A. Ducharne and K. Laval. Influence of the Realistic Description of Soil Water-Holding Capacity on the Global Water Cycle in a GCM. Journal of Climate, 13:4393-4413, December 2000. [ bib | DOI | ADS link ]
The sensitivity of the hydrological cycle to soil water-holding capacity (WHC) is investigated using the Laboratoire de Meteorologie Dynamique General Circulation Model (LMD GCM) coupled to a land surface model (LSM). A reference simulation (REF), with WHCs equal to 150 mm globally (except in deserts where it is set to 30 mm), is compared to two perturbation simulations using datasets with realistic WHC distributions:the `available WHC' (AWC) dataset is physically consistent with the definition of WHC in the LSM and has a global average close to 150 mm; the `total WHC' (TWC) dataset is used as a secondary reference for a large WHC increase (more than a doubling from 150 mm). The average impact over land of the increase in WHC (from REF to both AWC and TWC) is an increase in annual mean evaporation, split between increased annual precipitation and decreased annual mean moisture convergence. The regional responses, however, are more complex: precipitation increases in summer over the midlatitude landmasses through the recycling of increased evaporation; in the Tropics, moisture convergence and precipitation decrease in the intertropical convergence zone and precipitation increases in the surrounding areas, both behaviors being related to the sensitivity of tropical convection to surface energy fluxes in the LMD GCM.Two important conclusions arise from these numerical results: first, the changes in the hydrological cycle are driven through evaporation by the WHC changes realized in the hydrologically active regions (continental midlatitude and tropical rainbelts); second, WHC increase of 10% to 20% in the rainbelts induces changes in the hydrologic cycle with similar patterns and almost the same amplitude as changes resulting from an increase greater than 100%. These results are strongly conditioned to the land-atmosphere feedbacks, which can only be allowed in a GCM environment.
J. Haywood and O. Boucher. Estimates of the direct and indirect radiative forcing due to tropospheric aerosols: A review. Reviews of Geophysics, 38:513-543, November 2000. [ bib | DOI | ADS link ]
This paper reviews the many developments in estimates of the direct and indirect global annual mean radiative forcing due to present-day concentrations of anthropogenic tropospheric aerosols since Intergovernmental Panel on Climate Change . The range of estimates of the global mean direct radiative forcing due to six distinct aerosol types is presented. Additionally, the indirect effect is split into two components corresponding to the radiative forcing due to modification of the radiative properties of clouds (cloud albedo effect) and the effects of anthropogenic aerosols upon the lifetime of clouds (cloud lifetime effect). The radiative forcing for anthropogenic sulphate aerosol ranges from -0.26 to -0.82 W m-2. For fossil fuel black carbon the radiative forcing ranges from +0.16 W m-2 for an external mixture to +0.42 W m-2 for where the black carbon is modeled as internally mixed with sulphate aerosol. For fossil fuel organic carbon the two estimates of the likely weakest limit of the direct radiative forcing are -0.02 and -0.04 W m-2. For biomass-burning sources of black carbon and organic carbon the combined radiative forcing ranges from -0.14 to -0.74 W m-2. Estimates of the radiative forcing due to mineral dust vary widely from +0.09 to -0.46 W m-2; even the sign of the radiative forcing is not well established due to the competing effects of solar and terrestrial radiative forcings. A single study provides a very tentative estimate of the radiative forcing of nitrates to be -0.03 W m-2. Estimates of the cloud albedo indirect radiative forcing range from -0.3 to approximately -1.8 W m-2. Although the cloud lifetime effect is identified as a potentially important climate forcing mechanism, it is difficult to quantify in the context of the present definition of radiative forcing of climate change and current model simulations. This is because its estimation by general circulation models necessarily includes some level of cloud and water vapor feedbacks, which affect the hydrological cycle and the dynamics of the atmosphere. Available models predict that the radiative flux perturbation associated with the cloud lifetime effect is of a magnitude similar to that of the cloud albedo effect.
Z. X. Li. Influence of tropical Pacific El Niño on the SST of the Southern Ocean Through Atmospheric Bridge. Geophysical Research Letters, 27:3505-3508, November 2000. [ bib | DOI | ADS link ]
El Niño is a major interannual climate signal resulting from complex ocean-atmosphere interactions in the Tropical Pacific. Its impact on the SST (Sea Surface Temperature) of the Southern Ocean through an atmospheric bridge are investigated with an atmospheric general circulation model coupled to a slab mixed-layer ocean. Simulated results suggest that SST changes in the mid- and high-latitude oceans and the Tropical Indian Ocean can be explained by modifications of heat-flux exchange at the air-sea interface. For the Tropical Atlantic, however, the discrepancy is large, indicating that the oceans dynamics are not negligible.
