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2003 .

(14 publications)

S. Menon, J.-L. Brenguier, O. Boucher, P. Davison, A. D. Del Genio, J. Feichter, S. Ghan, S. Guibert, X. Liu, U. Lohmann, H. Pawlowska, J. E. Penner, J. Quaas, D. L. Roberts, L. Schüller, and J. Snider. Evaluating aerosol/cloud/radiation process parameterizations with single-column models and Second Aerosol Characterization Experiment (ACE-2) cloudy column observations. Journal of Geophysical Research (Atmospheres), 108:4762, December 2003. [ bib | DOI | ADS link ]

The Second Aerosol Characterization Experiment (ACE-2) data set along with ECMWF reanalysis meteorological fields provided the basis for the single column model (SCM) simulations, performed as part of the PACE (Parameterization of the Aerosol Indirect Climatic Effect) project. Six different SCMs were used to simulate ACE-2 case studies of clean and polluted cloudy boundary layers, with the objective being to identify limitations of the aerosol/cloud/radiation interaction schemes within the range of uncertainty in in situ, reanalysis and satellite retrieved data. The exercise proceeds in three steps. First, SCMs are configured with the same fine vertical resolution as the ACE-2 in situ data base to evaluate the numerical schemes for prediction of aerosol activation, radiative transfer and precipitation formation. Second, the same test is performed at the coarser vertical resolution of GCMs to evaluate its impact on the performance of the parameterizations. Finally, SCMs are run for a 24-48 hr period to examine predictions of boundary layer clouds when initialized with large-scale meteorological fields. Several schemes were tested for the prediction of cloud droplet number concentration (N). Physically based activation schemes using vertical velocity show noticeable discrepancies compared to empirical schemes due to biases in the diagnosed cloud base vertical velocity. Prognostic schemes exhibit a larger variability than the diagnostic ones, due to a coupling between aerosol activation and drizzle scavenging in the calculation of N. When SCMs are initialized at a fine vertical resolution with locally observed vertical profiles of liquid water, predicted optical properties are comparable to observations. Predictions however degrade at coarser vertical resolution and are more sensitive to the mean liquid water path than to its spatial heterogeneity. Predicted precipitation fluxes are severely underestimated and improve when accounting for sub-grid liquid water variability. Results from the 24-48 hr runs suggest that most models have problems in simulating boundary layer cloud morphology, since the large-scale initialization fields do not accurately reproduce observed meteorological conditions. As a result, models significantly overestimate optical properties. Improved cloud morphologies were obtained for models with subgrid inversions and subgrid cloud thickness schemes. This may be a result of representing subgrid scale effects though we do not rule out the possibility that better large-forcing data may also improve cloud morphology predictions.

Z. X. Li and S. Conil. Transient Response of an Atmospheric GCM to North Atlantic SST Anomalies. Journal of Climate, 16:3993--3998, December 2003. [ bib | DOI | ADS link ]

A high-resolution atmospheric general circulation model (GCM) is used to evaluate the atmospheric response to North Atlantic sea surface temperature (SST) anomalies. The transient evolution of the response is studied in detail. The linear and nonlinear effects can thus be contrasted and separated by their different time scales. Baroclinic patterns related directly to the surface anomalies quickly reach their maximum manifestation. However, barotropic patterns related to the mid- and upper troposphere eddy vorticity fluxes have longer time scales with much more important amplitude, able to gradually replace the initial baroclinic response. This study thus provides an evolutional picture of the two types of response in a GCM.

