Optical characterization of Polar winter aerosols and clouds

The Arctic region is particularly sensitive to climate change and has recently undergone major alterations including a dramatic decrease of sea-ice extent. Our ability to model and potentially mitigate climate change is limited, in part, by the uncertainties associated with radiative forcing due to direct and indirect aerosol effects which in turn are dependent on our understanding of aerosol and cloud processes. Aerosol loading can be characterized by aerosol optical depth (AOD) which is the most important (extensive or bulk) aerosol radiative parameter and arguably the most important regional indicator of aerosol behavior. One of the most important shortcomings in our understanding of Arctic aerosols is their behavior during the Polar winter. A major reason for this is the lack of night-time AOD measurements. In this work we use lidar and starphotometry instruments in the Arctic to obtain vertically resolved aerosol profiles and column in- tegrated representations of those profiles (AODs) respectively. In addition, data from a space-borne lidar (CALIOP) is used to provide a pan-Arctic context and seasonal statistics in support of ground based measurements

Characterisation of night-time aerosols using starphotometry

This is a study concerning the use of starphotometry to retrieve night-time aerosol optical depths (AODs). In the summer of 2007 a SPSTAR03 starphotometer was installed at a rural site at Egbert, Ontario for the purpose of the nighttime AOD measurements. Two series of daytime / nighttime AODs were acquired using the CIMEL CE 318 sunphotometer and the SPSTAR03 from Aug. 31 to Sept. 19 2007 and from June 30 to July 5, 2008. The measurements were complemented by vertical backscatter coefficient profiles acquired using a pulsed lidar. We found that starphotometer AOD estimates, based on the application of a two star method (TSM) to low and high elevation stars, are susceptible to atmospheric inhomogeneity effects. Starphotometer AOD estimates based on the one star method (OSM) reduce this sensitivity, but require absolute calibration values. A level of continuity was obtained between the daytime sunphotometry and nighttime starphotometry data. A continuity parameter (defined as the average difference between the measured nighttime and interpolated daytime values) was calculated over four distinct periods. It yielded the differences of 0.160, 0.053, 0.139 (total, fine and coarse mode optical depths) for the low star and 0.195, 0.070, 0.149 for the high star. We argue that cloud screening would have reduced the continuity parameter differences for the coarse and total optical depths. For 5 out of , 8 nights of lidar operation, a combination of the Angstrom and Spectral Deconvolution Algorithm (SDA) analysis provided an indication of the nature of the atmospheric features seen in the lidar data. Fine and coarse-mode events were detected during the measurement periods using the SDA. Lidar data was used to better understand complex atmospheric phenomena and was found especially effective for cloud detection and general signal increase/decrease analysis

