Faculty of Physics University of Warsaw > Events > Seminars > Atmospheric Physics Seminar

Atmospheric Physics Seminar

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room B0.14, Pasteura 5 at 13:15

prof. dr hab. Wojciech Grabowski (National Center for Atmospheric Research, Boulder, Colorado)

Shallow-to-deep transition of continental moist convection: cold pools, surface fluxes, and mesoscale organization

Diurnal cycle of convection over summertime continents starts with the formation and subsequent growth of a well-mixed convective boundary layer from the early morning temperature and moisture profiles as the surface sensible and latent heat fluxes increase after sunrise. It proceeds with the formation of shallow convective clouds as the convective boundary layer deepens, and leads to the eventual transition from shallow to deep precipitating convection around local noon. This problem is notoriously difficult to represent in large scale models of weather and climate, even with the so-called convection-permitting models that are becoming routinely used in NWP. This talk will present a model intercomparison case that I developed about a dozen years ago that is still being used to explore various aspects of the transition. Selected results from several studies using this setup, the most recent by Kurowski et al. (under review in J. Atmos. Sci.) will be discussed.

room B0.14, Pasteura 5 at 13:15

prof. dr hab. Piotr Flatau (Scripps Institution of Oceanography, University of California, San Diego)

Evidence for a Nimbostratus Uncinus in a Convectively Generated Mixed-Phase Stratiform Cloud Shield

The structure of a melting layer associated with a mesoconvective system is examined using a combination of in situ aircraft measurements and a unique Doppler radar operated by the U.S. Navy that has a range resolution as fine as 0.5 m. Interest in this case was motivated by ground-based all-sky camera images that captured the transient development of midlevel billow cloud structures within a precipitating trailing stratiform cloud shield associated with a passing deep convective system. A sequence of high-fidelity time–height radar measurements taken of this storm system reveal that the movement of the billow cloud structure over the radar site corresponded with abrupt transitions in the observed low-level precipitation structure. Of particular note is an observed transition from stratiform to more periodic and vertically slanted rain shaft structures that both radar and aircraft measurements indicate have the same temporal periodicity determined to arise visually between successive billow cloud bands. Doppler, balloon, and aircraft measurements reveal these transient bands are associated with a shallow circulation field that resides just above the melting level in a layer of moist neutral stability and strong negative vertical wind shear. The nature of these circulations and their impact on the evolving precipitation field are described in the context of known nimbostratus cloud types.

room B0.14, Pasteura 5 at 13:15

prof. dr hab. Wojciech Grabowski, prof. afiliowany UW (National Center for Atmospheric Research)

Can the impact of aerosols on deep convection be isolated from effects of meteorology in atmospheric observations?

Influence of atmospheric aerosols on clouds, deep convection in particular, continues to be a controversial topic. Arguably, only carefully designed numerical simulations can clearly separate impact of aerosols from effects of other factors, such as the atmospheric sounding, magnitude surface fluxes, or effects of larger-scale perturbations (e.g., waves) that affect atmospheric stability. This is virtually impossible using observations because of the insufficient accuracy of atmospheric measurements and the fundamental nature of the interaction between convection and its environment. This presentation will provide a general overview of the impact of aerosols on clouds and convection, and it will discuss results from numerical simulations supporting the above statements. In particular, I will argue that the answer to the question posed in the title is negative.

room B0.14, Pasteura 5 at 13:15

dr hab. Krzysztof Markowicz, prof. UW (Instytut Geofizyki, Wydział Fizyki, Uniwersytet Warszawski)

