Laboratoire De Meteorologie Dynamique Research Articles

The Antarctic surface mass balance in a stretched grid general circulation model

The Laboratoire de Météorologie Dynamique (LMD) variable-grid atmospheric general circulation model (AGCM) was used in this study for a five-year high-resolution simulation of the Antarctic climate. The horizontal resolution is about 100 km over a large part of the ice sheet. This study focuses on the simulated surface mass balance (precipitation-evaporation sublimation-melt) and on the spatial and temporal variability of snowfall in Antarctica. The simulated annual mean surface mass balance for the whole continent is close to the observed value, and the model simulates well the spatial distribution of the surface mass balance. The annual cycle of snowfall exhibits a clear minimum in summer over the high interior plateau as well as for Antarctica as a whole, in agreement with the observations. In the interior of the continent, the model produces a permanent light background snowfall that accounts for about 5% of the total annual precipitation. The bulk of the snowfall is produced irregularly during periods that generally last only two or three days that are caused by cyclones off the coast.

Ice-sheet mass balance during the last glacial maximum

In the framework of the Paleoclimate Modelling Intercomparison Project (PMIP), simulations of the Last Glacial Maximum (LGM) have- been performed. More than 10 different atmospheric general circulation models (AGCMs) have been used with the same boundary conditions: sea-surface temperatures prescribed by CLIMAP (1981), ice-sheet reconstruction provided by Peltier (1994), change in insolation, and reduced CO2 content. One of the major questions is to investigate whether the simulations of the LGM are in equilibrium with the prescribed ice-sheet reconstruction. To answer this question, we have used two different approaches. First, we analyze the results of a sel of LGM simulations performed with different versions of the Laboratoire de Meteorolo-gie Dynamique (LMD) AGCM and study the hydrologic and snow- budgets over the Laurcntide and Fennoscandian ice sheets. Second, we use the AGCM outputs to force an ice-sheet model in order to investigate its ability to maintain the ice sheets as reconstructed by CLIMAP (1981) or Peltier (1994).

Existence of an ice cap during the mid-Cretaceous period (120–90 Ma): an AGCM investigation

Many questions remain concerning whether or not an ice cap existed during the mid-Cretaceous period (120–90 Ma). Other than data and observations from ice-rafted materials, atmospheric general circulation models (AGCMs) may be appropriate tools to investigate whether changes in atmospheric composition, land-sea distribution, or oceanic circulation (used as boundary conditions to constrain the model), provide the climatic conditions that enable ice caps to be formed.This study uses an AGCM developed by the Laboratoire de Météorologie Dynamique (LMD) to perform a set of numerical sensitivity experiments to investigate plate tectonics (land–sea distribution and orography), CO2 partial pressure in the atmosphere and changes in prescribed sea-surface temperatures. The main goals are to quantify the effect of each forcing factor on the increase of Northern Hemisphere high-latitude temperatures, and to investigate whether combining these factors produce temperatures that would allow the ice-sheet formation.

Ice-sheet mass balance during the last glacial maximum

In the framework of the Paleoclimate Modelling Intercomparison Project (PMIP), simulations of the Last Glacial Maximum (LGM) have- been performed. More than 10 different atmospheric general circulation models (AGCMs) have been used with the same boundary conditions: sea-surface temperatures prescribed by CLIMAP (1981), ice-sheet reconstruction provided by Peltier (1994), change in insolation, and reduced CO2 content. One of the major questions is to investigate whether the simulations of the LGM are in equilibrium with the prescribed ice-sheet reconstruction. To answer this question, we have used two different approaches. First, we analyze the results of a sel of LGM simulations performed with different versions of the Laboratoire de Meteorolo-gie Dynamique (LMD) AGCM and study the hydrologic and snow- budgets over the Laurcntide and Fennoscandian ice sheets. Second, we use the AGCM outputs to force an ice-sheet model in order to investigate its ability to maintain the ice sheets as reconstructed by CLIMAP (1981) or Peltier (1994).

