Seminaires

Upcoming seminars

Date: June 26, 2018
Time: 11h
Location: TBA
By: L. Renault (IRD)
Title: Surface Current Interactions with the Atmosphere and their Consequences for the Ocean Dynamic and the Biogeochemical Variability
Abstract:

The Ocean-Atmosphere interactions have a large influence on the climate and on the ecosystems at the basin scale. The main climatic modes of variability (e.g., El Nino, NAO, …) are coupled modes between the Ocean and the Atmosphere. The ecosystems have a strong response to those variations through the influence of the wind, the light, and the temperature on the nutrient stock and, thus, on the primary production and the oxygen concentration. Systematic biases in sea surface temperature in global models have highlighted the limitations of studies based on the global models and have, thus, spurred the investigation of the Ocean-Atmosphere interactions based on the regional modeling approach. In the past few years,it has been demonstrated that the regional Ocean-Atmosphere interactions can strongly modulate the variability and the mean physical and biogeochemical state of the ocean. In this presentation, the focus will be on the influence of the surface current on the atmosphere (i.e., current feedback). Based on satellite observations and using a set of regional coupled ocean and atmosphere simulations carried out over different regions encompassing Eastern and Western boundary current systems, we will illustrate to which extent those interactions can control the exchange of energy between the Ocean and the Atmosphere, the mean, mesoscale, and submesoscale circulations, and the Western Boundary Currents Dynamic. Implications for the net primary production and oxygen concentration will be discussed.

Date: July 5, 2018
Time: 14h30
Location: TBA
By: Edriss S. Titi, Texas A&M University and The Weizmann Institute of Science
Title: Downscaling Data Assimilation Algorithm for Dissipative Evolution Models Employing Coarse Mesh Observables
Abstract:

One of the main characteristics of infinite-dimensional dissipative evolution equations, such as the Navier-Stokes equations and reaction-diffusion systems, is that their long-time dynamics is determined by finitely many parameters – finite number of determining modes, nodes, volume elements and other determining interpolants. In this talk I will show how to explore this finite-dimensional feature of the long-time behavior of infinite-dimensional dissipative systems to design nudging downscaling data assimilation algorithms for weather prediction based on discrete coarse mesh measurements. Moreover, I will also demonstrate uniform in time error estimates of the numerical discretization of these algorithms, which makes reliable upon implementation computationally. Furthermore, I will also present some recent results concerning a statistical version of these
algorithms.

Date: July 5, 2018
Time: 16h
Location: TBA
By: Ibrahim Hoteit, Sciences and Engineering, and Applied Mathematics & Computational Science​​​​​​​​, King Abdullah University of Science and Technology , Arabie saoudite
Title: On the Development of an Ensemble Forecasting System for the Red Sea
Abstract:
The talk will present our efforts to develop an ensemble forecasting system for the Red Sea. The system implements GCMs at Kilometers scales capable of assimilating available observations based on distributions estimated from large ensembles that could be fully executed in parallel. By ‘large’, we are targeting ensembles with several thousands of members. We are also actively working on developing more advanced assimilation schemes to enhance the system outputs in term of computational cost and performances. In this context, I will briefly present three new ensemble Bayesian filters sharing in common « targeted » sampling strategies; the first one introduces a deterministic sampling of the observations perturbations in the stochastic EnKF, the second exploits the idea of filtering with one-step-ahead-smoothing, and the third resorts to a Gaussian-mixture update step. Results from the current version of the system implementation will be presented and future plans will be discussed.

Past seminars

Date: October 3, 2017

Time: 14h

Location: E314

By: Ali R. Mohebalhojeh, Institute of Geophysics, University of Tehran

Title: On the quantification of imbalance and inertia-gravity waves generated in numerical simulations of moist baroclinic waves

 

Abstract:

 Quantification of inertia–gravity waves (IGWs) generated by upper-level jet-surface front systems and their parametrization in global models of the atmosphere relies on suitable methods to estimate the strength of IGWs. A harmonic divergence analysis (HDA) that has been previously employed for quantification of IGWs combines wave properties from linear dynamics with a sophisticated statistical analysis to provide such estimates. A question of fundamental importance that arises is how the measures of IGW activity  provided by the HDA are related to the measures coming from the wave–vortex decomposition (WVD) methods. The question is addressed by employing the nonlinear balance relations of the first- order delta-gamma , the Bolin–Charney, and the first- to third-order Rossby-number expansion to carry out WVD. The global kinetic energy of IGWs given by the HDA and WVD are compared in numerical simulations of moist baroclinic waves by the Weather Research and Forecasting (WRF) model in a channel on the f plane. The estimates of the HDA are found to be two to three times smaller than those of the optimal WVD. This is in part due to the absence of a well-defined scale separation between the waves and vertical flows, the IGW estimates by the HDA capturing only the dominant wave packets and with limited scales. It is also shown that the difference between the HDA and WVD estimates is related to the width of the IGW spectrum.

