Multi-angle Imaging SpectroRadiometer Research Articles

A study of 15-year aerosol optical thickness and direct shortwave aerosol radiative effect trends using MODIS, MISR, CALIOP and CERES

Abstract. By combining Collection 6 Moderate Resolution and Imaging Spectroradiometer (MODIS) and Version 22 Multi-angle Imaging Spectroradiometer (MISR) aerosol products with Cloud and Earth's Radiant Energy System (CERES) flux products, the aerosol optical thickness (AOT, at 0.55 µm) and shortwave (SW) aerosol radiative effect (SWARE) trends are studied over ocean for the near-full Terra (2000–2015) and Aqua (2002–2015) data records. Despite differences in sampling methods, regional SWARE and AOT trends are highly correlated with one another. Over global oceans, weak SWARE (cloud-free SW flux) and AOT trends of 0.5–0.6 W m−2 (−0.5 to −0.6 W m−2) and 0.002 AOT decade−1 are found using Terra data. Near-zero AOT and SWARE trends are also found for using Aqua data, regardless of the angular distribution models (ADMs) used. Regionally, positive AOT and cloud-free SW flux (negative SWARE) trends are found over the Bay of Bengal, the Arabian Sea, the Arabian/Persian Gulf and the Red Sea, while statistically significant negative trends are derived over the Mediterranean Sea and the eastern US coast. In addition, the global mean instantaneous SW aerosol direct forcing efficiencies are found to be ∼ −60 W m−2 AOT−1, with corresponding SWARE values of ∼ −7 W m−2 from both Aqua and Terra data, again regardless of CERES ADMs used. Regionally, SW aerosol direct forcing efficiency values of ∼ −40 W m−2 AOT−1 are found over the southwest coast of Africa where smoke aerosol particles dominate in summer. Larger (in magnitude) SW aerosol direct forcing efficiency values of −50 to −80 W m−2 AOT−1 are found over several other dust- and pollutant-aerosol-dominated regions. Lastly, the AOT and SWARE trends from this study are also intercompared with aerosol trends (such as active-based ones) from several previous studies. Findings suggest that a cohesive understanding of the changing aerosol skies can be achieved through the analysis of observations from both passive- and active-based analyses, as well as from both narrowband and broadband datasets.

Systemic Study of Aerosol Loading and Retention over Praia-Cape Verde: Satellite and Ground Observation Analysis

The situation in Praia may be critical at the moment because of its current aerosol loading and retention. In this study we obtained 14-year (2000-13) data set from satellite and ground observation from a multi-angle imaging spectro-radiometer (MISR) and an aerosol robotic network (AERONET). The analytic and statistical techniques were used to investigate the aerosol loading and retention over Praia-Cape Verde. The average annual aerosol retention over the study area increased by 18%. The highest aerosol retention rate of 39% was recorded in 2010. The aerosol loading was found to be higher between December to February. Since the ventilation over Praia is low, aerosol loading impact on the health and welfare of life forms may be threatened. The study also suggested other sectors of the economy that can be affected by the increased aerosol loading over Praia.

Relationship between MRPV Model Parameters from MISRL2 Land Surface Product and Land Covers: A Case Study within Mainland Spain

In this study, we showed that the multi-angle satellite remote sensing product, MISR L2 Land Surface (MIL2ASLS), which has a scale of 1.1 km, could be suitable for improving land-cover studies. Using seven images from this product, captured by the multi-angle imaging spectroradiometer sensor (MISR), we explored the values reached by the three parameters (ρ0, Θ, and k) of the Rahman–Pinty–Verstraete model, which was modified by Martonchick (MRPV). Thereafter, we compared the values and behaviors shown in seven Co-ordination of Information on the Environment (CORINE) land cover categories, in the red and near infrared (NIR) bands, over the seven MISR orbits captured in 2006 for Mainland Spain. Furthermore, we used Normalized Difference Vegetation Index (NDVI), Leaf Area Index (LAI), and Fraction of Photosynthetically Active Radiation (FPAR) ancillary data and the illumination angles from the same pixels, which made up the images. These ancillary data were also provided by the MISR products. An inferential statistic test was performed to evaluate the relationship between each parameter–band combination, and the land cover in every MISR orbit used. The results suggested that the ρ0 parameters of this product seemed to be the most related to photosynthetic activity, and it should be comparable with the widely-used NDVI. On the other hand, the k and Θ parameter values were not related, or at least not entirely related, to the phenology of land coverage. These seemed to be more influenced by the anisotropy behavior of the studied land cover pixels. Additionally, we observed, by constructing analysis of variance, how the mean of each MRPV parameter–band differed statistically (p < 0.01) by land covers and orbits. This study suggested that the MISR MRPV model parameter data product has great potential to be used to improve land cover applications.

