David L. Nelson

Satellite-derived aerosol optical depth over dark water from MISR and MODIS: Comparisons with AERONET and implications for climatological studies

[1]xa0Although the current Multiangle Imaging Spectroradiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite passive remote sensing midvisible aerosol optical thickness (AOT) products are accurate overall to about 0.05 or 20%, they differ systematically on a global, monthly average basis, by about 0.03 to 0.05. Some key climate change and other applications require accuracies of 0.03 or better. The instruments are sufficiently stable and well characterized, and have adequate signal-to-noise, to realize such precision. However, assumptions made in the current standard aerosol retrieval algorithms produce AOT biases that must be addressed first. We identify the causes of AOT discrepancies over dark water under typical, relatively low AOT conditions and quantify their magnitudes on the basis of detailed analysis. Examples were selected to highlight key issues for which there are coincident MISR, MODIS, and Aerosol Robotic Network (AERONET) observations. Instrument calibration and sampling differences, assumptions made in the MISR and MODIS standard algorithms about ocean surface boundary conditions, missing particle property or mixture options, and the way reflectances used in the retrievals are selected each contribute significantly to the observed differences under some circumstances. Cloud screening is also identified as a factor, though not fully examined here, as are the relatively rare high-AOT cases over ocean. Specific algorithm upgrades and further studies indicated by these findings are discussed, along with recommendations for effectively using the currently available products for regional and global applications.

Dynamics of fire plumes and smoke clouds associated with peat and deforestation fires in Indonesia

[1] During the dry season, anthropogenic fires in tropical forests and peatlands of equatorial Asia produce regionally expansive smoke clouds that have important effects on atmospheric radiation and air quality. Here we estimated the height of smoke on Borneo and Sumatra and characterized its sensitivity to El Nino and regional drought. We used Multiangle Imaging Spectroradiometer (MISR) satellite data and the MISR Interactive Explorer (MINX) software to estimate the heights of 317 smoke plumes on Borneo and 139 plumes on Sumatra during 2001–2009. In addition, we estimated the altitudes of larger smoke regions (smoke clouds) over Borneo using data from MISR and Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) products. Most smoke plumes on Borneo (83%) were observed during El Nino years. Annually averaged plume heights on Borneo were significantly higher during El Nino events. Mean MISR‐derived plume heights were 709 ± 14 m on Borneo and 749 ± 24 m on Sumatra during 2001–2009, with 96% of all plumes confined to within 500 m of the atmospheric boundary layer. Smoke clouds on Borneo were observed at altitudes between 1000 and 2000 m as measured by both MISR and CALIPSO. The difference in height between individual plumes and longer‐lived regional smoke clouds may be related to deeper planetary boundary layers and higher‐intensity fires later in the afternoon or other atmospheric mixing processes that occur on synoptic time scales. Our measurements and analyses suggested that direct injection of smoke into the free troposphere within fire plumes was not an important mechanism for vertical mixing of aerosols in equatorial Asia.

Stereoscopic Height and Wind Retrievals for Aerosol Plumes with the MISR INteractive eXplorer (MINX)

The Multi-angle Imaging SpectroRadiometer (MISR) instrument aboard the Terra satellite acquires imagery at 275-m resolution at nine angles ranging from 0° (nadir) to 70° off-nadir. This multi-angle capability facilitates the stereoscopic retrieval of heights and motion vectors for clouds and aerosol plumes. MISRs operational stereo product uses this capability to retrieve cloud heights and winds for every satellite orbit, yielding global coverage every nine days. The MISR INteractive eXplorer (MINX) visualization and analysis tool complements the operational stereo product by providing users the ability to retrieve heights and winds locally for detailed studies of smoke, dust and volcanic ash plumes, as well as clouds, at higher spatial resolution and with greater precision than is possible with the operational product or with other space-based, passive, remote sensing instruments. This ability to investigate plume geometry and dynamics is becoming increasingly important as climate and air quality studies require greater knowledge about the injection of aerosols and the location of clouds within the atmosphere. MINX incorporates features that allow users to customize their stereo retrievals for optimum results under varying aerosol and underlying surface conditions. This paper discusses the stereo retrieval algorithms and retrieval options in MINX, and provides appropriate examples to explain how the program can be used to achieve the best results.

Sensitivity of multiangle imaging to the optical and microphysical properties of biomass burning aerosols

[1] The treatment of biomass burning (BB) carbonaceous particles in the Multiangle Imaging SpectroRadiometer (MISR) Standard Aerosol Retrieval Algorithm is assessed, and algorithm refinements are suggested, based on a theoretical sensitivity analysis and comparisons with near-coincident AERONET measurements at representative BB sites. Over the natural ranges of BB aerosol microphysical and optical properties observed in past field campaigns, patterns of retrieved Aerosol Optical Depth (AOD), particle size, and single scattering albedo (SSA) are evaluated. On the basis of the theoretical analysis, assuming total column AOD of 0.2, over a dark, uniform surface, MISR can distinguish two to three groups in each of size and SSA, except when the assumed atmospheric particles are significantly absorbing (mid-visible SSA � 0.84), or of medium sizes (mean radius � 0.13 mm); sensitivity to absorbing, medium-large size particles increases considerably when the assumed column AOD is raised to 0.5. MISR Research Aerosol Retrievals confirm the theoretical results, based on coincident AERONET inversions under BB-dominated conditions. When BB is externally mixed with dust in the atmosphere, dust optical model and surface reflection uncertainties, along with spatial variability, contribute to differences between the Research Retrievals and AERONET. These results suggest specific refinements to the MISR Standard Aerosol Algorithm complement of component particles and mixtures. They also highlight the importance for satellite aerosol retrievals of surface reflectance characterization, with accuracies that can be difficult to achieve with coupled surface-aerosol algorithms in some higher AOD situations.