A. Garnier

LATMOS, Guyancourt, Île-de-France, France

Are you A. Garnier?

Claim your profile

Publications (54)81.57 Total impact

  • Journal of Applied Meteorological and Climatology. 07/2013;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Cirrus are cloud types that are recognized to have a strong impact on the Earth-atmosphere radiation balance. This impact is however still poorly understood, due to the difficulties in describing the large variability of their properties in global climate models. Consequently, numerous airborne and space-borne missions have been dedicated to their study in the last decades. The satellite constellation A-Train has for instance proven to be particularly helpful for the study of cirrus. More particularly, the Infrared Imaging Radiometer (IIR) carried onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite shows a great sensitivity to the radiative and microphysical properties of these clouds. Our study presents a novel methodology that uses the thermal infrared measurements of IIR to retrieve the ice crystal effective size and optical thickness of cirrus. This methodology is based on an optimal estimation scheme, which possesses the advantage of attributing precise uncertainties to the retrieved parameters. Two IIR airborne validation campaigns have been chosen as case studies for illustrating the results of our retrieval method. It is observed that optical thicknesses could be accurately retrieved but that large uncertainties may occur on the effective diameters. Strong agreements have also been found between the products of our method when separately applied to the measurements of IIR and of the airborne radiometer CLIMAT-AV, which consolidates the results of previous validation studies of IIR level-1 measurements. Comparisons with in situ observations and with operational products of IIR are also discussed and appear to be coherent with our results. However, we have found that the quality of our retrievals can be strongly impacted by uncertainties related to the choice of a pristine crystal model and by poor constraints on the properties of possible liquid cloud layers underneath cirrus. Simultaneous retrievals of liquid clouds radiative and microphysical properties and/or the use of different ice crystal models should therefore be considered in order to improve the quality of the results.
    Atmospheric Chemistry and Physics 01/2013; 13(2):5553-5599. · 4.88 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Two complementary case studies are conducted to analyse convective system properties in the region where strong cloud-top lidar backscatter anomalies are observed as reported by Platt et al. (2011). These anomalies were reported for the first time using in-situ microphysical measurements in an isolated continental convective cloud over Germany during the CIRCLE2 experiment (Gayet et al., 2012). In this case, quasi collocated in situ observations with CALIPSO, CloudSat and Meteosat-9/SEVIRI observations confirm that regions of backscatter anomalies represent the most active and dense convective cloud parts with likely the strongest core updrafts and unusual high values of the particle concentration, extinction and ice water content (IWC), with the occurrence of small ice crystal sizes. Similar spaceborne observations are then analyzed in a maritime mesoscale cloud system (MCS) on 20 June 2008 located off the Brazil coast between 0° and 3° N latitude. Near cloud-top backscatter anomalies are evidenced in a region which corresponds to the coldest temperatures with maximum cloud top altitudes derived from collocated CALIPSO/IIR and Meteosat-9/SEVIRI infrared brightness temperatures. The interpretation of CALIOP data highlights significant differences of microphysical properties from those observed in the continental isolated convective cloud. Indeed, SEVIRI retrievals in the visible confirm much smaller ice particles near-top of the isolated continental convective cloud, i.e. effective radius (Reff) ~15 μm against 22–27 μm in the whole MCS area. 94 GHz Cloud Profiling Radar observations from CloudSat are then used to describe the properties of the most active cloud regions at and below cloud top. The cloud ice water content and effective radius retrieved with the CloudSat 2B-IWC and DARDAR inversion techniques, show that at usual cruise altitudes of commercial aircraft (FL 350 or ~10 700 m level), high IWC (i.e. up to 2 to 4 g m−3) could be identified according to specific IWC-Z relationships. These values correspond to a maximum reflectivity factor of +18 dBZ (at 94 GHz). Near-top cloud properties also indicate signatures of microphysical characteristics according to the cloud-stage evolution as revealed by SEVIRI images to identify the development of new cells within the MCS cluster. It is argued that the availability of real time information of the km-scale cloud top IR brightness temperature decrease with respect to the cloud environment would help identify MCS cloud areas with potentially high ice water content and small particle sizes against which onboard meteorological radar may not be suitable to provide timely warning.