La caractérisation optique des aérosols et des nuages pendant l’hiver polaire

Résumé : L’Arctique est particulièrement sensible aux changements climatiques et a récemment subi des modifications majeures incluant une diminution dramatique de l’extension de la glace de mer. Notre capacité́ à modéliser et à potentiellement réduire les changements climatiques est limitée, en partie, par les incertitudes associées au forçage radiatif induit par les effets directs et indirects des aérosols, qui dépendent de notre compréhension des processus impliquant les nuages et les aérosols. La charge des aérosols est caractérisée par l’épaisseur optique des aérosols (AOD) qui est le paramètre radiatif extensif le plus important et l’indicateur régional du comportement des aérosols sans doute le plus décisif. Une de nos lacunes majeures dans la compréhension des aérosols arctiques est leur comportement durant l’hiver polaire. Cela est principalement dû au manque de mesures nocturnes d’AOD. Dans ce travail, on utilise des instruments (lidar et photomètre stellaire) installés en Arctique pour mesurer, respectivement, les profils verticaux des aérosols et une valeur intégrée dans la colonne (AOD) de ces profils. En outre, les données d’un lidar spatial (CALIOP) sont utilisées pour fournir un contexte pan-arctique et des statistiques saisonnières pour supporter les mesures au sol. Ces dernières ont été obtenues aux stations arctiques d’Eureka (80◦ N, 86◦ W) et de Ny Ålesund (79◦ N, 12◦ E) durant les hivers polaires de 2010-2011 et 2011-2012. L’importance physique des pe- tites variations d’amplitude de l’AOD est typique de l’hiver polaire en Arctique, mais suppose une vérification pour s’assurer que des artefacts ne contribuent pas à ces variations (par exemple un masque de nuage insuffisant). Une analyse des processus basée sur des événements (avec une résolution temporelle ≈ une minute) est essentielle pour s’assurer que les paramètres optiques et microphysiques extensifs (grossiers) et intensifs (par particules) sont cohérents et physiquement conformes. La synergie photomètre stellaire-lidar nous permet de caractériser plusieurs événements distincts au cours des périodes de mesures, en particulier : des aérosols, des cristaux de glace, des nuages fins et des nuages polaires stratosphériques (PSC). Dans l’ensemble, les modes fin (1μm) de l’AOD obtenus par photométrie stellaire (τ[indice inférieur f] et τ[indice inférieur c]) sont cohérents avec leurs analogues produits à partir des profils intégrés du lidar. Cependant certaines inconsistances causées par des facteurs instrumentaux et environnementaux ont aussi été trouvées. La division de l’AOD du photomètre stellaire τ[indice inférieur f] et τ[indice inférieur c] a été davantage exploitée afin d’éliminer les épaisseurs optiques du mode grossier (le filtrage spectral de nuages) et, par la suite, de comparer τ[indice inférieur]f avec les AODs obtenues par le filtrage de nuages traditionnel (temporel). Alors que les filtrages temporel et spectral des nuages des cas étudiés au niveau des processus ont conduit à des résultats bons à modérés en termes de cohérence entre les données filtrées spectralement et temporellement (les épaisseurs optiques des photomètres stellaires et lidars étant toutes deux filtrées temporellement), les résultats saisonniers semblent être encore contaminés par les nuages. En imposant un accord en utilisant un second filtre, plus restrictif, avec un critère de ciel clair ("enveloppe minimale du nuage"), les valeurs saisonnières moyennes obtenues étaient de 0.08 à Eureka et 0.04 à Ny Ålesund durant l’hiver 2010-2011. En 2011-2012, ces valeurs étaient, respectivement, de 0.12 et 0.09. En revanche les valeurs d’épaisseur optique de CALIOP (estimées entre 0 et 8 km) ont légèrement diminué de 2010-2011 à 2011-2012 (0.04 vs. 0.03). // Abstract : The Arctic region is particularly sensitive to climate change and has recently undergone major alterations including a dramatic decrease of sea-ice extent. Our ability to model and potentially mitigate climate change is limited, in part, by the uncertainties associated with radiative forcing due to direct and indirect aerosol effects which in turn are dependent on our understanding of aerosol and cloud processes. Aerosol loading can be characterized by aerosol optical depth (AOD) which is the most important (extensive or bulk) aerosol radiative parameter and arguably the most important regional indicator of aerosol behavior. One of the most important shortcomings in our understanding of Arctic aerosols is their behavior during the Polar winter. A major reason for this is the lack of night-time AOD measurements. In this work we use lidar and starphotometry instruments in the Arctic to obtain vertically resolved aerosol profiles and column integrated representations of those profiles (AODs) respectively. In addition, data from a space-borne lidar (CALIOP) is used to provide a pan-Arctic context and seasonal statistics in support of ground based measurements. The latter were obtained at the Eureka (80 ◦ N, 86 ◦ W) and Ny Ålesund (79 ◦ N, 12 ◦ E) high Arctic stations during the Polar Winters of 2010-11 and 2011-12. The physical significance of the variation of the small-amplitude AODs that are typical of the Arctic Polar Winter, requires verification to ensure that artifactual contributions (such as incomplete cloud screening) do not contribute to these variations. A process-level event-based analysis (with a time resolution of ≈ minutes), is essential to ensure that extracted extensive (bulk) and intensive (per particle) optical and microphysical indicators are coherent and physically consistent. Using the starphotometry-lidar synergy we characterized several distinct events throughout the measurement period: these included aerosol, ice crystal, thin cloud and polar stratospheric cloud (PSC) events. In general fine (1 μm )modeAODs from starphotometry ( τ[subscript f] and τ [subscript c] ) were coherent with their lidar analogues produced from integrated profiles : however several inconsistencies related to instrumental and environmental factors were also found. The division of starphotometer AODs into τ[subscript ]f and τ [subscript c] components was further exploited to eliminate coarse mode cloud optical depths (spectral cloud screening) and subsequently compare τ [subscript f] with cloud-screened AODs using a traditional (temporal based) approach. While temporal and spectral cloud screening case studies at process level resolutions yielded good to moderate results in terms of the coherence between spectrally and temporally cloud screened data (both temporally screened starphotometer and lidar optical depths), seasonal results apparently still contained cloud contaminated data. Forcing an agreement using a more restrictive, second-pass, clear sky criterion ("minimal cloud envelope") produced mean 2010-11 AOD seasonal values of 0.08 and 0.04 for Eureka and Ny Ålesund respectively. In 2011-12 these values were 0.12 and 0.09. Conversely, CALIOP AODs (0 to 8 km) for the high Arctic showed a slight decrease from 2010-2011 to 2011-2012 (0.04 vs 0.03)

