Gilles Bellon - Academia.edu (original) (raw)

Papers by Gilles Bellon

The size distribution of moist regions in the tropical atmosphere exhibits two modes of organizat... more The size distribution of moist regions in the tropical atmosphere exhibits two modes of organization, one at synoptic scales and the other at planetary scales. We show the existence of these two modes in two data sets: columnintegrated precipitable water and outgoing long-wave radiation. A simple two-column model of radiation and convection in the Tropics helps interpret the presence of these two modes. The coupling between radiative effects and low-level atmospheric dynamics provides a plausible mechanism for scale selection.

A model of intermediate complexity based on quasi-equilibrium theory --- a version of the Quasi-e... more A model of intermediate complexity based on quasi-equilibrium theory --- a version of the Quasi-equilibrium Tropical Circulation Model with a prognostic atmospheric boundary layer, as well as two free-tropospheric modes in momentum, and one each in moisture and temperature --- is used in a zonally-symmetric aquaplanet configuration to study the sensitivity of the Hadley circulation to the Sea Surface Temperature (SST) forcing. For equatorially symmetric SST forcing with large SST gradients in the tropics, the model simulates the classical double Hadley cell with one equatorial Intertropical Convergence Zone (ITCZ). For small SST gradients in the tropics, the model exhibits multiple equilibria, with one equatorially symmetric equilibrium and two asymmetric equilibria (mirror images of each other) with an off-equatorial ITCZ. Further investigation of the feedbacks at play in our model shows that the vertical structure of the wind is crucial to the existence and stability of the asymmetric equilibria in our model. The free-tropospheric moisture-convection feedback must also be sufficiently strong to sustain asymmetric equilibria. These results are used to improve our understanding of the convection pattern and Hadley circulation in the Eastern Pacific. We use our model to try to reproduce the observed regional Hadley cell and its seasonal variance. This sheds some light on the biases of General Circulation Models in this region.

Geophysical Research Letters, 2003

A four-box model of the tropical climate is used to assess its sensitivity to changes in the rela... more A four-box model of the tropical climate is used to assess its sensitivity to changes in the relative area of the moist and dry regions and to radiative perturbations. The feedbacks modulating this sensitivity are analyzed, particularly the dynamical feedbacks associated with the surface fluxes. The link between the large-scale circulation and these fluxes is found to play a crucial role in the tropics' sensitivity to radiative perturbations.

Journal of Climate, Oct 1, 2008

A simple coupled model is used in a zonally symmetric aquaplanet configuration to investigate the... more A simple coupled model is used in a zonally symmetric aquaplanet configuration to investigate the effect of ocean-atmosphere coupling on the Asian monsoon intraseasonal oscillation. The model consists of a linear atmospheric model of intermediate complexity based on quasi-equilibrium theory coupled to a simple, linear model of the upper ocean. This model has one unstable eigenmode with a period in the 30-60-day range and a structure similar to the observed northward-propagating intraseasonal oscillation in the Bay of Bengal/west Pacific sector.

Agu Fall Meeting Abstracts, Dec 1, 2008

We will present evidence, much of it old but some of it new, for the hypothesis that interactive ... more We will present evidence, much of it old but some of it new, for the hypothesis that interactive feedbacks involving surface heat fluxes are important to the dynamics of tropical intraseasonal oscillations. Both cloud- radiative feedbacks and surface turbulent flux feedbacks are probably important. Evidence in favor of this hypothesis includes the observed spatial distribution of intraseasonal variance in precipitation and outgoing longwave radiation, the observed relationship between intraseasonal latent heat flux and precipitation anomalies in regions where intraseasonal variability is strong, and sensitivity experiments performed with a small number of general circulation and idealized models. We will argue that it would be useful to assess the importance of surface fluxes to intraseasonal variability in a larger number of comprehensive numerical models, including the new generation of global high-resolution nonhydrostatic models. Such an assessment could provide insight into the relevance of interactive surface fluxes to real intraseasonal variability, perhaps making it possible to rule out either theoretical explanations in which surface fluxes are crucial, or those in which they are not.

The Atmospheric General Circulation Model ARPEGE-Climat is used in an aquaplanet configuration to... more The Atmospheric General Circulation Model ARPEGE-Climat is used in an aquaplanet configuration to study the response of a zonally symmetric atmosphere to a range of Sea Surface Temperature (SST) forcing. We impose zonally-symmetric SST distributions that are also symmetric ...