    Atmospheric Chemistry and Physics 01/2013; · 4.88 Impact Factor
  • Journal of Applied Meteorology and Climatology 07/2012; 51(7):1407-1425. · 2.02 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Polar Stratospheric Clouds (PSCs) induce a remarkable reddening of the sky at twilight. Such reddening has been observed regularly by the SAOZ UV-visible spectrometers operating in polar regions. The PSC detection is based on a Color Index (CI) derived from the ratio of the sunlight scattered at zenith at 550 to 350 nm. The cloud altitude is retrieved from the SZA (Sun Zenith Angle) of maximum CI, after calibration by comparison with PSCs observations from the CALIPSO lidar in orbit since 2006. The temperature threshold at which PSCs can form is then investigated using the ECMWF model at the cloud level. We will show statistics based on more than 20 years of observations in Sodankyla in Finland and in Dumont d'Urville in Antarctica. The PSC threshold temperature is found to be lower by about 5K in the Antarctic than in the Arctic, which is shown to be consistent with the expected stronger de-nitrification and dehydration of the southern winter vortex compared to the northern one.
    04/2012;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In the frame of validation of the spatial observations from the radiometer IIR on board CALIPSO, the two airborne campaigns Cirrus Cloud Experiment (CIRCLE)-2 and Biscay ‘08 took place in 2007 and 2008 in the western part of France, over the Atlantic Ocean. During these experiments, remote sensing measurements were made over cirrus clouds, right under the track of Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) in space and time collocation. For this purpose, a Falcon-20 aircraft was equipped with the Lidar pour l’Etude des InteractionsAe´rosols Nuages Dynamique Rayonnement et du Cycle de l’Eau (LEANDRE)-New Generation (NG) and the thermal infrared radiometer Conveyable Low-Noise Infrared Radiometer for Measurements of Atmosphere and Ground Surface Targets (CLIMAT)-Airborne Version (AV), whose spectral characteristics are strongly similar to those of the infrared imaging radiometer (IIR). In situ measurements were also taken in cirrus clouds during CIRCLE-2. After comparisons, consistent agreements are found between brightness temperatures measured by CLIMAT-AV and IIR. However, deviations in the brightness temperature measurements are still observed, mainly in the 8.6-mm channels. Simulations using a radiative transfer code are performed along a perfectly clear-sky area to show that these dissimilarities are inherent in slight differences between the spectral channels of both radiometers, and in differences between their altitudes. Cloudy and imperfectly clear areas are found to be harder to interpret, but the measurements are still coherent by taking into account experimental uncertainties. In the end, IIR measurements can be validated unambiguously.
    Journal of Atmospheric and Oceanic Technology 01/2012; 29:653-667. · 1.69 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ocean surface observations from the CloudSat radar and the spaceborne lidar aboard the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) platform are combined in the Synergized Optical Depth of Aerosol (SODA) algorithm and used to retrieve the optical depth of semitransparent single-layered cirrus clouds. In the operational CALIPSO data analysis, lidar-derived optical depths are typically estimated using a correction factor for multiple scattering effects and a single global mean lidar ratio. By combining the SODA approach with observations from the CALIPSO Imaging Infrared Radiometer, accurate values for both of these parameters can be derived directly from the measurements. Application of this approach yields a multiple scattering factor of 0.61 ± 0.15 sr, which is essentially identical to the value used operationally. However, the standard lidar ratio used in the CALIPSO daytime operational analysis is found to be biased low by around 25%. As a consequence, the lidar-derived optical depths retrieved from the daytime operational analyses are more than 30% smaller than those derived using SODA. The lidar ratio for semitransparent cirrus is found to be rather stable over ocean (33 ± 5 sr) with slight variations as a function of temperature and latitude. The geographic distribution shows a moderate decrease of average lidar ratio values over Indonesia during daytime, which may be attributed to a larger occurrence of high-altitude cirrus layers in this convectively active area.
    Journal of Geophysical Research 01/2012; · 3.17 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Cirrus clouds have an important influence on Earth's radiation balance through the scattering and absorption of solar and thermal radiation. Radiation absorbed by cirrus is also a significant contributor to atmospheric heating in the upper troposphere. These effects are often parameterized in terms of the cirrus visible optical depth, but current estimates of global cirrus optical depths - whether from models or from passive satellite sensors -contain significant uncertainties. Cirrus clouds are often semi-transparent and often occur as part of multi-layer cloud structures, which complicates retrievals by passive techniques. The CALIOP lidar, flying on the CALIPSO satellite since 2006, has the potential to provide a much improved global record of cirrus distribution and properties. CALIOP cirrus retrievals must be validated, however, and early comparisons with MODIS retrievals indicated significant discrepancies. There are multiple potential error sources, in both retrievals, which could be the source of these discrepancies. To validate and characterize uncertainties in CALIOP cirrus optical depths, intercomparisons have been performed with several other cirrus datasets. Intercomparisons with infrared retrievals from the IR radiometer carried on the CALIPSO satellite have been particularly informative. Intercomparisons between results from three different CALIOP cirrus retrieval techniques provide further insights. This presentation will discuss the quest to quantify the uncertainties in the CALIOP cirrus retrievals and develop an accurate dataset of cirrus optical depths.
