Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session L01: Mini-Symposium: Fluid Dynamics of Atmospheric CloudsLive Mini-Symposium
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Chair: Raymond Shaw, Michigan Technological University; Eckart Meiburg, University of California |
Monday, November 23, 2020 9:50AM - 10:16AM Live |
L01.00001: Numerical simulation of cloud droplets and turbulence Toshiyuki Gotoh, Izumi Saito, Takeshi Watanabe, Tatsuya Yasuda We have developed a numerical simulation code, CMS (Cloud Microphysics Simulator), to compute growth of the cloud droplets and turbulent flow and their interaction from the microscopic viewpoint. The evolution of the droplet spectrum was successfully computed from the single peak to emergence of the second peak showing the rain drop formation. Applying this code we have studied the effects of the number density and of turbulent fluctuations on the spectral broadening in the condensation process. It is found that the spectrum peak shifts smaller side and the width becomes narrower with the increase of the number density, in agreement with the experimental data of the turbulence-cloud chamber at Michigan Tech. One illuminating aspect of the CMS study is that in addition to the droplet statistics a large amount of turbulence data is obtained. One finding is that the variance spectra of the temperature and water vapor mixing ratio begins to deviate from the turbulence spectrum like $k^{-5/3}e^{-\alpha k\eta}$ at high wavenumbers and the modification propagates toward low wavenumbers with time. We study this modification by using a large scale simulation with simple model for scalar carried by particles and explore the physical explanation from the turbulence theory. [Preview Abstract] |
Monday, November 23, 2020 10:16AM - 10:42AM Live |
L01.00002: Numerical Simulations of Cumulus and Mammatus Clouds S. Ravichandran Cumulus, or `heap-like', clouds are crucial to mass and energy transport in the atmosphere. We model these clouds as transient diabatic plumes carrying water vapour, and show that their formation and evolution are governed by the balance between ambient stratification and the release of latent heat by condensation. The latter is governed by the ambient relative humidity and the boundary conditions at the plume base (1). We show how anomalous entrainment in cumulus clouds may be studied using the present formulation. Mammatus, or lobe-like, clouds develop underneath cloudy layers called cumulonimbus anvils. As water droplets settle out of the anvil, they evaporate and cool the layer of air immediately below the anvil, creating a density overhang and leading to an instability. The nature of this instability is governed by the density excess and depth of the overhang, which in turn are functions of the size and number concentration of liquid water droplets in the anvil. We show that the size of the water droplets plays a greater role in the formation of mammatus-like lobes, thus explaining why evaporative cooling is necessary but not sufficient for mammatus cloud formation (2). (1) SR and Narasimha, arXiv:2004.09631 (2) SR, Meiburg and Govindarajan, J. Fluid Mech. (2020), 899, [Preview Abstract] |
Monday, November 23, 2020 10:42AM - 11:08AM Live |
L01.00003: Inhomogeneous Mixing Processes in Clouds: Toward Mixed-Phase Clouds Fabian Hoffmann Clouds are one of the biggest unknowns in the climate system. This is especially true for mixed-phase clouds (MPCs), which consists of liquid and ice hydrometeors. This coexistence enables the Wegener-Bergeron-Findeisen (WBF) process, which describes the growth of the ice phase at the expense of the liquid phase and results in an accelerated dissipation of the cloud by precipitation. We will present modeling results employing a highly detailed Lagrangian cloud microphysical model, coupled to an idealized dynamical driver that represents fluid dynamics from the Kolmogorov scale to the scale of the largest entraining eddies. By explicitly simulating entrainment of cloud-free air and its subsequent mixing, we identify two MPC inhomogeneous processes. The first process addresses the nucleation of entrained aerosols to ice crystals, which is decelerated if the mixing process is sufficiently slow. The commensurate lower ice crystal concentration is disadvantageous to WBF. The second process is caused by the finite rate transport of water vapor from the liquid droplets to the ice crystals. An entrainment event may increase the distance between these particles, also decelerating WBF. Since both effects counteract WBF, they may contribute to the longevity of MPCs observed in the Arctic. [Preview Abstract] |
Monday, November 23, 2020 11:08AM - 11:34AM Live |
