J. Pelon

UPMC, Pittsburgh, Pennsylvania, United States

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Publications (181)243.09 Total impact

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    ABSTRACT: Recently, two Types of Ice Clouds (TICs) properties have been characterized using ISDAC airborne measurements (Alaska, April 2008). TIC-2B were characterized by fewer (<10 L-1) and larger (>110 μm) ice crystals, a larger ice supersaturation (>15%) and a fewer ice nuclei (IN) concentration (<2 order of magnitude) when compared to TIC-1/2A. It has been hypothesized that emissions of SO2 may reduce the ice nucleating properties of IN through acidification, resulting to a smaller concentration of larger ice crystals and leading to precipitation (e.g. cloud regime TIC-2B) because of the reduced competition for the same available moisture. Here, the origin of air masses forming the ISDAC TIC-1/2A (1 April 2008) and TIC-2B (15 April 2008) is investigated using trajectory tools and satellite data. Results show that the synoptic conditions favor air masses transport from the three potentials SO2 emission areas to Alaska: eastern China and Siberia where anthropogenic and biomass burning emission respectively are produced and the volcanic region from the Kamchatka/Aleutians. Weather conditions allow the accumulation of pollutants from eastern China/Siberia over Alaska, most probably with the contribution of acid volcanic aerosol during the TIC-2B period. OMI observations reveal that SO2 concentrations in air masses forming the TIC-2B were larger than in air masses forming the TIC-1/2A. Airborne measurements show high acidity near the TIC-2B flight where humidity was low. These results strongly support the hypothesis that acidic coating on IN are at the origin of the formation of TIC-2B.
    Atmospheric Chemistry and Physics 01/2014; · 5.51 Impact Factor
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    ABSTRACT: [1] This paper presents the implementation of a new version of the DARDAR (radar lidar) classification derived from CloudSat and CALIPSO data. The resulting target classification called DARDAR v2 is compared to the first version called DARDAR v1. Overall DARDAR v1 reports more cloud or rain pixels than DARDAR v2. In the low troposphere this is because v1 detects too many liquid cloud pixels, and in the higher troposphere this is because v2 is more restrictive in lidar detection than v1. Nevertheless, the spatial distribution of different types of hydrometeors show similar patterns in both classifications. The French airborne Radar-Lidar (RALI) platform carries a CloudSat/CALIPSO instrument configuration (lidar at a wavelength of 532nm and a 95GHz cloud radar) as well as an EarthCare instrument configuration (high spectral resolution lidar at 355nm and a 95GHz Doppler cloud radar). It therefore represents an ideal go-between for A-Train and EarthCare. The DARDAR v2 classification algorithm is adapted to RALI data for A-Train overpasses during dedicated airborne field experiments using the lidar at 532nm and the radar Doppler measurements. The results from the RALI classification are compared with the DARDAR v2 classification to identify where the classification should still be interpreted with caution. Finally, the RALI classification algorithm with lidar at 532nm is adapted to RALI with high spectral resolution lidar data at 355nm in preparation for EarthCare.
    Journal of Geophysical Research: Atmospheres. 07/2013; 118(14).
  • Journal of Applied Meteorological and Climatology. 07/2013;
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    ABSTRACT: The Eyjafjallajökull eruption during May 2010 is used as a case study to evaluate the consistency of retrievals from different thermal infrared instruments for the detection and characterization of volcanic ash plumes. In this study, the split window technique is used to estimate the optical thickness, the effective particle size and the mass concentration of volcanic particles from brightness temperatures measured in the infrared atmospheric window (8-12 μm). Retrievals are obtained for several mineral compositions whose optical properties are computed using Mie theory accounting for spectral variations of the refractive index. The method is applied similarly to data from MODIS, SEVIRI and IASI space-borne instruments, using two channels at 11 μm and 12 μm. Despite different instrumental characteristics, the results are in good agreement, which denotes the robustness of the retrieval method and the consistency of the observations. Nevertheless, the refractive index data and altitude used for the plume in the inversion may lead to large uncertainties in retrieved effective size and mass concentration in dense plumes and makes it difficult to estimate its composition. While it brings additional constrains, the use of a third channel (8.7 μm) does not allow determining the nature of the particles. As confirmed with high spectral resolution radiative transfer simulations, hyperspectral sensors, such as IASI, are well-suited to study the particle composition of volcanic plumes.