Date: October 19, 2017

Time: 14h

Location: E314

By: Nili Harnik (Tel Aviv Univ.)

Title: Gravity waves in a moist atmosphere: A mechanistic picture

 

Abstract:

Motivated by the need to interpret the influence of convection schemes on tropical variability, the interaction between gravity-driven waves and moisture in a shallow water model is analyzed with an emphasis on physical interpretation. A Betts-Miller type convective parameterization is used, and analytical solutions for the influence of moisture on wave speed and stability are obtained, both at the limit of a vanishing convective relaxation timescale (or “strict quasi-equilibrium” (SQE)) and for finite relaxation timescales. We show that the divergence and moisture fields are exactly out of phase only when the system is at the SQE limit. A relaxation timescale dependent “gross moist stability” and equivalent depth are derived for both one-dimensional gravity waves and Kelvin waves.
The wavenumber dependence of the effect of moisture is also analyzed, and it is seen that for any given value of the convective relaxation time, the larger scale waves are always closer to SQE than the smaller scale waves, as a natural consequence of the equivalence between SQE and the moisture-divergence phasing. The phasing between the height, divergence and moisture fields is calculated, and the behavior of moist gravity and Kelvin waves for finite relaxation timescales is explained using the phase differences between the various fields. Using this analysis, physically based explanations are provided for the results of prior GCM-based studies.

Date: October 19, 2017

Time: 15h

Location: E314

By: Pablo Zurita Gotor  (Univ. Madrid)

Title: Superrotation and the spinup of equatorial eddy-driven jets

 

Abstract:

This talk will discuss the spinup of equatorial westerly jets by tropical eddies, relevant for the problem of planetary superrotation. Motivated by the well-established phenomenology of extratropical eddy-driven jets, most superrotation models have focussed on the latitudinal propagation of the eddies and hence on meridional eddy momentum fluxes. However, it will be argued in this talk that because tropical vorticity dynamics makes meridional momentum advection inefficient accelerating the mean flow in the tropics, equatorial superrotation will be driven by vertical momentum advection in many cases of interest. We first consider the limit of small absolute vorticity in the tropics, relevant for the equilibration of Kelvin-Rossby instability. In this limit, vorticity fluxes are small and meridional momentum advection weak. More generally, it will be argued that meridional advection can only decelerate the mean flow with weak vorticity forcing. This is shown to be the case in the Earth’s deep tropics with the exception of the Northern monsoon season.

Date: October 25, 2017

Time: 14h

Location: E314

By: Cesar Rocha (Scripps Institution of Oceanography)

Title: Extraction of energy from balanced flow by near-inertial waves

 

Abstract:

Primitive-equation numerical simulations and analysis of reduced models suggest that stimulated generation—the transfer of energy from balanced flows to existing internal waves—is a leading contender for an ocean mesoscale energy sink. Here we study stimulated generation using an asymptotic model that couples barotropic quasi-geostrophic flow and near-inertial waves with the exp(imz) structure on the f-plane. A detailed description of the conservation laws of this vertical plane-wave model illuminates the mechanism of stimulated generation associated with vertical vorticity and lateral strain. In particular, there are two sources of wave potential energy, and corresponding sinks of balanced kinetic energy: (1) the refractive convergence of the wave action density into anticyclones (and divergence from cyclones) and (2) enhancement of wave-field gradients by geostrophic straining.

We quantify the energy conversion and describe the phenomenology of stimulated generation using numerical solutions of decaying ocean macroturbulence modified by near-inertial waves. The initial conditions are a uniform inertial oscillation and a two-dimensional turbulent field emergent from random initial conditions. In all solutions, stimulated generation co-exists with a transfer of balanced kinetic energy to large scales, which is associated with vortex merger. And geostrophic straining accounts for most of the generation of wave potential energy, which represents a sink of 10-20% of the initial balanced kinetic energy. But refraction is fundamental because it creates the initial eddy- scale lateral gradients in the near-inertial field that are then enhanced by advection. In these quasi-inviscid solutions, wave dispersion is the only mechanism that upsets stimulated generation: with barotropic balanced flow, lateral straining enhances the wave group velocity; the waves accelerate and thus rapidly escape from the straining regions. Because of this wave escape, the wave field does not suffer a direct cascade to dissipative scales.

Date: October 27, 2017

Time: 14h

Location: *L378*

By: Sebastian Schemm (ETH Zurich)

Title: Understanding ENSO’s influence on extratropical cyclones and cyclogenesis over the North Atlantic.