Spatial‐temporal characteristics of aerosol loading over the Yangtze River Basin during 2001–2015

ABSTRACTThe spatial‐temporal characteristics of aerosol loading over the Yangtze River Basin, China during 2001–2015 were investigated using moderate resolution imaging spectroradiometer (MODIS), multi‐angle imaging spectroradiometer (MISR), and ground‐level particulate matter (PM) data. Aerosol optical depth (AOD) >0.8 occurs in the Yangtze River Delta, central China, and the Sichuan Basin, while AOD <0.3 occurs over higher‐elevation areas in the western Sichuan Plateau and the source regions of the Yangtze River. The western Sichuan Plateau is characterized by fine‐mode natural aerosols, while the source region of the Yangtze River is more influenced by dust aerosols. The Sichuan Basin, central China, and the Yangtze River Delta are dominated by anthropogenic aerosols. The spatial distribution of ground‐level PM2.5 and its ratio to PM10 confirm the high aerosol loading and dominance of small particles over the middle and lower Yangtze Basin. Over the Yangtze River Delta, central China, and the Sichuan Basin, AOD varies seasonally from low in the cold months to high in the warm months, being suppressed by rainfall and wind during the Asian summer monsoon. Precipitation increases aerosol loading in the western Sichuan Plateau but reduces dust particles in the source region of the Yangtze River. There is no significant AOD trend over most areas of the Yangtze River Basin during 2001–2015, while strong decreasing trends are found over most of the middle and lower Yangtze Basin during 2011–2015. These decreasing trends may relate to changes in annual precipitation, wind speed, and air‐pollution control policies. The NO2 and SO2 emissions decreased by 16.51 and 23.40% over major provinces and cities of the Yangtze River Basin from 2011 to 2015. An increase in rainfall over the middle and lower Yangtze Basin and a better pollutant diffusion condition in the Sichuan Basin also favour the decreasing AOD during this period.

Distinguishing remobilized ash from erupted volcanic plumes using space-borne multi-angle imaging.

Volcanic systems are comprised of a complex combination of ongoing eruptive activity and secondary hazards, such as remobilized ash plumes. Similarities in the visual characteristics of remobilized and erupted plumes, as imaged by satellite-based remote sensing, complicate the accurate classification of these events. The stereo imaging capabilities of the Multi-angle Imaging SpectroRadiometer (MISR) were used to determine the altitude and distribution of suspended particles. Remobilized ash shows distinct dispersion, with particles distributed within ~1.5 km of the surface. Particle transport is consistently constrained by local topography, limiting dispersion pathways downwind. The MISR Research Aerosol (RA) retrieval algorithm was used to assess plume particle microphysical properties. Remobilized ash plumes displayed a dominance of large particles with consistent absorption and angularity properties, distinct from emitted plumes. The combination of vertical distribution, topographic control, and particle microphysical properties makes it possible to distinguish remobilized ash flows from eruptive plumes, globally.

IMPACTS OF RECIRCULATION EVENT ON AEROSOL DISPERSION AND RAINFALL PATTERNS IN PARTS OF NIGERIA

The peculiarity of the lower atmosphere of south west region of Nigeria was explored using theoretical and experimental approach. In the theoretical approach, the reconstruction of 1D model as an extraction from the 3D aerosol dispersion model was used to examine the physics of the recirculation theory. The experimental approach which consists of obtaining dataset from ground instruments was used to provide on-site guide for developing the new recirculation theories. The data set was obtained from the Davis weather station, Nigeria Meteorological agency and Multi-angle Imaging Spectro-radiometer (MISR). We looked at the main drivers of recirculation and propounded that recirculation is a complex process which triggers a reordering of the mixing layer- a key factor for initiating the type of rainfall in this region.

