The unprecedented 2017-2018 stratospheric smoke event : Decay phase and aerosol properties observed with the EARLINET

© Author(s) 2019. This open access work is distributed under the Creative Commons Attribution 4.0 License (https://creativecommons.org/licenses/by/4.0/).Six months of stratospheric aerosol observations with the European Aerosol Research Lidar Network (EARLINET) from August 2017 to January 2018 are presented. The decay phase of an unprecedented, record-breaking stratospheric perturbation caused by wildfire smoke is reported and discussed in terms of geometrical, optical, and microphysical aerosol properties. Enormous amounts of smoke were injected into the upper troposphere and lower stratosphere over fire areas in western Canada on 12 August 2017 during strong thunderstorm-pyrocumulonimbus activity. The stratospheric fire plumes spread over the entire Northern Hemisphere in the following weeks and months. Twenty-eight European lidar stations from northern Norway to southern Portugal and the eastern Mediterranean monitored the strong stratospheric perturbation on a continental scale. The main smoke layer (over central, western, southern, and eastern Europe) was found at heights between 15 and 20 km since September 2017 (about 2 weeks after entering the stratosphere). Thin layers of smoke were detected at heights of up to 22-23 km. The stratospheric aerosol optical thickness at 532 nm decreased from values > 0.25 on 21-23 August 2017 to 0.005-0.03 until 5-10 September and was mainly 0.003-0.004 from October to December 2017 and thus was still significantly above the stratospheric background (0.001-0.002). Stratospheric particle extinction coefficients (532 nm) were as high as 50-200 Mm-1 until the beginning of September and on the order of 1 Mm-1 (0.5- 5 Mm-1) from October 2017 until the end of January 2018. The corresponding layer mean particle mass concentration was on the order of 0.05-0.5 μg m-3 over these months. Soot particles (light-absorbing carbonaceous particles) are efficient ice-nucleating particles (INPs) at upper tropospheric (cirrus) temperatures and available to influence cirrus formation when entering the tropopause from above. We estimated INP concentrations of 50-500 L-1 until the first days in September and afterwards 5-50 L-1 until the end of the year 2017 in the lower stratosphere for typical cirrus formation temperatures of -55 ?C and an ice supersaturation level of 1.15. The measured profiles of the particle linear depolarization ratio indicated a predominance of nonspherical smoke particles. The 532 nm depolarization ratio decreased slowly with time in the main smoke layer from values of 0.15-0.25 (August-September) to values of 0.05-0.10 (October-November) and < 0.05 (December-January). The decrease of the depolarization ratio is consistent with aging of the smoke particles, growing of a coating around the solid black carbon core (aggregates), and thus change of the shape towards a spherical form. We found ascending aerosol layer features over the most southern European stations, especially over the eastern Mediterranean at 32-35? N, that ascended from heights of about 18-19 to 22-23 km from the beginning of October to the beginning of December 2017 (about 2 km per month). We discuss several transport and lifting mechanisms that may have had an impact on the found aerosol layering structures.Peer reviewe

The unprecedented 2017–2018 stratospheric smoke event: decay phase and aerosol properties observed with the EARLINET

Six months of stratospheric aerosol observations with the European Aerosol Research Lidar Network (EARLINET) from August 2017 to January 2018 are presented. The decay phase of an unprecedented, record-breaking stratospheric perturbation caused by wildfire smoke is reported and discussed in terms of geometrical, optical, and microphysical aerosol properties. Enormous amounts of smoke were injected into the upper troposphere and lower stratosphere over fire areas in western Canada on 12 August 2017 during strong thunderstorm–pyrocumulonimbus activity. The stratospheric fire plumes spread over the entire Northern Hemisphere in the following weeks and months. Twenty-eight European lidar stations from northern Norway to southern Portugal and the eastern Mediterranean monitored the strong stratospheric perturbation on a continental scale. The main smoke layer (over central, western, southern, and eastern Europe) was found at heights between 15 and 20 km since September 2017 (about 2 weeks after entering the stratosphere). Thin layers of smoke were detected at heights of up to 22–23 km. The stratospheric aerosol optical thickness at 532 nm decreased from values > 0.25 on 21–23 August 2017 to 0.005–0.03 until 5–10 September and was mainly 0.003–0.004 from October to December 2017 and thus was still significantly above the stratospheric background (0.001–0.002). Stratospheric particle extinction coefficients (532 nm) were as high as 50–200 Mm−1 until the beginning of September and on the order of 1 Mm−1 (0.5–5 Mm−1) from October 2017 until the end of January 2018. The corresponding layer mean particle mass concentration was on the order of 0.05–0.5 µg m−3 over these months. Soot particles (light-absorbing carbonaceous particles) are efficient ice-nucleating particles (INPs) at upper tropospheric (cirrus) temperatures and available to influence cirrus formation when entering the tropopause from above. We estimated INP concentrations of 50–500 L−1 until the first days in September and afterwards 5–50 L−1 until the end of the year 2017 in the lower stratosphere for typical cirrus formation temperatures of −55 ∘C and an ice supersaturation level of 1.15. The measured profiles of the particle linear depolarization ratio indicated a predominance of nonspherical smoke particles. The 532 nm depolarization ratio decreased slowly with time in the main smoke layer from values of 0.15–0.25 (August–September) to values of 0.05–0.10 (October–November) and < 0.05 (December–January). The decrease of the depolarization ratio is consistent with aging of the smoke particles, growing of a coating around the solid black carbon core (aggregates), and thus change of the shape towards a spherical form. We found ascending aerosol layer features over the most southern European stations, especially over the eastern Mediterranean at 32–35∘ N, that ascended from heights of about 18–19 to 22–23 km from the beginning of October to the beginning of December 2017 (about 2 km per month). We discuss several transport and lifting mechanisms that may have had an impact on the found aerosol layering structures

