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J. Polcher

Abstract

This paper studies the sensitivity of large-scale tropical convection to land surface changes in the general circulation model of the LMD. Ale method of analysis is based on a conceptual model of the energetics of convection applied every day of the simulation to the grid points within the region of interest. The synoptic events are then classified by their potential energy divergence. This allows for the distinction between the frequency and the characteristic energy and water cycle of the events. The difference in precipitation found in the tropical deforestation experiments is explained by changes in the number of intense convective events while their characteristic precipitation remains largely unchanged. With a series of ten deforestation experiments the relation between land surface processes changes and variations of the frequency of convection is studied. The highest sensitivity is found for the sensible heat flux: its increase leads to more deep convective events. The reduction of evaporation associated with deforestation decreases precipitation only for subsident events but it is only of marginal importance for the regional rainfall. These results suggest a relatively simple mechanism for the sensitivity of tropical convection to land surface processes in our model.

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M. J. Best, A. Beljaars, J. Polcher, and P. Viterbo
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A. D. Culf, G. Fisch, J. Lean, and J. Polcher

Abstract

Climate data from the Anglo–Brazilian Amazonian Climate Observation Study have been compared with the simulations of three general circulation models with prognostic cloud schemes. Monthly averages of net all-wave radiation, incoming solar radiation, net longwave radiation, and precipitation obtained from automatic weather stations sited in three areas of Amazonia are compared with the output from the unified model of the Hadley Centre for Climate Prediction and Research, the operational forecasting model of the European Centre for Medium-Range Weather Forecasts (ECMWF), and the model of the Laboratoire de Météorologie Dynamique (LMD). The performance of the models is much improved when compared to comparisons of observations with the output from earlier, less sophisticated models. However, the Hadley Centre and LMD models tend to overpredict net and solar radiation, and the ECMWF model underpredicts net and solar radiation at two of the sites, but performs very well in Manaus. It is shown that the errors are mainly linked to the amount of cloud cover produced by the models, but also to the incoming clear sky solar radiation.

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M. J. Best, A. Beljaars, J. Polcher, and P. Viterbo

Abstract

A generalized coupling is proposed between atmospheric models and surface schemes (land and ocean). A set of input and output variables is defined for this purpose in such a way that it can be used by many current and future models, including mosaic or tile schemes. The basic concept is to pass atmospheric variables from the lowest model level and their relation to corresponding fluxes to the surface scheme. The surface scheme returns the fluxes. In this framework, there is no need for the atmospheric model to have detailed information about the surface. Only the result of the surface computations is needed; namely, the fluxes, which are applied as a boundary condition. The equations for fully implicit coupling are derived, and the relevance for numerical stability is demonstrated. It is also shown that fully implicit coupling in a tile scheme leads to more robust results than partially implicit coupling.

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S. Vérant, K. Laval, J. Polcher, and M. De Castro

Abstract

In the broad context of the downscaling methods that are used to study climatic change impacts, the dependence of the surface hydrological processes simulated by the Organising Carbon and Hydrology in Dynamic Ecosystem (ORCHIDEE) land surface model, used in a stand-alone mode, on the spatial scale of the forcings is investigated over the Iberian Peninsula. These prescribed forcings are the outputs of a regional climate model, Pronóstico a Mesoescala (PROMES), with a high spatial resolution (20 km). In the first experiment, the PROMES outputs have been aggregated stepwise to the typical resolution of a general circulation model, and applied to ORCHIDEE, in order to analyze the impacts of the changing resolution on the simulated water balance. Then, subgrid-scale variability (SSV) for the different forcings has been progressively reintroduced. This second experiment is aimed at isolating the crucial elements of SSV that need to be preserved when a disaggregation is being performed.

The increase of interception loss when the spatial resolution goes beyond 100 km leads to unrealistic values of the interception loss ratio. In the northern humid region, the reduction of runoff frequency when the forcings are aggregated explains the decrease in runoff production, which can reach half the high-resolution runoff. These impacts drive the adjustment of the other hydrological components. The large increase of interception loss is compensated by a reduced transpiration in a dry climate, which induces a large change in soil moisture content, and by reduced runoff in humid regions. The second experiment underlines the dominant effect of precipitation SSV, and particularly the rainfall frequency, on the correct simulation of the water balance. The significant influence of the thermodynamic variables is also analyzed.

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K. Layal, R. Raghava, J. Polcher, R. Sadourny, and M. Forichon

Abstract

Results from 90-day simulations with the LMD GCM are described, where sea surface temperatures of 1987 or 1988 years are respectively prescribed. The initial states correspond to 1 June 1987 and 1 June 1988. The simulated precipitation rates over India show a strong contrast between the two years, with drought occurring during summer 1987 and abundant rainfall during summer 1988. The dry regime simulated during 1987 corresponds to an eastward displacement of the outflow at 200 mb and a weaker westerly flow at the surface as compared with 1988, both features being in agreement with reality. Because it is more difficult for models to simulate rainfall differences than to simulate wind variations between the two years, the changes in simulated rainfall over India are studied in more detail. In particular, more integrations are carried out to test the sensitivity of rainfall variations to initial conditions, and the result is that the decrease of rainfall in 1987 compared to 1988 is a robust feature of the model.