Radon 222 as a comparative tracer of transport and mixing in two general circulation models of the atmosphere

Radon 222 (222Rn) is used as a tracer to probe and intercompare transport, turbulent mixing, and convective mixing in the Laboratoire de Météorologie Dynamique (LMD) and Goddard Institute for Space Studies (GISS) atmospheric general circulation models (GCMs). Formulations for tracer transport and mixing and their control on the global distribution and time variability of 222Rn, as well as parameterizations for the continental surface source flux, are directly implemented into the two GCMs and run “in‐line.” Tracer formulations are largely inspired by climate variables (heat, moisture, momentum) formulations in the base GCMs. The comparison of model‐calculated 222Rn with observations of time (diurnal, seasonal, sporadic), variability, and spatial (horizontal, vertical) distribution shows partial agreement only. Uncertainties of the sources of 222Rn, in particular of the dependence of 222Rn emanation on soil freezing, are substantial, and the significance and reliability of some of the available observations are low. Model intercomparison is not subject to observation limitations, and it clearly indicates that the boundary layer is more homogeneously mixed in the LMD model, whereas deep convection is more efficient at carrying surface‐produced quantities to high tropospheric levels in the GISS model. Resolution also makes a large difference. The LMD model has a finer horizontal grid over most of the globe and is almost systematically better than the GISS model at reproducing sharp fluctuations of 222Rn and seasonal cycles. Our results support that 222Rn could provide an unequivocal absolute measure of the GCM's performances if a more comprehensive observational validation was available.

Radiation exchanges above West African moist savannas: Seasonal patterns and comparison with a GCM simulation

During a field campaign of the Savannas on the Long Term (SALT) Global Change and Terrestrial Ecosystems/International Geosphere Biosphere Program dealing with the energy budget of a West African moist savanna, particular attention was paid to the radiation budget because it is the driving force for all other surface fluxes. All components of the radiation budget were measured on the ground during one and a half year on a test site in Ivory Coast. The seasonal variations of the observed radiation at the surface and satellite data were compared to the values simulated for the 1978–1988 period by a general circulation model coupled to a land surface parameterization scheme, both developed by the Laboratoire de Météorologie Dynamique (LMD). Problems which appear when point observations of surface fluxes are compared with results from general circulation model simulations are addressed. Seasonal variations of the prescribed savanna albedo in the model differed significantly from the observed variations due to bush fire influence. However, the major discrepancy between model simulations and surface observations resulted from the overestimation of the incoming shortwave radiation at the surface. The major sources of discrepancy were found to be the simulated clear‐sky atmospheric absorption and/or scattering of the shortwave radiation and the simulation of cloud and/or cloud‐radiation interactions which lead to an overestimation of the absorbed energy at the surface by the model. It is concluded that improvement of the simulated energy budget in the humid tropics depends critically on a good simulation of the incoming shortwave radiation at the surface.

The albedo of temperate and boreal forest and the Northern Hemisphere climate: a sensitivity experiment using the LMD GCM

A deforestation experiment is performed using the Laboratoire de Meteorologie Dynamique Atmospheric General Circulation Model (LMD GCM) to determine the climatic role of the largest vegetation formation in the Northern Hemisphere, localized mostly north of latitude 45{degree}N, which is called the temperate and boreal forest. For this purpose, an iterative albedo scheme based on vegetation type, snow age, snowfall rate and area of snow cover, is developed for snow-covered surfaces. The results show a cooling of Northern Hemisphere soil and an increase in the snow cover when the forest is removed, as found by previous similar experiments. In our study this cooling is related to different causes, depending on the season. It is linked to modifications in the soil radiative properties, like surface albedo, due to the disappearance of forest, and consequently, to a greater exposure of the snow-covered soil underneath. It is also related to alterations in the hydrological cycle, observed mainly in summer and autumn at middle latitudes. The model shows a strong sensitivity to the coupled surface albedo - soil temperature - fractional snow cover response in the spring. A later and longer snowmelt season is also detected. This study adds to our understanding of climatic variationmore » on longer time scales, since it is widely accepted that the formation and disappearance of different vegetation formations is closely related to climatic evolution patterns, in particular on the time scale of the glacial oscillations. 32 refs., 12 figs.« less

A statistical study of the regional impact of deforestation on climate in the LMD GCM

The present study uses the general circulation model of the Laboratoire de Meteorologie Dynamique (LMD-GCM) coupled to the land-surface, vegetation model SECHIBA. The impact of deforestation on climate is discussed. Replacing tropical forests by degraded pastures changes albedo, the roughness length and the hydrological properties of the surface. The experiment was carried out over eleven years using the observed sea surface temperature from 1978 to 1988, which includes two major El Nino events. The discussion of the results in this study is limited to the regional impact of deforestation. The changes found for the surface fluxes in Amazonia, Africa and Indonesia are examined in detail and compared in order to understand the impact on temperature. Special attention is paid to feedback mechanisms which compensate for the surface changes and to the statistical significance of these results within the tropical variability of climate. It is shown that the relatively small regional impact of deforestation in this study is statistically significant and largely independent of the El Nino-Southern Oscillation phenomenon.