    AGU Fall Meeting Abstracts. 12/2011;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: During the CIRCLE-2 experiment carried out over Western Europe in May 2007, combined in situ and remote sensing observations allowed to describe microphysical and optical properties near-top of an overshooting convective cloud (11 080 m/−58 C). The airborne measurements were performed with the DLR Falcon aircraft specially equipped with a unique set of instruments for the extensive in situ cloud measurements of microphysical and optical properties (Polar Nephelometer, FSSP-300, Cloud Particle Imager and PMS 2-D-C) and nadir looking remote sensing observations (DLR WALES Lidar). Quasi-simultaneous space observations from MSG/SEVIRI, CALIPSO/CALIOP-WFC-IIR and CloudSat/CPR combined with airborne RASTA radar reflectivity from the French Falcon aircraft flying above the DLR Falcon depict very well convective cells which overshoot by up to 600m the tropopause level. Unusual high values of the concentration of small ice particles, extinction, ice water content (up to 70 cm−3, 30 km−1 and 0.5 gm−3, respectively) are experienced. The mean effective diameter and the maximum particle size are 43 μm and about 300 μm, respectively. This very dense cloud causes a strong attenuation of the WALES and CALIOP lidar returns. The SEVIRI retrieved parameters confirm the occurrence of small ice crystals at the top of the convective cell. Smooth and featureless phase functions with asymmetry factors of 0.776 indicate fairly uniform optical properties. Due to small ice crystals the power-law relationship between ice water content (IWC) and radar reflectivity appears to be very different from those usually found in cirrus and anvil clouds. For a given equivalent reflectivity factor, IWCs are significantly larger for the overshooting cell than for the cirrus. Assuming the same prevalent microphysical properties over the depth of the overshooting cell, RASTA reflectivity profiles scaled into ice water content show that retrieved IWC up to 1 gm−3 may be observed near the cloud top. Extrapolating the relationship for stronger convective clouds with similar ice particles, IWC up to 5 gm−3 could be experienced with reflectivity factors no larger than about 20 dBZ. This means that for similar situations, indication of rather weak radar echo does not necessarily warn the occurrence of high ice water content carried by small ice crystals. All along the cloud penetration the shape of the ice crystals is dominated by chainlike aggregates of frozen droplets. Our results confirm previous observations that the chains of ice crystals are found in a continental deep convective systems which are known generally to generate intense electric fields causing efficient ice particle aggregation processes. Vigorous updrafts could lift supercooled droplets which are frozen extremely rapidly by homogeneous nucleation near the −37 C level, producing therefore high concentrations of very small ice particles at upper altitudes. They are sufficient to deplete the water vapour and suppress further nucleation as confirmed by humidity measurements. These observations address scientific issues related to the microphysical properties and structure of deep convective clouds and confirm that particles smaller than 50 μm may control the radiative properties in convective-related clouds. These unusual observations may also provide some possible insights regarding engineering issues related to the failure of jet engines commonly used on commercial aircraft during flights through areas of high ice water content. However, large uncertainties of the measured and derived parameters limit our observations.
