Seminaires

Upcoming seminars

Date: December 18, 2018
Time: 14h
Location: E314
By: Tom Beucler (MIT)
Title: Interaction between water vapor, radiation and convection in the Tropics
Abstract:

The interaction between convection and large-scale dynamics is a primary source of uncertainty in numerical simulations of the atmosphere, impeding our understanding of the climate. In large-scale atmospheric models, this uncertainty can be attributed to improperly-simulated interactions between atmospheric heating and water vapor across scales. Water vapor has a central role in the atmosphere: it is the most abundant greenhouse gas in the atmosphere, the main absorber of solar radiation in the troposphere. Water vapor is also intimately connected to atmospheric convection which lifts it, leading it to condense into clouds. Once formed, these clouds have even larger radiative effects. *However, we still lack a robust conceptual framework connecting water vapor and clouds to convection and radiation.

Using fluid dynamics, thermodynamics and spectral analysis tools, we investigate the interaction between water vapor, radiation and convection in observations of the tropical atmosphere and in high-resolution models of radiative-convective equilibrium. Radiative-convective equilibrium is the simplest model of the tropical atmosphere, in which convective heating balances radiative cooling in the absence of horizontal energy transport. We introduce a framework relating the evolution of the length scale at which convection organizes to the spatial spectra of radiative cooling, surface enthalpy fluxes, and horizontal energy transport. The cloud longwave radiative effect is most important, stretching humid and dry regions to scales of several thousand kilometers in the Tropics. These findings suggest that resolving the coherence between high, ice-cloud radiation and water vapor across the 1-10,000 km scale range is key to modeling tropical dynamics, and may considerably reduce our biases in modeling large-scale tropical precipitation patterns that are relevant for human activity.

Date: January 10, 2018
Time: TBA
Location: TBA
By: J. McWilliams (UCLA)
Title: TBA
Abstract:

TBA

Date: January 22, 2018
Time: 2pm
Location: E314
By: F. Ragone (Università degli Studi di Milano-Bicocca)
Title: Studying extreme climatic events with rare event algorithms applied to numerical climate models
Abstract:

A reliable quantification of the risk associated with extreme climatic events is crucial for policymakers, civil protection agencies and insurance companies. Studying extremes on a robust statistical basis with complex numerical climate models is however computationally challenging, since extreme events are rare, and thus very long simulations are needed to sample a significant number of them. I will discuss how the problem of sampling extremes in climate models can be tackled using rare event algorithms. Rare event algorithms are numerical tools developed in the past decades in mathematics and statistical physics, dedicated to the reduction of the computational effort required to sample rare events in dynamical systems. Typically they are designed as genetic algorithms, in which a set of cloning rules are applied to an ensemble simulation in order to focus the computational effort on the trajectories leading to the events of interest. I will present a rare event algorithm developed in the context of large deviation theory, and I will show how it can be used to sample very efficiently extreme European heat waves in simulations with the climate model Plasim. This allows to characterise the statistics of heat waves with return times up to millions of years, with computational costs three orders of magnitude smaller than with direct sampling. This allows to sample a large number of trajectories leading to very rare events, which can be used to study their characteristic dynamics, and also to observe ultra rare events that would have never been observed in a normal simulation. I will then discuss how these techniques can be applied to study a wide range of different processes with complex climate models.

Date: January 29 2019)
Time: TBA
Location: TBA
By: Hye-Yeong Chun (Yonsei University, South Korea)
Title: TBA
Abstract:

TBA

Past seminars

Date: September 25, 2018
Time: 13h
Location: E314
By: O. Pauluis (NYU)
Title: Atmospheric thermodynamics : The atmosphere as a heat engine
Abstract:

TBA

Date: October 1, 2018
Time: 14h30
Location: E314
By: O. Pauluis (NYU)
Title: Isentropic analysis for tropical cyclones
Abstract:

TBA

Date: October 10, 2018
Time: 11h
Location: E314
By: Hiro Masunaga (Nagoya University, Japan)
Title: A Mechanism for the Maintenance of Sharp Tropical Margins
Abstract:

The deep tropics characterized by moist air and deep convection are separated from the dry, quiescent subtropics often by a sharp horizontal gradient of moisture only loosely tied to SST or other geographical constraints. Mapes et al. (GRL, 2018) showed that this margin of the moist tropics is a true PDF minimum (a regime separatrix), along a column water vapor (CWV) contour around 48 mm in instantaneous data. Quasi-meridional statistical composites of observations across the poleward-most excursion of this sinuous contour retain the sharpness of the margin while increasing signal to noise ratio. Observations primarily from a suite of the A-Train satellites show the meridional structure of thermodynamic state and budget terms across the margin of the moist tropics. Composites are computed around the PDF-minimum CWV value of 48 mm as well as a range of other thresholds from 35 mm to 60 mm for comparison.

Major findings are summarized as follows. (1) CWV increases equatorward from the subtropics for all CWV thresholds but eventually converges to ~48 mm deep into the tropical side. Precipitation abruptly intensifies on the tropical side of the margin but declines equatorward to ~3 mm/d regardless of the CWV thresholds. (2) The diabatic forcing to the atmosphere (radiative heating plus surface heat flux) changes its sign across the CWV=48 mm border, being positive on the tropical side and negative on the subtropics. This contrast is owing to the meridional gradient of radiative heating, principally the longwave effect of high clouds. (3) Vertical mode decomposition applied to vertical moisture advection implies that the second -mode moistening is sharply enhanced on the subtropical side of the margin, suggesting that an efficient “congestus moistening” process may be at work as inflowing lower-tropospheric air masses approach the margin. This second-mode moistening changes abruptly to weaker first-mode advective moistening (with a modest fraction of the drying due to the abrupt jump of precipitation) once the air mass enters the tropics. These observed features are interpreted in terms of a simple theory from the moisture and heat budget perspectives.

