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Atmospheric Physics Seminar

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room B4.58, Pasteura 5 at 13:15

Professor Lian-Ping Wang (Department of Mechanical Engineering, University of Delaware, USA)

Mesoscopic simulation methods of multiphase and turbulent flows: An overview and recent developments

Since the 1980s, direct numerical simulations have served as a vital research tool to probe flow structures and nonlinear dynamics in complex flows such as multiphase flows and turbulent flows. Most of these simulations were performed based on the continuum (conventional or macroscopic) Navier-Stokes equation. In recent years, mesoscopic methods based on the Boltzmann equation, such as the lattice Boltzmann method and gas kinetic schemes, have been developed and applied to these complex flows. In this talk, I will discuss recent advances in applying mesoscopic methods for rigorous simulations of such complex flows. Three specific examples will be considered: (a) turbulent channel flow laden with finite-size moving particles, (b) hydrodynamic interactions of cloud droplets, and (c) compressible turbulent flow. A few implementation issues in these simulations will be discussed. The purpose is to expose the capabilities of these mesoscopic methods, open research issues, and their potentials for atmospheric physics problems. I will also comment on the physical accuracy and computational efficiency of mesoscopic methods relative to conventional simulation methods such as pseudo-spectral and finite-volume methods.

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room B0.14, Pasteura 5 at 13:15

dr Shin-ichiro Shima (Graduated School of Simulation Studies, University of Hyogo, Kobe, Jp)

Grid convergence of the large-eddy simulation of shallow maritime cumuli field and a comparison of the super-droplet and bulk cloud microphysics model

A series of large-eddy simulations (LES) are carried out to investigate the grid resolution necessary to obtain an accurate numerical solution of a shallow maritime cumuli field. The experimental setup is based on Barbados Oceanographic and Meteorological Experiment (BOMEX). For the LES model, Scalable Computing for Advanced Library and Environment (SCALE) is used. For the cloud microphysics, the super-droplet method (SDM) and the two-moment bulk scheme of Seiki and Nakajima (2014) are adopted. The results indicate that for both cloud microphysics schemes, a gridresolution less than 10 m is necessary to accurately simulate a shallow maritime cumuli field. Our grid refinement analysis also shows that there is a significant contribution from small cumulus clouds to the total cloud cover, and such cumuli are generated by small-scale updrafts that can only be resolved at a fine resolution. This is a collaboration work with Yousuke Sato (Nagoya U) and Hirofumi Tomita (RIKEN).

room B0.14, Pasteura 5 at 13:15

Prof. Bjorn Stevens (Max Planck Institute for Meteorology, Hamburg)

How Clouds Respond to Global Warming

One of the big questions occupying climate science is how clouds respond to warming. Their response acts as a feedback on the radiative forcing of the climate system and paces uncertainty in future projections of warming. In this talk I review the evolution of our thinking regarding this question, and why it is important. In so doing I illustrate how vague concepts have been made more precise, to the point where are now putting together field campaigns designed to test specific physical hypotheses. Whereas this, and most, work has emphasized the role of low (stratiform) cloud, increasingly we are understanding how more convective clouds set the stage upon which low-stratiform clouds act out their part. In extremely warm climates, as might have been characteristic of the late Eocene, these interactions can give rise to quite surprising swings in the tropical climate. These findings suggest that, as the Earth warms, the behavior of the tropics may well be a source of surprises.

room 1.01, Pasteura 5 at 12:15

Prof. Erland Källén (Director of Research at European Centre for Medium range Weather Forecasts - ECMWF, Reading, UK)

Global weather forecasting for Europe

ECMWF was set up more than 40 years ago to deliver medium range (3-10 days) weather predictions for Europe. ECMWF is an intergovernmental organisation working for its European member and co-operating states. A global, numerical modelling approach has been adopted and today ECMWF is the world leader in medium range global weather prediction. The research done at ECMWF delivers new methods that continuously improves the quality of the predictions. Data assimilation methods that enables the use of satellite as well as in situ observations to define accurate initial states as well as improved modelling of Earth System components with an ever increasing spatial resolution all contribute to the improved predictions. ECMWF also acts as a European focal point for the science of numerical weather prediction. Through research collaborations, visitor programmes, training events and seminars and workshops ECMWF attracts world class scientists. A high performance computing facility is available for operational forecast production, research experimentation and member state usage. In addition to medium range weather forecasts ECMWF also produces extended range forecasts (monthly to seasonal), atmospheric composition forecasts and climate reanalyses.