    Atmospheric Chemistry and Physics 01/2011; 12(2012):727-744. · 4.88 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The evolution of aerosols in the tropical upper troposphere/lower stratosphere between June 2006 and October 2009 is examined using the observations of the space borne CALIOP lidar aboard the CALIPSO satellite. Superimposed on several volcanic plumes and soot from an extreme biomass-burning event in 2009, the measurements reveal the existence of fast cleansing episodes of the lower stratosphere to altitudes as high as 20 km. The cleansing of the full 14–20 km layer takes place within 1–4 months. Its coincidence with the maximum of convective activity in the southern tropics, suggests that the cleansing is the result of a large number of overshooting towers, injecting aerosol-poor tropospheric air into the lower stratosphere. The enhancements of aerosols at the tropopause level during the NH summer may be due to the same transport process but associated with intense sources of aerosols at the surface. Since, the tropospheric air flux derived from CALIOP observations during North Hemisphere winter is 5–20 times larger than the slow ascent by radiative heating usually assumed, the observations suggest that convective overshooting is a major contributor to troposphere-to-stratosphere transport with concommitant implications to the Tropical Tropopause Layer top height, chemistry and thermal structure.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2011; · 5.51 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The variability of stratospheric aerosol loading between 1985 and 2010 is explored with measurements from SAGE II, CALIPSO, GOMOS/ENVISAT, and OSIRIS/Odin space-based instruments. We find that, following the 1991 eruption of Mount Pinatubo, stratospheric aerosol levels increased by as much as two orders of magnitude and only reached "background levels" between 1998 and 2002. From 2002 onwards, a systematic increase has been reported by a number of investigators. Recently, the trend, based on ground-based lidar measurements, has been tentatively attributed to an increase of SO(2) entering the stratosphere associated with coal burning in Southeast Asia. However, we demonstrate with these satellite measurements that the observed trend is mainly driven by a series of moderate but increasingly intense volcanic eruptions primarily at tropical latitudes. These events injected sulfur directly to altitudes between 18 and 20 km. The resulting aerosol particles are slowly lofted into the middle stratosphere by the Brewer-Dobson circulation and are eventually transported to higher latitudes. Citation: Vernier, J.-P., et al. (2011), Major influence of tropical volcanic eruptions on the stratospheric aerosol layer during the last decade, Geophys. Res. Lett., 38, L12807, doi:10.1029/2011GL047563.
    Geophysical Research Letters 01/2011; 38. · 3.98 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Important volcanic material injections in the troposphere can induce significant environmental and meteorological perturbations. Several volcanic eruptions have been recently observed before the eruption of Eyjafjallajökull in Iceland, but this one was an important source of problems to European air transport, as this volcano was directly injecting material over northern Europe. It has thus become of major importance to precisely characterize transported material (identify silicate and sulfate aerosol and their properties) from observations and provide such information as inputs and control to transport models. IR observations have long been used to characterize volcanic emitted material. It is one of the advantages of the CALIPSO mission to combine IR and lidar instruments to provide co-located observations directly exploitable to improve the characterization of aerosol and clouds. CALIPSO, as part of the A-Train, further benefits from an unprecedented observational environment, which enables combined analyses with CloudSat and AQUA. Furthermore, the overpasses of the A-Train provide a snapshot into observations from geostationary platforms, such as MSG over Europe and Africa, which can be of valuable interest to follow the dispersion and modifications of plumes on a regular basis. In this presentation, we will focus on observations made after the eruption of the Chaiten volcano in Chile in 2008, and of the Icelandic volcano in 2010. Emitted plumes were transported over areas possibly covered with low clouds, so that potential observations from the surface need to be complemented, and that radiometric observations in the visible are largely perturbed. We show that the complementary observations of A-Train and geostationary sensors allow to better identify and characterize volcanic ash properties, and to follow the evolution of the plumes. More specifically, we focus on IR observations to characterize the size of silicate particles, and on combined CALIPSO IIR and lidar observations, involving newly developed research products, to analyze aerosol optical depths over clouds and ocean.
    AGU Fall Meeting Abstracts. 12/2010;
  • [Show abstract] [Hide abstract]
    ABSTRACT: This work deals with the study of aerosols in the tropical UTLS and transport processes responsible for their time evolution from the first two years observations of the CALIOP lidar carried on the French-US CALIPSO satellite launched in May 2006. Stratospheric aerosols retrieval algorithms developed have shown: a) the impact of medium-scale volcanic eruptions which represent an important source of aerosols in the stratosphere, ignored until now; b) the decoupling of the mid- and lower stratosphere at 20km separated by a region of zero vertical velocity, surmounted by the slow ascent of the Brewer-Dobson circulation modulated by the Quasi-Biennal Oscillation; c) the injection of clean air until 20km in the equatorial region during the boreal winter, likely washed out in the troposphere and rapidly transport in the most convective regions located above the tropical continents of the south hemisphere. d) the occurrence of aerosols between 15 and 18km above West Africa and Asia during their respective monsoon season, which could be small mineral dust lifted by convection from Sahara and Gobi deserts.