Date: October 18, 2018
Time: 14h
Location: L369
By: Colin Grudzien, NERSC, Bergen, Norway
Title: A dynamical systems framework for ensemble based filtering: a problem partially solved
Abstract:

Séminaire SAMA (Groupe Statistiques pour l’Analyse, la Modélisation et l’Assimilation).
In physical applications, dynamical models and observational data play dual roles in prediction and uncertainty quantification, each representing sources of incomplete and inaccurate information. In data rich problems, first-principle physical laws constrain the degrees of freedom of massive data sets, utilizing our prior insights to complex processes. Respectively, in data sparse problems, dynamical models fill spatial and temporal gaps in observational networks. The dynamical chaos characteristic of these process models is, however, among the primary sources of forecast uncertainty in complex physical systems. Observations are thus required to update predictions where there is sensitivity to initial conditions and uncertainty in model parameters. Broadly referred to as data assimilation, or stochastic filtering, the techniques used to combine these disparate sources of information include methods from Bayesian inference, dynamical systems and optimal control. While the butterfly effect renders the forecasting problem inherently volatile, chaotic dynamics also put strong constraints on the evolution of errors. It is well understood in the weather prediction community that the growth of forecast uncertainty is confined to a much lower dimensional subspace corresponding to the directions of rapidly growing perturbations. The Assimilation in the Unstable Subspace (AUS) methodology of Trevisan et al. has offered understanding of the mechanisms governing the evolution of uncertainty in ensemble forecasting, exploiting this dimensional reduction prescribed by the dynamics. With my collaborators, I am studying the mathematical foundations of ensemble based filtering in the perspective of smooth and random dynamical systems.

Date: November 15, 2018
Time: 15h
Location: L378
By: Jun-Ichi Yano (Meteo France, Toulouse)
Title: Tropical Atmospheric Madden-Julian Oscillation: Strongly-Nonlinear Free Solitary Rossby Wave?
Abstract:

The Madden-Julian oscillation (MJO), a planetary-scale eastward propagating coherent structure with periods of 30–60 days, is a prominent manifestation of intraseasonal variability in the tropical atmosphere. It is widely presumed that small-scale moist cumulus convection is a critical part of its dynamics. However, the recent results from high-resolution modeling as well as data analysis suggest that the MJO may be understood by dry dynamics to a leading-order approximation. Simple, further theoretical considerations presented herein suggest that if it is to be understood by dry dynamics, the MJO is most likely a strongly nonlinear solitary Rossby wave. Under a global quasi-geostrophic equivalent-barotropic formulation, modon theory provides such analytic solutions. Stability and the longevity of the modon solutions are investigated with a global shallow water model. The preferred modon solutions with the greatest longevities compare overall well with the observed MJO in scale and phase velocity within the factors.

Date: December 5, 2018
Time: 14h
Location: E314
By: Guillermo Scheffler, Centro de Investigaciones del Mar y la Atmosfera, Buenos Aires
Title: Optimization of stochastic parameters using nested ensemble Kalman filters
Abstract:

Séminaire SAMA (Groupe Statistiques pour l’Analyse, la Modélisation et l’Assimilation).
Stochastic parameterizations have been successfully used to represent the uncertainty associated with unresolved scale processes for ensemble forecasting and data assimilation systems. In order to accurately describe the uncertainty associated to the dynamical model and data assimilation system, stochastic parameters have to be optimized. Such parameters are related to the stochastic perturbations amplitude and their spatial covariance structure. A novel technique based on hierarchical ensemble Kalman filters is introduced, aiming to infer these type of parameters. The technique is proposed to be applied offline as part of an a priori optimization of the data assimilation system and could in principle be extended to the estimation of other hyperparameters of a data assimilation system.

Date: December 12, 2018
Time: 11h
Location: Salle serre
By: G. Bellon (University of Auckland)
Title: Oscillation de Madden-Julian dans les modèles de climat :encore et toujours problématique
Abstract:
Abstract: La plupart des modèles de climat ont toujours des difficultés à simuler des évènements réalistes de l’Oscillation de Madden-Julian(OMJ). Deux hypothèses ont été avancées pour expliquer cette déficience généralisée des modèles. La première avance que les modèles ne simule pas correctement le profil dechauffage diabatique dû au dégagement de chaleur latente, à l’effet radiatif et au mélange vertical dans les nuages ou ensembles de nuages, et que cette erreur des modèles influe sur la réponse dynamique de l’atmosphère tropicale qui permet le développement et la propagation de la perturbation convective de l’OMJ. La seconde hypothèse considère que les modèles quasi-hydrostatique sont incapables desimuler l’organisation spatiale de la convection profonde nécessaire à l’initiation d’un évènement del’OMJ. Cette incapacité serait due à l’absence de représentation des processus sous-maille et inter-maille qui sont essentiels cette organisation. Ce séminaire présentera quelques travaux qui s’attachent à évaluer le mérite de ces deux hypothèses: une évaluation des profils de chauffage diabatique simulés par les modèles et une exploration théorique de ce qu’on peut attendre de l’organisation spatiale de la convection.

Date: December 14, 2018
Time: 14h
Location: Salle serre
By: R. Brecht (Memorial University of Newfoundland)
Title: A variational discretization of the rotating shallow water equations on the sphere
Abstract:

We develop a variational integrator for the shallow-water equations on a rotating sphere. The variational integrator is built around a discretization of the continuous Euler–Poincaré reduction framework for Eulerian hydrodynamics. We describe the discretization of the continuous Euler–Poincaré equations on arbitrary simplicial meshes. Standard numerical tests are carried out to verify the accuracy and the excellent conservational properties of the discrete variational integrator.