    Atmospheric Measurement Techniques Discussions. 03/2013; 6(2):2793-2828.
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    ABSTRACT: The Saharan Air Layer (SAL) influences large scale environment from West Africa to eastern tropical America, by carrying large amounts of dust aerosols. However, the vertical distribution of the SAL is not well established due to a lack of systematic measurements away from the continents. This can be overcome by using the observations of the space lidar CALIOP on board CALIPSO. By taking advantage of CALIOP capability to distinguish dust aerosols from other types of aerosols through depolarization, the seasonal vertical distribution of the SAL is analysed at 1 degree horizontal resolution over a period of 5 yr (June 2006-May 2011). This study shows that SAL can be identified all year round displaying a clear seasonal cycle. It occurs higher in altitude and more northern in latitude during summer than during winter, but with similar latitude extent near Africa for the four seasons. The south border of the SAL is determined by the Intertropical Convergence Zone (ITCZ), which either prohibits dust layers to penetrate it or reduces significantly the number of dust layers seen in or south of it, as over the eastern tropical Atlantic. Spatially, near Africa, it is found between 5° S-15° N in winter going at 5-30° N in summer. Towards America (50° W), SAL is observed between 5° S-10° N in winter and 10-25° N in summer. During spring and fall, SAL is found between the position of winter and summer not only spatially, but also vertically. In winter, SAL occurs in the altitude range 0-3 km off West Africa, decreasing to 0-2 km close to South America. During summer, SAL is found to be thicker and higher near Africa at 1-5 km, reducing to 0-2 km in the Gulf of Mexico, farther west than during the other seasons. SAL is confined to one layer, of which the mean altitude is decreasing with westward transport by 13 m deg-1 during winter and 28 m deg-1, after 30&deg W, during summer. Its mean geometrical thickness is decreasing by 25 m deg-1 in winter and 9 m deg-1 in summer. Spring and fall present similar characteristics for both mean altitude and geometrical thickness. Wind plays a major role not only for the transport of dust within the SAL, but also by sculpting it. During winter, the trade winds transport SAL towards South America, while in spring and summer they scavenge dust aerosols below it by bringing maritime air masses from North Atlantic up to about 50° W. The North Atlantic westerlies, with their southern border occurring between 15° N and 30° N (depending on the season, the longitude and the altitude), prevent the SAL to develop further northward. In addition, their southward shift with altitude gives SAL its characteristic oval shape in the northern part. The effective dry deposition velocity of dust particles is estimated to be 0.07-0.08 cm s-1 in winter, 0.13-0.15 cm s-1 in spring and fall, and 0.2 cm s-1 in summer. Finally, the African Easterly Jet (AEJ) is observed to collocate with the maximum dust load of the SAL and this might promote the differential advection for SAL parts, especially during summer.
    Atmospheric Chemistry and Physics 02/2013; 13(2):4727-4784. · 4.88 Impact Factor
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    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
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    ABSTRACT: A statistical linear relationship between NO2 surface concentration and its integrated content in the atmospheric boundary layer (ABL) is established in urban conditions, using ABL depth as an ancillary parameter. This relationship relies on a unique data set including 20 months of observations from a ground-based UV-visible light spectrometer and from an aerosol lidar, both located in Paris inner city center. Measurements show that in all seasons, large vertical gradients of NO2 concentration exist in Paris developed ABL, explaining why the average concentration retrieved is only about 25% of NO2 surface concentration. This result shows that the commonly used hypothesis of constant mixing ratio in the ABL is not valid over urban areas, where large NOx emissions occur. Moreover, the relationship obtained is robust, and the studied area lacks of any particular orographic features, so that our results should be more widely applicable to pollution survey from space-borne observations.
