H. Douville

The CNRM-CM5.1 global climate model: description and basic evaluation

A new version of the general circulation model CNRM-CM has been developed jointly by CNRM-GAME (Centre National de Recherches Météorologiques—Groupe d’études de l’Atmosphère Météorologique) and Cerfacs (Centre Européen de Recherche et de Formation Avancée) in order to contribute to phase 5 of the Coupled Model Intercomparison Project (CMIP5). The purpose of the study is to describe its main features and to provide a preliminary assessment of its mean climatology. CNRM-CM5.1 includes the atmospheric model ARPEGE-Climat (v5.2), the ocean model NEMO (v3.2), the land surface scheme ISBA and the sea ice model GELATO (v5) coupled through the OASIS (v3) system. The main improvements since CMIP3 are the following. Horizontal resolution has been increased both in the atmosphere (from 2.8° to 1.4°) and in the ocean (from 2° to 1°). The dynamical core of the atmospheric component has been revised. A new radiation scheme has been introduced and the treatments of tropospheric and stratospheric aerosols have been improved. Particular care has been devoted to ensure mass/water conservation in the atmospheric component. The land surface scheme ISBA has been externalised from the atmospheric model through the SURFEX platform and includes new developments such as a parameterization of sub-grid hydrology, a new freezing scheme and a new bulk parameterisation for ocean surface fluxes. The ocean model is based on the state-of-the-art version of NEMO, which has greatly progressed since the OPA8.0 version used in the CMIP3 version of CNRM-CM. Finally, the coupling between the different components through OASIS has also received a particular attention to avoid energy loss and spurious drifts. These developments generally lead to a more realistic representation of the mean recent climate and to a reduction of drifts in a preindustrial integration. The large-scale dynamics is generally improved both in the atmosphere and in the ocean, and the bias in mean surface temperature is clearly reduced. However, some flaws remain such as significant precipitation and radiative biases in many regions, or a pronounced drift in three dimensional salinity.

Evaluation of forest snow processes models (SnowMIP2)

Thirty-three snowpack models of varying complexity and purpose were evaluated across a wide range of hydrometeorological and forest canopy conditions at five Northern Hemisphere locations, for up t ...

A new snow parameterization for the Météo-France climate model: Part I: validation in stand-alone experiments

Both observational studies and numerical experiments demonstrate the sensitivity of the atmosphere to variations in the extent and mass of snow cover. There is therefore a need for simple but realistic snow parameterizations in forecast and climate models. This study describes a new physically-based snow hydrology for use in the Meteo-France climate model, together with the ISBA land-surface scheme. A restricted number of parameters has been added, while preserving a single surface energy budget. The ageing process of the snow pack has been introduced through prognostic equations for snow density and snow albedo. Snowmelt computation has been modified over partially snow-covered and vegetated areas. The new scheme has been validated against field measurements in stand-alone simulations forced by observed meteorological conditions. The results show a strong improvement in the models performance, thereby suggesting that a simple one-layer snow model is able to reproduce the main physical mechanisms governing the snow pack evolution. Part II of the present study will concern the validation in a 3-D experiment within the Meteo-France climate model.

A new snow parameterization for the Météo-France climate model

Both observational studies and numerical experiments demonstrate the sensitivity of the atmosphere to variations in the extent and mass of snow cover. There is therefore a need for simple but realistic snow parameterizations in forecast and climate models. This study describes a new physically-based snow hydrology for use in the Météo-France climate model, together with the ISBA land-surface scheme. A restricted number of parameters has been added, while preserving a single surface energy budget. The ageing process of the snow pack has been introduced through prognostic equations for snow density and snow albedo. Snowmelt computation has been modified over partially snow-covered and vegetated areas. The new scheme has been validated against field measurements in stand-alone simulations forced by observed meteorological conditions. The results show a strong improvement in the models performance, thereby suggesting that a simple one-layer snow model is able to reproduce the main physical mechanisms governing the snow pack evolution. Part II of the present study will concern the validation in a 3-D experiment within the Météo-France climate model.