A. Mathieu, G. Sèze, A. Lahellec, C. Guerin, and A. Weill. Characterization of the Cloud-Topped Boundary Layer at the Synoptic Scale Using AVHRR Observations during the SEMAPHORE Experiment. Journal of Applied Meteorology, 42:1720--1730, December 2003. [ bib | DOI | ADS link ]

Satellite platforms NOAA-11 and -12 Advanced Very High Resolution Radiometer (AVHRR) data are used during the daytime to study large sheets of stratocumulus over the North Atlantic Ocean. The application concerns an anticyclonic period of the Structure des Echanges Mer Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherché Expérimentale (SEMAPHORE) campaign (10 17 November 1993). In the region of interest, the satellite images are recorded under large solar zenith angles. Extending the SEMAPHORE area, a region of about 3000 × 3000 km2 is studied to characterize the atmospheric boundary layer. A statistical cloud classification method is applied to discriminate for low-level and optically thick clouds. For AVHRR pixels covered with thick clouds, brightness temperatures are used to evaluate the boundary layer cloud-top temperature (CTT). The objective is to obtain accurate CTT maps for evaluation of a global model. In this application, the full-resolution fields are reduced to match model grid size. An estimate of overall temperature uncertainty associated with each grid point is also derived, which incorporates subgrid variability of the fields and quality of the temperature retrieval. Results are compared with the SEMAPHORE campaign measurements. A comparison with DX products obtained with the same dataset, but at lower resolution, is also presented. The authors claim that such instantaneous CTT maps could be as intensively used as classical SST maps, and both could be efficiently complemented with gridpoint error-bar maps. They may be used for multiple applications: (i) to provide a means to improve numerical weather prediction and climatological reanalyses, (ii) to represent a boundary layer global characterization to analyze the synoptic situation of field experiments, and (iii) to allow validation and to test development of large-scale and mesoscale models.

D. Luz, F. Hourdin, P. Rannou, and S. Lebonnois. Latitudinal transport by barotropic waves in Titan's stratosphere.. II. Results from a coupled dynamics-microphysics-photochemistry GCM. Icarus, 166:343--358, December 2003. [ bib | DOI | ADS link ]

We present a 2D general circulation model of Titan's atmosphere, coupling axisymmetric dynamics with haze microphysics, a simplified photochemistry and eddy mixing. We develop a parameterization of latitudinal eddy mixing by barotropic waves based on a shallow-water, longitude-latitude model. The parameterization acts locally and in real time both on passive tracers and momentum. The mixing coefficient varies exponentially with a measure of the barotropic instability of the mean zonal flow. The coupled GCM approximately reproduces the Voyager temperature measurements and the latitudinal contrasts in the distributions of HCN and C 2H 2, as well as the main features of the zonal wind retrieved from the 1989 stellar occultation. Wind velocities are consistent with the observed reversal time of the North-South albedo asymmetry of 5 terrestrial years. Model results support the hypothesis of a non-uniform distribution of infrared opacity as the cause of the Voyager temperature asymmetry. Transport by the mean meridional circulation, combined with polar vortex isolation may be at the origin of the latitudinal contrasts of trace species, with eddy mixing remaining restricted to low latitudes most of the Titan year. We interpret the contrasts as a signature of non-axisymmetric motions.

D. Luz and F. Hourdin. Latitudinal transport by barotropic waves in Titan's stratosphere.. I. General properties from a horizontal shallow-water model. Icarus, 166:328--342, December 2003. [ bib | DOI | ADS link ]

We present a numerical study of barotropic waves in Titan's stratosphere based on a shallow-water model. The forcing of the zonal flow by the mean meridional circulation is represented by a relaxation towards a barotropically unstable wind profile. The relaxation profile is consistent with observations and with previous results from a 3D general circulation model. The time constant of the forcing that best matches the northward eddy-transport of zonal momentum from the 3D model is τ5 Titan days. The eddy wind field is a zonal wavenumber-2 wave with a peak amplitude about 10% of the mean wind speed. The latitudinal transport of angular momentum by the wave tends to keep the flow close to marginal stability by carrying momentum upgradient, from the core of the jets into the low latitudes. Although the strongest eddy motions occur at the latitudes of the wind maxima, the strongest mixing takes place at the barotropically unstable regions, close to 30deg and spanning about 30deg in latitude. An eddy-mixing time constant of the order of 1 Titan day is inferred within these regions, and of a few tens of days within regions of stable flow. Horizontal gradients in transient tracer fields are less than 10% of the latitudinal gradient of the meridional tracer profile. Cassini's detection of such waves could provide a direct observation of wind speeds at stratospheric levels.