Tropospheric and stratospheric smoke over Europe as observed within EARLINET/ACTRIS in summer 2017

For several weeks in summer 2017, strong smoke layers were observed over Europe at numerous EARLINET stations. EARLINET is the European research lidar network and part of ACTRIS and comprises more than 30 ground-based lidars. The smoke layers were observed in the troposphere as well as in the stratosphere up to 25 km from Northern Scandinavia over whole western and central Europe to the Mediterranean regions. Backward trajectory analysis among other tools revealed that these smoke layers originated from strong wild fires in western Canada in combination with pyrocumulus convection. An extraordinary fire event in the mid of August caused intense smoke layers that were observed across Europe for several weeks starting on 18 August 2017. Maximum aerosol optical depths up to 1.0 at 532 nm were observed at Leipzig, Germany, on 22 August 2017 during the peak of this event. The stratospheric smoke layers reached extinction coefficient values of more than 600 Mm−1 at 532 nm, a factor of 10 higher than observed for volcanic ash after the Pinatubo eruption in the 1990s. First analyses of the intensive optical properties revealed low particle depolarization values at 532 nm for the tropospheric smoke (spherical particles) and rather high values (up to 20%) in the stratosphere. However, a strong wavelength dependence of the depolarization ratio was measured for the stratospheric smoke. This indicates irregularly shaped stratospheric smoke particles in the size range of the accumulation mode. This unique depolarization feature makes it possible to distinguish clearly smoke aerosol from cirrus clouds or other aerosol types by polarization lidar measurements. Particle extinction-to-backscatter ratios were rather low in the order of 40 to 50 sr at 355 nm, while values between 70-90 sr were measured at higher wavelengths. In the western and central Mediterranean, stratospheric smoke layers were most prominent in the end of August at heights between 16 and 20 km. In contrast, stratospheric smoke started to occur in the eastern Mediterranean (Cyprus and Israel) in the beginning of September between 18 and 23 km. Stratospheric smoke was still visible in the beginning of October at certain locations (e.g. Evora, Portugal), while tropospheric smoke was mainly observed until the end of August within Europe. An overview of the smoke layers measured at several EARLINET sites will be given. The temporal development of these layers as well as their geometrical and optical properties will be presented

Developpement d'un radiometre infrarouge thermique multicanal: Qualification du prototype large champ CLIMAT

