Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session M23: Geophysical: Atmospheric III |
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Chair: Brad Marston, Brown University Room: 318 |
Tuesday, November 26, 2013 8:00AM - 8:13AM |
M23.00001: Statistically Steady-State Large Eddy Simulations of Subtropical Clouds With Time Varying Large Scale Forcing Kyle Pressel, Tapio Schneider, Joao Teixeira, Zhihong Tan Despite the substantial effort directed towards understanding the role played by clouds in determining perturbed climate states, the combined effects of clouds remain a major, if not the most major, source of uncertainty in predictions of perturbed climate states. Here we discuss a new set of tools brought to bear on the cloud-climate problem, in particular statistically steady-state Large Eddy Simulations (LES) with dynamically consistent time-varying forcing provided by an atmospheric general circulation model (GCM). The simulations are used to investigate the response of subcloud-scale dynamics to variations in large scale dynamics for various perturbed climate scenarios. The results of this investigation are used to characterize the response of subtropical clouds to climate perturbations. [Preview Abstract] |
Tuesday, November 26, 2013 8:13AM - 8:26AM |
M23.00002: Shear effects in the evaporatively driven cloud-top mixing layer Juan Pedro Mellado A stably stratified shear layer destabilized locally by moist convection is studied using direct numerical simulations as a model to investigate the role of evaporative cooling at the top of stratocumulus clouds in the presence of vertical mean shear. Velocity and time scales are obtained from the study of the vertical structure. It is found that, overlapping with the background shear layer that has been often documented in the cloud-free cases, with a thickness $(1/3)(\Delta u)^2/\Delta b$, where $\Delta u$ and $\Delta b$ are the velocity and buoyancy increments across the cloud top, the system develops a turbulence layer that is dominated by free convection inside the cloud and by shear production inside the relatively thin overlap region. As turbulence intensifies, the turbulence layer encroaches upwards into the background shear layer and defines thereby the entrainment velocity. This encroachment is well characterized by the penetration length formed with the in-cloud convective velocity and the buoyancy frequency inside the background shear layer. Consistently, the turbulence intensity inside the overlap region follows a mixed scaling combining the background mean shear and the in-cloud convective velocity. [Preview Abstract] |
Tuesday, November 26, 2013 8:26AM - 8:39AM |
M23.00003: Sub-layers inside the entrainment zone of a dry, shear-free convective boundary layer Jade Rachele Garcia, Juan Pedro Mellado The entrainment zone of a dry, shear-free convective boundary layer growing into a homogeneously stably-stratified fluid is studied using direct numerical simulation. Based on the self-similar analysis of the mean and variance buoyancy profiles, we identify two sub-layers within the entrainment zone, defined as the region of negative buoyancy flux: i) an upper sub-layer with a thickness comparable to the penetrative length scale based on the convective velocity and the buoyancy frequency of the free troposphere and ii) a lower sub-layer acting as a transition towards the mixed layer, with a thickness equal to a constant fraction of the boundary layer height. The capping region of the penetrative thermals belongs to the upper sub-layer of the entrainment zone, and the troughs between the penetrating thermals belong to the lower sub-layer of the entrainment zone. Correspondingly, different buoyancy scales are identified in the different regions; parametrizations thereof are provided and explained. This multiplicity of characteristic scales inside the entrainment zone helps to explain the uncertainty associated with previous analysis of entrainment zone properties and the difficulty to parametrize them based on a single length scale and a single buoyancy scale. [Preview Abstract] |
Tuesday, November 26, 2013 8:39AM - 8:52AM |