room B0.14, Pasteura 5 at 13:15

prof. dr hab. Wojciech W. Grabowski, prof. afil. UW (National Center for Atmospheric Research, Boulder, USA, on leave at the Institute of Geophysics, Faculty of Physics, University of Warsaw, Warsaw, Poland)

Diffusional growth of cloud droplets in turbulent clouds

This presentation will discuss spectral broadening of the droplet size distribution through a mechanism referred to as eddy hopping. The key idea, suggested in late 1980ies by Al Cooper, is that droplets arriving at a given location within a turbulent cloud follow different trajectories and thus have different growth histories, and that this leads to a significant spectral broadening. In this study, the adiabatic parcel model with super-droplets is used to contrast droplet growth with and without turbulence. Turbulence inside the parcel is described by two parameters: i) the dissipation rate of the turbulent kinetic energy ε, and ii) the linear extent of the parcel L. As expected, adiabatic parcel without turbulence produces extremely narrow droplet spectra. In the turbulent parcel, a stochastic scheme is used to account for vertical velocity fluctuations that lead to local supersaturation fluctuations for each super-droplet. These fluctuations are argued to mimic the impact of droplets hopping turbulent eddies in a natural cloud. For L smaller than a few meters, noticeable spectral broadening is possible only for strong turbulence, say, ε > 100 cm2 s-3. For L typical for grid lengths of large eddy simulation (LES) models (say, L between 10 and 100 m), the impact is significant even with relatively modest turbulence intensities. The impact increases with both L and ε. The representation of eddy hopping presented in this talk can be included in a straightforward way in a subgrid-scale scheme of a Lagrangian LES cloud model. Preliminary results show a noticeable impact on the width of the droplet spectra. This effect has likely a significant influence on simulated rain formation through collision/coalescence, an aspect that needs to be investigated in the future.

room B0.14, Pasteura 5 at 13:15

prof. dr hab. Piotr Smolarkiewicz, prof afil. UW (European Centre for Medium-Range Weather Forecasts)

Simulation of all-scale atmospheric dynamics on unstructured meshes

The advance of massively parallel computing in the nineteen nineties and beyond encouraged finer grid intervals in numerical weather-prediction models. This has improved resolution of weather systems and enhanced the accuracy of forecasts, while setting the trend for development of unified all-scale atmospheric models. This talk first outlines the historical background to a wide range of numerical methods advanced in the process. Next, the trend is illustrated with a technical review of a versatile nonoscillatory forward-in-time finite-volume (NFTFV) approach, proven effective in simulations of atmospheric flows from small-scale dynamics to global circulations and climate. The outlined approach exploits the synergy of two specific ingredients: the MPDATA methods for the simulation of fluid flows based on the sign-preserving properties of upstream differencing; and the flexible finite-volume median-dualunstructured-mesh discretisation of the spatial differential operators comprising PDEs of atmospheric dynamics. The paper consolidates the concepts leading to a family of generalised nonhydrostatic NFTFV flow solvers that include soundproof PDEs of incompressible Boussinesq, anelastic and pseudo-incompressible systems, common in large-eddy simulation of small- and meso-scale dynamics, as well as all-scale compressible Euler equations. Such a framework naturally extends predictive skills of large-eddy simulation to the global atmosphere, providing a bottom-up alternative to the reverse approach pursued in the weather-prediction models. Theoretical considerations are substantiated by calculations attesting to the versatility and efficacy of the NFTFVapproach. Some prospective developments are also discussed.

room B0.14, Pasteura 5 at 13:15

dr Dariusz Baranowski, IGF UW (-Joint Institute for Regional Earth System Science & Engineering, University of California, Los Angeles, California, USA, -Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA)

Multi-scale interactions over the Maritime Continent: effects on propagation of atmospheric convection and model fidelity

Modern management of extreme weather events depends on forecasts reliable for extended time periods. Much of the predictability of global weather patterns lays within the intraseasonal variability of the tropical circulation, in particular the Madden-Julian Oscillation (MJO). The Maritime Continent is arguably the most important region in weather and climate system, in terms of forcing global atmospheric variability on sub-seasonal to decadal time scales. However, the multi-scale interactions of the atmosphere, ocean and land surfaces in the MC are sparsely observed, poorly understood and badly represented in models.In this presentation I will show examples of two-way interactions between eastward propagating convection (MJO and convectively coupled Kelvin waves) and vivid diurnal cycle of convection over the Maritime Continent. Implications of these multi-scale interactions for both propagating and stationary modes of convection will also be addressed.I will also talk about fidelity of modern General Circulation Models in representation of the key components of the diurnal cycle of convection over the Maritime Continent region and its intraseasonal variability. Models performance with respect to the metrics of the diurnal cycle, such as daily mean precipitation, amplitude and phase of the diurnal cycle of precipitation, will be validated with satellite observations from TRMM mission.