 

Abstract:

El Niño-Southern Oscillation (ENSO) is arguably one of the strongest internal drivers of global weather variability.
While the North Atlantic was traditionally assumed to be one of the regions which are only marginally affected by ENSO, there is a renewed interested in how ENSO affects the North Atlantic circulation on the scale of individual weather systems. I present an overview of our recent research on variability in extratropical cyclogenesis downstream of the Rocky Mountains, over the Gulf Stream and at Greenland and how ENSO-related large-scale circulation anomalies affect cyclone formation in these regions. Surprisingly, there is a significant relationship between extratropical cyclogenesis over the Gulf Stream and leeward of the Rocky Mountains. During La Niña winters, the more active polar jet creates conditions amenable for cyclogenesis leeward of the Rocky Mountains chain. The more active polar jet is a consequence of stationary wave propagation from tropical sources and the stronger La Niña ridge in the northeast Pacific. The downstream propagation of equatorward oriented upper-level eddies, which are reinforced by the polar jet, cause cyclogenesis-friendly conditions at Greenland by pushing the jet poleward. The eddies cause an overall positive NAO response to La Niña as described in the previous literature. During El Niño winters, cyclogenesis occurs more frequently over the Gulf Stream, as a consequence of a more extended Pacific jet. There is an interesting difference between central Pacific and eastern Pacific El Niño-variantes. During the former, Gulf Stream cyclogenesis occurs below the jet exit region, during the latter it preferentially occurs below the jet’s entrance region. Accordingly, during central Pacific El Niño winters, the storm track anchor over the Gulf Stream shifts poleward.

Date: November 10, 2017

Time: 11h

Location: E314

By: Xiyue (Sally) Zhang (Environmental Science and Engineering, California Institute of Technology, Pasadena, CA)

Title: Sensitivities of Arctic clouds to climate change

 

Abstract:

The Arctic has been experiencing rapid changes in recent decades. In addition to the prominent sea ice loss, Arctic amplification is a robust feature in climate models under greenhouse warming and has also been observed over the past decades. Cloud feedbacks and the trapping of heat under the stable inversion are thought to contribute to this Arctic amplification of global warming. However, the sign of any cloud feedback in high latitudes is uncertain. Understanding and constraining Arctic cloud feedback is difficult because of the ubiquitous temperature inversion in high latitudes and the presence of mixed-phase clouds, both of which are challenging to capture in global climate models (GCMs). Here we use high-resolution large-eddy simulations (LES) with a one-moment mixed-phase microphysics scheme to investigate how Arctic clouds respond to climate changes. To represent changing large-scale conditions such as meridional moisture advection in a realizable way, we drive the LES with output from a GCM: temperature and moisture tendencies from a grid cell at 82N are used as forcing terms in the LES. We find that low-cloud fraction decreases with increasing temperature across a wide range of climates. The cloud fraction changes are associated with a change in cloud regimes from stratocumulus in the coldest climate, to cumulus in the warmest climate. This can be understood from changes in the saturation deficit, the difference between specific humidity and saturation specific humidity, which increases with temperature because relative humidity changes in the boundary layer are modest. We also investigate the relationship between lower-tropospheric stability and cloud fraction. A sensitivity experiment that uniformly increases temperature in an atmospheric columns shows that decreased cloud fraction can be associated with increased lower tropospheric stability, contrary to common beliefs. Implications for cloud feedbacks in the Arctic are discussed.

Date: November 21, 2017

Time: 14h

Location: E314

By: P. Yiou (LSCE)

Title: Analogues of circulation and stochastic weather generators

 

Abstract: 

Analogues of circulation have found many innovative uses in the past few years, including climate reconstructions, attribution of event and data assimilation. After presenting the gist of the methodology of analogues and its link with recurrences in dynamical systems, I will show how this methodology can be used to design stochastic weather generators with coherent spatial features. I will also argue that such tools can also be used to perform ensemble predictions of climate variables such as temperature. I will give several examples of such applications.

Date: November 29, 2017
Time: 14h
Location: E314
By: Hanin Binder (LMD)
Title: Warm conveyor belts: cloud structure and role for cyclone dynamics and extreme events

Abstract:

Warm conveyor belts (WCBs) are strongly ascending, cloud and precipitation producing airstreams in extratropical cyclones. The intense cloud-diabatic processes lead to tropospheric potential vorticity (PV) modifications in the lower and upper troposphere, which can have a profound impact on the evolution of the synoptic- and large-scale flow. In the early phase of the WCB ascent diabatic PV production leads to the formation of a strong low-level positive PV anomaly, while at upper levels a negative PV anomaly is generated in the WCB outflow. In this talk we will first look at WCBs in satellite observations from CloudSat and CALIPSO, in order to gain an observational perspective on their vertical cloud structure. The second part will address the question how WCBs and the associated low-level positive PV anomalies influence cyclone intensification. For a large climatological set of cyclones in Northern Hemisphere winter, it will be shown that diabatic PV production in WCBs is essential for the intensification of many explosively developing cyclones. Finally, we will turn our attention to the WCB-related negative PV anomalies produced in the upper troposphere. Based on the analysis of an extreme wintertime Arctic warm event, it will be illustrated that the low-PV air in the WCB outflow can interact with the extratropical waveguide and thereby contribute essentially to the formation of high-impact weather.
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Date: December 1, 2017
Time: 16h15
Location: L369
By: Louis-Philippe Nadeau (Institut des Sciences de la Mer de Rimouski)
Title: Antarctic Sea Ice Control on the Depth of the North Atlantic Deep Water

