Characterizing the vertical profile of aerosol particle extinction and linear depolarization over Southeast Asia and the Maritime Continent: The 2007–2009 view from CALIOP

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Abstract

Vertical profiles of 0.532 μm aerosol particle extinction coefficient and linear volume depolarization ratio are described for Southeast Asia and the Maritime Continent. Quality-screened and cloud-cleared Version 3.01 Level 2 NASA Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) 5-km Aerosol Profile datasets are analyzed from 2007 to 2009. Numerical simulations from the U.S. Naval Aerosol Analysis and Predictive System (NAAPS), featuring two-dimensional variational assimilation of NASA Moderate Resolution Imaging Spectroradiometer and Multi-angle Imaging SpectroRadiometer quality-assured datasets, combined with regional ground-based lidar measurements, are considered for assessing CALIOP retrieval performance, identifying bias, and evaluating regional representativeness. CALIOP retrievals of aerosol particle extinction coefficient and aerosol optical depth (AOD) are high over land and low over open waters relative to NAAPS (0.412/0.312 over land for all data points inclusive, 0.310/0.235 when the per bin average is used and each is treated as single data points; 0.102/0.151 and 0.086/0.124, respectively, over ocean). Regional means, however, are very similar (0.180/0.193 for all data points and 0.155/0.159 when averaged per normalized bin), as the two factors offset one another. The land/ocean offset is investigated, and discrepancies attributed to interpretation of particle composition and a-priori assignment of the extinction-to-backscatter ratio ("lidar ratio") necessary for retrieving the extinction coefficient from CALIOP signals. Over land, NAAPS indicates more dust present than CALIOP algorithms are identifying, indicating a likely assignment of a higher lidar ratio representative of more absorptive particles. NAAPS resolves more smoke over water than identified with CALIOP, indicating likely usage of a lidar ratio characteristic of less absorptive particles to be applied that biases low AOD there. Over open waters except within the Bay of Bengal, aerosol particle scattering is largely capped below 1.5 km MSL, though ground-based lidar measurements at Singapore differ slightly from this finding. Significant aerosol particle presence over land is similarly capped near 3.0 km MSL over most regions. Particle presence at low levels regionally, except over India, is dominated by relatively non-depolarizing particles. Industrial haze, sea salt droplets and fresh smoke are thus most likely present.

Highlights

► Significant aerosol particle extinction over water in Southeast Asia capped below 1.5 km. ► Significant aerosol particle extinction over land in Southeast Asia land below 3.0 km. ► Elevated particle outflow from land over open water appears suppressed. ► CALIOP AOD high over land and low over water compared with NAAPS. ► Aerosol particles in Southeast Asian boundary layer predominantly spherical.