    Geophysical Research Letters. 01/2013;
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    ABSTRACT: Extensive measurements from ground-based sites and satellite remote sensing (CloudSat and CALIPSO) reveal the existence of two types of ice clouds (TICs) in the Arctic during the polar night and early spring. The first type (TIC-2A), being topped by a cover of nonprecipitating very small (radar unseen) ice crystals (TIC-1), is found more frequently in pristine environment, whereas the second type (TIC-2B), detected by both sensors, is associated preferentially with a high concentration of aerosols. To further investigate the microphysical properties of TIC-1/2A and TIC-2B, airborne in situ and satellite measurements of specific cases observed during Indirect and Semi-Direct Aerosol Campaign (ISDAC) have been analyzed. For the first time, Arctic TIC-1/2A and TIC-2B microstructures are compared using in situ cloud observations. Results show that the differences between them are confined in the upper part of the clouds where ice nucleation occurs. TIC-2B clouds are characterized by fewer (by more than 1 order of magnitude) and larger (by a factor of 2 to 3) ice crystals and a larger ice supersaturation (of 15-20%) compared to TIC-1/2A. Ice crystal growth in TIC-2B clouds seems explosive, whereas it seems more gradual in TIC-1/2A. It is hypothesized that these differences are linked to the number concentration and the chemical composition of aerosols. The ice crystal growth rate in very cold conditions impinges on the precipitation efficiency, dehydration and radiation balance. These results represent an essential and important first step to relate previous modeling, remote sensing and laboratory studies with TICs cloud in situ observations.
    Journal of Geophysical Research 01/2013; 117. · 3.17 Impact Factor
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    ABSTRACT: The Development of Methodologies for Water Vapour Measurement (DEMEVAP) project aims at assessing and improving humidity sounding techniques and establishing a reference system based on the combination of Raman lidars, ground-based sensors and GPS. Such a system may be used for climate monitoring, radiosonde bias detection and correction, satellite measurement calibration/validation, and mm-level geodetic positioning with Global Navigation Satellite Systems. A field experiment was conducted in September-October 2011 at Observatoire de Haute Provence. Two Raman lidars, a stellar spectrometer (SOPHIE), a differential absorption spectrometer (SAOZ), a sun photometer (AERONET), 5 GPS receivers and 4 types of radiosondes (Vaisala RS92, MODEM M2K2-DC and M10, and Meteolabor Snow-White) participated in the campaign. A total of 26 balloons with multiple radiosondes were flown during 16 clear nights. This paper presents preliminary findings from the analysis of all these datasets. Several classical Raman lidar calibration methods are evaluated which use either Vaisala RS92 measurements, point capacitive humidity measurements, or GPS integrated water vapour (IWV) measurements. A novel method proposed by Bosser et al. (2010) is also tested. It consists in calibrating the lidar measurements during the GPS data processing. The methods achieve a repeatability of 4-5%. A drift in the IGN-LATMOS Raman lidar calibration of 15% over the 45 days of the experiment is evidenced but not yet explained. When this drift is removed, the precision of the calibration factors improves to 2-3%. However, the variations in the absolute calibration factor between methods and types of reference data remain at the level of 7%. The intercomparison of radiosonde measurements shows good agreement between RS92 and Snow-White measurements up to 12 km. An overall dry bias is found in the measurements from both MODEM radiosondes. Investigation of situations with low RH values (<10%) in the lower and middle troposphere reveals, on occasion, a lower RH detection limit in the Snow-White measurements compared to RS92 due to a saturation of the Peltier device. However, on other occasions, a dry bias is found in RS92, instead. Raman lidar water vapour measurements were useful to distinguish between which of the radiosondes was biased. On average, both RS92 and Snow-White measurements show a slight moist bias at night-time compared to GPS IWV, while the MODEM measurements show a large dry bias. The spectrometer IWV measurements contained a large bias that is currently under investigation. The sun photometer (daytime) and calibrated Raman lidar (night-time) IWV measurements showed excellent agreement with the GPS IWV measurements.
    Atmospheric Measurement Techniques Discussions. 01/2013; 6(2):3439-3509.
