upper bound of Goluskin and Doering (J. Fluid Mech. 804, 2016) who prove that $Nu$ can grow no faster than ${\cal O} (Ra^{1/2})$ as $Ra \rightarrow \infty$, and thus the concept that roughness facilitates the attainment of the so-called ultimate regime. When $\lambda \ll \lambda_{\text{opt}}$ and $\lambda \gg \lambda_{\text{opt}}$, the planar case is recovered. The enhancement in the heat transport is shown to be due the increased production of plumes at the rough walls, thus manipulating the interaction between the boundary layers and the core flow. [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:15PM |
F12.00006: Stochastic Wave Breaking Dynamics Juan Restrepo, Jorge Ramirez Wave breaking dynamics in the Lagrangian frame, using numerically-generated data as well as laboratory data, is described and analyzed. Models that combine deterministic macroscale dynamics and stochastic microscale process are proposed. The dependency of irreversible processes on the frequency and the amplitude of the waves is revealed. The long term goal of this research enterprise is to produce a model for dissipation of wave and current energy at large spatio-temporal scales, due to the microscale breaking and whitecapping of waves. [Preview Abstract] |
Tuesday, March 14, 2017 1:15PM - 1:27PM |
F12.00007: Are Geophysical Jets Protected Topologically? Brad Marston, Antoine Venaille Atmospheric and oceanic jets can be surprisingly robust to perturbations. Do dynamics alone account for this stability, or are there deeper principles at work? A clue may be provided by classical systems with topologically protected chiral modes. These optical, acoustic, and mechanical systems realize physics that was first discovered in the context of condensed matter such as the quantum Hall effect and topological insulators. They share the common feature that low-energy waves propagate in only one direction with no backscattering. We address the question of whether or not such topological protection of chiral modes can be found at geophysical or even astrophysical scales by studying idealized models of geophysical jets. [Preview Abstract] |
Tuesday, March 14, 2017 1:27PM - 1:39PM |
F12.00008: The Madden-Julian Oscillation of the Earth's Atmosphere David Raymond, Zeljka Fuchs The Madden-Julian oscillation (MJO) is the largest propagating weather disturbance on the earth, circling the Tropics to the east with a period of 30-60 days. It is intimately coupled to atmospheric convection, is the most widespread weather maker in the tropics, and has significant influence on mid-latitude weather. It is also known to trigger El Ni\~{n}o events. Because of the uncertainty in the treatment of convection in global models, its representation in such models is poor. Discovered only in 1971, there is still debate over the fundamental mechanism of the MJO. A pair of papers in 1987 proposed that the energy source for the MJO was surface heat fluxes from warm ocean waters. The existence of mean equatorial surface winds toward the west biases these fluxes in favor of eastward-moving disturbances. These models were abandoned because they did not reproduce the global scale of the MJO and for various other reasons. However, advances in our understanding of how convection interacts with tropical weather disturbances has led to linear instability models that predict the greatest MJO intensification rates to occur at global scales, suggesting that the abandonment of the biased surface flux hypothesis was premature. I shall outline our recent work on the subject. [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 1:51PM |
F12.00009: Topographic influences on vegetation patterns in semi-arid regions Punit Gandhi, Karna Gowda, Sarah Iams, Lucien Werner, Mary Silber Regular spatial patterns in vegetation growth appear at a community scale in semi-arid ecosystems across the globe. Such patterns have been attributed to various kinds of positive feedback between the individual plants and water availability. Incorporating topographic information into modeling efforts has the potential to improve our understanding of the role that water transport plays in the formation and dynamics of the vegetation patterns. [Preview Abstract] |
Tuesday, March 14, 2017 1:51PM - 2:03PM |
F12.00010: Identifying Meteorological Controls on Open and Closed Mesoscale Cellular Convection as Associated with Marine Cold Air Outbreaks Isabel McCoy, Robert Wood, Jennifer Fletcher Marine low clouds are key influencers of the climate and contribute significantly to uncertainty in model climate sensitivity due to their small scale and complex processes. Many low clouds occur in large-scale cellular patterns, known as open and closed mesoscale cellular convection (MCC), which have significantly different radiative and microphysical properties. Investigating MCC development and meteorological controls will improve our understanding of their impacts on the climate. We conducted an examination of time-varying meteorological conditions associated with satellite-determined open and closed MCC. The spatial and temporal patterns of MCC clouds were compared with key meteorological control variables calculated from ERA-Interim Reanalysis to highlight dependencies and major differences. This illustrated the influence of environmental stability and surface forcing as well as the role of marine cold air outbreaks (MCAO, the movement of cold air from polar-regions across warmer waters) in MCC cloud formation. Such outbreaks are important to open MCC development and may also influence the transition from open to closed MCC. Our results may lead to improvements in the parameterization of cloudiness and advance the simulation of marine low clouds. [Preview Abstract] |
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