[Show abstract][Hide abstract] ABSTRACT: The Arctic radiation balance is strongly affected by clouds and surface albedo. Prior work has identified Arctic cloud liquid water path (LWP) and surface radiative flux biases in the Community Atmosphere Model, version 5 (CAM5), and reductions to these biases with improved mixed-phase ice nucleation schemes. Here, CAM5 net top-of-atmosphere (TOA) Arctic radiative flux biases are quantified along with the contributions of clouds, surface albedos, and new mixed-phase ice nucleation schemes to these biases. CAM5 net TOA all-sky shortwave (SW) and outgoing longwave radiation (OLR) fluxes are generally within 10 W m−2 of Clouds and the Earth’s Radiant Energy System Energy Balanced and Filled (CERES-EBAF) observations. However, CAM5 has compensating SW errors: Surface albedos over snow are too high while cloud amount and LWP are too low. Use of a new CAM5 Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar simulator that corrects an error in the treatment of snow crystal size confirms insufficient cloud amount in CAM5 year-round. CAM5 OLR is too low because of low surface temperature in winter, excessive atmospheric water vapor in summer, and excessive cloud heights year-round. Simulations with two new mixed-phase ice nucleation schemes—one based on an empirical fit to ice nuclei observations and one based on classical nucleation theory with prognostic ice nuclei—improve surface climate in winter by increasing cloud amount and LWP. However, net TOA and surface radiation biases remain because of increases in midlevel clouds and a persistent deficit in cloud LWP. These findings highlight challenges with evaluating and modeling Arctic cloud, radiation, and climate processes.
Journal of Climate 07/2014; 27(13):5174–5197. · 4.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  Level 1 measurements, including cross-polarized backscatter, from the Cloud-Aerosol Lidar with Orthogonal Polarization lidar, have been used to document the vertical structure of the cloud thermodynamic phase at global scale. We built a cloud phase identification (liquid, ice, or undefined) in the Global Climate Model (GCM)–oriented Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) Cloud Product (GOCCP) and analyzed the spatial distribution of liquid and ice clouds in five January, February, March (JFM) seasons of global-scale observations (2007–2011). We developed a cloud phase diagnosis in the Cloud Feedback Model Intercomparison Program Observation Simulator Package to evaluate the cloud phase description in the LMDZ5B climate model. The diagnosis in the simulator is fully consistent with the CALIPSO-GOCCP observations to ensure that differences between the observations and the “model + simulator” ensemble outputs can be attributed to model biases. We compared the liquid and ice cloud vertical distributions simulated by the model with and without the simulator to quantify the impact of the simulator. The model does not produce liquid clouds above 3 km and produces ice instead of liquid at low and middle altitudes in polar regions, as well as along the Intertropical Convergence Zone. The model is unable to replicate the observed coexistence of liquid and ice cloud between 0°C and −40°C. Liquid clouds dominate T > −21°C in the observations, T > −12°C in the model + simulator, and T > −7.5°C in the model parameterization. Even if the simulator shifts the model cloud phase parameterization to colder temperature because of the lidar instrument peculiarities, the cloud phase transition remains too warm compared to the observations.
Journal of Geophysical Research: Atmospheres. 07/2013; 118(14).
[Show abstract][Hide abstract] ABSTRACT: Evaluation of clouds in climate models is essential judging from the
strong impact of clouds on the earth's radiation budget. Cloud phase
influences many cloud properties and leads to different cloud
interactions with the climate system. Since june 2006, CALIPSO satellite
has provided new measurements of backscattered lidar profiles, which
have been used to evaluate the cloud description in CMIP5 climate models
through a lidar simulator (CFMIP Observation Simulator Package, COSP).