Satellite validation of GCM‐simulated annual cycle of the earth radiation budget and cloud forcing

Earth Radiation Budget Experiment (ERBE) data are used to validate radiative fluxes and cloud radiative forcing (CRF) simulated by the Laboratoire de Météorologie Dynamique (LMD) general circulation model (GCM). The emphasis of the work is on the development of new tests to obtain more significant elements of comparison between model simulations and satellite observations. These tests are applied to the clear‐sky fluxes and the cloud radiative forcing. The validation of the CRF described by a model requires to test the consistency between the solar or shortwave (SW: 0.2 to 5 μm) and longwave (LW: 5 to 50 μm) cloud forcing. For this purpose, we compute the mean cloud perturbation of the planetary albedo as a function of the LW cloud forcing for both model results and ERBE observations. In the SW spectral domain, the consideration of total fluxes does not provide very constraining elements of validation because most of the observed variations are prescribed (incoming solar radiation, solar zenith angle). We therefore distinguish the part of the SW seasonal variations related only to the variation of external parameters (mainly the insolation) from the part which arises from the combined variation of internal climate parameters (mainly cloud albedo and snow/ice cover) with the insolation. Fourier analysis is used to study the seasonal amplitude and phase of the CRF. The seasonal variation of the cloudiness is, respectively, out of phase (in phase) with the insolation in mid‐latitudes (in low and high latitudes). We show that this acts to enhance (to reduce) the seasonal amplitude of the absorbed SW flux in mid‐latitudes (in low and high latitudes). Finally, we show that the impact of the seasonal variation of the cloudiness on the variation of the net CRF is less than 10 W m−2.

Use of satellite observations for the study of mesoscale systems in polar regions

Abstract Atmospheric and surface parameters retrieved from observations by satellites in the TIROS-N/NOAA series provide valuable information for meteorological investigations in the data-sparse polar regions. A pattern-recognition method for processing these observations, the Improved Initialization Inversion (3I) system, has been developed at Laboratoire de Meteorologie Dynamique (LMD). This technique has been applied to several satellite passes, and the capability of the TIROS-N Operational Vertical Sounder (TOVS) to provide a description of mesoscale atmospheric phenomena has been demonstrated. In addition, the Special Sensor Microwave/Imager (SSM/I) launched in June 1987 on a satellite in the United States Defense Meteorological Satellite Program (DMSP) allows a new insight into the mesoscale structures of cyclones over the ocean by providing estimates of atmospheric water vapor, cloud water and surface winds. TOVS and SSM/I-inferred parameters are presented and discussed for several cases, including polar lows, and the mutual contribution of these two types of satellite data is shown.

Global Data Assimilation and Forecast Experiments Using the Improved Initialization Inversion Method for Satellite Soundings

This is an evaluation of a retrieval, data assimilation, and forecast experiment on satellite sounding data from the NOAA-9 and NOAA-10 polar orbiting satellites during a 15.5 day period in January–February 1987. Retrievals were produced globally through the 3I (improved initialization inversion) scheme developed at Laboratoire de Météorologie Dynamique, Paris. In this paper we use the European Centre for Medium Range Weather Forecasts (ECMWF) global data assimilation system to evaluate the global performance of the 3I retrieval scheme. The 3I-retrieved temperature profiles have also been compared with the operational retrievals produced by NESDIS (National Environmental Satellite, Data, and Information Service), Washington. It is shown that the large airmass-dependent biases, characteristic of the NESDIS retrievals, also occur with 3I, but with smaller amplitude. Some large local 3I retrieval errors are identified and explained, and their impact on analysis and forecasts demonstrated. The forecast scores with the 3I retrievals are worse than the scores with the NESDIS retrievals. This may be due to the much larger volume of soundings used in the 3I experiment. The use of a global assimilation system has identified aspects of the 3I system which can be improved.