Very early, the importance of evapotranspiration in simulating land rainfall was emphasized. Additional integrations are performed in order to study the impact of the new vegetation scheme introduced in the LMD GCM. It is shown that the contrast in rainfall between the two years is better simulated when the evapotranspiration rate of vegetation cover is represented. When vegetation is not represented in the model, the model does not simulate accurately the interannual variation of the precipitation rates.

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M. Sabre, K. Hodges, K. Laval, J. Polcher, and F. Désalmand

Abstract

The monsoon depressions that form over India during the summer are analyzed using simulations from the Laboratoire de Météorologie Dynamique general circulation model. This type of synoptic system often occurs with a frequency of one to two per month and can produce a strong Indian rainfall. Two kinds of analyses are conducted in this study. The first one is a subjective analysis based on the evolution of the precipitation rate and the pattern of the sea level pressure. The second one is an objective analysis performed using the TRACK program, which identifies and tracks the minima in the sea level pressure anomaly field and computes the statistics for the distribution of systems.

The analysis of a 9-yr control run, which simulates strong precipitation rates over the foothills of the Himalayas and over southern India but weak rates over central India, shows that the number of disturbances is too low and that they almost never occur during August, when break conditions prevail. The generated disturbances more often move north, toward the foothills of the Himalayas. Another analysis is performed to study the effect of the Tibetan Plateau elevation on these disturbances with a 9-yr run carried out with a Tibetan Plateau at 50% of its current height. It is shown that this later integration simulates more frequent monsoon disturbances, which move rather northwestward, in agreement with the current observations. The comparison between the two runs shows that the June–July–August rainfall difference is in large part due to changes in the occurrence of the monsoon disturbances.

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N. Gedney, P. M. Cox, H. Douville, J. Polcher, and P. J. Valdes

Abstract

The impact of land surface representation on GCM simulations of climate change is analyzed using eight climate change experiments, carried out with four GCMs each utilizing two different land surface schemes (LSSs). In the regions studied (Amazonia, the Sahel, and southern Europe) the simulations differ markedly in terms of their predicted changes in evapotranspiration and soil moisture. These differences are only partly as a result of differences in the predicted changes in precipitation and available energy. A simple “bucket model” characterization of each LSS demonstrates that the different hydrological sensitivities are also strongly dependent on properties of the LSS, most notably the runoff, which occurs when evaporation is marginally soil moisture limited. This parameter, “Y c,” varies significantly among the LSSs, and influences both the soil moisture in the 1 × CO2 control climate, and the sensitivity of both evaporation and soil moisture to climate change. It is concluded that uncertainty in the predicted changes in surface hydrology is more dependent on such gross features of the runoff versus soil moisture curve than on the detailed treatment of evapotranspiration.

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J-P. Schulz, L. Dümenil, J. Polcher, C. A. Schlosser, and Y. Xue

Abstract

Three different land surface schemes that are designed for use in atmospheric general circulation models are compared. They were run in offline mode with identical atmospheric forcing values that were observed at Cabauw. This procedure allows one to analyze differences in the simulations that are not caused by different atmospheric conditions and to relate them to certain model characteristics. The intercomparison shows that the models produced similar results for surface temperature and total net radiation, which are also in good agreement with the observations. But they underestimate latent heat flux and overestimate sensible heat flux in summer. Differences in the components of energy and hydrological cycle as simulated by the schemes can be related to differences in model structures. The calculation of the surface temperature is of major importance, particularly on a diurnal timescale. Depending on the scheme chosen, the simulated surface temperature is closer to the observed radiative surface temperature or the observed soil temperature at a depth of a few centimeters. If a land surface scheme is going to be coupled to an atmospheric model, this needs to be considered. The simulation of the surface energy fluxes can be improved by careful calibration of the relevant parameters according to the conditions at the observational site. The stomatal resistance was found to be an essential parameter in determining the evolution of evapotranspiration for the Cabauw simulations.

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Jean-Luc Redelsperger, Chris D. Thorncroft, Arona Diedhiou, Thierry Lebel, Douglas J. Parker, and Jan Polcher

African Monsoon Multidisciplinary Analysis (AMMA) is an international project to improve our knowledge and understanding of the West African monsoon (WAM) and its variability with an emphasis on daily-to-interannual time scales. AMMA is motivated by an interest in fundamental scientific issues and by the societal need for improved prediction of the WAM and its impacts on West African nations. Recognizing the societal need to develop strategies that reduce the socioeconomic impacts of the variability of the WAM, AMMA will facilitate the multidisciplinary research required to provide improved predictions of the WAM and its impacts. This will be achieved and coordinated through the following five international working groups: i) West African monsoon and global climate, ii) water cycle, iii) surface-atmosphere feedbacks, iv) prediction of climate impacts, and v) high-impact weather prediction and predictability.

AMMA promotes the international coordination of ongoing activities, basic research, and a multiyear field campaign over West Africa and the tropical Atlantic. AMMA is developing close partnerships between those involved in basic research of the WAM, operational forecasting, and decision making, and is establishing blended training and education activities for Africans.

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