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    ABSTRACT: We analyzed different models to estimate absorption at W-band by gaseous species by taking advantage of the collocated CloudSat-Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) measurements. We used the power backscattered by the surface in the green visible wavelength of the lidar of CALIPSO as a reference to infer CloudSat's 94-GHz ocean surface backscatter in clear air and infer the attenuation introduced by gaseous absorption. Different millimeter-wave propagation models (MPMs) and different sources to determine the profile of atmospheric thermodynamic state are used to estimate CloudSat attenuation. These estimates are compared to the observations to calculate the residual dispersion. We show here that we need to adjust the empirical constants of preexisting water vapor absorption models to minimize the dispersion. Our results indicate an overestimation of absorption by the water vapor continuum at 94 GHz in Liebe-based MPM. We also propose a new empirical model to better represent the absorption of the water vapor continuum near 94 GHz. When this model is used in combination with the Advanced Microwave Scanning Radiometer for the Earth Observing System water vapor path and the Global Modeling and Assimilation Office water vapor vertical profile distribution, it leads to the lowest dispersion of the data on a statistical basis (global data over one month). The improved model is expected to optimize water vapor correction applied to CloudSat data and, potentially, also to improve interpretation of brightness temperature measurements in the W-band (e.g., 85- and 98-GHz radiometric channels).
    IEEE Transactions on Geoscience and Remote Sensing 01/2013; · 3.47 Impact Factor
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    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 Discussions (ACPD). 01/2013;
  • Journal of Atmospheric and Oceanic Technology 07/2012; 29:911-921. · 1.69 Impact Factor
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    ABSTRACT: FENNEC is an international program aiming at better understanding the Saharan climate system. In June 2011, about 18 flights of the French Falcon-20 equipped with a lidar and a dropsonde unit were operated over northern Mauritania and Mali. This allowed us to fully document the installation of the 6-km deep heat low over the western Sahara together with Atlantic inflows, Mediterranean surges and other mesoscale features. The aircraft was guided by a unique effort using numerical weather prediction (NWP) models fully coupled with the dust entrainment and deposition scheme. Four daily forecasts over domains covering most of the Sahara included two Meteo-France operational models, ALADIN (24 km, 72 h) and AROME (5 km, 48 h), and the research model Meso-NH operating at low (20 km, 48 h) and high (5 km, 24 h) resolutions. Verification of predicted temperature, water vapor and wind profiles against dropsonde data demonstrated the high quality of the forecasts. This was also shown for dust forecasts through direct comparison between the observed and forecasted vertical structure of the backscattered lidar signal. The benefits of high-resolution forecasts are highlighted here with observed and predicted mesoscale features related to local emission sources, low-level jets, convective downdrafts and oceanic inflows.
    04/2012;
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    ABSTRACT: Recent studies using satellite observations show that numerous dust sources are located in the foothills of the Saharan mountains. Generally, dust emission is closely related to sediment supply and surface wind. Thus, dust emission can be inhibited by either lack of high wind speeds or by unsuitable surface characteristics. Significant rainfall and flash flood events have been proposed to lead to changes in pluvial sediment supplies in mountain drainage systems. These sediments are suitable for dust uplift and assumed to have a main contribution to the dust emission fluxes over these areas. This mechanism could help to explain the observed marked interannual variability of some dust sources, which is currently not well understood. This study uses a novel combination of airborne and space-borne measurements to explore dust sources within complex terrain. It consists of two main parts: First, dust emission forced by the break-down of nocturnal low-level jets is investigated by analysing data from the RAIN4DUST/FENNEC-France aircraft campaign in June 2011 based at Fuerteventura, Spain. Local dust emission over North Mauritania is observed using a combination of different measurement systems flying aboard the French Falcon FA20, such as high resolution aerial ground camera, high-resolution lidar instrument and drop-sondes. The orientation of the flight legs allows for the characterisation of the evolution of a developing dust plume in time and space combining information on ground surface structure and vertical dust distribution. Supplementary analysis of model simulations and satellite remote sensing products provide additional information on the location of dust sources and dust transport paths. Second, the role of pluvial sediment supply for dust emission in desert valleys is investigated. For a selected area over West Africa ENVISAT SAR (synthetic aperture radar) measurements from 2003-2010 are analysed to identify changes in surface sediments through loss of coherence between two consecutive images. Results from this study highlight the contribution of flash floods for dust sources located within complex terrain. Together the two approaches provide a detailed picture of dust emission from sources within complex terrain revealing controls on dust emission from both atmospheric factors and sediment supply.