P. de Rosnay, J. Polcher, K. Laval, and M. Sabre. Integrated parameterization of irrigation in the land surface model ORCHIDEE. Validation over Indian Peninsula. Geophysical Research Letters, 30:1986, October 2003. [ bib | DOI | ADS link ]

The work presented here describes a new modeling infrastructure of irrigation which is integrated in a land surface model and which will make this model a suitable tool for studies of interactions between irrigation water use and climate change. The model is presented and validated off-line over the Indian Peninsula. Numerical experiments are conducted with a 1 degree spatial resolution land surface model. Two 2-year simulations, forced by the ISLSCP (1987-88) data sets, are conducted with and without irrigation. The analysis focuses on irrigation modeling validation and briefly presents first results on irrigation's impact on the surface fluxes.

F. DéSalmand, A. Szantai, L. Picon, and M. Desbois. Systematic observation of westward propagating cloud bands over the Arabian Sea during Indian Ocean Experiment (INDOEX) from Meteosat-5 data. Journal of Geophysical Research (Atmospheres), 108:8004, September 2003. [ bib | DOI | ADS link ]

During the field experiment phase of the Indian Ocean Experiment (INDOEX), linear cloud formations parallel to the West Indian coast and propagating westward have been observed. Meteosat-5 images are used for the description of the life cycle of these events. Single cloud bands, or main cloud bands followed by (up to 10) secondary parallel cloud lines with a wavelike pattern, have been observed daily during four periods in the dry season of 1999 (15 January, 16-19 February, 27 February to 7 March, and 1-3 April 1999). During these four periods, one single cloud band or a packet of cloud bands is generated every day at nighttime. Their length reaches several hundreds of kilometers, their width lies below 40 km, and their duration in some cases exceeds 24 hours. The smaller cloud lines observed behind the leading cloud line are narrower and are separated by a distance of 2-10 km. Cloud tops are about 3-8degC colder than the Arabian Sea, corresponding to an altitude between 500 and 1200 m during the night and below 2500 m during daytime. Cloud bands travel westward over the Arabian Sea at a speed around 13 m/s, greater than the wind speed measured in the surrounding area. The motion of the cloud band presents similarities with the wavelike propagation of atmospheric phenomena such as the Australian morning glories. Common elements to the different cases are the following: weak low-level winds close to the southwestern Indian coast when the cloud bands appear, winds with a northerly component in the vicinity of the northwestern Indian coast, the presence of a sea-land breeze circulation along the West Indian coast, and the presence of high concentrations of pollutants over the northeastern Arabian Sea. However, existing physical links between these elements (if any) still have to be investigated.

J. Trentmann, B. Früh, O. Boucher, T. Trautmann, and M. O. Andreae. Three-dimensional solar radiation effects on the actinic flux field in a biomass-burning plume. Journal of Geophysical Research (Atmospheres), 108:4558, September 2003. [ bib | DOI | ADS link ]

Three-dimensional (3-D) solar radiative transfer models describe radiative transfer under inhomogeneous atmospheric conditions more accurately than the commonly used one-dimensional (1-D) radiative transfer models that assume horizontal homogeneity of the atmosphere. Here results of 3-D radiative transfer simulations for a biomass-burning plume are presented and compared with local one-dimensional (l-1-D) simulations, i.e., 1-D simulations in every column of the model domain. The spatial distribution of the aerosol particles was derived from a 3-D atmospheric transport simulation. We studied the impact of 3-D radiative effects on the actinic flux within the plume center. The differences in the actinic flux between results from the 3-D and the l-1-D simulations are considerable, ranging from -40% to more than +200%, depending on the wavelength, solar zenith angle, and the absorbing properties of the aerosol. The reason for this discrepancy is the neglect of horizontal photon transport in the 1-D simulation. These large 3-D effects on the actinic flux have the potential to influence significantly the in-plume photochemistry.

