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Oueslati B.,French National Center of Weather Research | Oueslati B.,University of Burgundy | Bellon G.,French National Center of Weather Research
Climate Dynamics | Year: 2015

The double intertropical convergence zone (ITCZ) bias still affects all the models that participate to CMIP5 (Coupled Model Intercomparison Project, phase 5). As an ensemble, general circulation models have improved little between CMIP3 and CMIP5 as far as the double ITCZ is concerned. The present study proposes a new process-oriented metrics that provides a robust statistical relationship between atmospheric processes and the double ITCZ bias, additionally to the existing relationship between the sea surface temperature (SST) and the double ITCZ bias. The SST contribution is examined using the THR-MLT index (Bellucci et al. in J Clim 5:1127–1145, 2010), which combines biases on the representation of local SSTs and the SST threshold leading to the onset of ascent in the double ITCZ region. As a metrics of a model’s bias in simulating the interaction between circulation and precipitation, we propose to use the Combined Precipitation Circulation Error (CPCE). It is computed as the quadratic error on the contribution of each vertical regime to the total precipitation over the tropical oceans. CPCE is a global measure of the circulation-precipitation coupling that characterizes the model physical parameterizations rather than the regional characteristics of the eastern Pacific. A linear regression analysis shows that most of the double ITCZ spread among CMIP5 coupled ocean–atmosphere models is attributed to SST biases, and that the precipitation large-scale dynamics relationship explains a significant fraction of the bias in these models, as well as in the atmosphere-only models. © 2015, Springer-Verlag Berlin Heidelberg.

Houet T.,French National Center for Scientific Research | Pigeon G.,French National Center of Weather Research
Environmental Pollution | Year: 2011

Facing the concern of the population to its environment and to climatic change, city planners are now considering the urban climate in their choices of planning. The use of climatic maps, such Urban Climate Zone-UCZ, is adapted for this kind of application. The objective of this paper is to demonstrate that the UCZ classification, integrated in the World Meteorological Organization guidelines, first can be automatically determined for sample areas and second is meaningful according to climatic variables. The analysis presented is applied on Toulouse urban area (France). Results show first that UCZ differentiate according to air and surface temperature. It has been possible to determine the membership of sample areas to an UCZ using landscape descriptors automatically computed with GIS and remote sensed data. It also emphasizes that climate behavior and magnitude of UCZ may vary from winter to summer. Finally we discuss the influence of climate data and scale of observation on UCZ mapping and climate characterization. © 2011 Elsevier Ltd. All rights reserved.

Bellon G.,French National Center of Weather Research
Climate Dynamics | Year: 2011

A simple coupled model is used in a zonally-symmetric configuration to investigate the effect of land-atmosphere coupling on the Asian monsoon intraseasonal oscillation. The atmospheric model is 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. The land model is the simple one-layer model SLand. The complete nonlinear version and a linear version of the model are used to understand how land-atmosphere interaction influences the northward-propagating intraseasonal oscillation that has been documented in the atmospheric model (Bellon and Sobel in J Geophys Res 113, 2008a, J Atmos Sci 65:470-489, 2008b). Our results show that this interaction damps the intraseasonal variability in most cases. The small heat capacity of land surfaces is the main factor that intervenes directly in the dynamics of the intraseasonal oscillation and explains the damping of intraseasonal variability. But in a few peculiar cases, the small heat capacity of land can also cause a strong interaction between the intraseasonal oscillation and the mean state via the nonlinearity of precipitation, that enhances the monsoon intraseasonal variability. High land albedo indirectly influences the intraseasonal variability by setting the seasonal mean circulation to conditions unfavorable for the monsoon intraseasonal oscillation. © 2010 Springer-Verlag.

Gueremy J.F.,French National Center of Weather Research
Tellus, Series A: Dynamic Meteorology and Oceanography | Year: 2011

A new and consistent convection scheme, providing continuous treatment of this atmospheric process, is described. The main concept ensuring the consistency of the whole system is buoyancy, a key element of any convective vertical motion. The buoyancy constitutes the forcing term of the convective vertical velocity, which is then used to define the triggering condition, the mass flux and the rates of entrainment-detrainment. The buoyancy is also used in its vertically integrated form to express the closure condition as a CAPE relaxation. The continuous treatment of convection from dry thermals to deep precipitating cumulus is made possible through the use of a continuous formulation of the entrainment-detrainment rates and CAPE relaxation time, together with an embedded precipitation scheme. This convection scheme is first evaluated with the help of single-column model simulations of specific case studies encompassing a variety of convective situations. Second, a coupled general-circulation model multiyear simulation is provided as a means to assess the model climate with respect to observations. ©2011 The Author Tellus A©2011 John Wiley & Sons A/S.