    04/2012;
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    ABSTRACT: The central Sahara has one of the most extreme climates on Earth. During the northern summer months, a large low pressure system caused by intense solar heating develops over a huge, largely uninhabited expanse of northern Mali, southern Algeria and eastern Mauritania. This Saharan heat low plays a pivotal role in the West African Monsoon. Based on this, the interested French, British and German communities have decided to propose the FENNEC project which aims at (i) characterizing the Saharan atmospheric boundary layer, (ii) evaluating its representation in regional and global models, and (iii) improving "aerosol" products issued from space-borne observations. A key element of this programme was the organization of an international field campaign in June 2011 over the Saharan heat low region, which will include both ground-based and airborne detachments. The Special Observing Period component of FENNEC-France included the implementation of the SAFIRE Falcon 20 to conduct research on the atmospheric boundary layer and the dust cycle of the Sahara, the installation of a remote sensing station in southern Spain, equipped with a backscatter lidar and a sunphotometer, to study the transport of desert dust to Europe, as well as a couple of GPS stations installed in southern Morocco to investigate the moisture inflow from the Atlantic Ocean into the Sahara. For the first time, the ALADIN and AROME models (5 and 24 km grid spacing, respectively) have been implemented operationally to provide forecasts of dust events over the Sahara and parts of the Sahel in June 2011 to assist in planning for airborne operations. This effort was complemented by the forecasts made with the Meso-NH model (5 and 20 km resolution). During the SOP period, the ground-based, airborne and space-borne observations have documented the evolution of dynamic properties of thermodynamic and the atmospheric boundary layer Saharan Africa (Mauritania and Mali) during the installation phase of the Saharan heat low west of the continent as well as the increase in aerosol loading associated with the phase shift of the heat low from east to west. During this period, episodes of intense uplift of desert aerosols associated with various dynamic phenomena (fronts, "Mediterannean surges", "Atlantic inflow" of low-level jets, etc ...) have also been documented as well as the export of dust over the Atlantic Ocean. An overview of implementation plan and of the first observational and modelling results acquired during the time of the SOP will be presented.
    04/2012;
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    ABSTRACT: A central question to understanding the climate of North Africa is to better comprehend and explain the existence of the Central Saharan dust loading maxima in summer. This dust 'hotspot' has been apparent in many satellite products. However, The actual physical processes involved are not clear. Indeed, qualitative maps of dust emissions suggest that emission sources are not co-located with the aerosol optical depth maximum. Therefore, it is not known to what extent the hotspot represents local emission and/or transport from remote emission sources. In this work, we aim to address this question by analysis of the long-term six year Level 1 and 2 Caliop data archive. The vertical profiles of aerosol from the caliop lidar potentially enables identification and quantification of active dust emission events. We then classify emission events according to the driving meteorological processes (e.g. harmatan flow, monsoon flow, mixing of low level jets, cold pools, etc.) from coincident analysis/reanalysis fields. This identification from caliop and classification of processes is based on a set of 'rules' informed by analysis of dust events during the Fennec project observational campaign.
