Frédéric Hourdin

LMDZ5B: the atmospheric component of the IPSL climate model with revisited parameterizations for clouds and convection

Based on a decade of research on cloud processes, a new version of the LMDZ atmospheric general circulation model has been developed that corresponds to a complete recasting of the parameterization of turbulence, convection and clouds. This LMDZ5B version includes a mass-flux representation of the thermal plumes or rolls of the convective boundary layer, coupled to a bi-Gaussian statistical cloud scheme, as well as a parameterization of the cold pools generated below cumulonimbus by re-evaporation of convective precipitation. The triggering and closure of deep convection are now controlled by lifting processes in the sub-cloud layer. An available lifting energy and lifting power are provided both by the thermal plumes and by the spread of cold pools. The individual parameterizations were carefully validated against the results of explicit high resolution simulations. Here we present the work done to go from those new concepts and developments to a full 3D atmospheric model, used in particular for climate change projections with the IPSL-CM5B coupled model. Based on a series of sensitivity experiments, we document the differences with the previous LMDZ5A version distinguishing the role of parameterization changes from that of model tuning. Improvements found previously in single-column simulations of case studies are confirmed in the 3D model: (1) the convective boundary layer and cumulus clouds are better represented and (2) the diurnal cycle of convective rainfall over continents is delayed by several hours, solving a longstanding problem in climate modeling. The variability of tropical rainfall is also larger in LMDZ5B at intraseasonal time-scales. Significant biases of the LMDZ5A model however remain, or are even sometimes amplified. The paper emphasizes the importance of parameterization improvements and model tuning in the frame of climate change studies as well as the new paradigm that represents the improvement of 3D climate models under the control of single-column case studies simulations.

Impact of the LMDZ atmospheric grid configuration on the climate and sensitivity of the IPSL-CM5A coupled model

The IPSL-CM5A climate model was used to perform a large number of control, historical and climate change simulations in the frame of CMIP5. The refined horizontal and vertical grid of the atmospheric component, LMDZ, constitutes a major difference compared to the previous IPSL-CM4 version used for CMIP3. From imposed-SST (Sea Surface Temperature) and coupled numerical experiments, we systematically analyze the impact of the horizontal and vertical grid resolution on the simulated climate. The refinement of the horizontal grid results in a systematic reduction of major biases in the mean tropospheric structures and SST. The mid-latitude jets, located too close to the equator with the coarsest grids, move poleward. This robust feature, is accompanied by a drying at mid-latitudes and a reduction of cold biases in mid-latitudes relative to the equator. The model was also extended to the stratosphere by increasing the number of layers on the vertical from 19 to 39 (15 in the stratosphere) and adding relevant parameterizations. The 39-layer version captures the dominant modes of the stratospheric variability and exhibits stratospheric sudden warmings. Changing either the vertical or horizontal resolution modifies the global energy balance in imposed-SST simulations by typically several W/m2 which translates in the coupled atmosphere-ocean simulations into a different global-mean SST. The sensitivity is of about 1.2xa0K per 1xa0W/m2 when varying the horizontal grid. A re-tuning of model parameters was thus required to restore this energy balance in the imposed-SST simulations and reduce the biases in the simulated mean surface temperature and, to some extent, latitudinal SST variations in the coupled experiments for the modern climate. The tuning hardly compensates, however, for robust biases of the coupled model. Despite the wide range of grid configurations explored and their significant impact on the present-day climate, the climate sensitivity remains essentially unchanged.

The Present and Future of the West African Monsoon: A Process-Oriented Assessment of CMIP5 Simulations along the AMMA Transect

AbstractThe present assessment of the West African monsoon in the models of the Coupled Model Intercomparison Project (CMIP) phase 5 (CMIP5) indicates little evolution since the third phase of CMIP (CMIP3) in terms of both biases in present-day climate and climate projections.The outlook for precipitation in twenty-first-century coupled simulations exhibits opposite responses between the westernmost and eastern Sahel. The spread in the trend amplitude, however, remains large in both regions. Besides, although all models predict a spring and summer warming of the Sahel that is 10%–50% larger than the global warming, their temperature response ranges from 0 to 7 K.CMIP5 coupled models underestimate the monsoon decadal variability, but SST-imposed simulations succeed in capturing the recent partial recovery of monsoon rainfall. Coupled models still display major SST biases in the equatorial Atlantic, inducing a systematic southward shift of the monsoon. Because of these strong biases, the monsoon is further ...

Superrotation of Venus' atmosphere analyzed with a full general circulation model

[1]xa0A general circulation model (GCM) has been developed for the Venus atmosphere, from the surface up to 100 km altitude, based on the GCM developed for Earth at our laboratory. Key features of this new GCM include topography, diurnal cycle, dependence of the specific heat on temperature, and a consistent radiative transfer module based on net exchange rate matrices. This allows a consistent computation of the temperature field, in contrast to previous GCMs of Venus atmosphere that used simplified temperature forcing. The circulation is analyzed after 350 Venus days (111 Earth years). Superrotation is obtained above roughly 40 km altitude. Below, the zonal wind remains very small compared to observed values, which is a major pending question. The meridional circulation consists of equator-to-pole cells, the dominant one being located within the cloud layers. The modeled temperature structure is globally consistent with observations, though discrepancies persist in the stability of the lowest layers and equator-pole temperature contrast within the clouds (10 K in the model compared to the observed 40 K). In agreement with observational data, a convective layer is found between the base of the clouds (around 47 km) and the middle of the clouds (55–60 km altitude). The transport of angular momentum is analyzed, and comparison between the reference simulation and a simulation without diurnal cycle illustrates the role played by thermal tides in the equatorial region. Without diurnal cycle, the Gierasch-Rossow-Williams mechanism controls angular momentum transport. The diurnal tides add a significant downward transport of momentum in the equatorial region, causing low latitude momentum accumulation.