room 17, Pasteura 7 at 11:15

dr Paul Field (UK Met Office)

The Challenge of Atmospheric Ice

Ice is important for the formation of precipitation and the radiative balance of the atmosphere. Consequently, the ability to predict the formation and evolution of ice in the atmosphere is important for the hydrological cycle and weather and climate prediction. This talk explores the challenges of trying to observe ice, the representation of ice in models and at least one unresolved mystery.

room 17, Pasteura 7 at 13:15

dr Juan Pedro Mellado (Max Planck Institute for Meteorology)

Resolving Turbulence in Planetary Boundary Layers

Turbulence is key for the vertical mixing across the planetary boundary layer. However, our understanding of how turbulence interacts with other phenomena, such as density stratification, radiation or clouds, remains limited in crucial ways, particularly at meter and submeter scales. During the last decades, high-resolution simulations have provided further insight into these small-scale processes. In particular, direct numerical simulation has become a powerful complement to large-eddy simulations, laboratory experiments and field observations, since it removes the uncertainty associated with turbulence models and numerical algorithms. In this talk, I will use two examples to illustrate recent developments based on direct numerical simulation. The first example considers the stable boundary layer in the strongly stratified regime. We will see that turbulence collapse in the stable boundary layer can occur intermittently in space without the need of external triggers, such as surface heterogeneity. It suffices that intrinsic large-scale flow structures have space and time to develop. This finding reconciles previous results and explains the difficulty to reproduce this intermittent behavior in simulations, where domains are often too small or grid resolutions too coarse. The second example is cloud-top entrainment in stratocumulus. We will see that centimeter scales are important to faithfully represent the effects of evaporative cooling and gravitational settling on radiative cooling. This finding helps to explain the observed variability across large-scale models when droplet evaporation strongly affects the cloud dynamics, like for buoyancy-reversal conditions. Besides, these results indicate that data from simulations could complement current high-resolution measurements to further advance our understanding of cloud entrainment at meter and submeter scales.

room 17, Pasteura 7 at 13:15

dr Tomasz Wacławczyk (Wydział Mechaniczny Energetyki i Lotnictwa Politechniki Warszawskiej)

A statistical model of the interface in the phase equilibrium state

This presentation concerns modeling of the two phase flows separated by immiscible gas/liquid interfaces. In particular, it demonstrates a relation between the re-initialization equation of the levelset functions derived by Wacławczyk [J.Comp.Phys., 299, (2015)] and the condition for the phase equilibrium provided by the functional derivative of the modified Ginzburg-Landau functional representing the interfacial density of Helmholtz free energy of the non-flat interface, compare with work of Allen and Cahn [Acta Metall., 27, (1979)]. As a consequence of the aforementioned relation, the statistical model of the non-flat interface in the phase equilibrium state is postulated and justified. This model is derived based on the statistical picture of the sharp interface disturbed by the field of stochastic forces; it introduces the relation between the sharp and diffusive interface models of the gas/liquid interfaces. During the seminar, derivation of the model equations, their physical interpretation and the results of numerical simulations will be presented and discussed.

room 17, Pasteura 7 at 13:15

dr inż. Małgorzata Werner (Zakład Klimatologii i Ochrony Atmosfery Uniwersytetu Wrocławskiego)

Application of WRF-Chem and EMEP for the air quality modelling in Poland

The presentation will focus on air quality in Poland and air quality modelling for Poland and especially application of two different Eulerian atmospheric transport models - the Weather Research and Forecasting (WRF) coupled online with chemistry (WRF-Chem) and EMEP coupled offline with the meteorology from WRF. Both models have been used in the reanalysis mode and the WRF-Chem model has been also applied for forecasting of air pollution concentrations for the Lower Silesia region. The presentation will also cover issues related to preparation of input data for modelling, verification of the results and evaluation of the modelled results in the context of the air quality management.

room 17, Pasteura 7 at 13:15

dr Gustavo Coelho Abade (IGF UW)