Fizyka smogu

Smog jest zjawiskiem, które budzi w ostatnim czasie w Polsce wiele kontrowersji. Głośno o smogu zrobiło się, gdy na rynku pojawiły się proste czujniki pomiarowe służące do pomiaru jakości powietrza. Umożliwiło to pomiar PM10/PM2.5 w miejscach gdzie do tej pory nie były prowadzone profesjonalne pomiary. Z drugie strony pojawiło się wiele kontrowersji, co do jakości danych pozyskanych z bardzo prostych i tanich czujników. W ramach seminarium dyskutowany będzie ww. problem, w szczególności ograniczenia i wiarygodność sensorów optycznych. Jednak głównym tematem wykładu będą procesy fizyczne, które są odpowiedzialne za powstawanie, rozwój oraz zanik smogu. Omawiane będą w szczególności warunki meteorologiczne, które sprzyjają pojawianiu się epizodów aerozolowych w Polsce w chłodnej porze roku. Poruszona będzie kwestia różnicy pomiędzy smogiem a mgłą w kontekście wielkości mikrofizycznych i optycznych. Ponadto, związany z tym tematem problem wzrost higroskopijnego cząstek aerozolu w warunkach wysokiej wilgotności powietrza. W ramach wykładu omawiane będą również długookresowe zmiany zanieczyszczenia powietrza w Polsce w oparciu o pomiary fotometryczne, monitoring jakości powietrza oraz bazy danych zawierające informacje o emisjach zanieczyszczeń.

room B0.14, Pasteura 5 at 13:15

mgr Jakub Nowak (Instytut Geofizyki, Wydział Fizyki, Uniwersytet Warszawski)

Measurement campaign ACORES 2017

Azores stratoCumulus measurements Of Radiation, turbulEnce and aeroSols (ACORES) was a helicopter-borne campaign carried out in June-July 2017 around the archipelago of Azores in the northeastern Atlantic with the base located on the island of Graciosa, next to the permanent observatory run by ARM (Atmospheric Radiation Measurement) programme. The project aim at investigating the small-scale structure of stratocumulus-topped marine boundary layer, in particular the entrainment interface layer at its top. High resolution collocated in-situ airborne measurements of dynamics, thermodynamics, microphysics and aerosols were performed with the use of two instrumental platforms flown as an external cargo under the helicopter: ACTOS (Airborne Cloud Turbulence Observation System) and SMART-HELIOS (Spectral Modular Airborne Radiation measurement sysTem HELIcopter-borne Observations of Spectral Radiation). University of Warsaw contributed with a dedicated model of a new UltraFast Thermometer (UFT-2) which served on ACTOS to record temperature fluctuations. The presentation will describe the goals of the campaign, summarize research flights and discuss performance of the thermometer.

room B0.14, Pasteura 5 at 13:15

dr Dariusz Baranowski (Instytut Geofizyki PAN)

About tropical cyclones

Each year natural hazards put millions of people in distress, creating enormous social and economical impacts. Recently, 2017 Atlantic hurricane season drew a lot of attention due to high frequency of hurricanes developing over the region and devastating Caribbean islands. In 2013, typhoon Haiyan, strongest system ever recorded to hit the land, devastated central Philippines causing over 6000 casualties and many more leaving roof-less. Last years have seen tremendous progress in observations, understanding and forecasting of tropical cyclones world wide, and particularly in Atlantic sector after Katrina disaster. However, better tools in forecasting and information dissemination are required to better address modern societal needs.

During the seminar, I will talk about physics of tropical cyclone development and evolution, overview progress in our understanding of the key processes responsible for both local and regional variability, and show examples of advances in observations techniques and forecasting. The 2017 Atlantic hurricane season will be analyzed in context of large scale (ENSO, MJO) and regional (SST anomaly, Azores high) features. Finally, I will talk about extended (subseasonal-to-seasonal) forecasting and present a seasonal forecast based outlook into 2018 hurricane season.

room 1.01, Pasteura 5 at 13:15

dr hab. Bogdan Rosa (Institute of Meteorology and Water Management – National Research Institute)

Dynamics of cloud droplets in turbulent flows

In recent years, pseudo-spectral direct numerical simulations (DNS) have emerged as an important research tool for studying statistics, structure and dynamics of small-scale turbulence and transport of the dispersed phase. Such studies are used, among others, to characterize the effects of air turbulence on the growth of cloud droplets during warm rain initiation. Modeling of these processes is a quite challenging task due to the wide range of scales involved (both spatial and temporal).

