Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H46: Multi-Scale Flows and Pathways in the Climate SystemFocus
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Sponsoring Units: GPC DFD Chair: Hussein Aluie, Rochester University Room: LACC 506 |
Tuesday, March 6, 2018 2:30PM - 3:06PM |
H46.00001: Multi-Scale Flows and Pathways in the Gulf of Mexico and South China Sea: implications of ocean submesoscale turbulence for oil dispersion, coral evolution and carbon uptake Invited Speaker: Annalisa Bracco In the ocean forcing acts at planetary scales and dissipation at microscales. In between there are the mesoscales, with characteristics akin to nearly two-dimensional, quasi-geostrophically, balanced turbulence. The dynamical structures typical of the mesoscales are eddies and fronts. They extend from few tens to hundred of kilometers, and act as weather systems of the ocean. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H46.00002: Nearshore Sticky Waters Juan Restrepo, Shankar Venkataramani, Clint Dawson Wind- and current-driven flotsam, oil spills, pollutants, and nutrients, approaching the nearshore will frequently |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H46.00003: Deep-inflow approach to mesoscale-organized and unorganized deep convection and the likely role of coherent structures Kathleen Schiro, J David Neelin, Fiaz Ahmed Representations of strongly precipitating deep-convective systems in climate models face two major challenges: 1) high sensitivity to approximations of turbulent entrainment of environmental air; 2) an unclear path to inclusion of mesoscale-organized systems. An alternative to traditional turbulent entrainment assumptions in deep convective parameterizations – Deep-Inflow Mixing – is presented, based on radar wind profiler observations of vertical mass flux from the Department of Energy GoAmazon2014/5 campaign. Updraft buoyancy computed with deep-inflow mixing and tropospheric thermodynamic properties yields predictive relationships to precipitation in both mesoscale-organized and unorganized convection. Results from reanalysis and satellite data show that this holds more generally: buoyancy from deep-inflow mixing yields a strong relation to precipitation across the tropics. Physical interpretation points to a strong role for coherent inflow typical of organized convection. This simultaneously provides a key step toward representing mesoscale-organized convection in climate models and removes a problematic dependence on traditional entrainment rates. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H46.00004: Understanding physical mechanisms associated with enhancement/reduction of extreme precipitation in a warming climate Jesse Norris, Gang Chen, J David Neelin As the climate warms, extreme precipitation may be enhanced or reduced, depending on changes to moisture (thermodynamical) and mass convergence (dynamical). Percentiles of precipitation are calculated using 6-hourly data from the Community Earth System Model (CESM) Large Ensemble (LENS) in both the present and a projected future (2071-2080) climate, along with the corresponding 3-D structures of water-vapor mixing ratio and mass convergence. This analysis allows an examination of how the 3-D moisture budget is projected to change for extreme events in a future climate. Precipitation extremes increase over most of the globe but decrease over some subtropical ocean regions. The sign of these changes to precipitation extremes is determined by the relative contributions of the dynamical and thermodynamical components of the moisture budget. The thermodynamical tendency is positive everywhere (i.e., it has a tendency to increase precipitation extremes) and relatively uniform with height, whereas the dynamical tendency changes between region, percentile of precipitation, and between different levels of the atmosphere. These results illustrate how thermodynamical and dynamical processes may offset or complement one another in a warming climate to alter precipitation extremes. |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H46.00005: Large Regional Shortwave Forcing by Anthropogenic Methane Informed by Jovian Observations William Collins, Daniel Feldman, Chaincy Kuo, Newton Nguyen The shortwave radiative forcing of methane is larger than the well-known longwave effects of many less important anthropogenic forcing agents routinely included in radiative forcing assessments. Recently it was recognized that the widely-used formulae to calculate CH4 radiative forcing systematically underestimated the global forcing by 25% since they did not include these effects. Here we show that shortwave forcing by methane can be accurately calculated despite considerable uncertainty and the existing large gaps in its shortwave spectroscopy. We demonstrate that the forcing is invariant, even when confronted with much more complete empirical methane absorption spectra extending to violet-light wavelengths derived from observations of methane-rich Jovian planetary atmospheres. We undertake the first global, spatially-resolved calculations of this forcing and find that localized annual-mean methane forcing can be as large as +0.25 W/m2, ten times the global annualized shortwave forcing and 41% of the total CH4 forcing. Our observationally-based estimates of shortwave forcing by anthropogenic methane are sufficiently large and accurate to warrant inclusion in analyses of historical climate, projections of future climate, and explorations of climate-change mitigation pathways. |
Tuesday, March 6, 2018 3:54PM - 4:30PM |