In this study, we present results of this evaluation focused on vertical
structure of clouds, global coverage and coverage above "hard to observe
regions" such as the desert and polar regions, and partition between
liquid and ice in clouds. The total cloud cover is underestimated in
all models and continental cloud covers (at low, mid, high altitudes)
are highly variable depending on the model. In the tropics, all models
underestimate the low cloud amount and none of them correctly simulate
the top of deep convective compare to observations. In the Arctic, the
modelled low cloud amounts are slightly biased compared to observations
and seasonal variation not reproduced. Thanks to a new cloud phase
diagnosis in the GCM-Oriented CALIPSO Cloud Product (GOCCP) and its
counterpart within the lidar simulator, we evaluate the cloud phase in
LMDZ5B model. Comparisons show that, contrary to ice clouds, liquid
clouds are largely underestimated in the model. Liquid clouds occur at
temperatures as cold as -40°C in the observations, but only as cold
as -20°C in the model. They are dominant at temperatures warmer than
-21°C in observations but only warmer than -12°C in the model.
In agreement with theory, statistical observations show that liquid and
ice co-exist between 0°C and -40°C, and that the cloud phase
depends not only on the temperature but also on humidity. For
comparison, ice and liquid do not co-exist in the model, and the
modelled cloud phase does not reproduce the observed sensitivity to the
atmospheric humidity. A specific focus on Arctic region shows that a
lack of liquid-containing Arctic clouds contributes to a lack of
"radiatively opaque" states in LMDZ5B model. The surface radiation
biases found in this one model are found in multiple models,
highlighting the need for improved modelling of Arctic cloud phase.
Further analysis will use learning from this study to investigate how
changes in the LMDZ5B cloud phase parameterization (and possibly other
models) impact cloud fractions, temperatures and fluxes for inter-annual
and long term period simulations.
[Show abstract][Hide abstract] ABSTRACT: A Saharan dust event affected the Rhine valley in southwestern Germany and eastern France on 1 August 2007 during the Convective and Orographically induced Precipitation Study (COPS) experiment. Prior to an episode of intense convection, a layer of dry, clean air capped by a moist, dusty layer was observed using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and airborne and ground-based lidar observations from North Africa to western Europe. The origin of the different layers was investigated using the regional model Meso-NH. For the purpose of modelling evaluation, a lidar simulator was developed for direct comparison of observed and simulated vertical structures of the lidar backscattered signal. Overall, the model reproduced the vertical structure of dust probed several times by the different lidar systems during its long-range transport. From Lagrangian back trajectories it was found that the dust was mobilized from sources in Mauritania six days earlier, while the dry layer subsided over the north Atlantic. Off the Moroccan coasts, the dry layer folded down beneath the dusty air mass and the two-layer structure was advected to the Rhine valley in about two days. By heating the atmosphere, the dust layer changed the static stability of the atmosphere and thus the occurrence of convection. A study of sensitivity to the radiative effect of dust showed a better prediction of precipitation when a dust prognostic scheme was used rather than climatology or when dust effects were ignored. This result suggests that dust episodes that occur prior to convective events might be important for quantitative precipitation forecasts
Quarterly Journal of the Royal Meteorological Society 01/2011; · 3.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A Saharan dust event affected the Rhine valley in southwestern Germany and eastern France on 1 August 2007 during the Convective and Orographically-induced Precipitation Study (COPS) experiment. Prior to an episode of intense convection, a layer of dry, clean air capped by a moist, dusty layer was observed using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite
Observation (CALIPSO), airborne and ground-based lidar observations from North Africa to western Europe. The origin of the different layers was investigated using the regional model Meso-NH. For the purpose of modeling evaluation, a lidar simulator was
developed for directly comparing observed and simulated vertical structures of the lidar backscattered signal. Overall, the model reproduced the vertical structure of dust probed at several times by the different lidar systems during its long-range transport. From Lagrangian back trajectories it was found that the dust was mobilized from sources inMauritania six days earlier, while the dry layer subsided over the north Atlantic. Off the Moroccan coasts, the dry layer folded down beneath the dusty airmass and the two-layer
structure was advected to the Rhine valley in about two days. By heating the atmosphere, the dust layer changed the static stability of the atmosphere and thus the occurrence of convection. A sensitivity study to the radiative effect of dust indeed shows a better prediction of precipitation when a dust prognostic scheme was used rather than climatology or when dust effects were ignored. This result suggests that dust episodes that occur prior to convective events might be important for quantitative precipitation forecasts.