Abstract:

Changes in deep ocean circulation and stratification have been argued to contribute to climatic shifts between glacial and interglacial climates by affecting the atmospheric carbon dioxide concentrations. Recently, two hypotheses have suggested that changes in deep ocean circulation and stratification during glacial times are linked to changes in Antarctic sea ice: an increased latitudinal extent of Antarctic sea ice (Ferrari et. al., 2014) and an increased rate of Antarctic sea ice formation (Jansen and Nadeau, 2016). Both mechanisms rely on the upward shift of the Atlantic Meridional Overturning Circulation (AMOC) above depths where diapycnal mixing is strong (above 2000 m), thus decoupling the AMOC from the abyssal overturning circulation. Here, these two hypotheses are tested using a series of OGCM simulations in an idealized configuration using two basins connected by a channel to the south. In order to investigate independently the effect of an increased latitudinal ice extent from the effect of an increased ice formation rate, sea ice is parameterized as a latitude strip over which the buoyancy flux is negative. The results suggest that both mechanisms can effectively decouple the two cells of the MOC, and that their effects are additive. In order to illustrate the role of Antarctic sea ice in decoupling the AMOC and the abyssal overturning cell, the age of deep water masses are estimated. Both an increased latitudinal ice extent and increased sea ice formation rate yield a dramatic increase in water mass age at depth. Implications for a global warming scenario are also discussed.

Date: December 5, 2017
Time: 16h
Location: E350
By: Tonia Capuano (LMD)
Title: Small-scale ocean dynamics in the Cape basin and its impact on the regional circulation

Abstract:

This study addresses the role of oceanic small-scale processes in the formation and transformation of subsurface waters that participate in the Indo-Atlantic interocean exchange. We focus on the Cape Basin dynamics, characterized by a highly non-linear turbulence. We provide qualitative and quantitative evidence of the direct impact that meso- and submesoscale structures, their dynamical interactions and their seasonal variability have on the local thermocline and intermediate waters. A sequence of numerical simulations, ranging from ‘eddy-permitting’ to ‘submesoscale resolving’, underlines the importance of an adequate vertical resolution to correctly depict the water masses properties.

We point out that Agulhas eddies are mainly generated through baroclinic instabilities and are marked by a clear seasonality. This is linked to the seasonal occurrence of distinct meso-submesoscale instabilities in the upper layers: symmetric instabilities are at play during summer, while mixed-layer instabilities prevail in winter. We also found that Charney baroclinic instability connects these two submesoscale regimes and plays a major role in the seasonal formation of a newly-identified type of mode waters: Agulhas Rings Mode Water. Finally, we show that eddies of both polarity advect, stir and mix Antarctic Intermediate Water, via the mesoscale strain field producing filaments and T-S fine-scale structures.

Our results suggest the existence of two dynamical regimes affecting the upper and intermediate layers of the Cape Basin. Near the surface, the submesoscale-driven frontogenesis and their enhanced energetics lead to a predominance of ageostrophic dynamics. The intermediate depths are, instead, characterised by a quasi-gesotrophic regime due to the prevailing mesoscale effects.

Date: December 8, 2017
Time: 14h
Location: E314
By: P. Davini (ISAC-CNR, Torino, Italy)
Title: Winter Euro-Atlantic atmospheric blocking in climate models: from AMIP1 to present-day high-resolution simulations
Abstract:

The correct simulation of midlatitude atmospheric blocking has always been a main concern since the earliest days of numerical modeling of Earth’s atmosphere. We here assess 20 years of global climate model (GCM) developments by comparing with a common metric simulations from the AMIP1 (1992), the CMIP3 (2007), and the CMIP5 (2012) intercomparison projects. Although large improvements are seen over the Pacific Ocean, only minor advancements have been achieved over the Euro-Atlantic sector. Some of the most recent GCMs still exhibit the same negative bias as 20 years ago, associated with large geopotential height systematic errors. Some individual models, nevertheless, have improved and do show good performances in both sectors. Since increased horizontal resolution seems able to alleviate the Euro-Atlantic blocking bias, we thus analyze a set of atmosphere-only climate simulations produced with the EC-Earth GCM at five different horizontal resolutions (from 125 km to 16 km). Results show that the usual negative bias in blocking frequency over Europe becomes negligible at 40-km and 25-km resolution. A combined effect by the more resolved orography and by a change in tropical precipitation is identified as the source of an upper tropospheric planetary wave. At the same time, a weakening of the meridional temperature gradient reduces the upper level baroclinicity and the zonal mean winds. Following these changes, the high resolution configurations show a weakened Atlantic eddy-driven jet stream favoring the breaking of synoptic Rossby waves over the Atlantic ridge and thus increasing the simulated European blocking frequency. However, at high-resolution the simulated Atlantic jet stream is too weak and the blocking duration is still underestimated, suggesting that the optimal blocking frequencies are achieved through compensation of errors.