Simultaneous Estimation of Leaf Area Index, Fraction of Absorbed Photosynthetically Active Radiation, and Surface Albedo From Multiple-Satellite Data

Leaf area index (LAI), fraction of absorbed photosynthetically active radiation (FAPAR), and surface broadband albedo are three routinely generated land-surface parameters from satellite observations, which have been widely used in land-surface modeling and environmental monitoring. Currently, most global land products are retrieved separately from individual satellite data. Many issues, such as data gaps, spatial and temporal inconsistencies, and insufficient accuracy under certain conditions resulting from the inadequacies of single-sensor observations, have made the incorporation of multiple sensors a reasonable solution. In this paper, an approach to simultaneous estimation of LAI, broadband albedo, and FAPAR from multiple-satellite sensors is further refined. The method, improved from that proposed in an earlier study using Moderate Resolution Imaging Spectroradiometer (MODIS) data, consists of several steps. First, a coupled dynamic and radiative-transfer model based on MODIS, SPOT/VEGETATION, and Multiangle Imaging SpectroRadiometer data was developed to retrieve LAI values and use them to construct a time-evolving dynamic model. Second, an iteration process with predefined exit criteria was developed to obtain consistent gap-filled LAI estimates. Third, a spectral albedo based on the retrieved LAI values was simulated using a radiative-transfer model and then converted to a broadband albedo using empirical methods. Snow-covered pixels identified by normalized difference snow index thresholds were adjusted to the weighted average of the underlying albedo and the maximum snow albedo. Finally, the FAPAR of green vegetation was calculated as a combination of the albedo at the top of the canopy, the soil albedo, and the transmittance of the PAR down to the background. Validation of retrieved LAI, albedo, and FAPAR values obtained from multiple-satellite data over ten study sites has demonstrated that the proposed method can produce more accurate products than presently distributed global products.

The MERRA-2 Aerosol Reanalysis, 1980 Onward. Part II: Evaluation and Case Studies.

The Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), is NASA's latest reanalysis for the satellite era (1980 onward) using the Goddard Earth Observing System, version 5 (GEOS-5), Earth system model. MERRA-2 provides several improvements over its predecessor (MERRA-1), including aerosol assimilation for the entire period. MERRA-2 assimilates bias-corrected aerosol optical depth (AOD) from the Moderate Resolution Imaging Spectroradiometer and the Advanced Very High Resolution Radiometer instruments. Additionally, MERRA-2 assimilates (non bias corrected) AOD from the Multiangle Imaging SpectroRadiometer over bright surfaces and AOD from Aerosol Robotic Network sunphotometer stations. This paper, the second of a pair, summarizes the efforts to assess the quality of the MERRA-2 aerosol products. First, MERRA-2 aerosols are evaluated using independent observations. It is shown that the MERRA-2 absorption aerosol optical depth (AAOD) and ultraviolet aerosol index (AI) compare well with Ozone Monitoring Instrument observations. Next, aerosol vertical structure and surface fine particulate matter (PM2.5) are evaluated using available satellite, aircraft, and ground-based observations. While MERRA-2 generally compares well to these observations, the assimilation cannot correct for all deficiencies in the model (e.g., missing emissions). Such deficiencies can explain many of the biases with observations. Finally, a focus is placed on several major aerosol events to illustrate successes and weaknesses of the AOD assimilation: the Mount Pinatubo eruption, a Saharan dust transport episode, the California Rim Fire, and an extreme pollution event over China. The article concludes with a summary that points to best practices for using the MERRA-2 aerosol reanalysis in future studies.

Assessment and Improvement of MISR Angstrom Exponent and Single-Scattering Albedo Products Using AERONET Data in China