Quarterly Journal of the Royal Meteorological Society. 01/2011; 137(2011):236-251.
[Show abstract][Hide abstract] ABSTRACT: Observations of trace gases and aerosols from satellite remote sensing provide essential information on pollution emissions and transport. IASI/METOP allows the global monitoring of several key species for atmospheric chemistry analysis, with unprecedented spatial sampling and coverage. Its ability to detect a large series of species within fire plumes has recently been demonstrated, and could significantly improve current evaluation of the impact of such extreme pollution events on air quality. Aerosol observations from several remote sensors on board satellites of the A-Train (MODIS, POLDER/PARASOL, CALIOP/CALIPSO) are also now commonly used for the analysis of the long-range transport of pollution. In this presentation, we will discuss the information provided by the carbon monoxide (CO) retrievals, one of the main species measured by IASI, on fire emissions transport mechanisms and pathways. Therefore, IASI retrievals will be compared to simulations from the CHIMERE regional chemistry and transport model for the case study of the large fires which burned in Greece during August 2007. We will then present an analysis of the constraint provided by IASI on chemistry within the transported plumes using the retrievals for the shorter lived species and for ozone. Finally, the IASI observations will be coupled to the aerosol observations from the PARASOL mission in order to assess the impact of this specific fire event on air quality in the Euro-Mediterranean region (both PM2.5 and ozone).
[Show abstract][Hide abstract] ABSTRACT: We analyze optical signatures in 18 months of CALIOP layer-integrated backscatter and depolarization ratio to investigate the geographical and seasonal distribution of oriented crystals in ice clouds on a global scale. Oriented crystals are found to be rare: they appear in ∼6% of all ice cloud layers, and inside these layers the proportion of oriented crystals is estimated below 5%, even though they have a significant effect on the cloud optical properties. The geographical pattern of crystal orientation is very stable over the year, without any noticeable cycle. We investigate the atmospheric conditions which might lead to crystal orientation, including synoptic-scale dynamics and thermodynamic profiles. In the tropics, detections of crystal orientation are more numerous in areas dominated by convection on a monthly basis, and at midlatitudes less numerous in areas dominated by strong horizontal winds. Synoptic effects, however, appear secondary; orientation is primarily driven by temperature. Oriented crystals are mostly nonexistent in ice clouds colder than −30°C, and very frequent in warmer ice clouds, appearing in 30% of such clouds in the tropics and up to 50% at higher latitudes. The temperatures where oriented crystals are found (−30°C to −10°C) are conducive to the formation of planar crystals. Results suggest oriented crystals are more frequent just above cloud base in slightly thicker cloud layers, which might provide clues to how and why orientation takes place.
Journal of Geophysical Research Atmospheres 01/2010; 115. · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In their comment, Poole et al. (2009) aim to show it is highly improbable that the observations described in Chepfer and Noel (2009), and described as "NAT-like" therein, are produced by Nitric Acid Trihydrate (NAT) particles. In this reply, we attempt to show why there is, in our opinion, too little evidence to reject this interpretation right away.