S. Generoso, F.-M. Bréon, Y. Balkanski, O. Boucher, and M. Schulz. Improving the seasonal cycle and interannual variations of biomass burning aerosol sources. Atmospheric Chemistry & Physics, 3:1211--1222, August 2003. [ bib | ADS link ]

This paper suggests a method for improving current inventories of aerosol emissions from biomass burning. The method is based on the hypothesis that, although the total estimates within large regions are correct, the exact spatial and temporal description can be improved. It makes use of open fire detection from the ATSR instrument that is available since 1996. The emissions inventories are re-distributed in space and time according to the occurrence of open fires. Although the method is based on the night-time hot-spot product of the ATSR, other satellite biomass burning proxies (AVHRR, TRMM, GLOBSCAR and GBA2000) show similar distributions.<BR /> <BR /> The impact of the method on the emission inventories is assessed using an aerosol transport model, the results of which are compared to sunphotometer and satellite data. The seasonal cycle of aerosol load in the atmosphere is significantly improved in several regions, in particular South America and Australia. Besides, the use of ATSR fire detection may be used to account for interannual events, as is demonstrated on the large Indonesian fires of 1997, a consequence of the 1997-1998 El Niño. Despite these improvements, there are still some large discrepancies between the simulated and observed aerosol optical thicknesses resulting from biomass burning emissions.

N. Bellouin, O. Boucher, D. Tanré, and O. Dubovik. Aerosol absorption over the clear-sky oceans deduced from POLDER-1 and AERONET observations. Geophysical Research Letters, 30:1748, July 2003. [ bib | DOI | ADS link ]

We estimate aerosol absorption over the clear-sky oceans using aerosol geophysical products from POLDER-1 space measurements and absorption properties from ground-based AERONET measurements. Our best estimate is 2.5 Wm-2 averaged over the 8-month lifetime of POLDER-1. Low and high absorption estimates are 2.2 and 3.1 Wm-2 based on the variability in aerosol single-scattering albedo observed by AERONET. Main sources of uncertainties are the discrimation of the aerosol type from satellite measurements, and potential clear-sky bias induced by the cloud-screening procedure.

P. Guyon, B. Graham, J. Beck, O. Boucher, E. Gerasopoulos, O. L. Mayol-Bracero, G. C. Roberts, P. Artaxo, and M. O. Andreae. Physical properties and concentration of aerosol particles over the Amazon tropical forest during background and biomass burning conditions. Atmospheric Chemistry & Physics, 3:951--967, July 2003. [ bib | ADS link ]

We investigated the size distribution, scattering and absorption properties of Amazonian aerosols and the optical thickness of the aerosol layer under the pristine background conditions typical of the wet season, as well as during the biomass-burning-influenced dry season. The measurements were made during two campaigns in 1999 as part of the European contribution to the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA-EUSTACH). In moving from the wet to the dry season, median particle numbers were observed to increase from values comparable to those of the remote marine boundary layer (˜400 cm-3) to values more commonly associated with urban smog (˜4000 cm-3), due to a massive injection of submicron smoke particles. Aerosol optical depths at 500 nm increased from 0.05 to 0.8 on average, reaching a value of 2 during the dry season. Scattering and absorption coefficients, measured at 550 nm, showed a concomitant increase from average values of 6.8 and 0.4 Mm-1 to values of 91 and 10 Mm-1, respectively, corresponding to an estimated decrease in single-scattering albedo from ca. 0.97 to 0.91. The roughly tenfold increase in many of the measured parameters attests to the dramatic effect that extensive seasonal biomass burning (deforestation, pasture cleaning) is having on the composition and properties of aerosols over Amazonia. The potential exists for these changes to impact on regional and global climate through changes to the extinction of solar radiation as well as the alteration of cloud properties.

