Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A34: Geophysical Fluid Dynamics: Atmospheric I |
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Chair: Alfredo Wetzel, University of Wisconsin, Madison Room: Georgia World Congress Center B406 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A34.00001: Modeling of Atmospheric Convection with Phase Changes Leslie Smith, Samuel N. Stechmann, Alfredo Wetzel Turbulent convection in the atmosphere is characterized by phase changes of water and latent heat release, and may organize itself on multiple length and time scales. Under the simplifying assumptions of a Boussinesq dynamical core and asymptotically fast cloud microphysics, the possibility for intermediate-scale convective organization is explored numerically using the test case of a tropical squall line. Adding the constraint of strong rotation, a large-scale, precipitating quasi-geostrophic (PQG) model is derived, analogous to the dry quasi-geostrophic equations, which have been foundational for turbulence theory in the mid-latitude atmosphere. Theoretical and numerical results from the PQG model are discussed, e.g., (i) the existence of exact solutions that are propagating fronts with discontinuous temperature, winds and total water, and (ii) dependence on rainfall speed of the spectral scaling for the variance of rainwater. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A34.00002: Large-eddy simulations and turbulence statistics of the hurricane boundary layer Mostafa Momen, Marc B. Parlange, Marco Giometto The hurricane is one of the significant fluid mechanics problems, involving complex processes such as turbulent boundary layers, rotating stratified flows, and thermal convection. The boundary layer is one of the most important parts of hurricanes and remains poorly understood due to the lack of sufficient observations and high-resolution numerical simulations. In this work, we present a suite of large-eddy simulations of the hurricane boundary layer to improve our understanding of the momentum and turbulent kinetic energy balance in such flow systems. In this problem, pressure gradients, centrifugal, buoyancy, Coriolis, and friction forces interact with each other to determine the statistical moments of the flow. Our results indicate that the mean radial and tangential velocity profiles are in good agreement with observations, validating the chosen approach. Finally, we show the turbulent kinetic energy budget profiles in the atmospheric boundary layer of hurricanes and briefly discuss their implications – for example by evaluating the associated risks for the infrastructure. This study provides new insights into the mechanisms sustaining the mean wind velocity profile and turbulence structures in hurricane boundary layers, setting the groundwork for future analysis. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A34.00003: Effects of Air Turbulence on Snowfall Kristie L Smith, Alec J Peterson, Zachary J Lebo, Filippo Coletti A predictive understanding of snow settling is necessary for reliably forecasting snowfall. Laboratory and field measurements indicate that air turbulence can significantly enhance the settling velocity of inertial particles in general, and snowflakes in particular. We test whether this knowledge can improve the accuracy of numerical weather prediction tools. The Predicted Particle Properties (P3) cloud microphysics scheme is employed. Laboratory data obtained in a zero-mean flow turbulence chamber are incorporated into the model, and simulations are run with and without the influence of turbulence on the settling of snow crystals for a wintertime cyclone over the Mountain West of the United States. Contrary to expectations, the average precipitation decreases when including the enhancement in snowfall speed due to turbulence, the largest decreases coming from regions of heavier snowfall. Because the baseline simulation overestimates snowfall compared to observations, such decrease results in an improved forecast. Two mechanisms are explored to explain the resulting dichotomy: reduced cloud depths and enhanced entrainment in the simulations with. Both mechanisms act to limit the total condensate in the clouds, thus reducing the amount of precipitation that is generated. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A34.00004: Symmetry breaking in supercell thunderstorms Lin Li, Pinaki Chakraborty At its base, a supercell thunderstorm hosts twin vortices that spin about their vertical axes with the same circulation but with opposite directions of spin. As the twin vortices are advected vertically, one of them, which is spinning in the cyclonic direction, becomes dominant, while the other withers away. This symmetry breaking, which is called "preferential enhancement," must be caused by asymmetric factors affecting the dynamics of vortices. It is widely believed that this symmetry breaking stems from the steering of the ambient wind (the turning of the wind-shear vector with height), wherein the Coriolis force from Earth's rotation is thought not to directly affect the vortex dynamics. Here, using state-of-the-art, non-hydrostatic, three-dimensional simulations, we show that, even in the absence wind steering, Coriolis force can lead to symmetry breaking, where the asymmetry is proportional to the Coriolis force. Our analysis highlights the significant but hitherto ignored role of Coriolis force in the dynamics of supercell thunderstorms. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A34.00005: Radiation feedback and grid convergence in LES of stratocumulus clouds Georgios Matheou, Joao Teixeira Stratocumulus clouds (Sc) have a large impact on the Earth's radiative balance and even though is one of the most studied cloud systems, boundary layer parameterizations in global circulation models remain challenging. Typically, large-eddy simulations (LES) are sufficiently reliable and are used to gain insight into boundary layer physics and to inform the development and evaluation of coarse-grained models. However, LES of Sc has been challenging compared to other boundary layer types. The source of difficulty in LES of Sc is studied using a series of numerical experiments. A strong feedback between cloud liquid, cloud top radiative cooling, and turbulence leads to slow grid convergence of the turbulent fluxes. In contrast, when the liquid-radiation-buoyancy feedback is not present in simulations without radiation, the turbulence structure of the boundary layer remains essentially identical for grid resolutions between 20 and 1.25 m. The entrainment rate does not depend on grid resolution but shows strong dependence on physical processes. For fine grid resolutions, the LES results agree with observations, especially with respect to cloud liquid, the vertical velocity variance, and the vertical velocity triple correlation without any model tuning or ad hoc model choices. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A34.00006: Discontinuous Fronts as Exact Solutions to Precipitating Quasi-Geostrophy Alfredo N. Wetzel, Leslie M. Smith, Samuel N. Stechmann Atmospheric fronts may be idealized as boundaries between two air masses with different temperature, density, moisture, etc. In this presentation, we discuss exact discontinuous solutions of a simplified model for moist mid-latitude synoptic atmospheric flows, the precipitating quasi-geostrophic (PQG) equations. These simple discontinuous fronts extend the celebrated Margules' front slope formula to the case of propagating moist fronts and require both rainfall and a phase change of water at the front interface to exist. The fronts propagate at speeds related to the rainfall velocity, temperature/wind jump magnitudes, and front geometry. To assess the realism of these fronts, we use rough estimates of relevant physical parameters to show that cold, warm, and stationary fronts are sensibly captured by the model. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A34.00007: On the mechanism of low-level jets formation Mona Karimi, Arquimedes Ruiz-Colombie, Walter Gutierrez, Luciano Castillo Low-level jet (LLJ) is characterized by the maximum wind in the lower part of an atmospheric boundary layers. Given their relatively high speed and low turbulence intensity, LLJs provide an opportunity for the stable atmosphere to serve as a high energy density resource compared to the unstable atmosphere. In this study, we use direct numerical simulation of the atmospheric boundary layer based on the observational data from the meteorological tower to understand the mechanism by which LLJs are formed. Maintenance of turbulence at high Richardson numbers can be associated to submeso motions of unusually well-defined waves that are formed above the atmospheric boundary layer. The disturbances of such motions can induce surface pressure perturbations and contribute to the finite-amplitude wind fluctuations close to the surface. We examine whether the inflection-point instability can induce such wave-like motions above the atmospheric boundary layer and whether such a wave-turbulence mechanism can contribute to the intermittent bursts and consequently formation and evolution of LLJs. |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A34.00008: On source specifications in a stochastic gravity wave parameterization Bruno Ribstein, Christophe Millet, François Lott While stochastic Gravity Wave (GW) parameterizations are promising for improving longstanding climate predictions, there is no consensus regarding the values of tunable parameters. In this study, the GW field is represented using a multiwave scheme, in which the GWs are related to sources. Each source is characterized by the horizontal wavenumber, which is randomly chosen within a given interval, and the phase velocity, which is assumed to follow a centered Gaussian distribution with fixed standard deviation. A control simulation, using the LMDz global climate model together with the present-day parameterization is compared with two variants, that are based on alternative source specifications. These variants are designed using a smaller standard deviation for the phase velocity, which is shown to be a condition for the small scale content of the resulting GW fields to recover that observed in vertical soundings. From on-line simulations, is it found that a vertical spread of the sources allows to restore the inclination of the stratospheric jet and to compensate the warm bias compared to reanalyses. The off-line results obtained with vertically distributed sources reproduce vertical wavenumber spectral tail slopes which vary near the -3 value reported by observations. |
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