Journal of Geophysical Research: Atmospheres, 2015

ABSTRACT The “Vertical structure and physical processes of the Madden-Julian oscillation (MJO)” p... more ABSTRACT The “Vertical structure and physical processes of the Madden-Julian oscillation (MJO)” project comprises three experiments, designed to evaluate comprehensively the heating, moistening, and momentum associated with tropical convection in general circulation models (GCMs). We consider here only those GCMs that performed all experiments. Some models display relatively higher or lower MJO fidelity in both initialized hindcasts and climate simulations, while others show considerable variations in fidelity between experiments. Fidelity in hindcasts and climate simulations are not meaningfully correlated. The analysis of each experiment led to the development of process-oriented diagnostics, some of which distinguished between GCMs with higher or lower fidelity in that experiment. We select the most discriminating diagnostics and apply them to data from all experiments, where possible, to determine if correlations with MJO fidelity hold across scales and GCM states. While normalized gross moist stability had a small but statistically significant correlation with MJO fidelity in climate simulations, we find no link with fidelity in medium-range hindcasts. Similarly, there is no association between time step to time step rainfall variability, identified from short hindcasts and fidelity in medium-range hindcasts or climate simulations. Two metrics that relate precipitation to free-tropospheric moisture—the relative humidity for extreme daily precipitation and variations in the height and amplitude of moistening with rain rate—successfully distinguish between higher-fidelity and lower fidelity GCMs in hindcasts and climate simulations. To improve the MJO, developers should focus on relationships between convection and both total moisture and its rate of change. We conclude by offering recommendations for further experiments.

Climate Dynamics, 2015

The double Intertropical Convergence Zone (ITCZ) bias still affects all the models that partic-6 ... more The double Intertropical Convergence Zone (ITCZ) bias still affects all the models that partic-6 ipate to CMIP5 (Coupled Model Intercomparison Project, phase 5). As an ensemble, general circulation 7 models have improved little between CMIP3 and CMIP5 as far as the double ITCZ is concerned. 8 The present study proposes a new process-oriented metrics that provides a robust statistical relationship 9 between atmospheric processes and the double ITCZ bias, additionally to the existing relationship between 10 the sea surface temperature (SST) and the double ITCZ bias.

Journal of the Atmospheric Sciences, 2012

A simple framework to study the sensitivity of atmospheric boundary layer (ABL) models to the lar... more A simple framework to study the sensitivity of atmospheric boundary layer (ABL) models to the large-scale conditions and forcings is introduced. This framework minimizes the number of parameters necessary to describe the large-scale conditions, subsidence, and radiation. Using this framework, the sensitivity of the stationary ABL to the large-scale boundary conditions [underlying sea surface temperature (SST) and overlying humidity and temperature in the free troposphere] is investigated in large-eddy simulations (LESs). For increasing SST or decreasing free-tropospheric temperature, the LES exhibits a transition from a cloudfree, well-mixed ABL stationary state, through a cloudy, well-mixed stationary state and a stable shallow cumulus stationary state, to an unstable regime with a deepening shallow cumulus layer. For a warm SST, when increasing free-tropospheric humidity, the LES exhibits a transition from a stable shallow cumulus stationary state, through a stable cumulus-under-stratus stationary state, to an unstable regime with a deepening, cumulus-under-stratus layer. For a cool SST, when increasing the free-tropospheric humidity, the LES stationary state exhibits a transition from a cloud-free, well-mixed ABL regime, through a well-mixed cumulus-capped regime, to a stratus-capped regime with a decoupling between the subcloud and cloud layers.

Journal of the Atmospheric Sciences, 2005

Two-layer bulk models of the cloudy boundary layer are investigated, focusing on the consistency ... more Two-layer bulk models of the cloudy boundary layer are investigated, focusing on the consistency issues between common assumptions on the turbulent mixing and the thermodynamic profiles. Simplifications of the vertical structure of the conserved variables are shown to lead to inconsistent steady states. First, linear profiles in the cloud layer are incompatible with the common parameterizations of the turbulent mixing. Second, this approximation is inconsistent with simple models of an infinitely thin inversion layer. Simple mass-flux parameterizations of the turbulence are shown to produce reasonable profiles inside the cloud layer without solving the problem of the inversion. The cloud Ϫ environment difference and the vertical gradient of the conserved variables appear as interesting test variables for such models, as well as their ability to match the free-tropospheric boundary conditions. The latter certainly requires a unified approach for the cloud and inversion layers.

Journal of the Atmospheric Sciences, 2008

A model of intermediate complexity based on quasi-equilibrium theory—a version of the quasi- equi... more A model of intermediate complexity based on quasi-equilibrium theory—a version of the quasi- equilibrium tropical circulation model with a prognostic atmospheric boundary layer, as well as two free- tropospheric modes in momentum, and one each in moisture and temperature—is used in a zonally sym- metric aquaplanet configuration to simulate aspects of the South Asian monsoon and its variability. Key qualitative

Journal of the Atmospheric Sciences, 2009

This paper examines the influence of imposed drying, intended to represent horizontal advection o... more This paper examines the influence of imposed drying, intended to represent horizontal advection of dry air, on parameterized deep convection interacting with large-scale dynamics in a single-column model framework. Two single-column models, one based on the NASA Goddard Earth Observing System general circulation model version 5 (GEOS5) and the other developed by Bony and Emanuel, are run in weak temperature gradient mode. Drying is imposed by relaxation of the specific humidity field toward zero within a specified vertical layer. The strength of the drying is controlled by specifying either the relaxation time scale or the vertically integrated drying tendency; results are insensitive to which specification is used.