Sensitivity of the Asian summer monsoon to an anomalous Eurasian snow cover within the Météo-France GCM

Both observational and numerical studies suggest that the Eurasian winter snow cover has a strong influence on the subsequent summer monsoon in Asia. An updated version of the ARPEGE climate model of Meteo-France, including a simple but physically-based snow parameterization, is used to test the impact of an increased snow mass prescribed at the beginning of March on the simulated summer monsoon circulation and rainfall. The large-scale features of the Asian monsoon are reproduced in a realistic way in the control integration, which is a necessary premise of such a sensitivity test. In the heavy snow cover experiment, the anomalous persistence of the winter snow pack delays the springtime continental heating. This weakens the thermal low over northern India and Persia as well as the southwesterly winds over the monsoon area. There is also a significant decrease in the rainfall over western India and Bengal-Burma, which usually represent the centers of maximum precipitation. Radiative, turbulence transfer and hydrological processes seem to be involved in the snow-monsoon relationship. The changes in the monsoon precipitation are strongly related to changes in the atmospheric circulation and are not reinforced by a local evaporation/convection feedback in our experiment.

Importance of vegetation feedbacks in doubled-CO2 climate experiments

The rising atmospheric concentration of carbon dioxide resulting from the burning of fossil fuels and deforestation is likely to provoke significant climate perturbations, while having far-reaching consequences for the terrestrial biosphere. Some plants could maintain the same intake of CO2 for photosynthesis by reducing their stomatal openings, thus limiting the transpiration and providing a positive feedback to the projected surface warming. Other plants could benefit from the higher CO2 level and the warmer climate to increase their productivity, which would on the contrary promote the transpiration. The relevance of these feedbacks has been investigated with the Meteo-France atmospheric general circulation model. The model has been run at the T31 spectral truncation with 19 vertical levels and is forced with sea surface temperature and sea ice anomalies provided by a transient simulation performed with the Hadley Centre coupled ocean-atmosphere model. Besides a reference doubled-CO2 experiment with no modification of the vegetation properties, two other experiments have been performed to explore the impact of changes in the physiology (stomatal resistance) and structure (leaf area index) of plants. Globally and annually averaged, the radiative impact of the CO2 doubling leads to a 2°C surface warming and a 6% precipitation increase, in keeping with previous similar experiments. The vegetation feedbacks do not greatly modify the model response on the global scale. The increase in stomatal resistance does not systematically lead to higher near-surface temperatures due to changes in the soil wetness annual cycle and the atmospheric circulation. However, both physiological and structural vegetation feedbacks are evident on the regional scale. They are liable to modify the CO2 impact on the hydrological cycle, as illustrated for the case of the European summertime climate and the Asian summer monsoon. The strong sensitivity of the climate in these areas emphasizes the large uncertainties of climate change predictions for some of the most populated regions of the world and argues for the need to include more interactive land surface processes in current generation climate models.

Two-year global simulation of L-band brightness temperatures over land

This letter presents a synthetic L-band (1.4 GHz) multiangular brightness temperature dataset over land surfaces that was simulated at a half-degree resolution and at the global scale. The microwave emission of various land-covers (herbaceous and woody vegetation, frozen and unfrozen bare soil, snow, etc.) was computed using a simple model [L-band Microwave Emission of the Biosphere (L-MEB)] based on radiative transfer equations. The soil and vegetation characteristics needed to initialize the L-MEB model were derived from existing land-cover maps. Continuous simulations from a land-surface scheme for 1987 and 1988 provided time series of the main variables driving the L-MEB model: soil temperature at the surface and at depth, surface soil moisture, proportion of frozen surface soil moisture, and snow cover characteristics. The obtained global maps constitute a useful dataset for a first evaluation of the sensitivity of future satellite-based L-band radiometry data to soil moisture.

Present-day and future Amazonian precipitation in global climate models: CMIP5 versus CMIP3

The present study aims at evaluating and comparing precipitation over the Amazon in two sets of historical and future climate simulations based on phase 3 (CMIP3) and 5 (CMIP5) of the Coupled Model Intercomparison Project. Thirteen models have been selected in order to discuss (1) potential improvements in the simulation of present-day climate and (2) the potential reduction in the uncertainties of the model response to increasing concentrations of greenhouse gases. While several features of present-day precipitation—including annual cycle, spatial distribution and co variability with tropical sea surface temperature (SST)—have been improved, strong uncertainties remain in the climate projections. A closer comparison between CMIP5 and CMIP3 highlights a weaker consensus on increased precipitation during the wet season, but a stronger consensus on a drying and lengthening of the dry season. The latter response is related to a northward shift of the boreal summer intertropical convergence zone in CMIP5, in line with a more asymmetric warming between the northern and southern hemispheres. The large uncertainties that persist in the rainfall response arise from contrasted anomalies in both moisture convergence and evapotranspiration. They might be related to the diverse response of tropical SST and ENSO (El Niño Southern Oscillation) variability, as well as to spurious behaviours among the models that show the most extreme response. Model improvements of present-day climate do not necessarily translate into more reliable projections and further efforts are needed for constraining the pattern of the SST response and the soil moisture feedback in global climate scenarios.