Optische Charakterisierung von polaren Winteraerosolen und Wolken

The Arctic region is particularly sensitive to climate change and has recently undergone major alterations including a dramatic decrease of sea-ice extent. Our ability to model and potentially mitigate climate change is limited, in part, by the uncertainties associated with radiative forcing due to direct and indirect aerosol effects which in turn are dependent on our understanding of aerosol and cloud processes. Aerosol loading can be characterized by aerosol optical depth (AOD) which is the most important (extensive or bulk) aerosol radiative parameter and arguably the most important regional indicator of aerosol behavior. One of the most important shortcomings in our understanding of Arctic aerosols is their behavior during the Polar winter. A major reason for this is the lack of night-time AOD measurements. In this work we use lidar and starphotometry instruments in the Arctic to obtain vertically resolved aerosol profiles and column in- tegrated representations of those profiles (AODs) respectively. In addition, data from a space-borne lidar (CALIOP) is used to provide a pan-Arctic context and seasonal statistics in support of ground based measurements

Characterisation of night-time aerosols using starphotometry

This is a study concerning the use of starphotometry to retrieve night-time aerosol optical depths (AODs). In the summer of 2007 a SPSTAR03 starphotometer was installed at a rural site at Egbert, Ontario for the purpose of the nighttime AOD measurements. Two series of daytime / nighttime AODs were acquired using the CIMEL CE 318 sunphotometer and the SPSTAR03 from Aug. 31 to Sept. 19 2007 and from June 30 to July 5, 2008. The measurements were complemented by vertical backscatter coefficient profiles acquired using a pulsed lidar. We found that starphotometer AOD estimates, based on the application of a two star method (TSM) to low and high elevation stars, are susceptible to atmospheric inhomogeneity effects. Starphotometer AOD estimates based on the one star method (OSM) reduce this sensitivity, but require absolute calibration values. A level of continuity was obtained between the daytime sunphotometry and nighttime starphotometry data. A continuity parameter (defined as the average difference between the measured nighttime and interpolated daytime values) was calculated over four distinct periods. It yielded the differences of 0.160, 0.053, 0.139 (total, fine and coarse mode optical depths) for the low star and 0.195, 0.070, 0.149 for the high star. We argue that cloud screening would have reduced the continuity parameter differences for the coarse and total optical depths. For 5 out of , 8 nights of lidar operation, a combination of the Angstrom and Spectral Deconvolution Algorithm (SDA) analysis provided an indication of the nature of the atmospheric features seen in the lidar data. Fine and coarse-mode events were detected during the measurement periods using the SDA. Lidar data was used to better understand complex atmospheric phenomena and was found especially effective for cloud detection and general signal increase/decrease analysis