In this study, the effects of air turbulence on the growth of cloud droplets during warm rain initiation is quantified. Turbulence can enhance the rate of collision-coalescence and as such provides a mechanism to overcome the gap between the diffusional growth and the gravitational collision-coalescence mechanism. Several specific issues related to geometric collisions (without droplet–droplet aerodynamic interaction) of the same-size particles will be discussed. These include: the effect of the large-scale forcing mechanisms, the effect of the flow Reynolds number or equivalently the range of flow scales represented in DNS and the role of gravity. A thorough analysis of these effects is necessary for developing better parameterizations for numerical weather prediction models which, in turn, will allow to develop more accurate weather forecasts and deepen our knowledge of the climate change.

The research tool employed for modeling cloud processes is an innovative DNS code that allows to integrate the Navier-Stokes equations using pseudo-spectral method. The code is massively parallel MPI application, designed to run on supercomputers with distributed memory. It enables performing high-resolution DNS of turbulent collisions so the simulation results can be used to address the question of Reynolds number dependence of pair and collision statistics. Moreover, larger domains (equivalently larger Reynolds number) make the simulations closer to the physical conditions.
Bieżące informacje o seminarium: Dynamics of cloud droplets in turbulent flows

room 1.01, Pasteura 5 at 13:15

prof. dr hab. Wojciech Grabowski (National Center for Atmospheric Research [NCAR])

Modeling of cloud microphysics: can we do better?

Traditional cloud modeling methodologies apply the continuous medium approach for all thermodynamic variables, not only for the temperature and water vapor, but also for cloud condensate and precipitation. Continuous in time and space Eulerian variables used to represent cloud and precipitation particles are mass and sometimes number mixing ratios in bulk schemes and mass and/or number spectral density mixing ratios in bin schemes. Such a methodology has been the workhorse of cloud-scale modeling from its early days. However, there are challenges in applying such approaches due to numerical diffusion in the physical space and in the particle mass (or size) space for bin schemes, difficulty in representing aerosol processing by clouds, and inability to properly represent unresolved spatial scales that arguably play a significant role in the development of the particle size/mass spectra. This presentation will discuss problems with the Eulerian methodology and introduce a particle-based Lagrangian approach that is gaining popularity in cloud-scale modeling. Application of this approach to the problem of droplet spectral broadening in warm shallow clouds will illustrate key advantages of the method. Prospects of applying the Lagrangian particle-based methodology to more complex simulations involving clouds will be discussed.
Bieżące informacje o seminarium: Modeling of cloud microphysics: can we do better?

room 1.01, Pasteura 5 at 13:15

dr Gustavo Abade (Institute of Geophysics Faculty of Physics WU)

Broadening of cloud droplet spectra through turbulent entrainment and eddy hopping

This seminar is concerned with the effects of cloud turbulence and turbulent entrainment on the CCN activation and evolution of the cloud droplet-size spectrum. We simulate an ensemble of idealized turbulent cloud parcels that are subject to stochastic entrainment events.

Turbulent mixing inside the parcel, supersaturation fluctuations, and the resulting stochastic droplet activation and growth by condensation are simulated using a Monte Carlo scheme. We show that turbulence plays an important role in aiding entrained CCN to activate, broadening the droplet-size distribution towards both small and large sizes.
Bieżące informacje o seminarium: Broadening of cloud droplet spectra through turbulent entrainment and eddy hopping

room 1.01, Pasteura 5 at 13:15

dr Andrzej Wyszogrodzki (IMGW - PIB)

Numerical solver of the Navier-Stokes equations in an anelastic approximation and its application to multiscale geophysical flows

Because of the enormous range of spatial and temporal scales in geophysical flows, explicit integration of generic compressible equations with existing computational resources are prohibitively expensive for the majority of applications. In order to account for a broad range of scales, one has to invoke analytic or numerical approximations that allow for a relatively large time-step integrations of the governing equations. In effect, meteorological models encompass a variety of approximate systems of fluid equations tailored to the spatial and temporal scales of the investigated phenomena.