M23.00004: Effect of systematic mode reduction on cloud formation and buoyancy transport in a model of moist turbulent convection Joerg Schumacher, Thomas Weidauer The insufficient parametrization of low clouds which are caused by shallow convection remains one of the biggest sources of uncertainty in large-scale models of global atmospheric motion. One way to overcome this lack of understanding is to develop simplified models of moist convection which allow for systematic studies of the cloud formation in different dynamical regimes. They provide an ideal testing bed for systematic and stepwise reductions of degrees of freedom. Such systematic reductions are studied here for a recently developed moist Rayleigh-B\'{e}nard convection model in the conditionally unstable regime. Our analysis is based on the Proper Orthogonal Decomposition (POD). The resulting reduced-order dynamical systems which are obtained by a projection of the original equations of motion onto the most energetic POD modes are found to reproduce important statistical quantities such as the cloud cover, liquid water fluxes and the global buoyancy transport to a very good degree. The number of modes can be compressed significantly before the POD models break down and cause significant deviations of essential mean transport quantities from the original fully resolved simulation data. [Preview Abstract] |
Tuesday, November 26, 2013 8:52AM - 9:05AM |
M23.00005: Multiscale Eddy Simulation for Moist Atmospheric Convection Samuel Stechmann, Bjorn Stevens A multiscale computational framework is designed for simulating atmospheric convection and clouds. In this multiscale framework, large eddy simulation (LES) is used to model the coarse scales of 100 m and larger, and a stochastic, one-dimensional turbulence (ODT) model is used to represent the fine scales of 100 m and smaller. Coupled and evolving together, these two components provide a multiscale eddy simulation (MES). Through its fine-scale turbulence and moist thermodynamics, MES allows coarse grid cells to be partially cloudy and to encompass cloud--clear air mixing on scales down to 1 m; in contrast, in typical LES such fine-scale processes are not represented or are parameterized using bulk deterministic closures. To illustrate MES and investigate its multiscale dynamics, a shallow cumulus cloud field is simulated. In comparison to LES, many statistical mean quantities are essentially the same in MES, which indicates that the bulk properties of the cloud fields are similar in LES and MES. However, MES has significantly larger turbulent kinetic energy and variance. To illustrate the fine-scale variability, an individual cloud is considered in detail, and partially cloudy grid cells are seen to be prominent near the cloud edges. [Preview Abstract] |
Tuesday, November 26, 2013 9:05AM - 9:18AM |
M23.00006: Direct numerical simulation of stationary homogeneous moist turbulence Daniel Chung, Georgios Matheou Direct numerical simulation is reported of stationary and homogeneous, buoyancy-driven turbulence in moist air. Moist-air dynamics is more complex than its dry-air counterpart because of the possibility of latent-heat release during condensation or latent-heat absorption during evaporation. These phase changes depend on the local fluid composition and alters the buoyancy in a non-trivial way. In this study, moist air is modeled using equilibrium thermodynamics and the continuum approach in which the effect of phase changes is manifested through a nonlinear dependence of buoyancy upon the mixture fraction. A large-scale forcing is imposed on the the mixture-fraction equation to model the engulfing action of large eddies. This flow represents an idealisation of subgrid-scale moist processes that occur in the simulation of clouds, and is a first step toward improving subgrid condensation schemes. Statistics from this flow, including cloud fraction, mean liquid water content and subgrid buoyancy flux, will be compared with the predictions of the commonly used Sommeria and Deardorff (1977) scheme. [Preview Abstract] |
Tuesday, November 26, 2013 9:18AM - 9:31AM |
M23.00007: Laboratory study of orographic cloud-like flow Kanwar Nain Singh, K.R. Sreenivas Clouds are one of the major sources of uncertainty in climate prediction, listed in ``the most urgent scientific problems requiring attention'' IPCC. Also, convective clouds are of utmost importance to study the dynamics of tropical meteorology and therefore, play a key role in understanding monsoons. The present work is to study the dynamics of orographic clouds. Parameterization of these clouds will help in forecasting the precipitation accurately. Also, one could validate laboratory results from our study by actually measuring cloud development along a sloping terrain. In this context a planar buoyant turbulent wall jet is considered as an appropriate low order fluid-dynamical model for studying the turbulence and entrainment in orographic-clouds. Flow is volumetrically heated to mimic the latent heat release due to condensation in an actual cloud. This is the first step in studying the entrainment dynamics of the evolving orographic cloud. We are going to present some results on the cloud development using techniques that allows us to construct a 3-dimensional flow field at each instance and its development over the time. By combining velocity field from PIV and flow volume from PLIF at successive instances, we estimate the entrainment coefficient. Since the life-cycle of a cloud is determined by the entrainment of ambient air, these results could be extremely helpful in understanding the dynamics of the clouds. Detailed results will be presented at the conference. [Preview Abstract] |