Bellon G.,French National Center of Weather Research | Sobel A.H.,University of Applied and Environmental Sciences
Journal of Climate | Year: 2010

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) 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. Further investigation of the feedbacks at play in the model shows that the assumed vertical structure of temperature variations is crucial to the existence and stability of the asymmetric equilibria. The free-tropospheric moisture-convection feedback must also be sufficiently strong to sustain asymmetric equilibria. Both results suggest that the specific physics of a given climate model condition determine the existence of multiple equilibria via the resulting sensitivity of the convection to free-tropospheric humidity and the vertical structure of adiabatic heating. The symmetry-breaking mechanism and resulting multiple equilibria have their origin in the localmultiple equilibria that can be described by a single-columnmodel using theweak temperature gradient approximation. An additional experiment using an SST latitudinal distribution with a relative minimum at the equator shows that the feedbacks controlling these multiple equilibria might be relevant to the double-ITCZ problem. © 2010 American Meteorological Society.

Montmerle T.,French National Center of Weather Research
Monthly Weather Review | Year: 2012

This study focuses on the impact of using specific background error covariances in precipitating areas in the Application of Research to Operations at Mesoscale (AROME-France) numerical weather prediction (NWP) system that considers reflectivities and radial velocities in its assimilation system. Such error covariances are deduced from the application of geographical masks on forecast differences generated from an ensemble assimilation of various precipitating cases. The retrieved forecast error covariances are then applied in an incremental three-dimensional variational data assimilation (3D-Var) specifically in rainy areas, in addition to the operational climatological background error covariances that are used elsewhere. Such heterogeneous formulation gives better balanced and more realistic analysis increments, as retrieved from the assimilation of radar data. For instance, midlevel humidification allows for the reinforcement of the low-level cooling and convergence, the warming in clouds, and high-level divergence. Smaller forecast error horizontal lengths explain the smaller-scale structures of the increments and render possible the increase of data densities in rainy areas. Larger error variances for the dynamical variables give more weight to wind observations such as radial winds. Areduction of the spinup is also shown and is positively correlated to the size of the area where rainy forecast error covariances are applied. Positive forecast scores on cumulated rain and on low-level temperature and humidity are finally displayed. ©2012 American Meteorological Society.

Beck J.,French National Center of Weather Research | Weiss C.,Texas Tech University
Monthly Weather Review | Year: 2013

Idealized supercell modeling has provided a wealth of information regarding the evolution and dynamics within supercell thunderstorms. However, discrepancies in conceptual models exist, including uncertainty regarding the existence, placement, and forcing of low-level boundaries in these storms, as well as their importance in low-level vorticity development. This study offers analysis of the origins of low-level boundaries and vertical vorticity within the low-level mesocyclone of a simulated supercell. Low-level boundary location shares similarities with previous modeling studies; however, the development and evolution of these boundaries differ from previous conceptual models. The rear-flank gust front develops first, whereas the formation of a boundary extending north of the mesocyclone undergoes numerous iterations caused by competing outflow and inflow before a steady-state boundary is produced. A third boundary extending northeast of the mesocyclone is produced through evaporative cooling of inflow air and develops last. Conceptual models for the simulation were created to demonstrate the evolution and structure of the low-level boundaries. Only the rear-flank gust front may be classified as a "gust front," defined as having a strong wind shift, delineation between inflow and outflow air, and a strong pressure gradient across the boundary. Trajectory analyses show that parcels traversing the boundary north of the mesocyclone and the rear-flank gust front play a strong role in the development of vertical vorticity existing within the low-level mesocyclone. In addition, baroclinity near the rear-flank downdraft proves to be key in producing horizontal vorticity that is eventually tilted, providing a majority of the positive vertical vorticity within the low-level mesocyclone. © 2013 American Meteorological Society.