Geophysical Research Letters 12/2009; · 3.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The final budget of dust remaining in the atmosphere or deposited on the surface depends directly on the emissions, boundary layer turbulence, stability in the troposphere, and clouds properties. The modeling of these processes remains uncertain and mineral dust long-range transport constitutes a major unknown. To improve this transport, it is crucial to improve modeling of altitudes and thicknesses of mineral dust layers. The spaceborne lidar Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) collects new information about the aerosol vertical distribution. Here we diagnose the lidar profile from the outputs of the transport model CHIMERE-DUST and we compare those with their observed counterparts. During the periods June to September 2006 and January to March 2007, the occurrences and structures of dust layers are estimated from the observed and modeled lidar signals. Accounting for the daytime and nighttime periods, the seasonal variability, and CALIPSO flight tracks, it is shown that the presence/absence of dust is correctly reproduced by the model in 70% of the 170,000 vertical profiles studied. The mineral dust horizontal distribution is quite correctly reproduced by the model, while the vertical one shows a vertical overspread which is more pronounced during winter (+100% compared to observations) than summer (+50%). The maximum value of the modeled lidar signal is underestimated with respect to the measured one by typically 30%. Multilayered dust situations are more frequent in the observations (30% of the total data set) than in the model (10%). Despite these errors, the model is able to catch the seasonal variations of the dust layers: the increases of the dust load and of the dust altitudes during summer and the northward shift of the maximum dust occurrence.
Journal of Geophysical Research Atmospheres 01/2009; 114. · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper presents statistics of polar stratospheric clouds (PSCs) above Antarctica from June to October 2006 using observations from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) spaceborne lidar, part of the CALIPSO mission. Synoptic-scale changes in geographic and temporal distribution are documented weekly and correlated with temperature fields. A high spatial and temporal variability tends to contradict the hypothesis that PSCs are mostly created via slow processes mainly governed by large-scale temperature changes. Linear depolarization ratios reveal strongly typed PSCs with distinct characteristics (implying different microphysics), but unique cloud compositions cannot be singled out. A west/east imbalance is observed in the depolarization distribution, symptomatic of microphysical disparities. A classification based on depolarization and scattering ratios suggests more than 60% of mixed PSCs, followed by more than 20% of STS, and a roughly equal concentration of nitric
acid trihydrate (NAT)-based and pure ice PSCs (8%). Up to the beginning of August, supercooled ternary solution (STS) PSCs experience a steady decrease in concentration correlated with an increase in ice-based and mixed PSCs; this tendency gets reversed after the first week of August, hinting at the existence of a large-scale seasonal cycle in PSC population.
Journal of Geophysical Research Atmospheres 01/2008; 113:D02205. · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: New space-borne active sensors make it possible to observe the three-dimensional structure of clouds. Here we use CALIPSO lidar observations together with a lidar simulator to evaluate the cloudiness simulated by a climate model: modeled atmospheric profiles are converted to an ensemble of subgrid-scale attenuated backscatter lidar signals from which a cloud fraction is derived. Except in regions of persistent thick upper-level clouds, the cloud fraction diagnosed through this procedure is close to that actually predicted by the model. A fractional cloudiness is diagnosed consistently from CALIPSO data at a spatio-temporal resolution comparable to that of the model. The comparison of the model's cloudiness with CALIPSO data reveals discrepancies more pronounced than in previous model evaluations based on passive observations. This suggests that space-borne lidar observations constitute a powerful tool for the evaluation of clouds in large-scale models, including marine boundary-layer clouds.
[Show abstract][Hide abstract] ABSTRACT: A dust event was observed in Europe on 23-25 March 2007. Surface observations in Central Europe showed huge concentrations of particulate matter (PM). At the same time, dust models diagnosed a Saharan dust outbreak flowing from Sahara to Europe. However, lidar measurements and surface stations in Eastern Europe diagnosed a dust event originating from Ukraine related to chernozemic-erodible lands. Using surface and satellite measurements with modeling results, it is demonstrated that the finally huge surface concentrations recorded in the Netherlands, Belgium, and the north of France were mostly due to the extremely rare Ukraine dust event, whereas Saharan dust events usually produce only midtroposphere plumes. To investigate this episode, the chemistry-transport model CHIMERE is modified to account for the erodibility of chernozemic soil inside Europe. A size distribution for chernozemic dust emission is proposed. Over Western Europe, the model reproduces the observed PM concentration peaks up to 200 mug m-3, with a large contribution of Ukraine dust, up to 170 mug m-3. This first model study of dust emissions due to European arable land shows that it is possible to fairly retrieve the magnitude of surface concentrations far away from the emission sources.