    04/2010; 12:12268.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Various dynamical, chemical and radiative processes control the thermal structure of the middle atmosphere. The middle atmosphere plays an important role in the radiative budget of the Earth and contains the stratospheric ozone layer protecting us against harmful solar UV radiation. Observations are thus fundamental to have a good knowledge of this region and to predict correctly its future evolution in relation with global climate changes. In situ and remote sensing ground-based measurements provide a detailed but localized description of the state of the atmosphere. On the contrary, space borne instruments allow the observation of atmospheric parameters on a global scale. The goal of this article is to describe the available space instruments for the study of the temperature and dynamics of the stratosphere and the mesosphere and to show how ground-based and satellite observations complement each other. (C) 2010 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
    Comptes Rendus Geosciences 01/2010; 342(4-5, SI):323-330. · 1.70 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A multi-platform field measurement campaign involving aircraft and balloons took place over West Africa between 26 July and 25 August 2006, in the frame of the concomitant AMMA Special Observing Period and SCOUT-O3 African tropical activities. Specifically aiming at sampling the upper troposphere and lower stratosphere, the high-altitude research aircraft M55 Geophysica was deployed in Ouagadougou (12.3° N, 1.7° W), Burkina Faso, in conjunction with the German D-20 Falcon, while a series of stratospheric balloon and sonde flights were conducted from Niamey (13.5° N, 2.0° E), Niger. The stratospheric aircraft and balloon flights intended to gather experimental evidence for a better understanding of large scale transport, assessing the effect of lightning on NOx production, and studying the impact of intense mesoscale convective systems on water, aerosol, dust and chemical species in the upper troposphere and lower stratosphere. The M55 Geophysica carried out five local and four transfer flights between southern Europe and the Sahel and back, while eight stratospheric balloons and twenty-nine sondes were flown from Niamey. These experiments allowed a characterization of the tropopause and lower stratosphere of the region. We provide here an overview of the campaign activities together with a description of the general meteorological situation during the flights and a summary of the observations accomplished.
    Atmospheric Chemistry and Physics. 01/2010; 10(2010):2237-2256.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper presents a comparison of combined Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) extinction retrievals with airborne lidar and in situ cirrus cloud measurements. Specially oriented research flights were carried out in western Europe in May 2007 during the Cirrus Cloud Experiment (CIRCLE‐2) with the German Deutsches Zentrum für Luft‐ und Raumfahrt (DLR) and the French Service des Avions Français Instrumentés pour la Recherche en Environnement (SAFIRE) Falcon aircraft equipped for remote and in situ measurements, respectively. Four cirrus cloud situations including thin cirrus layers and outflow cirrus linked to midlatitude fronts and convective systems were chosen to perform experimental collocated observations along the satellite overpasses. The measurements were carried out with temperatures ranging between −38°C and −60°C and with extinction coefficients no larger than 2 km−1. Comparisons between CALIOP and airborne lidar (LEANDRE New Generation (LNG)) attenuated backscatter coefficients reveal much larger CALIOP values for one frontal cirrus situation which could be explained by oriented pristine ice crystals. During the four selected cases the CALIOP cirrus extinction profiles were compared with in situ extinction coefficients derived from the Polar Nephelometer. The results show a very good agreement for two situations (frontal and outflow cases) despite very different cloud conditions. The slope parameters of linear fittings of CALIOP extinction coefficients with respect to in situ measurements are 0.90 and 0.94, with correlation coefficients of 0.69 and only 0.36 for the latter case because of a small number of measurements. On the contrary, significant differences are evidenced for two other situations. In thin frontal cirrus at temperatures ranging between −58°C and −60°C, systematic larger CALIOP extinctions can be explained by horizontally oriented ice crystals with prevalent planar‐plate shape as revealed by the Cloud Particle Imager instrument. This nicely explains the disagreements between CALIOP and LNG observations for that case. For the last cirrus situation related to dense outflow cirrus, CALIOP extinctions are systematically lower than the in situ observations. No clear explanations can be drawn to assess this feature, but the shattering of ice crystals on probe tips may enhance the measured extinction because numerous large ice crystals are observed during this cirrus situation. Finally, relationships between the ice water content and the extinction coefficient, the effective diameter, and the temperature are determined from this in situ measurements data set.
