Descriptions are provided of the aerosol classification algorithms and the extinction-to-backscatter ratio (lidar ratio) selection schemes for the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) aerosol products. One year of CALIPSO level 2 version 2 data are analyzed to assess the veracity of the CALIPSO aerosol-type identification algorithm and generate vertically resolved distributions of aerosol types and their respective optical characteristics. To assess the robustness of the algorithm, the interannual variability is analyzed by using a fixed season (June–August) and aerosol type (polluted dust) over two consecutive years (2006 and 2007). The CALIPSO models define six aerosol types: clean continental, clean marine, dust, polluted continental, polluted dust, and smoke, with 532-nm (1064 nm) extinction-to-backscatter ratios Sa of 35 (30), 20 (45), 40 (55), 70 (30), 65 (30), and 70 (40) sr, respectively. This paper presents the global distributions of the CALIPSO aerosol types, the complementary distributions of integrated attenuated backscatter, and the volume depolarization ratio for each type. The aerosol-type distributions are further partitioned according to surface type (land/ocean) and detection resolution (5, 20, and 80 km) for optical and spatial context, because the optically thick layers are found most often at the smallest spatial resolution. Except for clean marine and polluted continental, all the aerosol types are found preferentially at the 80-km resolution. Nearly 80% of the smoke cases and 60% of the polluted dust cases are found over water, whereas dust and polluted continental cases are found over both land and water at comparable frequencies. Because the CALIPSO observables do not sufficiently constrain the determination of the aerosol, the surface type is used to augment the selection criteria. Distributions of the total attenuated color ratios show that the use of surface type in the typing algorithm does not result in abrupt and artificial changes in aerosol type or extinction.

The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite was launched in April 2006 to provide global vertically resolved measurements of clouds and aerosols. Correct discrimination between clouds and aerosols observed by the lidar aboard the CALIPSO satellite is critical for accurate retrievals of cloud and aerosol optical properties and the correct interpretation of measurements. This paper reviews the theoretical basis of the CALIPSO lidar cloud and aerosol discrimination (CAD) algorithm, and describes the enhancements made to the version 2 algorithm that is used in the current data release (release 2). The paper also presents a preliminary assessment of the CAD performance based on one full day (12 August 2006) of expert manual classification and on one full month (July 2006) of the CALIOP 5-km cloud and aerosol layer products. Overall, the CAD algorithm works well in most cases. The 1-day manual verification suggests that the success rate is in the neighborhood of 90% or better. Nevertheless, several specific layer types are still misclassified with some frequency. Among these, the most prevalent are dense dust and smoke close to the source regions. The analysis of the July 2006 data showed that the misclassification of dust as cloud occurs for ,1% of the total tropospheric cloud and aerosol features found. Smoke layers are misclassified less frequently than are dust layers. Optically thin clouds in the polar regions can be misclassified as aerosols. While the fraction of such misclassifications is small compared with the number of aerosol features found globally, caution should be taken when studies are performed on the aerosol in the polar regions. Modifications will be made to the CAD algorithm in future data releases, and the misclassifications encountered in the current data release are expected to be reduced greatly.

We would like to acknowledge the support and successful cooperation of NASA and CNES in the development and operation of CALIPSO and the advocacy of Gérard Mégie for the mission. We thank Bill Hunt and the team at Ball Aerospace for CALIOP and payload integration; the teams at SODERN and Thales Alenia Space for the IIR and platform integration, respectively; the operations teams at NASA and CNES; and the support of the ASDC and ICARE data centers, who all made essential contributions to the success of the CALIPSO mission. The work described in "The occurrence of marine stratus and stratocumulus" was carried out by T. Kubar in collaboration with D. E.

[1] An extensive dust storm originating on 17 August 2006 in North Africa was observed and tracked by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar. Over the next several days, the dust layer moved westward across the Atlantic Ocean and into the Gulf of Mexico. The initial stages of the event were examined using a sequence of CALIPSO measurements. The first of these was acquired very near the source on 18 August. Successive measurements were made over the Atlantic Ocean on 19 and 20 August, at respective locations approximately $1300 km and $2400 km from the source region. The later stages of the event were assessed using measurements acquired by the NASA Langley Research Center airborne HSRL over the Gulf of Mexico on 28 August. Within the free troposphere, the intrinsic optical properties of the dust remain relatively unchanged for the first 3 d of transport over the Atlantic Ocean. This is consistent with previous in situ measurements that have shown that there is little change in the size distribution of dust as it crosses the Atlantic. After the 10 d journey to the Gulf of Mexico, some changes are seen in the lidar ratios, the backscatter color ratio, and the optical depth ratio. The linear depolarization ratio appears to remain essentially constant ($0.32) at all four locations mentioned above, demonstrating a notable consistency in the dust particle nonsphericity. The measured 532 nm lidar ratios are 41 ± 3, 41 ± 4, 41 ± 6 and 45.8 ± 0.8 sr, respectively, at locations near the source, over the Atlantic Ocean, and in the Gulf of Mexico. The corresponding 1064 nm lidar ratios are 52 ± 5, 55 ± 5, 54 ± 13 and 44 ± 8.3 sr. The 532 nm lidar ratios are consistent with previous measurements and with CALIPSO's prelaunch models. The lidar ratios retrieved at 1064 nm are somewhat larger than would be expected on the basis of existing modeling studies. The backscatter color ratios are 0.74 ± 0.07, 0.75 ± 0.08, 0.72 ± 0.04 and 0.62 ± 0.01, and the optical depth ratios are 0.97 ± 0.02, 1.01 ± 0.05, 0.93 ± 0.17 and 0.62 ± 0.13, respectively.Citation: Liu, Z., et al. (2008), CALIPSO lidar observations of the optical properties of Saharan dust: A case study of long-range transport,

[1] This study examines seasonal variations of the vertical distribution of aerosols through a statistical analysis of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar observations from June 2006 to November 2007. A data-screening scheme is developed to attain good quality data in cloud-free conditions, and the polarization measurement is used to separate dust from non-dust aerosol. The CALIPSO aerosol observations are compared with aerosol simulations from the Goddard Chemistry Aerosol Radiation Transport (GOCART) model and aerosol optical depth (AOD) measurements from the MODerate resolution Imaging Spectroradiometer (MODIS). The CALIPSO observations of geographical patterns and seasonal variations of AOD are generally consistent with GOCART simulations and MODIS retrievals especially near source regions, while the magnitude of AOD shows large discrepancies in most regions. Both the CALIPSO observation and GOCART model show that the aerosol extinction scale heights in major dust and smoke source regions are generally higher than that in industrial pollution source regions. The CALIPSO aerosol lidar ratio also generally agrees with GOCART model within 30% on regional scales. Major differences between satellite observations and GOCART model are identified, including (1) an underestimate of aerosol extinction by GOCART over the Indian sub-continent, (2) much larger aerosol extinction calculated by GOCART than observed by CALIPSO in dust source regions, (3) much weaker in magnitude and more concentrated aerosol in the lower atmosphere in CALIPSO observation than GOCART model over transported areas in midlatitudes, and (4) consistently lower aerosol scale height by CALIPSO observation than GOCART model. Possible factors contributing to these differences are discussed.