Date: December 12, 2017
Time: 9h30-11h30
Location: Salle Serre
Title: Advancing the understanding of the tropical tropopause composition and the forcing by the Asian summer monsoon dynamics: first results of the recent StratoClim and BATAL campaigns

Program:

09h30-10h00: Bernard Legras, LMD-ENS – Pathways of Asian pollution to the tropical tropopause region by the Asian summer monsoon and an introduction to the StratoClim aircraft campaign

10h00-10h30: Corinna Kloss, Forschungszentrum Jülich – Measurements of different surface pollutants (including sulphur and carbon compounds) by integrated cavity output spectroscopy with the AMICA instrument onboard the StratoClim aircraft: preliminary results

10h30-11h00: Silvia Bucci, LMD-ENS – Surface pollutant measurements and analysis of air masses origin for the StratoClim campaign

11h00-11h30: Gwenaël Berthet, LPCEE Orléans – First results of the balloon campaign BATAL

 

Date: December 14, 2017

Time: 14h

Location: E314

By: E. Horne (Ladhyx)

Title: Energetics aspects and irreversible mixing in stratified turbulence: numerical study

 

Abstract:

The local mixing produced by turbulence in the ocean interior plays a crucial role in its global energy budget. This mixing partially drives large scale dynamics, as evidence in the meridional overturning circulation (MOC). The circulation is produced thanks to the downward transport of energy from the surface to the deep bottom of the ocean, possible thanks to vertical mixing. Many processes produce mixing in the ocean, mostly forced by interior tides and winds. In addition, fine measurements of the density in the ocean show that the stratification can vary quite abruptly at small scales. Nevertheless, the proportion of energy transferred from turbulent structures to effective mixing is very difficult to measure in the ocean, and the details of the distribution of the injected energy is yet not fully understood. In order to answer these questions, a set of 3D Direct Numerical Simulations (DNS) of a turbulent stratified flow are performed by solving the Navier-Stokes equations under Boussinesq approximation. A classical Fourier pseudo-spectral method is used with 1024^3 grid points. A porous penalization region is introduced to take into account non-flux conditions at the bottom and at the top of the box. A turbulent velocity field is introduced at t=0 and perturbs the initially linear buoyancy profile which is then free to evolve in time.
The instantaneous irreversible mixing is compute by comparing the potential energy and the background potential energy of the system. The mixing efficiency, which is a form to measure how the injected energy is partitioned between available potential energy and kinetic energy, is computed for a broad range of degrees of stratification (characterized by the non-dimensional Richardson number, Ri). Our results indicate that the mixing efficiency presents a non trivial, yet simple, dependency with respect to the Richardson number. In addition, these results are captured by a statistical model developed by Venaille 2016. These results allow to improve notoriously the historical approach to model the mixing efficiency in oceanic conditions, which is often taken to be constant.

Date: January 25, 2018
Time: 14h
Location: E350
By: Paolo Davini, (ISAC-CNR, Torino, Italy)
Title: Stratocumulus clouds dynamics: coherent structures and the role of evaporative cooling.
Abstract:

The representation of stratocumulus clouds in Global Climate Models (GCM) is still problematic. Indeed, the different spatial scales at play and the complexity of the large set of phenomena that governs the Stratocumulus Topped Boundary Layer (STBL) still challenge present-day GCM parameterizations.
We here discuss two process-oriented approaches that – making use of high-resolution large eddy simulations – aims at improving our physical understanding and eventually at improving the representation of stratocumulus clouds in GCM.

First we show that the main convective coherent structures, namely updrafts, downdrafts and entrainment, are equally important to describe the STBL dynamics in terms of mass, heat and moisture fluxes. This is done with a novel approach based on two passive tracers able to detect and track each convective structure in the STBL. It is thus possible to estimate the sensitivity of the turbulent fluxes to the intensity of the cloud-top cooling, to the surface latent and sensible fluxes or to the strength of the wind shear.

In second place, we explore the role of evaporative cooling on the dynamics of a non-precipitating STBL. This is done by inhibiting the dynamical contribution of evaporative cooling to the buoyancy term in the vertical momentum equation, without directly affecting the conserved variables. By comparing the results from this simulation with a reference twin one we find that evaporative cooling is on average negligible and thus it does not influence boundary layer entrainment.