    Journal of Geophysical Research 01/2010; 115(2010-06-26-D00H25):1-17. · 3.17 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The possible impact of deep convective overshooting over land has been explored by six simultaneous soundings of water vapour, particles and ozone in the lower stratosphere next to Mesoscale Convective Systems (MCSs) during the monsoon season over West Africa in Niamey, Niger in August 2006. The water vapour measurements were carried out using a fast response FLASH-B Lyman-alpha hygrometer. The high vertical resolution observations of the instrument show the presence of accumulation of enhanced water vapour layers between the tropopause at 370 K and the 420 K level. Most of these moist layers are shown connected with overshooting events occurring upwind as identified from satellite IR images over which the air mass probed by the sondes passed during the three previous days. In the case of a local overshoot identified by echo top turrets above the tropopause by the MIT C-band radar also in Niamey, tight coincidence was found between enhanced water vapour, ice crystal and ozone dip layers indicative of fast uplift of tropospheric air across the tropopause. The water vapour mixing ratio in the enriched layers exceeds frequently by 1–3 ppmv the average 6 ppmv saturation ratio at the tropopause and by up to 7 ppmv in the extreme case of local storm in coincidence with the presence of ice crystals. The presence of such layers strongly suggests hydration of the lower stratosphere by geyser-like injection of ice particles over overshooting turrets. The pile-like increase of water vapour up to 19 km seen by the high-resolution hygrometer during the season of maximum temperature of the tropopause, suggests that the above hydration mechanism may contribute to the summer maximum moisture in the lower stratosphere. If this interpretation is correct, hydration by ice geysers across the tropopause might be an important contributor to the stratospheric water vapour budget.
    Atmospheric Chemistry and Physics 01/2009; · 4.88 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The evolution of the aerosols in the tropical stratosphere since the beginning of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission in June 2006 is investigated using Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) lidar data. It is shown that the current operational calibration requires adjustment in the tropics. Indeed, on the basis of the assumption of pure Rayleigh scattering between 30 and 34 km the current calibration leads to an average underestimation of the scattering ratio by 6% because of the significant amount of aerosols up to 35 km altitude in the tropics, in contrast to midlatitudes. A better result is obtained by adjusting the calibration to higher altitudes, 36-39 km, where past Stratospheric Aerosol and Gas Experiment (SAGE) II extinction measurements showed an almost complete absence of aerosols. After recalibration the tropical stratospheric aerosol picture provided by CALIOP during the first 2 years of the mission reveals significant changes in the aerosol concentration associated with different transport processes. In the stratosphere the slow ascent of several volcanic layers and their meridional transport toward the subtropics are very consistent with the Brewer-Dobson circulation. The near-zero vertical velocity observed around 20 km during the Northern Hemisphere (NH) summer is in good agreement with radiative heating calculation. In the Tropical Tropopause Layer (TTL), weak depolarizing particles are observed during land convective periods, particularly intense over South Asia during the monsoon season. Finally, seasonal fast occurrence of apparent clean air in the TTL during the NH winter requires more investigations to understand its origin.
    Journal of Geophysical Research 01/2009; 114. · 3.17 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A multi-platform field measurement campaign involving aircraft and balloons took place over West Africa between 26 July and 25 August 2006, in the frame of the concomitant AMMA Special Observing Period and SCOUT-O3 African tropical activities. Specifically aiming at sampling the upper troposphere and lower stratosphere, the high-altitude research aircraft M55 Geophysica was deployed in Ouagadougou (12.3° N, 1.7° W), Burkina Faso, in conjunction with the German D-20 Falcon, while a series of stratospheric balloon and sonde flights were conducted from Niamey (13.5° N, 2.0° E), Niger. The stratospheric aircraft and balloon flights intended to gather experimental evidence for a better understanding of large scale transport, assessing the effect of lightning on NOx production, and studying the impact of intense mesoscale convective systems on water, aerosol, dust and chemical species in the upper troposphere and lower stratosphere. The M55 Geophysica carried out five local and four transfer flights between southern Europe and the Sahel and back, while eight stratospheric balloons and twenty-nine sondes were flown from Niamey. These experiments allowed a characterization of the tropopause and lower stratosphere of the region. We provide here an overview of the campaign activities together with a description of the general meteorological situation during the flights and a summary of the observations accomplished.
    Atmospheric Chemistry and Physics Discussions. 01/2009;
  • CloudSat / CALIPSO Joint Meeting; 06/2007

Publication Stats

430 Citations
81.57 Total Impact Points

Institutions

  • 2013
    • LATMOS
      Guyancourt, Île-de-France, France
  • 2010–2012
    • Pierre and Marie Curie University - Paris 6
      Lutetia Parisorum, Île-de-France, France
  • 2009–2011
    • Université de Versailles Saint-Quentin
      Versailles, Île-de-France, France
  • 2006
    • ENEA
      Roma, Latium, Italy
  • 1989–2002
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France