L. Menut, R. Vautard, C. Flamant, C. Abonnel, M. Beekmann, P. Chazette, P. H. Flamant, D. Gombert, D. Guédalia, D. Kley, M. P. Lefebvre, B. Lossec, D. Martin, G. Mégie, P. Perros, M. Sicard, and G. Toupance. Measurements and modelling of atmospheric pollution over the Paris area: an overview of the ESQUIF Project. Annales Geophysicae, 18:1467-1481, November 2000. [ bib | DOI | ADS link ]
The Étude et Simulation de la QUalité de l'air en Ile de France (ESQUIF) project is the first integrated project dedicated to the study of the processes leading to air pollution events over the Paris area. The project was carried out over two years (summer 1998 to winter 2000) to document all types of meteorological conditions favourable to air quality degradation, and in particular to photo oxydant formation. The goals of ESQUIF are (1) to improve our understanding of the relevant chemical and dynamical processes and, in turn, improve their parametrizations in numerical models, and (2) to improve and validate existing models dedicated to pollution analysis, scenarios and/or forecasting, by establishing a comprehensive and thorough database. We present the rationale of the ESQUIF project and we describe the experimental set-up. We also report on the first experiments which took place during the summer of 1998 involving surface networks, and remote sensing instruments as well as several aircraft. Focusing on three days of August 1998, the relative contributions of long-range transported and locally-produced ozone to the elevated ozone concentrations observed during this period are discussed and chemistry-transport model preliminary results on this period are compared to measurements.
J.-L. Bertaux, D. Fonteyn, O. Korablev, E. Chassefière, E. Dimarellis, J. P. Dubois, A. Hauchecorne, M. Cabane, P. Rannou, A. C. Levasseur-Regourd, G. Cernogora, E. Quemerais, C. Hermans, G. Kockarts, C. Lippens, M. de Maziere, D. Moreau, C. Muller, B. Neefs, P. C. Simon, F. Forget, F. Hourdin, O. Talagrand, V. I. Moroz, A. Rodin, B. Sandel, and A. Stern. The study of the martian atmosphere from top to bottom with SPICAM light on mars express. Planetary and Space Science, 48:1303-1320, October 2000. [ bib | DOI | ADS link ]
SPICAM Light is a small UV-IR instrument selected for Mars Express to recover most of the science that was lost with the demise of Mars 96, where the SPICAM set of sensors was dedicated to the study of the atmosphere of Mars (Spectroscopy for the investigation of the characteristics of the atmosphere of mars). The new configuration of SPICAM Light includes optical sensors and an electronics block. A UV spectrometer (118-320 nm, resolution 0.8 nm) is dedicated to Nadir viewing, limb viewing and vertical profiling by stellar occultation (3.8 kg). It addresses key issues about ozone, its coupling with H 2O, aerosols, atmospheric vertical temperature structure and ionospheric studies. An IR spectrometer (1.2- 4.8 μm, resolution 0.4-1 nm) is dedicated to vertical profiling during solar occultation of H 2O, CO 2, CO, aerosols and exploration of carbon compounds (3.5 kg). A nadir looking sensor for H 2O abundances (1.0- 1.7 μm, resolution 0.8 nm) is recently included in the package (0.8 kg). A simple data processing unit (DPU, 0.9 kg) provides the interface of these sensors with the spacecraft. In nadir orientation, SPICAM UV is essentially an ozone detector, measuring the strongest O 3 absorption band at 250 nm in the spectrum of the solar light scattered back from the ground. In the stellar occultation mode the UV Sensor will measure the vertical profiles of CO 2, temperature, O 3, clouds and aerosols. The density/temperature profiles obtained with SPICAM Light will constrain and aid in the development of the meteorological and dynamical atmospheric models, from the surface to 160 km in the atmosphere. This is essential for future missions that will rely on aerocapture and aerobraking. UV observations of the upper atmosphere will allow study of the ionosphere through the emissions of CO, CO +, and CO 2+, and its direct interaction with the solar wind. Also, it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere. The SPICAM Light IR sensor is inherited from the IR solar part of the SPICAM solar occultation instrument of Mars 96. Its main scientific objective is the global mapping of the vertical structure of H 2O, CO 2, CO, HDO, aerosols, atmospheric density, and temperature by the solar occultation. The wide spectral range of the IR spectrometer and its high spectral resolution allow an exploratory investigation addressing fundamental question of the possible presence of carbon compounds in the Martian atmosphere. Because of severe mass constraints this channel is still optional. An additional nadir near IR channel that employs a pioneering technology acousto-optical tuneable filter (AOTF) is dedicated to the measurement of water vapour column abundance in the IR simultaneously with ozone measured in the UV. It will be done at much lower telemetry budget compared to the other instrument of the mission, planetary fourier spectrometer (PFS).
