Publication acceptée, avril 2013

Level1 measurements, including cross-polarized backscatter, from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) 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, undefined) in the GCM-Oriented Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) Cloud Product (GOCCP) and analysed the spatial distribution of liquid and ice clouds in 5 JFM seasons of global scale observations (2007-2011). We developed a cloud phase diagnosis in the Cloud Feedback Model Intercomparison Program Observation Simulator Package (COSP) 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 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 mid-altitudes in polar regions, as well as along the Inter-Tropical Convergence Zone. The model is unable to replicate the observed co-existence of liquid and ice cloud between 0 and -40 °C. Liquid clouds dominate for 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 observations.

 

Publications acceptées, mars 2013

Quelques publications reliées aux thématiques d’EECLAT ayant été acceptées récemment :

Tropical islands, such as Reunion Island (21°S, 55.5°E) in the southwestern Indian Ocean, have significant solar resource that is highly variable in both spatial and temporal scales because of heterogeneous and rapidly changing cloudiness. The characterization of this variability is essential to enhance penetration of solar energy systems, such as photovoltaic or thermal farms. This work focuses on the large-scale, meso-scale and local-scale variability in cloudiness and surface solar irradiance at different temporal scales. Vertical velocity at 500 hPa from ERA-Interim reanalyses are used to study large-scale subsidence. CALIPSO, MODIS and Meteosat-7 satellite observations are used to study cloud properties and associated irradiances at the meso-scale. Solar irradiance measurements at seven Meteo-France stations around Reunion Island are used to investigate three meteorologically-distinct regions, namely the windward and leeward coasts, and the coasts parallel to the general trade wind direction. Day-to-day variations in daily irradiation values and diurnal-scale variability of solar irradiances are seasonally dependent. Winter seasons are characterized by large-scale atmospheric subsidence and broken low-level cloudiness, while in summer, clouds are found both at low and high altitudes. Three parameters are introduced to characterize solar irradiance diurnal cycle regimes. Five physically-sensible regimes are found, identified as clear, morning clear, overcast, afternoon clear, and random cloudiness. Regime occurrences have marked seasonal dependencies and vary significantly between the windward, leeward and lateral coasts. In winter and early summer, when cloudiness is driven predominantly by local (thermal, orographic) processes, the aggregated solar daily irradiation (average of the seven ground stations) remains near 80% of the clear-sky irradiation. In late summer, this values drops below 65% as large-scale overcast systems regularly affect the entire island. The station-to-station dis-correlation distance in terms of daily clear-sky index, defined as the distance for which the correlation coefficient drops to 0.5, is 62 km in summer and 29 km in winter. For hourly clear-sky indexes, the station-to-station dis-correlation distances is 4 and 3 km, for summer and winter, respectively. Accordingly, aggregate analyses show that compensation effects are more important at hourly than daily time scales. The measurements also reveal that the atmosphere over the island tends to be clearer in the morning than in the afternoon while over the ocean surrounding the island, the opposite is true. These results provide insights and tools to help develop improved diurnal-scale solar irradiance forecast systems for tropical islands

Four years (2007-2010) of co-located 94 GHz CloudSat radar reflectivities and 532 nm CALIPSO Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) backscattering coefficients are used to globally characterize snow precipitating clouds. CALIOP is particularly useful for the detection of mixed and supercooled liquid water (SLW) layers. Liquid layers are ubiquitous in snow precipitating clouds: overall/over sea/over land 49%/57%/33% of the snowy profiles present SLW or mixed-phase layers. The spatial and seasonal dependencies of our results -with snowing clouds more likely to be associated with mixed phase during summer periods- are related to snow layer top temperatures. SLW occurs within the majority (> 80%) of snow-precipitating clouds with cloud tops warmer than 250 K, and is present 50% of the time when the snow-layer top temperature is about 240 K. There is a marked tendency for such layers to occur close to the top of the snow-precipitating layer (75% of the times within 500 m). Both instruments can be synergetically used for profiling ice-phase-only snow, especially for light snow (Z < 0 dBZ, S < 0.16 mm/h) when CALIOP is capable of penetrating, on average, more than half of the snow layer depth. These results have profound impact for deepening our understanding of ice nucleation and snow growth processes, for improving active and passive snow remote sensing techniques and for planning snow precipitation missions.