Adiabatic nonlinear normal mode initialization for a grid point global model

The baroclinic primitive equation model used for short and medium range forecasting admits high frequency as well as desirable Rossby modes. These high frequency oscillations are excited by initial imbalances between the observed mass and wind fields. In this paper we determine and describe the normal modes of the linearized version of the general circulation model of Laboratoire de Meteorologie Dynamique, Paris. These normal modes are then used to initialize the model through Machenhauer’s nonlinear correction scheme. The adiabatic nonlinear normal mode initialization technique is shown to be superior to dynamic initialization in terms of elimination of high-frequency oscillations in the forecast. Normal modes of a particular model depend on the finite difference scheme chosen to approximate the governing system of model equations. The results presented correspond to enstrophy-conserving finite difference scheme.

Extended Memory of the Initial Conditions in Long-Range Forecasts of the January 1983 Atmospheric Circulation

Abstract We compare four ensembles of long-range numerical forecasts made with the Laboratoire de Meteorologie Dynamique general circulation model. These ensembles make use of mid-December initial conditions taken from distinct “non-El Nino” years and 1-day lagged initial conditions of mid-December 1982, a specific El Nino year. Both climatological sea surface temperatures (SSTs) and observed SSTs for 1982/83 winter are used to force these ensembles. The comparison of both ensembles using mid-December 1982 initial conditions reveals a rapid impact of the SST anomalies on the forecast. On the other hand, at the end of the 45-day forecasts, namely, more than one month after their beginning, the ensembles making use of the same boundary conditions still display significant differences. The noteworthy feature is that the ensemble using both mid-December 1982 initial conditions and observed SST anomalies leads to an ensemble average January-mean predicted circulation that shows a much better agreement with the...

Simulation of Airborne Impurity Cycles Using Atmospheric General Circulation Models

Atmospheric general circulation models are believed to be appropriate tools for studying airborne impurity cycles in order to supplement observations and to improve our knowledge of gaseous and particulate pollutant cycles in the atmosphere. The main aspects of the modelling of tracer cycles are reviewed and illustrated by two particular examples: desert dust particles in the 1 μm range and water isotope species HDO and H218O. Some results from a first simulation including desert dust and water isotope cycles using the model developed at the Laboratoire de Météorologie Dynamique (LMD) are presented and compared to observations, with particular emphasis on ice-sheet data. The relatively good agreement with observations obtained so far is encouraging and should stimulate further applications to other types of tracers.

Simulation of Airborne Impurity Cycles Using Atmospheric General Circulation Models

Atmospheric general circulation models are believed to be appropriate tools for studying airborne impurity cycles in order to supplement observations and to improve our knowledge of gaseous and particulate pollutant cycles in the atmosphere. The main aspects of the modelling of tracer cycles are reviewed and illustrated by two particular examples: desert dust particles in the 1 μm range and water isotope species HDO and H2 18O. Some results from a first simulation including desert dust and water isotope cycles using the model developed at the Laboratoire de Météorologie Dynamique (LMD) are presented and compared to observations, with particular emphasis on ice-sheet data. The relatively good agreement with observations obtained so far is encouraging and should stimulate further applications to other types of tracers.

Methode de representation de la turbulence d'echelle inferieure a la maille pour un modele tri-dimensionnel de convection nuageuse

Significant improvements are occurring in the representation of physical processes in atmospheric convection models. They should go along with parallel improvements in the parameterization of subgrid scale turbulent processes. This problem appears to be particularly delicate in the presence of clouds, due to the local release of latent heat. Two important points are the choice of adequate turbulent thermodynamic variables and of the method for truncating the statistical moment equations. These topics are discussed here within the framework of the three-dimensional convection model under development at the Laboratoire de Meteorologie Dynamique. Assuming the need for at least a simplified second-order closure, two improvements are tested on a numerical simulation of the Porto Rico experiment conducted by the National Center for Atmospheric Research (U.S.A.) in 1972. They concern the use of a rate equation for sub-grid scale turbulent kinetic energy and of specific variables which are approximately conserved in the condensation process.