Oueslati B.,French National Center of Weather Research | Bellon G.,French National Center of Weather Research
Journal of Climate | Year: 2013

The spurious double intertropical convergence zone (ITCZ) is a systematic bias affecting state-of-the-art coupled general circulation models (GCMs). Modeling studies show that the ITCZ structure is very sensitive to moist convection parameterization and in particular, to the vertical profile of convective heating and freetropospheric moistening. To further explore this sensitivity, the authors focus in this study on the influence of lateral entrainment in convective plumes on the simulated tropical precipitation and large-scale circulation. Sensitivity studies to the entrainment parameter were performed in a hierarchy of models (coupled ocean- atmosphere GCM, atmospheric GCM, and aquaplanet GCM), in order to mitigate the double ITCZ problem in the Centre National de Recherches Météorologiques Coupled Global Climate Model, version 5 (CNRMCM5). The sensitivity of the ITCZ structure to lateral entrainment is robust across our hierarchy of models. In response to increased entrainment, the realistic simulations exhibit a weakening of the southern side of the double ITCZ over the southeastern Pacific Ocean and a better representation of the South Pacific convergence zone (SPCZ). However, as a result of stronger moisture-convection feedbacks, precipitation is overestimated in the center of convergence zones. The change in ITCZ configuration is associated with a more realistic representation of the large-scale vertical regimes, explained by a decreased frequency of weak-tomoderate ascending regimes and an enhanced frequency of subsidence regimes. Mechanisms at play in this circulation change are examined by analyzing the vertically integrated dry static energy budget. This energetic analysis suggests that the feedback between large-scale dynamics and deep convection is crucial in controlling the probability distribution function (PDF) of midtropospheric vertical wind. This PDF, in turn, controls the precipitation distribution and, in particular, the double ITCZ bias. © 2013 American Meteorological Society.

Bergot T.,French National Center of Weather Research
Quarterly Journal of the Royal Meteorological Society | Year: 2013

In order to study the small-scale structure of radition fog, large-eddy simulations (LESs) of a fog case are analysed. The LESs were performed at very high resolution -2 m in the horizontal and 1 m in the vertical. Despite uncertainties in the measurements, particularly for advection, the main characteristics of the fog layer were well captured by the model. Radiation fog forms in statically stable stratification near the ground. During the formation phase, small stripes occur in the middle of the fog layer, associated with a significant burst in the turbulent kinetic energy (TKE). During the development phase, the dynamics of the fog layer change significantly. The maximum of the variance moves to the top of the fog layer where horizontal rolls appear clearly. These eddies have their centre near the mean top of the fog layer and have a depth corresponding to about one third of the fog layer height. This leads to a maximum of TKE at the top of fog, and to very strong scatter on the liquid water content. The energy is clearly produced at a length-scale corresponding to the fog height. The turbulence is 3D homogeneous inside the fog layer, while it is better characterized by 2D turbulence near the top. During the dissipation phase, the radiative heating of the surface increases the convective structure of the fog. The dissipation of fog at ground level takes a long time (about 2 h), even if the soil conditions are homogeneous. The top of the stratus layer is homogeneous, while the spread of the base height reaches a value typical of the cloud thickness. © 2012 Royal Meteorological Society.

Oueslati B.,French National Center of Weather Research | Bellon G.,French National Center of Weather Research
Climate Dynamics | Year: 2013

The atmospheric general circulation models ARPEGE-climate and LMDz are 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 about the equator, with varying off-equatorial SST gradients. In both models, we obtain the characteristic inter-tropical convergence zone (ITCZ) splitting that separates two regimes of equilibrium (in terms of precipitations): one with one ITCZ over the equator for large SST gradients in the tropics, and one with a double ITCZ for small tropical SST gradients. Transition between these regimes is mainly driven by changes in the low-level convergence that are forced by the SST gradients. Model-dependent, dry and moist feedbacks intervene to reinforce or weaken the effect of the SST forcing. In ARPEGE, dry advective processes reinforce the SST forcing, while a competition between sensible heat flux and convective cooling provides a complex feedback on the SST forcing in the LMDz. It is suggested that these feedbacks influence the location of the transition in the parameter range. © 2012 Springer-Verlag.

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