H46.00006: Multiscale processes and instabilities in Earth's clouds: Why we must and how we can make progress in modeling them Invited Speaker: Tapio Schneider How Earth's low clouds respond to climate change is the most important unsolved problem in the physical climate sciences. It is the source of the largest uncertainties in climate projections. The reason is the multiscale nature of clouds: scales from the micrometers or droplet formation, to the meters of turbulent cloud dynamics, to the thousands of kilometers of large-scale atmospheric circulations are intricately coupled in clouds. Explicitly resolving this large a range of scales in numerical simulations will remain out of reach for the foreseeable future. Here I show that the interplay of radiative and dynamical processes can give rise to instabilitites in stratocumulus clouds, which have the potential to dramatically alter climate. Such instabilities are not captured by current climate models because they inadequately represent the multiscale physics of clouds. I lay out a blueprint for climate models that can overcome these difficulties and provide more accurate projections of climate changes. |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H46.00007: A Rational Approach to Cumulus Parameterization David Raymond, Sharon Sessions Deep atmospheric convection is treated parametrically in global weather and climate models. The division of labor between explicit and parameterized calculations is traditionally based on the horizontal scale of grid cells. This becomes problematic when the grid size decreases to the scale of convection. An alternative division of labor computes the balanced part of the flow explicitly and parameterizes the unbalanced part of the flow. (``Balance'' refers to a dynamical condition in rotating fluids relating the winds to thermodynamic variables. The simplest case is geostrophic balance in which the horizontal pressure gradient in the fluid balances the Coriolis force.) The advantage of this approach is that balanced dynamics is robust, with far fewer degrees of freedom than convection, and represents the actual large-scale flow with reasonable fidelity. The challenge is then to understand how the balanced flow controls the unbalanced flow, i.e., convection and related circulations, and conversely, how the unbalanced flow affects the balanced component. We discuss recent progress in this program based on numerical modeling and observation of deep convection in tropical weather systems over the ocean. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H46.00008: Evaluating Lagrangian Model Simulations of the Madden-Julian Oscillation with Metrics for Balanced Dynamics Sharon Sessions, K Ryder Fox, Stipo Sentic, Patrick Haertel, David Raymond The Madden-Julian Oscillation (MJO) is a planetary-scale disturbance that occurs on intraseasonal time scales and influences weather and climate globally. Predicting the MJO is possible if it is a response to large-scale balanced flow. In the balanced dynamics framework, a large scale horizontal rotation modifies the thermodynamic environment which contributes to the development and maintenance of a secondary circulation associated with the convective system. Observational data suggest that the MJO exhibits characteristics consistent with balanced dynamics. Namely, the mid-tropospheric potential vorticity is strongly correlated with moist atmospheric instability and parameters related to the secondary circulation, including mid- and low-tropospheric convergence. These are also well correlated with precipitation. These correlations can be used to evaluate models which simulate the MJO. We apply this strategy to two simulations using a Lagrangian model. The first simulation has a robust MJO signature, while the second poorly reproduces the MJO. Comparing correlations of parameters relevant to the balanced dynamics can help identify where the correlations break down in the model, and thus identify physical mechanisms important for convective organization on the MJO scale. |
Tuesday, March 6, 2018 4:54PM - 5:06PM |
H46.00009: Reduced-Order Quasilinear Dynamics of Ocean Surface Boundary-Layer Flows John Marston, Joseph Skitka, Baylor Fox-Kemper In an effort to develop new physically-based sub-grid representations of unresolved processes in global climate models, the combined effectiveness of quasilinear approximations and model reduction at reproducing oceanic surface boundary-layer turbulence is studied. Two different averaging operations are tested (horizontal and ensemble). Dimensional reduction is achieved with a Galerkin projection of the quasilinear equations of motion onto an energetically optimized subset of modes as determined by proper orthogonal decomposition. Test problems of horizontally homogeneous surface-forced thermal convection and Langmuir turbulence are examined. A reduced quasilinear model that employs the horizontal mean is able to reproduce vertical profiles of certain energetically important turbulent transports and energies to within 30% error with less than 0.2% of modes retained. Statistical descriptions based upon second-order closures that correspond to the quasilinear approximations are investigated. |
Tuesday, March 6, 2018 5:06PM - 5:18PM |
H46.00010: Reduced-Order Models for the Large-Scale Atmospheric Turbulence Pedram Hassanzadeh, Ashesh Chattopadhyay Identifying the key spatio-temporal characteristics of the large-scale atmospheric turbulence plays an important role in understanding the multi-scale dynamics of the large-scale circulation. In this study, we aim at extracting these key characteristics using data-driven reduced-order modeling techniques. We apply methods that are based on the Fluctuation-Dissipation Theorem, Koopman spectral analysis, and Green’s functions to data from idealized global circulation models and reanalysis to develop a robust and accurate reduced-order modeling framework for the large-scale atmospheric turbulence. We discuss the connections of the extracted spatio-temporal characteristics to those obtained using the conventional empirical orthogonal function analysis. We further demonstrate how the calculated reduced-order models can be used to quantify eddy-mean flow interactions in the large-scale atmospheric circulation by computing the eddy-jet feedback in the extratropical low-frequency variability. |
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