This talk summarizes overall author's research presented as the habilitation achievement, i.e. the development of computational methods for solving Navier-Stokes equations in the anelastic approximation. Methods these are based on accurate non-oscillatory forward-in-time (NFT) algorithms formulated on structured refined grid or unstructured meshes within the framework of EULAG model and applied to many research problems in a broad range - from micro O(10-2 m) to planetary (104 m) - of physical scales.
Bieżące informacje o seminarium: Numerical solver of the Navier-Stokes equations in an anelastic approximation and its application to multiscale geophysical flows

room 1.01, Pasteura 5 at 13:15

dr Jędrzej Bojanowski (Centrum Teledetekcji, Instytut Geodezji i Kartografii w Warszawie)

Satellite cloud climatology: retrieval, evaluation and climate analysis

The talk will focus on retrieval and validation of cloud fraction climatologies derived from polar-orbiting (AVHRR) and geostationary sensors (MVIRI, SEVIRI) in a frame of ESA Cloud_cci and CMSAF projects. Validation of both datasets requires a high frequency of ground-based observations to allow for: timely collocations (for polar-orbiting satellites) and resolved diurnal cycle (for geostationary satellites). We will present a new dataset of 10-minute cloud amount estimates calculated from ground measurements of long-wave incoming radiation by means of the Bayesian Automatic Cloud Detection Algorithm (BACADA) at the Baseline Surface Radiation Network. Finally, we will look at climatological changes of cloud fraction in the last 3 decades, as well as at the consistency among trends and variability of cloud fraction, surface solar radiation and top-of-atmosphere radiation over Europe.

room 1.01, Pasteura 5 at 13:15

dr Piotr Dziekan (IGF UW)

Stochastic coalescence in Lagrangian cloud microphysics

Coalescence of hydrometeors is commonly modeled using the Smoluchowski equation. It is a mean-field equation that does not capture the stochastic nature of coalescence. More exact methods include the DNS and the master equation. The super-droplet method (SDM), which is a Lagrangian method for modeling cloud microphysics, is another alternative. It is shown that the SDM with multiplicities equal to 1 is in agreement with the master equation.

Next, we use SDM simulations to determine validity of more approximate methods: the Smoluchowski equation and the SDM with mulitplicities greater than 1. In the latter, we determine how many computational droplets are necessary to correctly model the expected number and the standard deviation of the autoconversion time.

Then, SDM is used to study stochastic effects relevant for rain formation: fluctuations in the autoconversion time and lucky droplets. Size of the coalescence cell is found to strongly affect system behavior. In small cells, correlations in droplet sizes and droplet depletion slow down rain formation. In large cells, collisions between rain drops are more frequent and this also can slow down rain formation. The increase in the rate of collision between rain drops may be an artefact caused by assuming a too large well-mixed volume. The highest ratio of rain water to cloud water is found in cells of intermediate sizes. Maximal size of a volume that is turbulently well-mixed with respect to coalescence is estimated at V = 1.5·10−2 cm3. The Smoluchowski equation is not valid in such small volumes. Implications for LES modeling are discussed.

room 1.01, Pasteura 5 at 13:15

dr Jesper G. Pedersen (IGF UW)

Anisotropy of observed and simulated turbulence in the upper part of the stratocumulus-topped boundary layer