A ground-based Rayleigh lidar has provided continuous observations of tropospheric water vapour profiles and cirrus cloud using a preliminary Raman channels setup on an existing Rayleigh lidar above La Reunion over the period 2002–2005. With this instrument, we performed a first measurement campaign of 350 independent water vapour profiles. A statistical study of the distribution of water vapour profiles is presented and some investigations concerning the calibration are discussed. Analysis regarding the cirrus clouds is presented and a classification has been performed showing 3 distinct classes. Based on these results, the characteristics and the design of a future lidar system, to be implemented at the new Reunion Island altitude observatory (2200 m) for long-term monitoring, is presented and numerical simulations of system performance have been realised to compare both instruments.

Spaceborne lidar observations have recently revealed a previously undetected significant population of Subvisible Cirrus (SVC). We show them to be colder than −74 °, with an optical depth below 0.0015 on average. The formation and persistence over time of this new cloud population could be related to several atmospheric phenomena. In this paper, we investigate if these clouds follow the same formation mechanisms as the general tropical cirrus population (including convection and in-situ ice nucleation), or if specific nucleation sites and trace species play a role in their formation. The importance of three scenarios in the formation of the global SVC population is investigated through different approaches that include comparisons with data imaging from several spaceborne instruments and back-trajectories that document the history and behavior of air masses leading to the point in time and space where subvisible cirrus were detected. In order to simplify the study of their formation, we singled out SVC with coherent temperature histories (mean variance lower than 4 K) according to back-trajectories along 5, 10 or 15 days (respectively 58, 25 and 11% of SVC). Our results suggest that external processes, including local increases in liquid and hygroscopic aerosol concentration (either through biomass burning or volcanic injection forming sulfate-based aerosols in the troposphere or the stratosphere) have very limited short-term or mid-term impact on the SVC population. On the other hand, we find that ~20% of air masses leading to SVC formation interacted with convective activity 5 days before they led to cloud formation and detection, a number that climbs to 60% over 15 days. SVC formation appears especially linked to convection over Africa and Central America, more so during JJA than DJF. These results support the view that the SVC population observed by CALIOP is an extension of the general upper tropospheric ice clouds population with its extreme thinness as its only differentiating factor.

Clouds cover about 70% of the Earth’s surface and play a dominant role in the energy and water cycle of our planet. Only satellite observations provide a continuous survey of the state of the atmosphere over the entire globe and across the wide range of spatial and temporal scales that comprise weather and climate variability. Satellite cloud data records now exceed more than 25 years; however, climate data records must be compiled from different satellite datasets and can exhibit systematic biases. Questions therefore arise as to the accuracy and limitations of the various sensors and retrieval methods. The Global Energy and Water cycle Experiment (GEWEX) Cloud Assessment, initiated in 2005 by the GEWEX Radiation Panel (GEWEX Data and Assessment Panel since 2011), provides the first coordinated intercomparison of publically available, standard global cloud products (gridded, monthly statistics) retrieved from measurements of multi-spectral imagers (some with multi-angle view and polarization capabilities), IR sounders and lidar. Cloud properties under study include cloud amount, cloud height (in terms of pressure, temperature or altitude), cloud thermodynamic phase, cloud radiative and bulk microphysical properties (optical depth or emissivity, effective particle radius and water path). Differences in average cloud properties, especially in the amount of high-level clouds, are mostly explained by the inherent instrument measurement capability for detecting and/or identifying optically thin cirrus, especially when overlying low-level clouds. The study of long-term variations with these datasets requires consideration of many factors. The monthly, gridded database presented here facilitates further assessments, climate studies and the evaluation of climate models.

EECLAT 2013 workshop – post-mortem

The EECLAT 2013 workshop took place between January 21st and 23rd, 2013 in Besse. Below is the final agenda, with links to the scientific presentations. You can also download all the presentations in a single zip file, if that’s how you roll. You will find here the meeting notes.

Thanks to all participants for braving the snow ! Also thanks to O. Jourdan, N. Montoux and all the people from LAMP who helped organize this meeting (especially to those who brought the video projector on tuesday…). Thanks to M. Seydi and S. Sportouch for helping with organizing the workshop.

 

dehors : ça caille.