These two results are important for the representation of stratocumulus in climate models, as they suggest that in STBL 1) the turbulent transport from downdrafts is as important as the one from updrafts and should be included in any parametrization 2) only the slab-average cooling associatedwith evaporative cooling is relevant but that the exact spatial structure is not.

Date: January 26, 2018
Time: 14h
Location: E350
By: Achim Wirth (LEGI)
Title: A fluctuation-dissipation relation for the ocean subject to turbulent atmospheric forcing
Abstract:

We establish the fluctuation-dissipation relation for a turbulent fluid layer (ocean) subject to frictional forcing by a superposed lighter fluid layer (atmosphere), using a hierarchy of mathematical models. The fluctuation-dissipation relation reflects the fact that air-sea interaction not only injects energy in the ocean but also dissipates it. Energy injection and dissipation must therefore be related. The competition between the two processes determines the oceanic energy budget. When applying the fluctuation-dissipation relation to a two-dimensional two-layer Navier-Stokes model with turbulent dynamics, in the atmosphere and the ocean, coupled by a quadratic friction law, the friction parameter is estimated within 8 percent of the true value, while the estimation of the mass ratio between the atmosphere and the ocean fails, as the forcing time-scale is not faster than the characteristic time-scale of the atmospheric dynamics.

Date: February 1, 2018
Time: 14h
Location: E314
By: Benoit Semin (ESPCI)
Title: A model experiment of the quasi-biennial oscillation
Abstract:

The quasi-biennial oscillation is the periodic reversal of the wind in the lower equatorial stratosphere. The period of the oscillation is 28 months on average, and is not linked to the year duration. This wind is known to be generated by atmospheric waves, in particular internal gravity waves. We have set up an experiment which reproduces this phenomenon. Linearly stratified salty water is located between two plexiglas cylinders. Internal gravity waves are generated in the fluid using 16 membranes located at the top of the fluid. Each membrane oscillates sinusoidally in the vertical direction, in opposition of phase with its two neighbors: the wave is stationary in the azimuthal direction. When the amplitude of the forcing is large enough, a mean flow is generated, and oscillates with a period which is much larger than the wave period. This oscillation of the mean flow is similar to the one observed in the atmosphere. We have shown that the bifurcation can be either sub-critical or super-critical, depending on the parameters.

Date: February 8, 2018
Time: 14h
Location: E314
By: Peter Hitchcock (LMD)
Title: The dynamics of stratosphere-troposphere coupling
Abstract:

Variability and trends in the circumpolar stratospheric jets that form in the winter hemisphere has been increasingly recognized as having a significant influence on the eddy driven jets within the troposphere below (and consequently on surface weather and climate). This has been observed in both hemispheres on various time scales, and contributes to uncertainty in projections of circulation changes in the coming century.

The downward influence of extratropical stratospheric variability is also robustly observed in model simulation, yet a clear dynamical understanding of this coupling remains elusive. I will discuss what is known about the dynamics of this coupling and what remains unclear, with an emphasis on the broader value of resolving these questions.

Date: March 12, 2018
Time: 14h
Location: E314
By: Keshav J RAJA (LEGI)
Title: Forcing of mean flows by the reflection of three-dimensional internal wave beams
Abstract:

Internal waves are waves that occur inside a stratified medium such as oceans or atmosphere. Internal waves play an important role in many processes in oceans. The interaction between internal waves and ocean topography has been an active field of research for long. Yet there are many questions remaining on the topic. I am going to present our work on a specific process namely, the reflection of 3D internal wave beams on a slope, showing results from laboratory experiments and numerical simulations. The reflection of plane internal waves or two-dimensional internal wave beams on a slope has been studied a lot in the past. However, the case of a finite width three-dimensional internal wave beam has gained attention only very recently. In the case of reflection of three-dimensional internal wave beams, a strong mean horizontal flow is found to be induced by the wave beam, which perturbs the wave field and weakens the second harmonics. The generation and growth of this wave-induced mean flow are examined in detail using results from experiments and three-dimensional numerical simulations. Furthermore, the effects of wave parameters such as wave amplitude and wave beam width, and the influence of fluid viscosity on the induced mean flow are examined. Finally, the forcing of mean flow by a reflecting inertia-gravity wave beam is also examined for various values of Coriolis frequencies.