Introduction

With the proliferation of satellite-based atmospheric remote sensors used for applied research (e.g., King et al., 1999), embodied notably through the suite of instruments supported by the National Aeronautics and Space Administration Earth Observing System (NASA EOS) program (Chuvieco and Justice, 2008), understanding of the role that aerosol particles play in the planetary radiation budget has improved (IPCC, 2007). Of all significant anthropogenic atmospheric components contributing to global radiative equilibrium, however, uncertainties regarding aerosol particle presence and microphysical variability remain highest (Schwartz and Andreae, 1996, Rotstayn and Penner, 2001, Anderson et al., 2003a, Intergovernmental Panel on Climate Change, 2007). Inadequate characterization of aerosol optical properties has a far-reaching consequence. Numerical models incapable of resolving significant aerosol particle presence and/or their microphysical characteristics may accurately constrain surface and top-of-the-atmosphere radiative fluxes, particularly as such measurements are increasingly available and model assimilation schemes are maturing (Loeb et al., 2003, Loeb et al., 2009). They do so, however, at the risk of inaccurately deriving column heating/cooling rates, and thus thermodynamic structure, which can significantly degrade skill for depicting regional circulation and oscillatory regimes governing climate (Lau et al., 2008, Tian et al., 2008, Luo et al., 2009, Kuhlmann and Quass, 2010).
Whereas many passive-based satellite instruments measure scattered visible and near-infrared atmospheric radiances, which are devolved into column-integrated aerosol particle optical and microphysical properties like aerosol optical depth (AOD) and fine/course mode partitioning fractions (e.g., Tanré et al., 1997, Kaufman et al., 1997a, Martonchik et al., 1997), these instruments are limited at best from collecting vertically-resolved information. In response, NASA in collaboration with the French Centre National d'Études Spatiales (CNES), developed the three-channel Cloud Aerosol Lidar with Orthogonal Polarization instrument (0.532 μm with linear polarization diversity, and 1.064 μm; CALIOP) flown aboard the EOS Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite (Winker et al., 2009). Lidar instruments are uniquely tuned to the detection of aerosol particle and optically-thin hydrometeor cloud scattering, and collect range-resolved information with relatively high vertical resolution (~ 1–100 m; Measures, 1984). Sensitivity to the linear polarization plane of backscatter signal provides ancillary information for helping characterize aerosol particle type, composition and likely surface source (Sassen, 2000, Liu et al., 2009). With global coverage, diurnal profiling capabilities, and set within the EOS "A-Train" constellation of complimentary passive and active remote sensors (Stephens et al., 2002), thus contributing to an unmatched synergy for joint cloud and aerosol observation, CALIOP profiling has revolutionized global aerosol particle research (Winker et al., 2006, Winker et al., 2010).
Perhaps in no other climate regime is the necessity for comprehensive satellite observation of aerosol particles more critical than that of Southeast Asia (SA) and the nearby islands and waters of the Maritime Continent (MC). Reid et al. (2012) and Reid et al. (2013--this issue), considered in context with Reid et al. (2012), establish the present state of understanding for annual and inter-seasonal SA/MC aerosol particle two-dimensional (x, y; 2D) distributions, corresponding surface source activity, including the influence of regional biomass burning and its covariance with synoptic climate, and the efficacy of satellite-based algorithms designed for retrieving aerosol particle optical properties. They describe the SA/MC aerosol system as a unique confluence of urban and industrial anthropogenic sources, domestic, agricultural and natural burning of regional grasslands, deltas, savannahs and forests, with subsequent chemical processing over the salt-rich marine waters. The region is further subject to transport of aeolian dusts from the north-central sub-continent and even Sahara (e.g., Lee et al., 2006). In spite of a reasonably stable tropical climate, SA/MC aerosol particle composition and microphysical variability are highly complex.
Reid et al. (2012) and Reid et al. (2013--this issue) further explain why SA/MC aerosol particle physical properties are critically under-constrained for parameterization in radiative/climate models. From an observability standpoint, isolating the aerosol particle signal in SA/MC satellite datasets is daunting. Marine layer cumulus and stratocumulus clouds, widespread convective activity with cumulonimbus cloud towers and ice-crystal anvil outflow (e.g., Kang et al., 1999), and the close proximity of the tropical western Pacific warm pool and subsequent tropical tropopause transition layer, where optically-thin cirrus clouds propagate with increasing persistence toward the regional southeast (McFarquhar et al., 2000, Gettelman and Forster, 2002, Riihimaki and McFarlane, 2010, Virts and Wallace, 2010), all combine to create a viewing scene from space that is regularly cloud-contaminated (Hsu et al., 2003). With limited surface infrastructure, and thus a limited distribution of ground-based observing sites, the challenges of characterizing the SA/MC aerosol system are considerable.
Overcoming system observability challenges through coordinated ground/satellite remote sensing and the compilation of regionally representative datasets for climate study are the founding basis for the Seven Southeast Asian Studies (7SEAS) program (http://7-seas.gsfc.nasa.gov; http://www.nrlmry.navy.mil/flambe/7seas/7seas.html). These efforts are compulsory if the community is to resolve outstanding regional issues related to direct, semi-direct and indirect aerosol particle forcing. As such, this paper describes the critical third dimension for SA/MC aerosol particle presence, composition, and optical scattering efficiency. Vertical profiles of aerosol particle extinction and linear depolarization are described for a three-year cloud-cleared and quality-assured CALIOP data subset (2007–2009). Annual and seasonal mean AOD and particle composition are evaluated for representativeness versus a global forecast model equipped with 2D variational (2DVAR; x, y) assimilation of quality-screened AOD datasets collected by NASA Moderate Resolution Imaging Spectroradiometer instrument (MODIS; King et al., 1992) from both the Terra and Aqua satellites, and Multi-angle Imaging SpectroRadiometer instrument (MISR; Diner et al., 1998). Annual and seasonal mean aerosol particle extinction coefficients and linear volume depolarization ratio (LVDR), solved on a 1° ×ばつ 1° regional grid, are reported at five levels above mean sea level (MSL) and investigated for magnitude and variability, and interpreted for predominant composition and likely corresponding surface source. The analysis is refined for detailed quantitative comparison between CALIOP and the model within seven SA/MC sub-sectors. CALIOP-derived extinction coefficient profiles are compared with ground-based lidar observations in two sub-sectors. The fractional contribution to total extinction for each of six discrete CALIOP aerosol models used in constraining algorithm retrievals are investigated for regional representativeness. The result is a comprehensive three-year study of the physical attributes of SA/MC aerosol particles, as actively profiled from space.