La caractérisation optique des aérosols et des nuages pendant l’hiver polaire

Résumé : L’Arctique est particulièrement sensible aux changements climatiques et a récemment subi des modifications majeures incluant une diminution dramatique de l’extension de la glace de mer. Notre capacité́ à modéliser et à potentiellement réduire les changements climatiques est limitée, en partie, par les incertitudes associées au forçage radiatif induit par les effets directs et indirects des aérosols, qui dépendent de notre compréhension des processus impliquant les nuages et les aérosols. La charge des aérosols est caractérisée par l’épaisseur optique des aérosols (AOD) qui est le paramètre radiatif extensif le plus important et l’indicateur régional du comportement des aérosols sans doute le plus décisif. Une de nos lacunes majeures dans la compréhension des aérosols arctiques est leur comportement durant l’hiver polaire. Cela est principalement dû au manque de mesures nocturnes d’AOD. Dans ce travail, on utilise des instruments (lidar et photomètre stellaire) installés en Arctique pour mesurer, respectivement, les profils verticaux des aérosols et une valeur intégrée dans la colonne (AOD) de ces profils. En outre, les données d’un lidar spatial (CALIOP) sont utilisées pour fournir un contexte pan-arctique et des statistiques saisonnières pour supporter les mesures au sol. Ces dernières ont été obtenues aux stations arctiques d’Eureka (80◦ N, 86◦ W) et de Ny Ålesund (79◦ N, 12◦ E) durant les hivers polaires de 2010-2011 et 2011-2012. L’importance physique des pe- tites variations d’amplitude de l’AOD est typique de l’hiver polaire en Arctique, mais suppose une vérification pour s’assurer que des artefacts ne contribuent pas à ces variations (par exemple un masque de nuage insuffisant). Une analyse des processus basée sur des événements (avec une résolution temporelle ≈ une minute) est essentielle pour s’assurer que les paramètres optiques et microphysiques extensifs (grossiers) et intensifs (par particules) sont cohérents et physiquement conformes. La synergie photomètre stellaire-lidar nous permet de caractériser plusieurs événements distincts au cours des périodes de mesures, en particulier : des aérosols, des cristaux de glace, des nuages fins et des nuages polaires stratosphériques (PSC). Dans l’ensemble, les modes fin (1μm) de l’AOD obtenus par photométrie stellaire (τ[indice inférieur f] et τ[indice inférieur c]) sont cohérents avec leurs analogues produits à partir des profils intégrés du lidar. Cependant certaines inconsistances causées par des facteurs instrumentaux et environnementaux ont aussi été trouvées. La division de l’AOD du photomètre stellaire τ[indice inférieur f] et τ[indice inférieur c] a été davantage exploitée afin d’éliminer les épaisseurs optiques du mode grossier (le filtrage spectral de nuages) et, par la suite, de comparer τ[indice inférieur]f avec les AODs obtenues par le filtrage de nuages traditionnel (temporel). Alors que les filtrages temporel et spectral des nuages des cas étudiés au niveau des processus ont conduit à des résultats bons à modérés en termes de cohérence entre les données filtrées spectralement et temporellement (les épaisseurs optiques des photomètres stellaires et lidars étant toutes deux filtrées temporellement), les résultats saisonniers semblent être encore contaminés par les nuages. En imposant un accord en utilisant un second filtre, plus restrictif, avec un critère de ciel clair ("enveloppe minimale du nuage"), les valeurs saisonnières moyennes obtenues étaient de 0.08 à Eureka et 0.04 à Ny Ålesund durant l’hiver 2010-2011. En 2011-2012, ces valeurs étaient, respectivement, de 0.12 et 0.09. En revanche les valeurs d’épaisseur optique de CALIOP (estimées entre 0 et 8 km) ont légèrement diminué de 2010-2011 à 2011-2012 (0.04 vs. 0.03). // Abstract : The Arctic region is particularly sensitive to climate change and has recently undergone major alterations including a dramatic decrease of sea-ice extent. Our ability to model and potentially mitigate climate change is limited, in part, by the uncertainties associated with radiative forcing due to direct and indirect aerosol effects which in turn are dependent on our understanding of aerosol and cloud processes. Aerosol loading can be characterized by aerosol optical depth (AOD) which is the most important (extensive or bulk) aerosol radiative parameter and arguably the most important regional indicator of aerosol behavior. One of the most important shortcomings in our understanding of Arctic aerosols is their behavior during the Polar winter. A major reason for this is the lack of night-time AOD measurements. In this work we use lidar and starphotometry instruments in the Arctic to obtain vertically resolved aerosol profiles and column integrated representations of those profiles (AODs) respectively. In addition, data from a space-borne lidar (CALIOP) is used to provide a pan-Arctic context and seasonal statistics in support of ground based measurements. The latter were obtained at the Eureka (80 ◦ N, 86 ◦ W) and Ny Ålesund (79 ◦ N, 12 ◦ E) high Arctic stations during the Polar Winters of 2010-11 and 2011-12. The physical significance of the variation of the small-amplitude AODs that are typical of the Arctic Polar Winter, requires verification to ensure that artifactual contributions (such as incomplete cloud screening) do not contribute to these variations. A process-level event-based analysis (with a time resolution of ≈ minutes), is essential to ensure that extracted extensive (bulk) and intensive (per particle) optical and microphysical indicators are coherent and physically consistent. Using the starphotometry-lidar synergy we characterized several distinct events throughout the measurement period: these included aerosol, ice crystal, thin cloud and polar stratospheric cloud (PSC) events. In general fine (1 μm )modeAODs from starphotometry ( τ[subscript f] and τ [subscript c] ) were coherent with their lidar analogues produced from integrated profiles : however several inconsistencies related to instrumental and environmental factors were also found. The division of starphotometer AODs into τ[subscript ]f and τ [subscript c] components was further exploited to eliminate coarse mode cloud optical depths (spectral cloud screening) and subsequently compare τ [subscript f] with cloud-screened AODs using a traditional (temporal based) approach. While temporal and spectral cloud screening case studies at process level resolutions yielded good to moderate results in terms of the coherence between spectrally and temporally cloud screened data (both temporally screened starphotometer and lidar optical depths), seasonal results apparently still contained cloud contaminated data. Forcing an agreement using a more restrictive, second-pass, clear sky criterion ("minimal cloud envelope") produced mean 2010-11 AOD seasonal values of 0.08 and 0.04 for Eureka and Ny Ålesund respectively. In 2011-12 these values were 0.12 and 0.09. Conversely, CALIOP AODs (0 to 8 km) for the high Arctic showed a slight decrease from 2010-2011 to 2011-2012 (0.04 vs 0.03)