L. Picon, R. Roca, S. Serrar, J. L. Monge, and M. Desbois. A new METEOSAT “water vapor” archive for climate studies. Journal of Geophysical Research (Atmospheres), 108:4301, May 2003. [ bib | DOI | ADS link ]

Water vapor plays an important role in the climate system through a number of mechanisms spanning a wide range of space and timescale. Since 1977, the METEOSAT satellites are equipped with a radiometer dedicated to the measurements of upper tropospheric humidity (UTH) which covers a relevant range of scales for a better understanding of the water vapor role in the climate. Due to the changes of the satellites and the calibration techniques over the last 20 years, this water vapor METEOSAT archive is not homogeneous and cannot be directly used for climatic studies. Hence the authors present in this paper a newly homogenized METEOSAT water vapor channel archive. Two main types of anomalies entail the original METEOSAT archive. The first one corresponds to the successive improvements of the calibration procedure. In this case, a statistical correction technique based on comparisons between ECMWF-simulated brightness temperature (BT) and water vapor METEOSAT-observed BT is developed. The second type of anomaly concerns the METEOSAT radiometer changes over the time. While still measuring the UTH, the details of the filter function, indeed, evolved over the last 20 years. In this second case, the correction is based on a physical method implying simulations of the same scene by different radiometer filter functions. Two major cases are documented in detail for September 1987 and for February 1994. Sensitivity analysis of the techniques is conducted and the methods are shown to be robust with respect to the details of their implementations. The efficiency of the two methods is then evaluated. The resulting archive reveals water vapor seasonal cycle features in better agreement with climatological estimates. The new homogenized METEOSAT archive consists of 3-hourly total sky radiance at the 0.625deg × 0.625deg resolution over the July 1983 to February 1994 period, offering the opportunity to investigate the variability of the regional UTH from synoptic scales to interannual and interdecadal scales.

S. Lebonnois, F. Hourdin, P. Rannou, D. Luz, and D. Toublanc. Impact of the seasonal variations of composition on the temperature field of Titan's stratosphere. Icarus, 163:164--174, May 2003. [ bib | DOI | ADS link ]

We investigate the role of seasonal variations of Titan's stratospheric composition on the temperature. We use a general circulation model coupled with idealized chemical tracers that reproduce variations of ethane (C 2H 6), acetylene (C 2H 2), and hydrogen cyanide (HCN). Enhancement of the mole fractions of these compounds, at high latitudes in the winter hemisphere relative to their equatorial values, induces a relative decrease in temperature above approximately 0.2 mbar, with a peak amplitude around -20 K, and a relative increase in temperature below, around 1 mbar, with a peak amplitude around +7 K. These thermal effects are mainly due to the variations of the cooling to space induced by the varying distributions. The ethane, acetylene, and hydrogen cyanide variations affect the cooling rates in a similar way, with the dominant effect being due to ethane, though its latitudinal variations are small.

O. Boucher, C. Moulin, S. Belviso, O. Aumont, L. Bopp, E. Cosme, R. von Kuhlmann, M. G. Lawrence, M. Pham, M. S. Reddy, J. Sciare, and C. Venkataraman. DMS atmospheric concentrations and sulphate aerosol indirect radiative forcing: a sensitivity study to the DMS source representation and oxidation. Atmospheric Chemistry & Physics, 3:49--65, January 2003. [ bib | ADS link ]

The global sulphur cycle has been simulated using a general circulation model with a focus on the source and oxidation of atmospheric dimethylsulphide (DMS). The sensitivity of atmospheric DMS to the oceanic DMS climatology, the parameterisation of the sea-air transfer and to the oxidant fields have been studied. The importance of additional oxidation pathways (by O3 in the gas- and aqueous-phases and by BrO in the gas phase) not incorporated in global models has also been evaluated. While three different climatologies of the oceanic DMS concentration produce rather similar global DMS fluxes to the atmosphere at 24-27 Tg S yr -1, there are large differences in the spatial and seasonal distribution. The relative contributions of OH and NO3 radicals to DMS oxidation depends critically on which oxidant fields are prescribed in the model. Oxidation by O3 appears to be significant at high latitudes in both hemispheres. Oxidation by BrO could be significant even for BrO concentrations at sub-pptv levels in the marine boundary layer. The impact of such refinements on the DMS chemistry onto the indirect radiative forcing by anthropogenic sulphate aerosols is also discussed.

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