Date: March 15, 2018
Time: 14h
Location: E314
By: Hervé Herbin (LOA Lille)
Title: Etude des aérosols volcaniques par mesures infrarouges à haute résolution spectrale : du laboratoire à la télédétection.
Abstract:

Les aérosols sont des éléments clés du système climatique global, car ils jouent un rôle important dans de nombreux processus atmosphériques, tels que les propriétés radiatives, la chimie hétérogène, la formation des nuages ou les précipitations, dont beaucoup d’entre eux sont encore mal connus. En particulier, lors d’une éruption volcanique une quantité importante d’aérosols est émise dans l’atmosphère pouvant être transportée à longue distance, influencent grandement le bilan radiatif et pouvant également affecter les populations ou encore perturber le trafic aérien. Malgré d’importants efforts en observations et modélisation, leurs impacts sur le système Terre-Atmosphère est estimé jusqu’à présent avec une grande incertitude. Ceci s’explique par le fait qu’une meilleure connaissance des effets des aérosols nécessite une bonne détermination de nombreux paramètres tels que la composition chimique ou minéralogique, la distribution en taille, l’épaisseur optique, ou encore le régime de diffusion, qui varient avec le nombre d’ondes et le milieu environnent.
Bien qu’initialement exploitées pour les analyses quantitatives des espèces en phase gazeuse, les mesures IR à haute résolution spectrale ont récemment démontré leur potentiel pour la caractérisation des aérosols, bénéficiant de nombreux avantages tels qu’une bonne sensibilité à la nature des particules, la mesure possible de jour comme de nuit ou encore la possibilité de restituer simultanément la composition gazeuse. Notamment, il est possible, à partir d’instruments de type spectromètre à transformée de Fourier, de quantifier la nature, la granulométrie et la concentration des constituants atmosphériques liquides et solides. Toutefois, ces derniers sont encore très peu exploités à cause de la médiocre connaissance des propriétés optiques aérosols. Chaque type de particules possède à la fois sa propre structure vibrationnelle en absorption et son propre régime de diffusion, ce qui se traduit par une grande diversité et variabilité spectrale des indices de réfraction. De fait, l’indice complexe de réfraction, qui est le paramètre qui relie les propriétés physico-chimique et les propriétés optiques des aérosols, apparait alors comme la source principale d’incertitude. Or, les quelques bases de données qui regroupent des valeurs d’indices de refraction (GEISA, HITRAN, ARIA) mettent en lumière à quel point l’information sur les liquides et les solides est considérablement moindre que pour les espèces gazeuses.
Ainsi, au cours de ce séminaire sera présentée une méthodologie (expérimentale et théorique) permettant d’établir les propriétés optiques des aérosols, notamment d’origine volcanique. Puis, nous verrons comment ces informations peuvent être utilisées dans le cadre de la télédétection depuis le sol et l’espace.

Date: April 12, 2018
Time: 14h
Location: E314
By: Brian Arbic (University of Michigan)
Title: Global Modeling of Internal Tides and the Internal Gravity Wave Continuum
Abstract:

We discuss our efforts to model internal tides and the internal gravity wave continuum on a global scale. We discuss results from US Navy HYCOM simulations, NASA MITgcm simulations, and efforts to compare both to observations. We also discuss the relevance of these simulations to the CNES/NASA SWOT wide-swath altimeter mission.

Date: April 19, 2018
Time: 14h
Location: E314
By: B. Nadiga (Los Alamos National Lab)
Title: Multiscale interactions and non-hydrostatic modeling of the ocean and atmosphere
Abstract:

Improvements in both algorithms and high performance computing resources now permit global atmospheric simulations to be performed at resolutions at which non-hydrostatic effects become significant. As such, we consider HOMME-NH, a variable-resolution, efficient and architecture-aware, non-hydrostatic dynamical core that has been developed under the US DOE’s earth system modeling initiative. We study the nonlinear evolution of an unstable baroclinic wave in both the hydrostatic and non-hydrostatic settings with an aim of understanding how attendant dynamical interactions across scales differ in the two settings. Our aim in this study is to try to anticipate how biases in climate models may change when non-hydrostatic dynamics are resolved in the atmosphere.

Linkages between the larger scale balanced modes and smaller scale, imbalanced modes play an important role in ocean circulation as well. For example, the question of how ocean circulation equilibrates in the presence of continuous large-scale forcing and a tendency of geostrophic turbulence to confine energy to large and intermediate scales is related to such linkages. Again, by considering the nonlinear evolution of an unstable baroclinic wave at small Rossby and Froude numbers (and in a small aspect ratio domain) at high resolutions, we show that submesoscale instabilities provide an interior pathway between the energetic oceanic mesoscales and smaller unbalanced scales. Phenomenology-wise, mesoscale shear and strain resulting from the primary baroclinic instability drive frontogenesis; fronts in turn support ageostrophic secondary circulation and instabilities. These two processes together lead to a quick rise in dissipation rate which then reaches a peak and begins to fall as frontogenesis slows down; eventually balanced and imbalanced modes decouple. Dissipation of balanced energy by imbalanced processes is shown to scale exponentially with Rossby number of the base flow. Further, a break is seen in the total energy (TE) spectrum at small scales with a transition from −3 to −5/3 reminiscent of the atmospheric spectra of Nastrom and Gage.