Retrievals of ice-water content from an airborne elastic lidar in tropical convective clouds

Ice water content (IWC) is one of the critical parameters in determining the cloud radiative impact. In this work lidar-based IWC retrievals obtained in tropical mesoscale convective systems are evaluated in the context of an extensive in-situ and remote sensing instrumentation suite. Based on a test case of May 27, 2015 lidar-derived IWC values at 50 m above the aircraft were on average within 25% of the in-situ IWC measurements obtained using an isokinetic probe

Retrievals of ice-water content from an airborne elastic lidar in tropical convective clouds

Ice water content (IWC) is one of the critical parameters in determining the cloud radiative impact. In this work lidar-based IWC retrievals obtained in tropical mesoscale convective systems are evaluated in the context of an extensive in-situ and remote sensing instrumentation suite. Based on a test case of May 27, 2015 lidar-derived IWC values at 50 m above the aircraft were on average within 25% of the in-situ IWC measurements obtained using an isokinetic probe

Synchronized Starphotometry and Lidar measurements in the High Arctic

The sunphotometry-lidar synergy has proven to be effective for the characterization of aerosol events in the High Arctic. Sunphotometry measurements, however, are limited to the day-time periods. Starphotometry, based on the extinction of bright-star radiation, can mitigate the lack of aerosol optical depth (AOD) measurements during the Polar Night. In this work we present several examples of the coincident starphotometry-lidar measurements at Eureka, Canada (79°59'N, 85°56'W) obtained in Feb-Mar 2011. We show a correlation between fine (sub-micron) and coarse (super-micron) mode AOD dynamics from starphotometry and the backscatter profiles and depolarization ratio values from the lidar data