Mapping the components, size, and absorbing/scattering properties of particle pollution is of great interest in the environmental and public health fields. Although the Multi-angle Imaging SpectroRadiometer (MISR) can detect a greater number of aerosol microphysical properties than most other spaceborne sensors, the Angstrom exponent (AE) and single-scattering albedo (SSA) products are not widely utilized or as robust as the aerosol optical depth (AOD) product. This study focused on validating MISR AE and SSA data using AErosol RObotic NETwork (AERONET) data for China from 2004 to 2014. The national mean value of the MISR data (1.08) was 0.095 lower than that of the AERONET data. However, the MISR SSA average (0.99) was significantly higher than that of AERONET (0.89). In this study, we developed a method to improve the AE and SSA by narrowing the selection of MISR mixtures via the introduction of the following group thresholds obtained from an 11-year AERONET dataset: minimum and maximum values (for the method of MISR_Imp_All) and the top 10% and bottom 10% of the averaged values (for MISR_Imp_10%). Overall, our improved AE values were closer to the AERONET AE values, and additional samples (MISR_Imp_All: 28.04% and 64.72%, MISR_Imp_10%: 34.11% and 73.13%) had absolute differences of less than 0.1 and 0.3 (defined by the expected error tests, e.g., EE_0.1) compared with the original MISR product (18.46% and 50.23%). For the SSA product, our method also improved the mean, EE_0.05, and EE_0.1 from 0.99, 16.13%, and 56.45% (MISR original product) to 0.96, 40.32%, and 70.97% (MISR_Imp_All), and 0.94, 54.84%, and 90.32% (MISR_Imp_10%), respectively.

Handling outliers in model inversion studies: a remote sensing case study using MISR-HR data in South Africa

Outliers, which are essentially unexpected data items, affect the interpretation of model results and measurements alike. They are typically screened out through statistical procedures, though this approach does not provide a real justification for discarding values, other than they appear different from the rest of the data. On the other hand, remote sensing products are frequently derived by inverting a model against measurements, a procedure that naturally leads to the estimation of some ‘cost function’, a numerical value that quantitatively expresses the ability of the model to ‘fit’ the data. This paper argues that it may be more meaningful to identify and sift outliers on the basis of this cost function, than solely on the basis of being different from some measure of central tendency. One advantage of this approach is that it will filter out data points with an excessive mismatch between the model and the data, whether or not these appear to be outliers. This approach is demonstrated by analysing specific products derived from NASA’s Multi-angle Imaging SpectroRadiometer (MISR) data, though the method is applicable to any result generated through model inversion against observational data, and is therefore of general interest to a wide range of geographical applications.

Estimation of FAPAR over Croplands Using MISR Data and the Earth Observation Land Data Assimilation System (EO-LDAS)

The Fraction of Absorbed Photosynthetically-Active Radiation (FAPAR) is an important parameter in climate and carbon cycle studies. In this paper, we use the Earth Observation Land Data Assimilation System (EO-LDAS) framework to retrieve FAPAR from observations of directional surface reflectance measurements from the Multi-angle Imaging SpectroRadiometer(MISR) instrument. The procedure works by interpreting the reflectance data via the semi-discrete Radiative Transfer (RT) model, supported by a prior parameter distribution and a dynamic regularisation model and resulting in an inference of land surface parameters, such as effective Leaf Area Index (LAI), leaf chlorophyll concentration and fraction of senescent leaves, with full uncertainty quantification. The method is demonstrated over three agricultural FLUXNET sites, and the EO-LDAS results are compared with eight years of in situ measurements of FAPAR and LAI, resulting in a total of 24 site years. We additionally compare three other widely-used EO FAPAR products, namely the MEdium Resolution Imaging Spectrometer (MERIS) Full Resolution, the MISR High Resolution (HR) Joint Research Centre Two-stream Inversion Package (JRC-TIP) and MODIS MCD15 FAPAR products. The EO-LDAS MISR FAPAR retrievals show a high correlation with the ground measurements ( r 2 > 0.8), as well as the lowest average R M S E (0.14), in line with the MODIS product. As the EO-LDAS solution is effectively interpolated, if only measurements that are coincident with MISR observations are considered, the correlation increases ( r 2 > 0.85); the R M S E is lower by 4–5%; and the bias is 2% and 7%. The EO-LDAS MISR LAI estimates show a strong correlation with ground-based LAI (average r 2 = 0.76), but an underestimate of LAI for optically-thick canopies due to saturation (average R M S E = 2.23). These results suggest that the EO-LDAS approach is successful in retrieving both FAPAR and other land surface parameters. A large part of this success is based on the use of a dynamic regularisation model that counteracts the poor temporal sampling from the MISR instrument.