Parameterization of the Dry Convective Boundary Layer Based on a Mass Flux Representation of Thermals

Presented is a mass flux parameterization of vertical transport in the convective boundary layer. The formulation of the new parameterization is based on an idealization of thermal cells or rolls. The parameterization is validated by comparison to large eddy simulations (LES). It is also compared to classical boundary layer schemes on a

The Art and Science of Climate Model Tuning

AbstractThe process of parameter estimation targeting a chosen set of observations is an essential aspect of numerical modeling. This process is usually named tuning in the climate modeling community. In climate models, the variety and complexity of physical processes involved, and their interplay through a wide range of spatial and temporal scales, must be summarized in a series of approximate submodels. Most submodels depend on uncertain parameters. Tuning consists of adjusting the values of these parameters to bring the solution as a whole into line with aspects of the observed climate. Tuning is an essential aspect of climate modeling with its own scientific issues, which is probably not advertised enough outside the community of model developers. Optimization of climate models raises important questions about whether tuning methods a priori constrain the model results in unintended ways that would affect our confidence in climate projections. Here, we present the definition and rationale behind model...

AMMA-Model Intercomparison Project

The African Monsoon Multidisciplinary Analyses-Model Intercomparison Project (AMMA-MIP) was developed within the framework of the AMMA project. It is a relatively light intercomparison and evaluation exercise of both global and regional atmospheric models, focused on the study of the seasonal and intraseasonal variations of the climate and rainfall over the Sahel. Taking advantage of the relative zonal symmetry of the West African climate, one major target of the exercise is the documentation of a meridional cross section made of zonally averaged (10°W–10°E) outputs. This paper presents the motivations and design of the exercise, and it discusses preliminary results and further extensions of the project.

Sub‐surface nuclear tests monitoring through the CTBT Xenon Network

We present the first evaluation of the atmospheric xenon network to be installed as part of the International Monitoring System (IMS) in the frame of the Comprehensive Test Ban Treaty (CTBT). We show that this network should, by itself, provide a significant contribution to the total efficiency of the IMS. For this evaluation, we introduce an inverse approach based upon the time symmetry of the atmospheric transport of trace species. This approach may find applications in a variety of environmental problems.

Control of deep convection by sub-cloud lifting processes: the ALP closure in the LMDZ5B general circulation model

Recently, a new conceptual framework for deep convection scheme triggering and closure has been developed and implemented in the LMDZ5B general circulation model, based on the idea that deep convection is controlled by sub-cloud lifting processes. Such processes include boundary-layer thermals and evaporatively-driven cold pools (wakes), which provide an available lifting energy that is compared to the convective inhibition to trigger deep convection, and an available lifting power (ALP) at cloud base, which is used to compute the convective mass flux assuming the updraft vertical velocity at the level of free convection. While the ALP closure was shown to delay the local hour of maximum precipitation over land in better agreement with observations, it results in an underestimation of the convection intensity over the tropical ocean both in the 1D and 3D configurations of the model. The specification of the updraft vertical velocity at the level of free convection appears to be a key aspect of the closure formulation, as it is weaker over tropical ocean than over land and weaker in moist mid-latitudes than semi-arid regions. We propose a formulation making this velocity increase with the level of free convection, so that the ALP closure is adapted to various environments. Cloud-resolving model simulations of observed oceanic and continental case studies are used to evaluate the representation of lifting processes and test the assumptions at the basis of the ALP closure formulation. Results favor closures based on the lifting power of sub-grid sub-cloud processes rather than those involving quasi-equilibrium with the large-scale environment. The new version of the model including boundary-layer thermals and cold pools coupled together with the deep convection scheme via the ALP closure significantly improves the representation of various observed case studies in 1D mode. It also substantially modifies precipitation patterns in the full 3D version of the model, including seasonal means, diurnal cycle and intraseasonal variability.

High resolution simulation of the South Asian monsoon using a variable resolution global climate model

This study examines the feasibility of using a variable resolution global general circulation model (GCM), with telescopic zooming and enhanced resolution (~35 km) over South Asia, to better understand regional aspects of the South Asian monsoon rainfall distribution and the interactions between monsoon circulation and precipitation. For this purpose, two sets of ten member realizations are produced with and without zooming using the LMDZ (Laboratoire Meteorologie Dynamique and Z stands for zoom) GCM. The simulations without zoom correspond to a uniform 1° × 1° grid with the same total number of grid points as in the zoom version. So the grid of the zoomed simulations is finer inside the region of interest but coarser outside. The use of these finer and coarser resolution ensemble members allows us to examine the impact of resolution on the overall quality of the simulated regional monsoon fields. It is found that the monsoon simulation with high-resolution zooming greatly improves the representation of the southwesterly monsoon flow and the heavy precipitation along the narrow orography of the Western Ghats, the northeastern mountain slopes and northern Bay of Bengal (BOB). A realistic Monsoon Trough (MT) is also noticed in the zoomed simulation, together with remarkable improvements in representing the associated precipitation and circulation features, as well as the large-scale organization of meso-scale convective systems over the MT region. Additionally, a more reasonable simulation of the monsoon synoptic disturbances (lows and disturbances) along the MT is noted in the high-resolution zoomed simulation. On the other hand, the no-zoom version has limitations in capturing the depressions and their movement, so that the MT zone is relatively dry in this case. Overall, the results from this work demonstrate the usefulness of the high-resolution variable resolution LMDZ model in realistically capturing the interactions among the monsoon large-scale dynamics, the synoptic systems and the meso-scale convective systems, which are essential elements of the South Asian monsoon system.