P. Defraigne, O. de Viron, V. Dehant, T. Van Hoolst, and F. Hourdin. Mars rotation variations induced by atmosphere and ice caps. Journal of Geophysical Research, 105:24563-24570, October 2000. [ bib | DOI | ADS link ]
Because of the conservation of angular momentum, the atmospheric winds and the mass exchange between the Martian ice caps and atmosphere, associated with the sublimation/condensation process (mainly CO2), induce seasonal effects on Mars' polar motion, nutation, and length of day (LOD). These effects are computed using the output of a global circulation model of the Martian atmosphere, providing atmospheric pressure fields, ice cap surface pressure fields, and zonal as well as meridional winds. For the LOD variations, total amplitudes (CO2 and wind effects) of 0.22 ms for the annual wave and of 0.38 ms for the semiannual wave are obtained. These amplitudes are more than one order of magnitude larger than the LOD variations induced by the zonal tides, which are at the level of 10 μs. For the induced polar motion the annual amplitude is ˜11 milliarcseconds (mas), and the semiannual amplitude is ˜3 mas. The effect on the nutations, related to the diurnal forcing, is at the level of 0.1 mas. The differences between the results for a liquid and for a solid core are examined and shown to be 1% of the total effects.
F. Chevallier, F. Chéruy, R. Armante, C. J. Stubenrauch, and N. A. Scott. Retrieving the Clear-Sky Vertical Longwave Radiative Budget from TOVS: Comparison of a Neural Network-Based Retrieval and a Method UsingGeophysical Parameters. Journal of Applied Meteorology, 39:1527-1543, September 2000. [ bib | DOI | ADS link ]
At a time when a new generation of satellite vertical sounders is going to be launched (including the Infrared Atmospheric Sounder Interferometer and Advanced Infrared Radiometric Sounder instruments), this paper assesses the possibilities of retrieving the vertical profiles of longwave clear-sky fluxes and cooling rates from the Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) radiometers aboard the polar-orbiting National Oceanic and Atmospheric Administration satellites since 1979. It focuses on two different methodologies that have been developed at Laboratoire de Météorologie Dynamique (France). The first one uses a neural network approach for the parameterization of the links between the TOVS radiances and the longwave fluxes. The second one combines the geophysical variables retrieved by the Improved Initialization Inversion method and a forward radiative transfer model used in atmospheric general circulation models. The accuracy of these two methods is evaluated using both theoretical studies and comparisons with global observations.
F. Chéruy and F. Chevallier. Regional and Seasonal Variations of the Clear Sky Atmospheric Longwave Cooling over Tropical Oceans. Journal of Climate, 13:2863-2875, August 2000. [ bib | DOI | ADS link ]
The vertical distribution of the clear sky longwave cooling of the atmosphere over tropical oceans is inferred from three different datasets. Two of the datasets refer to the TIROS-N Operational Vertical Sounder (TOVS) NOAA/NASA Pathfinder project, PathA and PathB, and the last one refers to the ECMWF reanalysis (ERA-15). Differences are identified originating from the temperature and water vapor fields. They affect the geographical distribution of the longwave fields to various degrees. However, the three datasets lead to similar conclusions concerning the sensitivity of the clear sky total longwave cooling to SST variations. For the highest values of the SST (greater than 27degC), positively correlated to the increased efficiency of the longwave trapping (super-greenhouse effect), the atmosphere shows a lesser efficiency to cool radiatively. The atmosphere does reradiate the longwave radiation toward the surface as efficiently as it traps it. This is verified on regional as well as on seasonal scales. Such longwave cooling behavior is due to an increased mid- and upper-tropospheric humidity resulting from convective transports. The three datasets agree with the vertical distribution of the radiative cooling variations from normal to favorable to super-greenhouse effect conditions, except in the boundary layer, where the coarse resolution of the TOVS-retrieved data makes them not reliable in it. In `normal' conditions the cooling uniformly increases over the vertical with the SST. Over 27degC, the cooling is intensified above 400 hPa and reduced between 900 and 400 hPa.