    04/2012;
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    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
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    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
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    ABSTRACT: This study reports on the optical and microphysical characterization of particles contained in volcanic ash clouds using thermal infrared radiometry. Several works have shown the potential of the split window technique for estimating the effective particle size and optical thickness of semi-transparent clouds from two channels in the infrared atmospheric window (8 - 12 μm). In the present study, this approach is applied to the characterization of volcanic particles. The inversion algorithm is based on LUTs built with an accurate radiative transfer code, including gaseous absorption as well as multiple scattering and absorption by particles. Realistic spectral variations for optical properties of several types of volcanic particle (Ash, H2SO4 ...) and water or ice clouds have been calculated from refractive indices and Mie theory. In addition, this inversion procedure needs to define the altitude, temperature and thickness of the ash cloud. Consequently, meteorological data relative to the atmosphere and the state of ash cloud have been calculated using the Regional Atmospheric Modeling System (RAMS). This model allows giving a 3D dynamical transport of the ash plume and its vertical distribution. Ash sources and scenes area have been studied in detail with higher resolution using nested grid system of the model. In a first step, a sensitivity study based on radiative transfer calculations is presented in order to illustrate the potential of this approach. This technique is then applied to volcanic plumes in April-May 2010. Brightness temperatures from the MODIS spectroradiometer, the Infrared Imaging Radiometer (IIR) and the SEVIRI radiometer are used for some scenes acquired simultaneously over North / West of Europe with similar spectral or spatial resolutions. Spatial distribution of the retrieved optical thickness and effective size of particles are presented and analyzed. Comparisons between retrievals from IIR, MODIS and SEVIRI are presented. Contributions of the spectral channels to retrievals, as well as the influence of the particle type are also discussed.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Because of their very important spatial and temporal coverage, cirrus clouds are nowadays recognized as having a major impact on the earth-atmosphere radiative budget. However, this impact is still badly evaluated due to our lack of knowledge on the wide range of their microphysical, optical and geometrical properties. Consequently, their study has become of primal importance. Many airborne campaigns have been performed during the last decades, but the high altitudes of these clouds make these precise observations too seldom for a good global comprehension of their properties. The use of spatial instruments such as the infrared radiometer IIR/CALIPSO thus becomes a great help in order to improve our knowledge on these clouds. In May 2007 and October 2008, the two airborne campaigns "CIRCLE-2" and "Biscay 08" were conducted in order to study cirrus clouds, but also to validate the IIR instrument using precise airborne radiometric and in situ measurements. During both campaigns, two aircrafts were equipped with the infrared radiometer CLIMAT-AV, the lidar LEANDRE, and different in situ instruments, and flew right under the track of CALIPSO over the Atlantic Ocean. CLIMAT-AV and LEANDRE were chosen to have characteristics very similar to IIR and CALIOP in order to make measurements comparable. In a first study, the aircraft and satellite measurements were directly compared and showed very good similarities, with a possible bias explained by small differences in scene observations and of instrumental characteristics. This study has now been pushed further by using an optimal estimation method in order to retrieve cirrus cloud microphysical properties such as the optical thickness of the cloud and the effective size of its particles. The retrievals were conducted separately on both instruments and then compared to find very strong correlations. A comparison with operational IIR retrievals, as well as with the in situ observations also shows good results. Finally a quick information constant study has been effectuated in order to quantify the amount of information on the retrieved parameters contained in IIR channels, depending on the cloud properties and the atmospheric conditions. This final study allows us to determine the strengths and weaknesses of IIR measurements, in the view of optimal synergies with other A-Train instruments.
    AGU Fall Meeting Abstracts. 12/2011;

Publication Stats

1k Citations
243.09 Total Impact Points

Institutions

  • 2014
    • UPMC
      Pittsburgh, Pennsylvania, United States
  • 2013
    • LATMOS
      Guyancourt, Île-de-France, France
  • 1989–2013
    • Pierre and Marie Curie University - Paris 6
      Lutetia Parisorum, Île-de-France, France
  • 1980–2013
    • French National Centre for Scientific Research
      • Laboratoire d'aérologie (LA)
      Lutetia Parisorum, Île-de-France, France
  • 2005–2008
    • Institut Pierre Simon Laplace
      Lutetia Parisorum, Île-de-France, France
  • 2006
    • University of Colorado
      Denver, Colorado, United States
  • 2003
    • Université du Littoral Côte d'Opale (ULCO)
      Dunkirk, Nord-Pas-de-Calais, France
    • Hampton VA Medical Center
      Hampton, Virginia, United States
  • 1985
    • Le Laboratoire de Météorologie Dynamique (LMD)
      Lutetia Parisorum, Île-de-France, France