Global off-line evaluation of the ISBA-TRIP flood model

This study presents an off-line global evaluation of the ISBA-TRIP hydrological model including a two-way flood scheme. The flood dynamics is indeed described through the daily coupling between the ISBA land surface model and the TRIP river routing model including a prognostic flood reservoir. This reservoir fills when the river height exceeds the critical river bankfull height and vice versa. The flood interacts with the soil hydrology through infiltration and with the overlying atmosphere through precipitation interception and free water surface evaporation. The model is evaluated over a relatively long period (1986–2006) at 1° resolution using the Princeton University 3-hourly atmospheric forcing. Four simulations are performed in order to assess the model sensitivity to the river bankfull height. The evaluation is made against satellite-derived global inundation estimates as well as in situ river discharge observations at 122 gauging stations. First, the results show a reasonable simulation of the global distribution of simulated floodplains when compared to satellite-derived estimates. At basin scale, the comparison reveals some discrepancies, both in terms of climatology and interannual variability, but the results remain acceptable for a simple large-scale model. In addition, the simulated river discharges are improved in term of efficiency scores for more than 50% of the 122 stations and deteriorated for 4% only. Two mechanisms mainly explain this positive impact: an increase in evapotranspiration that limits the annual discharge overestimation found when flooding is not taking into account and a smoothed river peak flow when the floodplain storage is significant. Finally, the sensitivity experiments suggest that the river bankfull depth is potentially tunable according to the river discharge scores to control the accuracy of the simulated flooded areas and its related increase in land surface evaporation. Such a tuning could be relevant at least for climate studies in which the spatio-temporal variations in precipitation are generally poorly represented.

Impact of doubled CO2 on global‐scale leaf area index and evapotranspiration: Conflicting stomatal conductance and LAI responses

[1] Current increase in atmospheric CO2 is expected to modify both climate and plant function, thereby impacting plant structure and gas exchange. We investigate the effects of doubled CO2 on leaf area index (LAI) and evapotranspiration (ETR) using a global vegetation model for present-day and doubled-CO2 conditions. The model assumes that adaptation of plants to the local climate leads to an equilibrium LAI, which depends on resource availability (minimizing water stress, canopy carbon cost and self-shading). The model compares reasonably well with remote sensing estimates of LAI. Four doubled-CO2 simulations are designed to investigate the role of climate, CO2-induced stomatal closure, enhanced photosynthesis, and a combination of these effects. These simulations show that plant physiological responses to doubled CO2 are potentially more important than climate changes for LAI, and equally important for ETR. In addition, even the sign of the simulated changes in LAI and ETR varies with the assumptions on plant responsiveness to CO2. A reduction of stomatal conductance moderates or cancels the water losses caused by a warmer and drier climate, but photosynthesis stimulation counteracts this stomatal effect, especially in the mid-to-high latitudes, because of enhanced LAI. Experimental evidence of LAI and ETR response to CO2 has been reviewed and compared to the different simulations. On the basis of this confrontation we argue that plant response to CO2 doubling may have a relatively small net impact on global ETR and may cause a moderate increase of LAI. Tree stomata may be less responsive to CO2 than was previously assumed, and stimulated plant growth partly cancels the water savings caused by stomatal closure. Understanding the responses of plant canopies to CO2 is therefore critical for land surface hydrology in a CO2 rich world. INDEX TERMS: 1818 Hydrology: Evapotranspiration; 1851 Hydrology: Plant ecology; 1655 Global Change: Water cycles (1836); 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; KEYWORDS: LAI, CO2, stomatal conductance, global, evapotranspiration Citation: Kergoat, L., S. Lafont, H. Douville, B. Berthelot, G. Dedieu, S. Planton, and J.-F. Royer, Impact of doubled CO2 on globalscale leaf area index and evapotranspiration: Conflicting stomatal conductance and LAI responses, J. Geophys. Res., 107(D24), 4808, doi:10.1029/2001JD001245, 2002.