F. Hourdin and J.-P. Issartel. Sub-surface nuclear tests monitoring through the CTBT Xenon Network. Geophysical Research Letters, 27:2245-2248, August 2000. [ bib | DOI | ADS link ]
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.
P. Goloub, M. Herman, H. Chepfer, J. Riedi, G. Brogniez, P. Couvert, and G. SéZe. Cloud thermodynamical phase classification from the POLDER spaceborne instrument. Journal of Geophysical Research, 105:14, June 2000. [ bib | DOI | ADS link ]
Cloud phase recognition is important for cloud studies. Ice crystals correspond to physical process and properties that differ from those of liquid water drops. The angular polarization signature is a good mean to discriminate between spherical and nonspherical particles (liquid and ice phase, respectively). POLDER (Polarization and Directionality of Earth Reflectances) has been launched on the Japanese ADEOS platform in August 1996. Because of its multidirectional, multispectral, and multipolarization capabilities this new radiometer gives useful information on clouds and their influence on radiation in the shortwave range. The POLDER bidirectional observation capability provides the polarization signatures within a large range of scattering angles in three spectral bands centered on 0.443, 0.670, and 0.865 μm with a spatial resolution of 6.2 km×6.2 km. These original features allow to obtain some information both on cloud thermodynamic phase and on cloud microphysics (size/shape). According to POLDER airborne observations, liquid cloud droplets exhibit very specific polarization features of a rainbow for scattering angles near 140deg. Conversely, theoretical studies of scattering by various crystalline particles and also airborne measurements show that the rainbow characteristics disappear as soon as the particles depart from the spherical shape. In the paper the POLDER algorithm for cloud phase classification is presented, as well as the physical principle of this algorithm. Results derived from the POLDER spaceborne version are also presented and compared with lidar ground-based observations and satellite cloud classification. This cloud phase classification method is shown to be reliable. The major limitation appears when thin cirrus clouds overlap the liquid cloud layer. In this case, if the cirrus optical thickness is smaller than 2, the liquid phase may be retrieved. Otherwise, the ice phase is correctly detected as long as cloud detection works.
S. Bony, W. D. Collins, and D. W. Fillmore. Indian Ocean Low Clouds during the Winter Monsoon. Journal of Climate, 13:2028-2043, June 2000. [ bib | DOI | ADS link ]
While low-level clouds over the Pacific and Atlantic Oceans have been investigated extensively, low clouds over the Indian Ocean are not as well characterized. This study examines the occurrence of nonoverlapped low clouds over the Indian Ocean during the northeast monsoon using several sources of data. Climatologies derived from surface observations and from the International Satellite Cloud Climatology Project are reviewed. Another cloud climatology is developed using infrared and visible imagery from the Indian geostationary satellite. The new climatology has better spatial and temporal resolution than in situ observations. The three datasets are generally consistent and show several persistent features in the cloud distribution. During January-April, maxima in the occurrence of low clouds occur at subtropical latitudes over the Arabian Sea, the Bay of Bengal, the China Sea, and the southern Indian Ocean. The predominant types of low clouds differ in the northern and southern areas of the Indian Ocean region and China Sea. The Arabian Sea and the Bay of Bengal are covered mostly by cumulus clouds, while the southern Indian Ocean and the China Sea are covered mostly by large-scale stratiform clouds such as stratocumulus. These observations are consistent with atmospheric analyses of temperature, humidity, and stability over the Indian Ocean.
M. Sabre, K. Hodges, K. Laval, J. Polcher, and F. Désalmand. Simulation of Monsoon Disturbances in the LMD GCM. Monthly Weather Review, 128:3752, May 2000. [ bib | DOI | ADS link ]
P. Bechtold, J. L. Redelsperger, I. Beau, M. Blackburn, S. Brinkop, J. Y. Grandpeix, A. Grant, D. Gregory, F. Guichard, C. Hoff, and E. Ioannidou. A GCSS model intercomparison for a tropical squall line observed during TOGA-COARE. II: Intercomparison of single-column models and a cloud-resolving model. Quarterly Journal of the Royal Meteorological Society, 126:865-888, April 2000. [ bib | DOI | ADS link ]
O. Boucher and D. Tanré. Estimation of the aerosol perturbation to the Earth's Radiative Budget over oceans using POLDER satellite aerosol retrievals. Geophysical Research Letters, 27:1103-1106, April 2000. [ bib | DOI | ADS link ]
POLDER satellite retrievals of aerosol properties over oceans are used to estimate a global-mean clear-sky aerosol shortwave flux perturbation of order -5 to -6 Wm-2. Uncertainties due to aerosol absorption and POLDER cloud screening algorithm are quantified. In order to bound the radiative forcing by anthropogenic aerosols, we attempt to remove the contribution of background aerosols from these estimates and present all-sky aerosol radiative effects for three regions and two methods. The results are sensitive to the thresholds used to define the background conditions.