Journal of Geophysical Research Atmospheres 01/2008; 113. · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper presents a study of ice crystal shapes in midlatitude ice clouds inferred from a technique based on the comparison of ray-tracing simulations with lidar depolarization ratio measured at 532 nm. This technique is applied to three years of lidar depolarization ratio observations from the Site Instrumental de Recherche par Télédétection Atmosphérique (SIRTA) observatory in Palaiseau, France, amounting to 322 different days of ice cloud observations. Particles in clouds are classified in three major groups: plates, columns, and irregular shapes with aspect ratios close to unity. Retrieved shapes are correlated with radiosounding observations from a close-by meteorological station: temperature, relative humidity, wind speed, and direction. Results show a strong dependence of the relative concentration of different crystal shapes to most atmospheric variables, such as the temperature, with a clear successive dominance by platelike (temperature above 20°C), irregular (temperatures between 60° and 40°C), and columnlike shapes (temperatures below 60°C). Particle shapes are almost exclusively columnlike below 75°C. This is in sharp contrast with previous results of the same classification applied to tropical clouds, and highlights the high geographic variability of the ice clouds distribution of microphysical properties. Results also suggest that ice clouds created by jet streams (identified by high wind speeds) are strongly dominated by columnlike shapes, while front-created ice clouds (identified by lower wind speeds) show a much more variable mix of shapes, with the dominant shapes depending on other factors. Results also suggest different microphysical properties according to the average direction source of air masses and winds. Following these results, a possible parameterization of ice crystal shapes in midlatitude ice clouds as a function of temperature is provided.
Journal of the Atmospheric Sciences 01/2006; 63(11):2978-2991. · 2.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The shapes of ice crystals have been derived from measurements in cirrus during the CRYSTAL/FACE experiment in July, 2002. The measurements were made in the size range from 5 – 45 mm with an optical particle spectrometer that measures forward (4 – 12°) and backscattered (168 – 176°) light from individual particles. The phase functions for ensembles of ideal crystal shapes were integrated over these collection angles to compare with the measured ratios of forward to backward scattered light from nine research flights. Approximately 90% of the crystals agreed with the theoretical model for a mixture of bullet rosettes, plates and hollow columns. Approximately 10% of the crystals were characterized as ice spheres. There was no significant dependency of shape on wind direction, temperature or relative humidity with respect to ice. Citation: Baumgardner, D., H. Chepfer, G. B. Raga, and G. L. Kok (2005), The shapes of very small cirrus particles derived from in situ measurements, Geophys. Res. Lett., 32, L01806, doi:10.1029/2004GL021300. 1. Background  Surface and projected area are two important charac-teristics of an ice crystal and both are dependent on crystal shape. The albedo of cirrus clouds is sensitive to the projected area of crystals, particularly those whose sizes are smaller than 50 mm [Arnott et al., 1994; Jensen et al., 1994; Heymsfield and McFarquhar, 1996; Yang et al., 2001] and that are frequently found in high concentrations [Heymsfield and Platt, 1984; Knollenberg et al., 1993; Heymsfield and McFarquhar, 1996; Boudala et al., 2002; Gayet et al., 2002; Garrett et al., 2003]. Very little shape analysis of sub-50 mm cirrus crystals has been done; however, since the majority of measurements are made with optical array probes, such as the PMS-2D spectrometer, that have a minimum resolution of 25 mm. Higher resolution instruments like formvar replicators, the Video Ice Particle Sampler [Miloshevich and Heymsfield, 1997] and the Cloud Particle Imager [Lawson, 2001] capture the crystal shape but analysis is labor intensive and only the projected area can be measured directly. An instrument capable of mea-suring the phase function directly of small, individual particles is the Polar Nephelometer [Gayet et al., 1997]; however, there have been no publications that analyze crystal shapes in the sub-50 mm size range. The images from a CPI of sub-50 mm ice crystals in Arctic stratiform and cirrus clouds have been analyzed for their habits [Korolev et al., 1999] and asphericity [Korolev and Isaac, 2003]. These investigators found that fewer than 50% of the ice crystals could be classified as spherical. These measurements were made in clouds warmer than 225 k, and tops lower than 10 Km.  Here we present a new bi-directional light scattering technique for the shape analysis of individual crystals smaller than 50 mm measured in tropical and sub-tropical clouds during the 2002 CRYSTAL-FACE field campaign.