Lundi 21 janvier

14h: Accueil

14h30 : Infos generales EECLAT. J. Delanoe, H. Chepfer

  • budget 2012: ce qui etait prevu, ce qui a ete engagé
  • retours des evaluations TOSCA et LEFE
  • infos CALIPSO, EarthCARE
  • CALIPSO-CloudSAT Science Meeting : 30 septembre au 4 octobre 2013 (NCAR, Boulder, USA)
  • debut de reflexion: post-EarthCARE, futur d’EECLAT

15h30 : T1 – Clouds at the regional scale

19h30 : diner

Mardi 22 janvier

9h : T2 – Clouds at the global scale

11h : pause

11h15 : T3 – Aerosols

12h30: dejeuner

16h : T0 – cal/val CALIOP IIR, T3, T4 – PSC, T5 – Radiative transfer tools, T6 – Airborne dataset for Earth-CARE

20h-21h: Repas

Mercredi 23 janvier

8h15 : T7 – Lidar IPRAL: IPSL Hi-Performance multi-wavelength Raman Lidar for Cloud Aerosol Water Vapor Research. M. Haeffelin

9h-12h : Discussions autour de la suite du projet.

EECLAT workshop 21-23 jan 2013

Le premier workshop EECLAT aura lieu du 21 au 23 janvier 2013 a Sancy, pres de Clermont. Plus de 30 participants sont prevus. Merci à Olivier Jourdan, Nadege Montoux et Stephane Sportouch pour leur aide.

Voici une ébauche d’agenda :

  • 21 janvier, apres-midi : petit point “news EECLAT” (CALIPSO, CloudSAT, Earth-CARE, SIRTA, reponse du TOSCA et peut-etre du LEFE, budget ?), puis : Science !
  • 22 janvier : Science !
  • 23 janvier, matin : organisation de la propal TOSCA printemps 2013.

La partie “Science” sera découpée en themes en fonction des participants (les vraies informations suivront).

Publications acceptées, octobre 2012

Quelques publications reliées aux thématiques d’EECLAT ayant été acceptées récemment :

The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite provides robust and global direct measurements of the cloud vertical structure. The GCM-Oriented CALIPSO Cloud Product is used to evaluate the simulated clouds in five climate models using a lidar simulator. The total cloud cover is underestimated in all models (51% to 62% vs. 64% in observations) except in the Arctic. Continental cloud covers (at low, mid, high altitudes) are highly variable depending on the model. In the tropics, the top of deep convective clouds varies between 14 and 18 km in the models versus 16 km in the observations, and all models underestimate the low cloud amount (16% to 25%) compared to observations (29%). In the Arctic, the modeled low cloud amounts (37% to 57%) are slightly biased compared to observations (44%), and the models do not reproduce the observed seasonal variation. It results that the total cloud cover is underestimated in all models (51% to 62% instead of 64% in observations) except in Arctic. The continental cloud covers (at low, mid, high altitudes) are highly variable depending on the model. In the tropics, the top of the deep convective clouds varies between 14 and 18 km in the models against 16 km in the observations, and all the models underestimate the low cloud amount (16% to 25%) compared to observations (29%). In Arctic, the modeled low cloud amounts (between 57% and 37%) are slightly biased compared to observations (44%), and the models hardly reproduce the observed seasonal variation.

Ground-based observations show that persistent liquid-containing Arctic clouds occur frequently and have a dominant influence on Arctic surface radiative fluxes. Yet, without a hemispheric multi-year perspective, the climate relevance of these intriguing Arctic cloud observations was previously unknown. In this study, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) observations are used to document cloud phase over the Arctic basin (60-82 {degree sign}N) during a five-year period (2006-2011). Over Arctic ocean-covered areas, low-level liquid-containing clouds are prevalent in all seasons, especially in Fall. These new CALIPSO observations provide a unique and climate-relevant constraint on Arctic cloud processes. Evaluation of one climate model using a lidar simulator suggests a lack of liquid-containing Arctic clouds contributes to a lack of “radiatively opaque” states. The surface radiation biases found in this one model are found in multiple models, highlighting the need for improved modeling of Arctic cloud phase.

