Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session G1: Geophysical: Atmospheric II |
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Chair: Alberto Aliseda, University of Washington Room: 22 |
Monday, November 19, 2012 8:00AM - 8:13AM |
G1.00001: Rain-induced dissipation in hurricanes Tapan Sabuwala, Gustavo Gioia, Pinaki Chakraborty Hurricanes originate from a potent mix of atmospheric and oceanic conditions, and manifest intensely swirling winds and torrential rains. The drag forces on the falling raindrops act to dissipate energy, which, in the context of global precipitation, has been shown to play a key role in global atmospheric circulation. And yet, the role of rain-induced dissipation in the energetics of a hurricane remains uncharted. Here, using dimensional analysis and satellite data assimilated from the Tropical Rainfall Measuring Mission, we propose a simple model of rain-induced dissipation in a hurricane. We modify Emanuel's idealized heat engine model of hurricanes by incorporating the rain-induced dissipation and predict the maximum intensity a hurricane can achieve for a given set of atmospheric and oceanic conditions. We find that the modified model predictions are closer to the observed data as compared with Emanuel's model. Further, we use the modified model to predict inter-annual trends in various metrics of hurricane activity in the North Atlantic basin and show that the model predictions compare well with the observed trends. We conclude that rain-induced plays a significant role in the energetics of hurricanes and should be incorporated in global climate models. [Preview Abstract] |
Monday, November 19, 2012 8:13AM - 8:26AM |
G1.00002: Impact of the asymmetric dynamical processes of the structure and intensity change of two-dimensional hurricane-like vortices Konstantinos Menelaou, M.K. Yau, Yosvany Martinez In this study, a simple two-dimensional (2D) unforced barotropic model is used to study the asymmetric dynamics of the hurricane-inner core region and assess their impact on the structure and intensity change. Two sets of experiments are conducted starting with a ring of enhanced vorticity to mimic intense mature hurricane-like vortices, that is perturbed by an external impulse. The theory of empirical normal modes (ENM), and the Eliassen-Palm (EP) flux theorem is then applied to extract the dominant wave modes from the dataset and diagnose their kinematics, structure, and impact on the hurricane-like vortex. From the first experiment, it is found that the evolution and the lifetime of an elliptical eyewall may be controlled by the inviscid damping of sheared vortex Rossby waves (VRWs) or quasimodes. The critical radius and the structure of the quasimode obtained by the ENM analysis is shown to be consistent with the predictions of a linear eigenmode analysis of small perturbations. From the second experiment, it is found that the outward propagating VRWs that arise due to barotropic instability and the inward mixing of high vorticity, organizes into secondary ring of enhance vorticity that contains a secondary wind maximum. Sensitivity tests performed on the spatial extent of the initial external impulse verifies the robustness of the results. The fact that the secondary eyewall occurs close to the critical radius of some of the dominant modes emphasize the important role played by the VRWs. [Preview Abstract] |
Monday, November 19, 2012 8:26AM - 8:39AM |
G1.00003: Radiatively Driven Turbulence at the Cloud Top Alberto de Lozar, Juan Pedro Mellado We use Direct Numerical Simulations to investigate a radiatively-driven smoke cloud-top mixing layer. This configuration mimics relevant aspects of stratocumuls clouds, in particular the mixing across an inversion that bounds a radiatively driven turbulent flow. A 1D formulation is employed for the radiation calculations. Below the inversion a convective boundary layer propagates downwards into the cloud-bulk. The convective boundary layer decouples from the inversion properties other than the injected buoyancy flux. This buoyancy flux is equal to the total radiative cooling minus the cooling of the inversion layer where the cloud mixes with the free atmosphere. An exact equation at a properly defined inversion point divides the inversion cooling into three components: a molecular flux, a turbulent flux and the direct radiative cooling by the smoke inside the inversion layer. The normalized turbulent flux levels to a constant value ($0.175\pm0.05$), which is independent of the stratification. As suggested by earlies studies, we observe that the turbulent entrainment only occurs at the small scales and that eddies larger than four optical lengths ($50\,$m in a typical DYCOMS-II cloud) perform little or no entrainment. [Preview Abstract] |
Monday, November 19, 2012 8:39AM - 8:52AM |
G1.00004: Large-eddy simulations of contrail-to-cirrus transition in atmospheric turbulence Roberto Paoli, Odile Thouron, Joris Picot, Daniel Cariolle Contrails are ice clouds that form by condensation of water vapor exhaust from aircraft engines and develop further in the wake as they are entrained by the airplane trailing vortices. When contrails spread to form cirrus clouds, they can persist for hours and become almost indistinguishable from natural cirrus. This talk focuses on the role of atmospheric turbulence in determining the characteristics of these ``contrail cirrus.'' Large-eddy simulations are carried out using the atmospheric model Meso-NH with the goal of identifying the processes driving the contrail-to-cirrus transition as a function of contrail age. To that end, the effects of atmospheric turbulence, microphysics, and radiative transfer are analyzed separately. Turbulent fields are first generated by means of a stochastic forcing technique that reproduces the atmospheric conditions encountered in the upper troposphere. Contrails generated by a model aircraft are then inserted on the top of these fields. Finally, ice microphysics and radiative transfer are activated to find out on which spatial and temporal scales the vertical motion prevails over the essentially horizontal motion induced by atmospheric turbulent diffusion. [Preview Abstract] |
Monday, November 19, 2012 8:52AM - 9:05AM |