Sedimentation of hydrodynamically interacting spheres

Collision-coalescence of cloud droplets driven by differential sedimentation is an important mechanism for formation of rain. Theoretical parametrizations (to be used in stochastic simulations of collision-coalescence processes) requires a statistical characterization of the spatial distribution of droplets sedimenting in the complex flow environment of a cloud. This talk is concerned with the spatial distribution statistics of hard spheres sedimenting in low-Reynolds number flow. In this regime small-scale turbulence is absent. Careful numerical simulations and theoretical considerations show that the widely used assumption of random (uncorrelated) distribution of spheres in space is unphysical.

room 17, Pasteura 7 at 13:15

prof dr hab. Zbigniew Sorbjan (Marquette University i Instytut Geofizyki PAN)

Assesment of the gradient-based similarity theory using Large Eddy Simulations and SHEBA data

Gradient-based similarity functions, evaluated based on data generated by a large-eddy simulation model of the stably stratified boundary layer, are compared with analogous similarity functions, derived from field observations in the stable surface layer during the SHEBA experiment in the Arctic. The comparison is performed in terms of explicit and implicit local scaling systems for the temperature and momentum fluxes, standard deviations of the vertical velocity and of temperature, as well as dissipation rates for the turbulent kinetic energy and for the temperature variance. The explicit scaling employs the semi-empirical mixing length l as the length scale. Within the implicit scaling systems, the Dougherty–Ozmidov length scale L, the Ellison length L, and the vertical overturning scale Lw, are used as length scales. A satisfactory agreement of numerically simulated and experimentally assessed similarity functions is obtained in the range of larger values of the Richardson number, 0.02 < Ri < 0.2. The agreement is poorer near the underlying surface (i.e. for small values of Ri) due to the imperfect performance of the sub-grid parameterization within the large-eddy simulation model.

room 17, Pasteura 7 at 13:15

dr Jacob Fugal (Institut for Atmospheric Physics, University of Mainz, Germany and Max Planck Institute for Chemistr)

Turbulent Mixing and Entrainment in Clouds examined via Digital Holographic Particle Measurements

Turbulent mixing and entrainment in clouds is readily visible feature of atmospheric clouds, especially at cloud edge. Atmospheric clouds have one of highest Reynolds numbers of any terrestrial phenomenon. Cloud particles, cloud droplets and cloud ice crystals, respond to turbulence and entrainment in a three-phase system via evaporation-condensation and other phase changes, collision and coalescence and riming and sedimentation. Cloud scales vary from roughly ~10 km in stratocumulus clouds, down to ~1 mm dissipation scale, and one way how clouds are structured on these scales can be examined is in how the cloud particles appear as ensembles in local sample volumes relative to their position in the whole cloud. While entire cloud volumes are beyond any imaginable human method of measurement, recent developments in digital holography allow local measurements on the cm-scale spaced every 10 to 50 m in a cloud. A significant breakthrough is algorithm improvement to process holograms and sift the real particles from noise and artifacts using among other things supervised machine learning. Sample results are shown of measurements in a variety of clouds, cloud particle sizes, and altitudes or temperatures.

room 17, Pasteura 7 at 13:15

prof. dr hab. Wojciech W. Grabowski, profesor afiliowany UW (IGF FUW, Mesoscale and Microscale Meteorology, NCAR)

Modeling condensation in cloud-scale models

Condensation of water vapor to form and grow cloud droplets is the most fundamental process of cloud and precipitation formation. It drives cloud dynamics through the release of latent heat and determines the strength of convective updrafts. Cloud-scale modelstypically simulate condensation by applying two drastically different methods. The first one is the bulk condensation, where cloud water is assumed to exist only in saturated conditions and evaporates instantaneously when the air becomes sub-saturated. The second approach involves prediction of the in-cloud super- or sub-saturation and is typically associated with models that predict not only condensate mass but also relevant features of the droplet size distribution (e.g., models with the 2-moment microphysics or with the bin microphysics). However, predicting in-cloud super- or sub-saturation is numerically cumbersome. This talk will address the question whether the difference between the two approaches has a noticeable impact on convective dynamics. To answer this question with confidence, we apply a novel modeling methodology, the microphysical piggybacking. Piggybacking simulations with the bin microphysics for shallow non-precipitating convection and the double-moment bulk microphysics for deep convection will be discussed. For the shallow convection, the differences in cloud fields simulated with bulk and bin schemes come not from small differences in the condensation but from more significant differences in the evaporation of cloud water near cloud edges as a result of entrainment and mixing with the environment. For the deep convection, the model results show a significant dynamical impact of finite supersaturations and a strong microphysical effect associated with upper-tropospheric anvils. Implications of these results for modeling of convective dynamics will be discussed and a possible intermediate modeling methodology will be suggested.