Date: April 20, 2018
Time: 14h
Location: E314
By: R. Caballero (Univ. Stockholm)
Title: Arctic warming and sea ice decline driven by atmospheric moist intrusions.
Abstract:

The Arctic is the region where the most dramatic climate change is expected. There is increasing observational evidence that the Arctic winter climate is strongly controlled by filamentary intrusions of moist, warm air–akin to the atmospheric rivers of lower latitudes–which cross the entire basin. I will discuss how the statistical distribution of these intrusions is affected by large-scale atmospheric circulation, particularly midlatitude storm track activity and blocking events; how the intrusions affect surface temperature and sea ice concentration; and how they contribute to Arctic extreme events.

Date: May 11, 2018
Time: 11h
Location: E314
By: Yanxu Chen (McGill)
Title: A shallow-water model forced by flow-dependent Ekman pumping
Abstract:

Date: May 24, 2018
Time: 14h
Location: E314
By: D. Flack (LMD)
Title: Convective-Scale Perturbation Growth: Ensembles and Beyond
Abstract:

Flooding from intense rainfall, resulting from convection, causes millions of euros of damage each year. However, convection has limited predictability often resulting in short lead times for warnings of such events. I present work from my PhD and subsequent post-doc which aimed to determine the spatial scale of perturbation growth across the UK from simple stochastic perturbations, and the impact of perturbation growth from a physically-based stochastic boundary layer scheme compared to initial and boundary condition perturbations. These themes are examined using convective-scale configurations of the Met Office Unified Model. The spatial perturbation growth is shown to be influenced by the environment in which it occurs, with environments leading to more organised convection having more localised growth compared to those resulting in scattered showers. Furthermore, through the use of a super-ensemble it is shown that growth from a stochastic boundary layer scheme can equal that of initial and boundary conditions. Evidence is also presented that shows neighbourhood post-processing techniques can artificially increase the ensemble size, allowing the production of more reliable probabilities of rainfall events being forecast. These results have implications both within operational forecasting centres and the research community in the interpretation of convective-scale ensembles for highly localised precipitating events.

Date: June 11, 2018
Time: 11h
Location: E314
By: G. Meneghello (MIT)
Title: Hurricane monitoring using controllable atmospheric ballon
Abstract:

In-situ observations of tropical storms (hurricanes, cyclones, and typhoons) are essential for improving both storm’s forecasts and our understanding of their genesis and development. Despite the continuous development of satellite based products, in-situ observations represent an essential contribution to the initialization of forecast models. They are also difficult, costly, and dangerous to obtain: the design of in-situ observational platforms often requires a tradeoff between persistency — the ability to observe for a long period of time — and controllability — the ability to direct observations to particularly interesting areas. We will talk about how swarms of buoyancy-controlled atmospheric balloons can be employed as an observational platform with the ability to bridge the gap between persistency and controllability.

Date: June 13, 2018
Time: 14h
Location: E314
By: A. Venaille (ENS Lyon)
Title: Topological origin of equatorial waves
Abstract:

The concept of topologically-protected transport along the edge of physical systems was born three decades ago in the context of quantum Hall electronics. Waves are protected from disorder and backscattering when emerging at the boundary separating bulk materials characterized by different topological invariants. These invariants are integers characterizing singularities in families of waves, and can be related to the number of unidirectional edge waves propagating in the system. Physicists realized recently that topological protection applies to virtually all areas of physics from photonics, to cold atoms, to classical mechanical systems, and have related these properties to the presence of broken discrete symmetries. However the interplay between discrete symmetries and topology have so far played little role in thinking about the fluid dynamics of oceans and atmospheres. We show that, as a consequence of the rotation of the Earth that breaks time reversal symmetry, equatorially trapped Kelvin and Yanai waves have a topological origin, manifesting as edge modes in rotating shallow water flows.

Date: June 21, 2018
Time: 14h
Location: E314
By: Sylvia C. Sullivan (Columbia)
Title: Structural and hydrological changes in mesoscale convective systems with ENSO phase
Abstract:

In the tropics, mesoscale convective systems (MCS) cause significant surface warming from their extensive anvils and heavy rainfall from their cores. But the adjustment of these radiative and hydrological impacts according to the phase of the El Niño Southern Oscillation remains unclear. To understand such changes, we construct tropical climatologies of MCS occurrence, lifetime, and extent between 1983 and 2008 using an ISCCP convective tracking database and differentiating for El Niño or La Niña months. System frequency, duration, and size overlap increase dramatically in regions of anomalously high sea surface temperature and convective available potential energy during El Niño. With an MCS “growth metric”, we show that horizontal spreading of MCS relative to their vertical deepening is largest in these same regions. This thermodynamic story can be modulated by the dynamics in a particular region, illustrated by the shear profiles and convergence in different subdomains. Whether their explanation is thermodynamic or dynamic, MCS structural changes are reflected in outgoing longwave radiation anomalies (OLRa) and varying responses of precipitation accumulation and intensity (PA). In particular, we see that more extensive systems during El Niño induce more negative OLRa and larger PA.