L01.00004: Seasonal cycle of idealized polar clouds: large eddy simulations driven by a GCM Xiyue Zhang, Tapio Schneider, Zhaoyi Shen, Kyle Pressel, Ian Eisenman The uncertainty in polar cloud feedbacks calls for process understanding of the cloud response to climate warming. As an initial step, we investigate the seasonal cycle of polar clouds in the current climate by adopting a novel modeling framework using large eddy simulations (LES), which explicitly resolve cloud dynamics. Resolved horizontal and vertical advection of heat and moisture from an idealized GCM are prescribed as forcing in the LES. The LES are also forced with prescribed sea ice thickness, but surface temperature, atmospheric temperature, and moisture evolve freely without nudging. A semigray radiative transfer scheme, without water vapor or cloud feedbacks, allows the GCM and LES to achieve closed energy budgets more easily than would be possible with more complex schemes; this allows the mean states in the two models to be consistently compared. We show that the LES closely follow the GCM seasonal cycle, and the seasonal cycle of low clouds in the LES resembles observations in the Arctic: maximum cloud liquid occurs in late summer and early autumn, and winter clouds are dominated by ice in the upper troposphere. Large-scale advection of moisture provides the main source of water vapor for the liquid clouds in summer, while a temperature advection peak in winter makes the atmosphere relatively dry and reduces cloud condensate. The framework we develop and employ can be used broadly for studying cloud processes and the response of polar clouds to climate warming. [Preview Abstract] |
Monday, November 23, 2020 11:34AM - 12:00PM Live |
L01.00005: Entrainment in a Simulated Supercell Thunderstorm Sonia Lasher-Trapp The turbulent motions of an active cumulus cloud introduce air from outside the cloud inward, in a process called \textit{entrainment}. The subsequent mixing of this air dilutes parts of the cloud, evaporating/sublimating cloud water/ice and reducing buoyancy. An understanding of entrainment is critical to understanding and predicting the initiation of storms and the precipitation they produce. Entrainment has often been studied in cumuli growing in environments lacking vertical wind shear, neglecting a class of thunderstorms called \textit{supercells} that last longer, and often produce copious precipitation. A quantitative evaluation of entrainment and its effects in the developing and mature stages of a simulated supercell thunderstorm is presented. The two stages entrain air by both non-turbulent and turbulent mechanisms. Entrainment rates increase throughout the developing stage, but successive thermals replenish some condensate and drive the nascent storm towards higher cloud tops and precipitation formation. During the mature stage, turbulent entrainment into a rotating supercell core does occur, often at rates greater than in the developing stage, in contrast to previous notions. ``Ribbons'' of overturning motions wrap around the rotating updraft and ascend in time. Entrainment also occurs from non-turbulent mechanisms. At this stage, entrainment is less effective at diluting the thunderstorm core, allowing significant precipitation production. [Preview Abstract] |
Monday, November 23, 2020 12:00PM - 12:26PM Live |
L01.00006: Cloud-turbulence interactions: Insights from moist Rayleigh-Benard convection experiments Prasanth Prabhakaran, Abu Sayeed Md Shawon, Gregory Kinney, Subin Thomas, Will Cantrell, Raymond Shaw Clouds are ubiquitous in nature and play an important role in atmospheric circulation and global climate change. One of the key processes in the evolution of a cloud is the interaction between aerosol particles and cloud droplets, called the aerosol-indirect effect. Aerosol particles become cloud droplets when the ambient relative humidity (saturation ratio) exceeds a critical value. In the traditional formulation of this problem, only average saturation ratios are considered. Using experiments and theory, we study the effects of turbulent fluctuations in the formation and growth of cloud droplets. The experiments were conducted in a turbulent Rayleigh-B\'{e}nard convection chamber. Cloudy conditions are created through a continuous injection of aerosols. A steady state is obtained when a dynamic equilibrium is established between droplet formation through activation of aerosols and droplet removal through sedimentation. Our measurements and the theoretical model show a clear transition from a regime in which the mean saturation ratio dominates to one in which the turbulent fluctuations determine cloud properties. [Preview Abstract] |
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