Tuesday, November 26, 2013 9:31AM - 9:44AM |
M23.00008: Study of microphysical and radiative properties of contrail cirrus using large-eddy simulations Roberto Paoli, Odile Thouron, 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 contrail cirrus, they can persist for hours resulting in additional (artificial) cloud cover that adds to the cover due to natural cirrus. This talk presents recent results from large-eddy situations (LES) of contrail cirrus dispersion that are carried out using the atmospheric model M\'eso-NH. The objective is to investigate whether and how the ambient conditions and the microphysical and optical properties of ice crystals (e.g. shape, albedo), affect the three-dimensional structure and the overall microphysical and radiative characteristics of the contrail. The analysis is carried out by changing the radiative properties of the atmosphere (e.g. day/night conditions) for a given level of atmospheric turbulence. The turbulent field is generated by means of a stochastic forcing technique that reproduces the atmospheric conditions encountered in the upper troposphere. In addition to helping understanding the physics of contrails, the LES data retrieved from this study may provide useful inputs to the parameterization of contrail cirrus into global or climate models. [Preview Abstract] |
Tuesday, November 26, 2013 9:44AM - 9:57AM |
M23.00009: Large eddy simulations of Arctic mixed-phase clouds Colleen M. Kaul, Joao Teixeira, Graeme L. Stephens Mixed-phase stratocumuli have been observed to persist in the Arctic for hours or even days, despite the inherent instability of liquid droplet--ice particle mixtures. Since mixed-phase and ice-only clouds have very different radiative effects, identifying the factors that allow the maintenance of mixed-phase clouds is an important component of understanding Arctic climate. Various feedbacks between turbulence, radiation, and microphysical processes are hypothesized to exist, but further information about these conjectured feedback mechanisms is needed. Prior large eddy simulation studies of Arctic mixed-phase clouds have largely focused on the details of their microphysical modeling, although microphysical processes alone cannot explain the longevity of mixed-phase Arctic stratocumuli. Therefore, this study investigates the representation of turbulence in large eddy simulations of such clouds, considering the effects of turbulence closure, grid resolution, and domain size on the predicted cloud characteristics in three different case studies. [Preview Abstract] |
Tuesday, November 26, 2013 9:57AM - 10:10AM |
M23.00010: Turbulent Mixing at the Edge of a Cloud Raymond Shaw, Matthew Beals, Jacob Fugal, Bipin Kumar, Jiang Lu, Joerg Schumacher, Jeffrey Stith Numerical and field experiments have been brought to bear on the question of how atmospheric clouds respond when they experience turbulent mixing with their environment. Simply put, we ask when a cloud is diluted, do all droplets evaporate uniformly (homogeneous mixing) or does a subset of droplets evaporate completely, leaving the remaining droplets unaffected (inhomogeneous mixing)? First, the entrainment of clear air and its subsequent mixing with a filament of cloudy air is studied in DNS that combine the Eulerian description of the turbulent velocity, temperature and vapor fields with a Lagrangian cloud droplet ensemble. The simulations provide guidance on the proper definition of the thermodynamic response time for the Damkoehler number, and demonstrate the transition from inhomogeneous to homogeneous mixing as mixing progresses within the inertial subrange. Second, an airborne digital holographic instrument (Holodec) shows that cloud edges are inhomogeneous at the centimeter scales. In local cloud volumes the droplet size distribution fluctuates strongly in number density but with a nearly unchanging mean droplet diameter, until the fluctuations finally cascade to the centimeter scale, when the droplet diameter begins to respond. [Preview Abstract] |
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