  • Chepfer H., G. Cesana, D. Winker, B. Getzewich, and M. Vaughan, 2012: Comparison of two different cloud climatologies derived from CALIOP Level 1 observations: the CALIPSO-ST and the CALIPSO-GOCCP, J. Atmos. Ocean. Tech., in press
  • Nam C., S. Bony, JL Dufresne, H. Chepfer, 2012: The ‘too few, too bright’ tropical low-cloud problem in CMIP5 models, Geophys. Res. Lett., idoi:10.1029/2012GL053421.
Previous generations of climate models have been shown to under-estimate the occurrence of tropical low-level clouds and to over-estimate their radiative effects. This study analyzes outputs from multiple climate models participating in the Fifth phase of the Coupled Model Intercomparison Project (CMIP5) using the Cloud Feedback Model Intercomparison Project Observations Simulator Package (COSP), and compares them with different satellite data sets. Those include CALIPSO lidar observations, PARASOL mono-directional reflectances and CERES radiative fluxes at the top of the atmosphere. We show that current state-of-the-art climate models predict overly bright low-clouds, even for a correct low-cloud cover. The impact of these biases on the Earth’ radiation budget, however, is reduced by compensating errors. Those include the tendency of models to under-estimate the low-cloud cover and to over-estimate the occurrence of mid- and high-clouds above low-clouds. Finally, we show that models poorly represent the dependence of the vertical structure of low-clouds on large-scale environmental conditions. The implications of this ‘too few, too bright low-cloud problem’ for climate sensitivity and model development are discussed.
We compare Gravity Waves (GW) and Polar Stratospheric Clouds (PSC) above the Antarctic Peninsula for winters (June to September) between 2006 and 2010. GW activity is inferred from stratospheric temperature and vertical winds from the Weather and Research Forecast mesoscale model (WRF), and documented as a function of time and geography for the studied period. Significant GW activity affects 36% of days and follows the Peninsula orography closely. Volumes of PSC, composed of ice and Nitric Acid Trihydrate (NAT), are retrieved using observations from the spaceborne lidar CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization). They are documented against GW activity as a function of time and longitude. Sixty-three percent of ice PSC are observed during GW events, when the average volume of PSC per profile doubles. Maximum ice PSC volumes are seen directly over the Peninsula (65°W), while maximum NAT PSC volumes appear downstream further East (∼35°W). Effects of GW events on NAT PSC are felt as far East as 40°E. Our results support the importance of gravity waves as a major mechanism driving the evolution of ice PSC in the area, but the effects on NAT PSC are harder to detect. After a GW event ends, volumes of ice PSC get back to their usual levels in less than 24 h, while this process takes more than 48 h for NAT PSC. Daily profiles of H2O and HNO3 mixing ratios, retrieved from MLS observations, are used to derive ice and NAT frost points with altitude and time. Combining these frost points with modeled stratospheric temperatures, the volumes of air able to support ice and NAT crystals are quantified and compared with PSC volumes. Correlation is high for ice crystals, but not for NAT, consistent with their much slower nucleation mechanisms. Observations of ice PSC over the domain are followed by a strong increase (+50–100%) in NAT PSC formation efficiency 2 to 6 h later. This increase is followed by a steep drop (6–10 h later) and a longer period of slow decline (10–24 h later), at the end of which the NAT PSC formation efficiency is less than half its initial value. The fact that these effects tend to cancel each other out, coupled to the important lag in NAT PSC reaction to GW activity, suggest why it is especially difficult to quantify how GW activity impacts NAT PSC cover.
During the POLARCAT-France airborne measurement campaign in spring 2008, several pollution plumes transported from mid-latitude regions were encountered. The study presented here focuses on air masses from two different geographic origins (Europe and Asia) and from 2 different source types (anthropogenic pollution and forest fires). A first case study is dedicated to a European air mass, which was repeatedly sampled and analysed during three consecutive days. Thereby, the evolution of the aerosol properties (size distributions, CO mixing ratio) is characterised and related processes are discussed. In particular, the role of coagulation, condensation and cloud processing in the evolution of the Aitken and the accumulation mode particles are contrasted. A second case study focuses on European air masses impacted solely by biomass burning emissions and Asian air masses with contributions from both biomass burning and anthropogenic emissions. The analysis of aerosol modes highlight a similar behaviour for particle originating from biomass burning (from Europe as well as Asia). In comparison to the predominating aged accumulation mode in biomass burning particles, a still larger aerosol accumulation mode related to Asian anthropogenic emissions can be isolated. These findings corroborate the external mixing of such kind of aerosol size distributions. An electron microscopy study (coupled to X-ray elemental analysis) of particles illustrated soot-like inclusions in several samples. Within samples attributed to forest fire sources, the chemical signature is highly associated with the presence of potassium, which is a characteristic tracer element for biomass burning plumes. The single particle images suggest an internal mixing of sampled individual aerosol particles. Thus, particles are found externally mixed as demonstrated from particle size distributions while they appear internally mixed at the particle scale.