A multichannel radiometer, which is designed to have up to six channels in the thermal infrared, in particular the atmospheric window centered on 10 µm, has been performed to realize ground measurements of radiance and brightness temperatures, and for aiborne measurements. The detector is a fast thermopile characterized by a very low temperature dependence of its responsivity. The CIMEL company has been in charge of the instrumental realization. The physical analyze and the qualification of the successives steps of the instrument development have been performed by the Laboratoire d Optique Atmosph´erique (LOA). A wide field of view prototype named CLIMAT (Conveyable Low noise Infrared radiometer for Measurements of Atmosphere and surface Targets) has been analyzed in laboratory and in realistic conditions. The essential characteristics of sensitivity, noise, field of view, thermal behavior, temporal stability have been studied with the prototype. The first results of brightness temperature collected in Lille in clear sky conditions, are then presented and compared with brightness temperature computed with a radiative code. The prototype behavior is besides compared with the one of a monochannel Barnes PRT-5 (8-14µm). The instrument has been also qualified for airborne measurements of brightness sea surface temperature off Calais, with the aid of satellite data (AVHRR) of sea temperature.Un radiomètre multicanal comportant jusqu' à six canaux dans la fenêtre spectrale de l infrarouge thermique a 10 µm, a été conçu pour réaliser des mesures, sur le terrain et aéroportées, de luminance et de température de brillance. Le détecteur est une thermopile rapide dont la réponse est faiblement dépendante de la température. La réalisation instrumentale a été conduite par la société CIMEL. L'analyse physique et la qualification des différentes étapes du développement de l'instrument ont été effectuées au Laboratoire d'Optique Atmosphérique (LOA). Un prototype nomme CLIMAT (Conveyable Low noise Infrared radiometer for Measurements of Atmosphere and surface Targets) dont le champ de visée est de 10 degrés a été qualifie en laboratoire puis en conditions réelles. Les caractéristiques essentielles de sensibilité, bruit, champ, comportement thermique, stabilité temporelles ont été étudiées sur le prototype. Les premiers résultats de mesures de la température de brillance d'un ciel clair, réalisées a Lille, sont présentes et compares avec les résultats de simulations obtenus avec un code radiatif. Le comportement du prototype est en outre comparé avec celui d un radiomètre monocanal Barnes PRT-5 (8-14µm). L instrument a enfin été qualifie lors de mesures aéroportées de température de brillance de la surface marine au large de Calais, a l aide des données satellitaires (AVHRR) de température de la mer

Evaluation of a planetary boundary layer subgrid-scale model that accounts for near-surface turbulence anisotropy

In numerical modelling, far from regions of large gradients, the resolved scales contain most of the energy of turbulent motion whilst on subgrid scales (SGS), motions are less energetic. However, SGS contribution becomes larger than the resolved part near the surface. Recent studies have shown that near-surface turbulence anisotropy has a dramatic effect on the mixing length to be used in SGS models which are generally derived for free-stream isotropic turbulence or use the standard Prandtl mixing length z. Using the flux measurements collected at the SIRTA observatory located near Paris (France), this paper shows that a SGS model suitable for both the surface layer and free-stream turbulence must discard the Prandtl mixing length and account for near-surface turbulence anisotropy. It also shows that such an SGS scheme, which is suited for all numerical models of the atmosphere, improves the representation of planetary boundary layer processes in most stability conditions

Leveraging Upcoming 355-nm Channels in Space Lidars for Cloud Science: Comparison With 532-nm and Case Studies from the IPRAL Ground-Based Lidar

International audienceAfter ten years of CALIPSO lidar record, there are now no doubt that lidar is one of the most powerful tool to study cloud physics. Recent study (Chepfer et al. 2018) also shows that providing a long enough time series, lidar allows studying cloud trends. CALIPSO which works at 532 nm is at the end of life, but other lidar mission with 355-nm laser are planned or just launched: ADM-Aeolus, while not designed for clouds (only few vertical levels) could be very useful (among others) to make junction between CALIPSO and future other lidar missions, EarthCare, and other future lidar missions that are under study in the agencies and institutes. It is then necessary to study how continuity can be ensured between CALIPSO cloud products and upcoming space lidars that have different wavelengths, and what is the added value of this new wavelength for clouds.Here, the objective is then to study how leveraging upcoming 355-nm channels in space lidars for cloud science.To address this question, we use IPRAL lidar located in the SIRTA ground-based observatory (near Paris, France): IPRAL is a high-performance backscatter lidar that works at three wavelengths including 532- and 355-nm channels. It crosses atmosphere from ground to 20 km at a 15-m vertical resolution. We will present work on: (i) the creation of the equivalent of GOCCP CALIPSO cloud product from IPRAL 532-nm channel, and the adaptation of the method for the IPRAL 355-nm channel in order to make them as consistent as possible; (ii) comparison to CALIPSO for some collocated cases study in order to qualify and quantify the limits of a possible 532/355 continuity cloud product; (iii) new diagnostics relevant for cloud science based on the 355-nm IPRAL signal analysis

Long-term study of coherent structures in the atmospheric surface layer

A long-term study of coherent turbulence structures in the atmospheric surface layer has been carried out using 10 months of turbulence data taken on a 30-m tower under varying meteorological conditions. We use an objective detection technique based on wavelet transforms. The applied technique permits the isolation of the coherent structures from small-scale background fluctuations which is necessary for the development of dynamical models describing the evolution and properties of these phenomena. It was observed that coherent structures occupied 36% of the total time with mean turbulent flux contributions of 44% for momentum and 48% for heat. The calculation of a transport efficiency parameter indicates that coherent structures transport heat more efficiently than momentum. Furthermore, the transport efficiency increases with increasing contribution of the structures to the overall transport