G1.00005: On the evolution of stratified turbulent clouds Andrea Maffioli, Peter Davidson, Stuart Dalziel, Nedunchezhian Swaminathan The evolution of a turbulent cloud in a stratified fluid is studied by means of direct numerical simulations. The focus of the study is on the edge dynamics occurring between the turbulence and the quiescent region surrounding it. By comparing isosurface plots of the materially conserved potential vorticity $\Pi$ and the $u_x$ velocity component ($x$ is horizontal) it is possible to divide the edge flow into fluid intrusions and horizontally-travelling wave-packets. The 3D structure of the intrusions and the wave-packets is similar, both structures being pancake-like, and the only difference is in their extension away from the turbulent cloud. Individual wave-packets were tracked and it was found that their group speed agrees with the theoretical group speed relation for linear internal gravity waves. The wave-packets can therefore be thought of as quasi-linear finite-amplitude internal gravity waves. The kinetic energy radiated away by the waves was measured and it was found to be 15--20\% of the total kinetic energy in the numerical domain. As a result, horizontally-travelling waves set off during a localized turbulence episode in the atmosphere could be important in the context of meteorology as they alter the energy and momentum budgets in different regions of the atmosphere. [Preview Abstract] |
Monday, November 19, 2012 9:05AM - 9:18AM |
G1.00006: Effect of inertia of water droplets on a turbulent cloud: a toy model Rama Govindarajan, Ravichandran Sivaramakrishnan We ask whether the fact that water droplets are inertial can affect the upward trajectory of a cloud. We answer in the affirmative using a toy model, where the turbulent cloud is represented by a distribution of point vortices. Viscosity is neglected and the water droplets are assumed to be point particles whose inertia only gives rise to a drag against the flow. The growing water droplets are shown to cluster in a curtain-like structure along the sides of the cloud as it rises through the atmosphere, causing repeated cycles of nucleation, growth and departure of water droplets in the central region of the cloud. The net effect is a slowing down of the loss of water vapour, and the resulting ``slow release'' of buoyancy allows the cloud to attain a higher height. [Preview Abstract] |
Monday, November 19, 2012 9:18AM - 9:31AM |
G1.00007: Preferential accumulation, enhanced relative velocity and gravitational settling due to inertial droplet interactions with turbulence Colin Bateson, Alberto Aliseda We are exploring the hypothesis that, during warm-rain formation, turbulence-induced collisions can explain the size gap between the limit of condensational growth and the onset of gravitational collisions and sedimentation. We use wind tunnel experiments to study the evolution of water droplets in homogeneous, isotropic, slowly decaying grid turbulence. We analyze the preferential concentration and the enhanced relative velocity of droplets in the $10-200~\mu m$ range due to their inertial interactions with the underlying turbulence. Data from Phase Doppler Particle Analysis and flow visualizations provide insight into the droplet relative velocity and settling velocity fields. We focus on those fields' dependency on droplet Stokes number and local concentration. Recent improvements to our experimental setup allow for high-magnification, high-speed imaging of the flow inside the wind tunnel. We use these images to observe near-droplet dynamics and collision events, with the ultimate goal of formulating a model for droplet collision-coalescence efficiency that can be used in numerical simulations and parameterizations of turbulence-induced droplet collisions. [Preview Abstract] |
Monday, November 19, 2012 9:31AM - 9:44AM |
G1.00008: E-epsilon turbulence closure for sea spray-laden marine atmospheric boundary layer in high wind conditions Yevgenii Rastigejev, Sergey Suslov In-depth understanding and accurate modeling of the interaction between sea spray and a turbulent airflow under high-wind conditions is essential for improving intensity forecasts of hurricanes and severe storms. Here we consider the E-epsilon turbulence closure for the spray-stratified atmospheric marine boundary layer. Our mathematical model accounts for turbulent kinetic energy transport in the vertical direction, the dependence of the turbulent mixing length on the spray stratification and the spray inertia. It is shown that accounting for all these physical factors is important since none of them dominate for all possible hurricane conditions. The obtained analytical and numerical solutions show significant differences between the current E-epsilon model and the lower order Turbulent Kinetic Energy (TKE) and Monin-Obukhov (MO) similarity models considered previously. It is demonstrated that the air turbulence suppression by the spray causes an acceleration of the airflow and a reduction of air-sea drag coefficient that is qualitatively consistent with resent experimental observations. [Preview Abstract] |
Monday, November 19, 2012 9:44AM - 9:57AM |
G1.00009: Wave-induced mean flow at an interface John McHugh, Robert Sharman A vertical packet of internal waves that are horizontally periodic will generate a mean flow that has the same sense as the group velocity of the incident waves. When the wave packet impinges on a density-gradient interface the waves are partially reflected and the wave-induced mean flow is enhanced just under the interface. A density-gradient interface has continuous density but discontinuous buoyancy frequency, and is an idealization of Earth's tropopause. Here we consider waves generated by flow past an isolated object, and maintain a vertical packet by introducing the obstacle gradually. The resulting waves are confined horizontally over a narrow interval and hence are not slowly varying in the horizontal. Nonlinear simulations show that the mean flow at the interface has a component with the same sense as the wave group velocity above the mean position of the interface, but also a component with the opposite sense just below the mean interfacial position. This combination establishes a wave-induced circulation at the interface. [Preview Abstract] |
Monday, November 19, 2012 9:57AM - 10:10AM |
G1.00010: Modes in rotating troposphere with a leak Lyubov Chumakova, Rodolfo Rosales, Esteban Tabak We study how the rigid lid modes change, if the effect of waves leaking into the stratosphere from the troposphere is taken into account in the linear, hydrostatic, incompressible, rotating case. We present an effective boundary condition which accounts for the leak and the resulting ``leaky modes.'' This work is partially supported by NSF 0903008 (LC), DMS 1007967 and DMS 0907955 (RR), and DMS 0908077 (ET). [Preview Abstract] |
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