Journal of the Atmospheric Sciences, 2013

The adjustment of the trade-wind atmospheric boundary layer to an 1 abrupt sea-surface warming is... more The adjustment of the trade-wind atmospheric boundary layer to an 1 abrupt sea-surface warming is investigated using an LES and two simple bulk 2 models: a mixed-layer model (MLM), and a model based on the mixing-line 3 hypothesis (XLM). The near-surface temperature adjusts in a few hours, faster 4 than can be expected from the characteristic timescales associated with the 5 physical processes at play. The near-surface humidity adjusts more slowly, 6 with a timescale of about a day, and it exhibits an initial decrease before 7 increasing to its equilibrium value.

Journal of Geophysical Research, 2008

1] A linear model of intermediate complexity based on quasi-equilibrium theory is used in a zonal... more 1] A linear model of intermediate complexity based on quasi-equilibrium theory is used in a zonally symmetric aquaplanet configuration to investigate the stability of the mean summer monsoon flow in the south Asian region. In the control case, the mean state has one linearly unstable mode that corresponds in period and structure to the variability in the nonlinear model described by and to the observed 30-to 60-d mode of intraseasonal variability. This mode features propagation of the precipitation anomalies from the equator to the monsoon trough around 20°N. Both the period of this mode and its direction of propagation are determined by the direction and magnitude of the mean meridional baroclinic flow. The wind-induced surface heat fluxes associated with the surface westerlies in the northern tropics are an important contributor to the instability of the mode. The mechanisms of propagation and reinitiation of convection are further described. (1) Meridional advection of vorticity by the mean flow is responsible for the creation of free-tropospheric vorticity north of the convection center. This vorticity in turn creates boundary layer convergence via Coriolis acceleration and subsequent northward movement of the convection. (2) Meridional advection of zonal momentum creates equatorial free-tropospheric easterlies on the equator. These easterlies in turn generate low-level convergence via the b term of the Coriolis acceleration and cause the reemergence of convection at the equator. Citation: Bellon, G., and A. H. Sobel (2008), Instability of the axisymmetric monsoon flow and intraseasonal oscillation, J. Geophys.

Journal of Climate, 2008

Journal of Climate, 2010

Tropical Circulation Model with a prognostic atmospheric boundary layer, as well as two free-trop... more Tropical Circulation Model with a prognostic atmospheric boundary layer, as well as two free-tropospheric modes in momentum, and one each in moisture and temperature-is used in a zonally symmetric aquaplanet configuration to study the sensitivity of the Hadley circulation to the sea surface temperature (SST) latitudinal distribution. For equatorially symmetric SST forcing with large SST gradients in the tropics, the model simulates the classical double Hadley cell with one equatorial intertropical convergence zone (ITCZ). For small SST gradients in the tropics, the model exhibits multiple equilibria, with one equatorially symmetric equilibrium and two asymmetric equilibria (mirror images of each other) with an off-equatorial ITCZ.

Journal of Climate, 2013

This is the first part of a series of two articles analyzing the global thermal properties of atm... more This is the first part of a series of two articles analyzing the global thermal properties of atmosphere-ocean coupled general circulation models (AOGCMs) within the framework of a two-layer energy-balance model (EBM). In this part, the general analytical solution of the system is given and two idealized climate change scenarios, one with a step forcing and one with a linear forcing, are discussed. These solutions give a didactic description of the contributions from the equilibrium response and of the fast and slow transient responses during a climate transition. Based on these analytical solutions, a simple and physically based procedure to calibrate the two-layer model parameters using an AOGCM step-forcing experiment is introduced. Using this procedure, the global thermal properties of 16 AOGCMs participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) are determined. It is shown that, for a given AOGCM, the EBM tuned with only the abrupt 43CO 2 experiment is able to reproduce with a very good accuracy the temperature evolution in both a step-forcing and a linear-forcing experiment. The role of the upper-ocean and deep-ocean heat uptakes in the fast and slow responses is also discussed. One of the main weaknesses of the simple EBM discussed in this part is its ability to represent the evolution of the top-of-the-atmosphere radiative imbalance in the transient regime. This issue is addressed in Part II by taking into account the efficacy factor of deepocean heat uptake.