Stratocumulus clouds play a central role in the radiative balance of the Earth, and thorough understanding of their structure and dynamics is essential for accurate climate- and weather-prediction. Laboratory and numerical experiments have in combination with field measurements over the last few decades greatly improved our knowledge about how the stratocumulus-topped boundary layer (STBL) evolves, but important questions still remain open; e.g. regarding the role of turbulence. In this study, we use results from large-eddy simulations and observations from the POST and DYCOMS-II field campaigns to examine properties of turbulence near the cloud top. More specifically, we determine the level of anisotropy across a range of scales and investigate how this may affect cloud-top entrainment - the process in which the STBL grows upwards over time. Focusing on turbulence ~100 m below the cloud top, we see remarkable similarity between daytime and nocturnal flight data covering different inversion strengths and free-tropospheric conditions. Typical-resolution LES of the STBL (based on POST flight 13 and DYCOMS-II flight 1) captures the observed characteristics of below-cloud-top turbulence reasonably well, but the simulation results are sensitive to changes in grid spacing and the choice of subgrid-scale mixing length. Using a fixed vertical grid spacing of 5 m, we find that decreasing the horizontal grid spacing and increasing the subgrid-scale mixing length leads to increased dominance of vertical fluctuations, increased entrainment velocity and decreased liquid water path. Our analysis supports the suggestion that entrainment parametrizations (used in e.g. climate models) could potentially be improved by accounting more accurately for anisotropic deformation of turbulence in the cloud-top region.

room 1.01, Pasteura 5 at 13:15

dr Dariusz Baranowski (IGF UW)

South Asia monsoon: contribution of atmosphere – ocean interactions over Bay of Bengal to seasonal variability in atmospheric circulation

Monsoon refers to rainy phase of seasonally reversing winds and precipitation anomalies in a presence of asymmetric heating of land and ocean surfaces. Globally, major monsoon systems consist of West African and Asia-Australian monsoons as well as North and South American monsoons. However, the latter one is associated with incomplete reversal of winds.

South Asian monsoon occurs from July to September. Weather systems that form over the Bay of Bengal (BoB) and then move over land contribute a significant amount of the rainfall received over India, Bangladesh and Myanmar annually. Monsoon depressions that form over the BoB and then move along the monsoon trough provide about 50% of the total rainfall over central India. Air-sea interaction over the BoB has been known to play a central role in nurturing such systems, owing to its ability to sustain a high SST through the season. A hitherto unexplored region important to the spatial patterns of the summer monsoon is the southern BoB where the features of the ocean as well as the overlying atmosphere are unique in many respects effectively weakening or intensifying monsoon circulation on intraseasonal time scale.

Bay of Bengal Boundary Layer Experiment (BoBBLE) was designed to obtain high quality in situ data sets of the ocean, air-sea interface and atmosphere during the summer monsoon in the southern BoB. Observational array was created across BoB at 8N and served by CTD stations, uCTD transects, 5 gliders and 7 Argo floats, as well as ship-borne surface fluxes, radiosondes and ADCP measurements.

This presentation will provide an overview of the BoBBLE field campaign conducted on board RV Sindhu Sadhana, during break phase of 2016 South Asia monsoon (June-July, 2016) and preliminary results thus obtained.

room B0.14, Pasteura 5 at 13:15

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

Perturbation equations for all-scale atmospheric dynamics

An important characteristic of the atmospheric dynamics is that it constitutes a relatively small perturbation about dominant balances of hydrostacy, geostrophy and thermal winds established in effect of the Earth gravity, rotation, stably-stratified thermal structure of its atmosphere and the incoming flux of solar energy. Given this specificity, it is compelling to formulate the governing partial differential equations (PDEs) in terms of perturbation variables, defined with respect to an arbitrary "ambient" state of the atmosphere that already satisfies these dominant balances. The role of ambient states is to enhance the efficacy of numerical solution---e.g. by simplifying the design of the initial and boundary conditions and/or improving the conditioning of elliptic boundary value problems---without resorting to linearisation of the system. This talk introduces a new generalised theory, formulated for velocity (vector) perturbations as well as entropy and pressure perturbations about an arbitrary ambient state. The theoretical development is complete, from analytic equations to non-oscillatory forward-in-time finite-volume integrators, including all entries of the linear operator and associated coefficients of the Helmholtz problem arising from the semi-implicit solution procedure.