Validation of two‐channel VIRS retrievals of aerosol optical thickness over ocean and quantitative evaluation of the impact from potential subpixel cloud contamination and surface wind effect

TRMM/CERES‐VIRS Single Satellite Footprint (SSF) data and AERONET Sun/sky radiometer observations from 1998 have been combined to validate SSF aerosol optical thickness (τ) retrievals over ocean along with a quantitative evaluation of the effects of potential subpixel cloud contamination and surface wind on the satellite τ retrievals. Potential subpixel cloud contamination is verified in Visible/Infrared Scanner (VIRS) SSF aerosol retrievals and constitutes a major source of systematic and random errors of the retrieval algorithm as determined from comparisons with AERONET observations. A positive correlation between the surface wind speed (which determines the roughness of the ocean surface) and the SSF τ has been observed for large surface wind speed. The validation results imply this correlation represents the real relationship between the surface wind and the wind‐driven aerosols rather than the disturbing effect of the surface reflectance associated with the rough ocean surface. After the potential subpixel cloud contamination is minimized and the effects of large surface wind are removed in the τ match‐ups, the positive biases in the SSF τ (compared to AERONET τ) for mean conditions have been reduced from 0.05 to 0.02 in VIRS channel 1 (0.63 μm) and 0.05 to 0.03 in channel 2 (1.61 μm). Random errors have also been reduced from 0.09 to 0.06 at 0.63 μm, and from 0.06 to 0.05 at 1.61 μm. The validation results support the application of the SSF aerosol data in radiation and climate studies as well as supply useful guidance for the adjustment and improvement of the aerosol retrieval algorithm

SeaWIFS Postlaunch Technical Report Series

This report documents the scientific activities that took place at the Acqua Alta Oceanographic Tower (AAOT) in the northern Adriatic Sea off the coast of Italy from 2-6 August 1999. The ultimate objective of the field campaign was to evaluate the capabilities of a new instrument called the SeaWiFS Photometer Revision for Incident Surface Measurements (SeaPRISM). SeaPRISM is based on a CE-318 sun photometer made by CIMEL Electronique (Paris, France). The CE-318 is an automated, robotic system which measures the direct sun irradiance plus the sky radiance in the sun plane and in the almucantar plane. The data are transmitted over a satellite link, and this remote operation capability has made the device very useful for atmospheric measurements. The revision to the CE-318 that makes the instrument potentially useful for SeaWiFS calibration and validation activities is to include a capability for measuring the radiance leaving the sea surface in wavelengths suitable for the determination of chlorophyll a concentration. The initial evaluation of this new capability involved above- and in-water measurement protocols. An intercomparison of the water-leaving radiances derived from SeaPRISM and an in-water system showed the overall spectral agreement was approximately 8.6%, but the blue-green channels intercompared at the 5% level. A blue-green band ratio comparison was at the 4% level

Regional evaluation of an advanced very high resolution radiometer (AVHRR) two‐channel aerosol retrieval algorithm

Advanced Very High Resolution Radiometer (AVHRR) aerosol optical thickness retrieval over the ocean is one of the two existing sources of long‐term global satellite aerosol measurements (Total Ozone Mapping Spectrometer aerosol data set is the other). To make this 20‐year historical data more useful for climate studies, the quality of the data (or the performance of the retrieval algorithm) has to be systematically evaluated. In this paper, as a continuation of our previous global validation effort, we present regional validation results for an AVHRR independent two‐channel aerosol retrieval algorithm by comparing the retrievals with observations from the Aerosol Robotic Network (AERONET). The bias and the random errors of the retrieval algorithm applied to NOAA‐14/AVHRR observations were determined and documented for key aerosol types (including biomass‐burning, urban/industrial, desert dust, and marine). As a by‐product of the validation, effective refractive indexes of the key aerosol types were also statistically determined through sensitivity analysis. The global and regional validations indicate that the new independent two‐channel algorithm (with a globally unified aerosol model) performs well in the sense of the global mean. However, improvements are necessary to make the retrieval sensitive to aerosol types and to capture aerosol regional variations. The results will facilitate the utilization of long‐term AVHRR aerosol products in climate studies and will provide guidance for improving aerosol retrievals from future NOAA satellite instruments