D. Cruette, A. Marillier, J. L. Dufresne, J. Y. Grandpeix, P. Nacass, and H. Bellec. Fast Temperature and True Airspeed Measurements with the Airborne Ultrasonic Anemometer Thermometer (AUSAT). Journal of Atmospheric and Oceanic Technology, 17:1020, 2000. [ bib | DOI | ADS link ]
F. Chevallier, J.-J. Morcrette, A. Chédin, and F. Cheruy. TIGR-like atmospheric-profile databases for accurate radiative-flux computation. Quarterly Journal of the Royal Meteorological Society, 126:777-785, January 2000. [ bib | DOI | ADS link ]
F. Chevallier, J.-J. Morcrette, F. Chéruy, and N. A. Scott. Use of a neural-network-based long-wave radiative-transfer scheme in the ECMWF atmospheric model. Quarterly Journal of the Royal Meteorological Society, 126:761-776, January 2000. [ bib | DOI | ADS link ]
F. Codron, A. Vintzileos, and R. Sadourny. An Improved Scheme for Interpolating between an Atmospheric Model and Underlying Surface Grids near Orography and Ocean Boundaries. Monthly Weather Review, 128:1177, 2000. [ bib | DOI | ADS link ]
C. Covey, A. Abe-Ouchi, G. J. Boer, B. A. Boville, U. Cubasch, L. Fairhead, G. M. Flato, H. Gordon, E. Guilyardi, X. Jiang, T. C. Johns, H. Le Treut, G. Madec, G. A. Meehl, R. Miller, A. Noda, S. B. Power, E. Roeckner, G. Russell, E. K. Schneider, R. J. Stouffer, L. Terray, and J.-S. von Storch. The seasonal cycle in coupled ocean-atmosphere general circulation models. Climate Dynamics, 16:775-787, 2000. [ bib | DOI | ADS link ]
We examine the seasonal cycle of near-surface air temperature simulated by 17 coupled ocean-atmosphere general circulation models participating in the Coupled Model Intercomparison Project (CMIP). Nine of the models use ad hoc “flux adjustment” at the ocean surface to bring model simulations close to observations of the present-day climate. We group flux-adjusted and non-flux-adjusted models separately and examine the behavior of each class. When averaged over all of the flux-adjusted model simulations, near-surface air temperature falls within 2K of observed values over the oceans. The corresponding average over non-flux-adjusted models shows errors up to 6K in extensive ocean areas. Flux adjustments are not directly applied over land, and near-surface land temperature errors are substantial in the average over flux-adjusted models, which systematically underestimates (by 5K) temperature in areas of elevated terrain. The corresponding average over non-flux-adjusted models forms a similar error pattern (with somewhat increased amplitude) over land. We use the temperature difference between July and January to measure seasonal cycle amplitude. Zonal means of this quantity from the individual flux-adjusted models form a fairly tight cluster (all within 30% of the mean) centered on the observed values. The non-flux-adjusted models perform nearly as well at most latitudes. In Southern Ocean mid-latitudes, however, the non-flux-adjusted models overestimate the magnitude of January-minus-July temperature differences by 5K due to an overestimate of summer (January) near-surface temperature. This error is common to five of the eight non-flux-adjusted models. Also, over Northern Hemisphere mid-latitude land areas, zonal mean differences between July and January temperatures simulated by the non-flux-adjusted models show a greater spread (positive and negative) about observed values than results from the flux-adjusted models. Elsewhere, differences between the two classes of models are less obvious. At no latitude is the zonal mean difference between averages over the two classes of models greater than the standard deviation over models. The ability of coupled GCMs to simulate a reasonable seasonal cycle is a necessary condition for confidence in their prediction of long-term climatic changes (such as global warming), but it is not a sufficient condition unless the seasonal cycle and long-term changes involve similar climatic processes. To test this possible connection, we compare seasonal cycle amplitude with equilibrium warming under doubled atmospheric carbon dioxide for the models in our data base. A small but positive correlation exists between these two quantities. This result is predicted by a simple conceptual model of the climate system, and it is consistent with other modeling experience, which indicates that the seasonal cycle depends only weakly on climate sensitivity.