Section snippets

CALIOP Level 2 aerosol particle profiles and quality-assurance screening

Version 3.01 CALIOP Level 2 aerosol profile products (L2-AProf) include 0.532 μm extinction coefficients and LVDR profiles derived in 5-km along-track segments at 60 m vertical resolution and separated into contiguous daytime and nighttime granule files. Retrievals for extinction coefficient from CALIOP signal profiles are indirect. That is, since CALIOP is an elastic-backscatter lidar instrument, the equation for lidar scattering contains two distinct unknown terms: particulate extinction and

Aerosol particle composition: comparing CALIOP retrievals versus NAAPS speciation

Aerosol particle composition identified by the CALIOP algorithms is designated in the L2-AProf data file by the Atmospheric_Volume_Description variable for every 60 m profile bin. The six CALIOP aerosol particle types identified above are distinguished based on spectral scattering properties identified from both the visible and infrared instrument channels and linear depolarizing efficiencies at 0.532 μm (National Aeronautics and Space Administration, 2010, Omar et al., 2009). For 2007–2009, mean

Vertical profiles of CALIOP 0.532 μm extinction coefficient compared with ground-based lidar measurements

Returning to the seven designated SA/MC sub-sectors, ground-based lidar measurements of aerosol particle extinction coefficient are considered from February 2008 to January 2009 at Phimai, Thailand (0.532 μm; 15.2° N, 102.6° E, 212 m MSL) and November 2009–December 2010 at Singapore (0.527 μm; 1.30° N, 103.77° E, 79 m MSL; Fig. 4). Mean annual and seasonal CALIOP 0.532 μm extinction coefficient profiles are thus shown for the T and SN sub-sectors, respectively in Fig. 15, Fig. 16, versus the

Conclusions and impact

In this paper, a comprehensive assessment of the 0.532 μm vertical profile for aerosol particle extinction coefficient and linear depolarization, as well as aerosol composition and regional sources, are described from 2007 to 2009 over Southeast Asia and the surrounding waters and islands of the Maritime Continent (SA/MC). A quality-screened and cloud-cleared subset of Version 3.01 Level 2 NASA Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) 5-km Aerosol Profile datasets is studied.

Acknowledgments

This research was funded by the Office of Naval Research Code 35. Author JRC acknowledges the support of NASA Interagency Agreement NNG11HG12I on behalf of the Micropulse Lidar Network (MPLNET). MPLNET is operated with the support of the NASA Radiation Sciences Program. The authors thank Mark A. Vaughan at NASA Langley Research Center with his help interpreting the CALIOP Level 2.0 Aerosol Profile datasets. The group acknowledges the NASA AERONET program, their contributing principal
Dr. James R. Campbell is a Meteorologist assigned to the Aerosol and Radiation Section at the Naval Research Laboratory in Monterey, CA. He received a Ph.D. in Atmospheric Sciences from the University of Alaska Fairbanks in 2006. He received B.S. and M.S. degrees in Meteorology from the University of Utah in 1993 and 1997, respectively. Dr. Campbell's scientific experience and research interests are in atmospheric remote sensing, aerosol and cloud physics and polar meteorology. The primary

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    Dr. James R. Campbell is a Meteorologist assigned to the Aerosol and Radiation Section at the Naval Research Laboratory in Monterey, CA. He received a Ph.D. in Atmospheric Sciences from the University of Alaska Fairbanks in 2006. He received B.S. and M.S. degrees in Meteorology from the University of Utah in 1993 and 1997, respectively. Dr. Campbell's scientific experience and research interests are in atmospheric remote sensing, aerosol and cloud physics and polar meteorology. The primary focus of his work is the characterization of aerosol and cloud physical properties using lidar (light detection and ranging) instruments.
    Mr. Jason L. Tackett is an analyst for the CALIPSO Lidar Science Working Group at NASA Langley Research Center in Hampton, VA through Science Systems and Applications, Inc. He received an M.S. in Atmospheric Sciences from the University of Illinois at Urbana-Champaign in 2009 and a B.S in Physics from Kansas State University in 2007. His research interests include aerosol–cloud interactions, Saharan dust transport and aerosol optical property retrieval methods using lidar.
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