Geophysical Research Letters 01/2005; 32:L01806. · 3.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The goal of this paper is to retrieve information about ice particle orientation in cirrus clouds. This is achieved by comparing simulations of sunlight reflection on a cirrus cloud with measurements of polarized radiances from the spaceborne instrument Polarization and Directionality of the Earth's Reflectance (POLDER-1) on Advanced Earth Observing Satellite-1 (ADEOS-1). Results show that horizontal orientation of crystals can be spotted by the presence of a local maximum of polarized radiance in the direction of specular reflection. The angular width of the local maximum is shown to contain information on the particle maximum deviation angle, while the maximum intensity can provide information on particle shape and relative concentrations of ice crystals, horizontally and randomly oriented. The study of 31 ice cloud cases show that in 80% of them, the deviation angle is less than 3°. Also, the relative concentration of horizontally oriented crystals is less than 21%, depending on the angular distribution used for crystal deviation.
Journal of the Atmospheric Sciences 08/2004; 61(16):2073-2081. · 2.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A shape classification technique for cirrus clouds that could be applied to future spaceborne lidars is presented. A ray-tracing code has been developed to simulate backscattered and depolarized lidar signals from cirrus clouds made of hexagonal-based crystals with various compositions and optical depth, taking into account multiple scattering. This code was used first to study the sensitivity of the linear depolarization rate to cloud optical and microphysical properties, then to classify particle shapes in cirrus clouds based on depolarization ratio measurements. As an example this technique has been applied to lidar measurements from 15 mid-latitude cirrus cloud cases taken in Palaiseau, France. Results show a majority of near-unity shape ratios as well as a strong correlation between shape ratios and temperature: The lowest temperatures lead to high shape ratios. The application of this technique to space-borne measurements would allow a large-scale classification of shape ratios in cirrus clouds, leading to better knowledge of the vertical variability of shapes, their dependence on temperature, and the formation processes of clouds.
[Show abstract][Hide abstract] ABSTRACT: This study develops and examines a multiangle, multisatellite method for determining effective cloud particle shapes from reflectances observed at visible wavelengths. The technique exploits the significant differences in the various cloud particle shape phase functions near the backscatter direction to infer particle shape from a combination of views from a near-backscatter angle and a side scattering angle. Adding-doubling calculations confirm that the optimal viewing combinations include one near-backscatter angle and another between 60" and 150". Sensitivity to shape increases with solar zenith angle. A total of 28 collocated, visible images from pairs of currently operating meteorological satellites with the desired viewing combinations were analyzed for particle shape. Matching reflectances from images with optimal viewing angles clearly separates water droplet from ice crystal clouds. Reflectance pairs from matched pixels containing ice crystals can be explained by the range of selected microphysical models. The most common retrieved shapes correspond to combinations of hexagonal compacts (aspect ratio of unity), hexagonal columns, and bullet rosettes. Although no single microphysical model can account for the observed variability, taken together, the models used for retrieving cloud particle size by the Clouds and the Earth's Radiant Energy System and the Moderate Resolution Imaging Spectroradiometer Projects can account for most of the reflectance variability observed in this limited data set. Additional studies are needed to assess the uncertainties in retrieved shapes due to temporal and spatial mismatches, anisotropic and bright background reflectances, and calibration errors and to validate the retrieved shapes. While applicable to a limited number of dual-satellite viewing combinations for current research and operational meteorological satellites, this approach could be used most extensively to derive effective particle size, shape, and optical depth from a combination of an imaging satellite in an L1 orbit, like Triana, and any other lower Earth orbiting Satellites.