Si vous souhaitez voir apparaitre vos publications acceptées, n’hésitez pas a nous les communiquer : vincent dot noel at lmd.polytechnique.fr.

Meeting EECLAT le 2 avril 2012

La reunion de bilan et de coordination pour la propal EECLAT 2012-2013 a eu lieu lundi 2 avril 2012 a l’UPMC. Etaient presents: G. Ancellet, S. Bastin, D. Bouniol, G. Cesana, H. Chepfer, E. Defer, J. Delanoe, C. Hoareau, N. Montoux, V. Noel, J. Pelon, M. Reverdy, E. Riviere, C. Stubenrauch, S. Turquety.

Durant la matinée, apres une introduction de J. Delanoe reprenant les points des evaluations TOSCA et LEFE de la propal 2011-2012, les responsables des 6 themes de la propal ont présenté les résultats obtenus dans le cadre de chaque work package :
– theme 1, nuages a l’echelle regionale (D. Bouniol)
– theme 2, nuages a l’echelle globale (V. Noel)
– theme 3, aerosols (S. Turquety, G. Ancellet)
– theme 4, PSC (V. Noel)
– theme 5, Radiative transfer tools for A-Train & Earth-Care (H. Chepfer)
– theme 6, Ground-based and airborne dataset for Earth-Care preparation (J. Delanoe)
Les transparents des presentations associes seront tres prochainement mis a disposition sur le site web EECLAT.

M. Haeffelin (IPSL), responsable du SIRTA, est intervenu pour decrire le projet IPRAL de construction d’un lidar Raman sol installe au SIRTA. Ce lidar pourrait (entre autres applications) etre d’une grande utilite pour produire un jeu de donnees test permettant de mettre au point des algorithmes destinés a l’analyse des observations Earth-Care (theme 6 de la propal 2011-2012). Durant son evaluation du projet IPRAL (courant 2011), le CNES a promis de le soutenir financierement (a hauteur de ~110Keuros) s’il etait inclus dans la prochaine propal EECLAT, afin que son interet scientifique soit valide par le TOSCA. Il a ete decide 1) d’inclure la description du projet IPRAL dans le theme 6 de la nouvelle propal, en y incluant les documents decrivant les promesses et demandes du CNES, 2) d’ajouter des arguments soutenant l’interet d’IPRAL dans le reste de la propal, et 3) de souligner clairement dans le texte que l’eventuel soutien financier du CNES pour IPRAL ne devait pas se faire au detriment du reste de la propal (surtout aux depends du financement de post-doc).

Durant l’apres-midi, il a ete decide que la version mise a jour de la propal soumise au TOSCA cette annee (mi-avril 2012) conservera la structure generale de la propal precedente et son decoupage en WP. Chaque WP sera repris en sortant les activites prevues et realisees, et en les regroupant avec les autres resultats obtenus dans une section “bilan”. Les activites prevues restantes seront mises a jour, en supprimant ce qui n’est plus d’actualité, etc. La liste des personnes impliquees dans chaque WP sera mise a jour.

Concernant le budget, un tour de table a ete fait pour distribuer le budget de frais de missions alloue par le LEFE en fonction des besoins de chacun. Pour sa revision future, il a ete demande que chaque WP separe clairement ce qui est demande au TOSCA de ce qui est demande au LEFE, afin d’eviter la confusion du tour precedent.

Il a ete egalement prevu l’annee prochaine d’organiser une reunion de discussion scientifique autour des objectifs de la propal, afin entre autres d’envisager des reorganisations, regroupements ou eclatements de WP, etc. Cette reunion serait financee par le budget d’animation scientifique alloue par le LEFE, et aurait probablement lieu en janvier 2013, suffisamment en avance de la soumission de la propal 2013. Elle aurait probablement lieu hors region parisienne et s’etalerait sur deux jours.

Les gens souhaitant participer au workshop EarthCare/Calipso/CloudSat (ECCC ? EC3 ?), qui aura lieu du 18 au 22 juin a Paris ont ete invites a soumettre des abstracts en tres grand nombre sur le site du workshop.