Microphysical Characteristics of Atmospheric Particulate Matter from NASA’s MODIS, MISR, and AERONET Observations

We present a comparative study of atmospheric particulate matter (also known as aerosols) observed by satellite remote sensing and ground-based observations. We compare satellite measurements obtained by NASA’s Moderate Resolution Imaging Spectro-Radiometer (MODIS) and Multi-angle Imaging Spectro-Radiometer (MISR) instruments against the ground-based aerosol sun-photometer data from the Aerosol Robotic Network (AERONET) station in Cairo, Egypt from 2003 to 2014 to build a long-term database for climatological studies and to improve upon the accuracy and coverage achievable from the satellite data. We deduce microphysical and geometrical properties about the dominant aerosols based on key optical properties including aerosol optical depth (AOD), single scattering albedo (SSA), and Ångström exponent (AE). This has allowed us to place important constraints on the type of aerosols (natural, anthropogenic, and biogenic).

Size-resolved particulate matter concentrations derived from 4.4 km-resolution size-fractionated Multi-angle Imaging SpectroRadiometer (MISR) aerosol optical depth over Southern California

There is significant public health interest in gaining a better understanding of the health effects associated with particulate matter (PM) of differing sizes and compositions. Unfortunately the PM monitoring data needed for such studies are limited. Satellite observations of aerosol optical depth (AOD) have been used to overcome this limitation, but existing techniques using AOD to estimate surface concentrations of PM have limited ability to differentiate particle size and type, and the spatial resolution is often too coarse to evaluate PM variability on local scales. Fortunately there has been a significant breakthrough in aerosol product from the Multi-angle Imaging SpectroRadiometer (MISR) instrument onboard the NASA Terra satellite. Recent computational advances have made it possible to derive a “local mode” MISR aerosol product at a spatial resolution of 4.4km that maintains the quality of the 17.6km resolution Version 22 operational aerosol product. MISR is uniquely capable of providing total AOD as well as AOD fractionated by size (small, medium, and large), enabling estimation of size-resolved surface PM concentrations.Total and size-fractionated AOD from the MISR 4.4km aerosol product were used to develop spatio-temporal models to estimate surface concentrations of PM2.5, PM10, and PM2.5 chemical species over Southern California. We found, and confirmed by leave-one-site-out cross validation, that PM2.5 was best estimated with a spatio-temporal model of AOD small+medium that also included adjustment for relative humidity and wind speed (R2=0.67, CV R2=0.51), while PM10 was best estimated from AOD large with adjustment for dew point and wind speed (R2=0.76, CV R2=0.44). Total AOD was most strongly associated with PM2.5 components SO42− (R2=0.74, CV R2=0.69) and NO3− (R2=0.72, CV R2=0.39). The best fitting models were applied to all available MISR aerosol retrievals over the region, generating surfaces of estimated size-resolved PM concentrations that will be a great asset to the environmental science and public health communities.

Decadal-scale trends in regional aerosol particle properties and their linkage to emission changes

Understanding long-term trends in aerosol loading and properties is essential for evaluating the health and climatic effects of these airborne particulates as well as the effectiveness of pollution control policies. While many studies have used satellite data to examine the trends in aerosol optical depth (AOD), very few have investigated the trends in aerosol properties associated with particle size, morphology, and light absorption. In this study, we investigate decadal-scale (13–15 year) trends in aerosol loading and properties during 2001–2015 over three populous regions: the Eastern United States (EUS), Western Europe (WEU), and Eastern and Central China (ECC). We use observations from MISR (Multi-angle Imaging SpectroRadiometer) and MODIS (Moderate resolution Imaging Spectroradiometer). Relationships between aerosol property trends and air pollutant emission changes are examined. We find that annual mean AOD shows pronounced decreasing trends over EUS and WEU regions, as a result of considerable emission reductions in all major pollutants except for mineral dust and ammonia (NH3). Over the ECC region, AOD increases before 2006 due to emission increases induced by rapid economic development, fluctuates between 2006 and 2011, and subsequently decreases after 2011 in conjunction with effective emission reduction in anthropogenic primary aerosols, sulfur dioxide (SO2), and nitrogen oxides (NOx). The fraction of small-size AOD (<0.7 μm diameter), Ångstrom exponent and single-scattering albedo have generally decreased, while the fractions of large-size (>1.4 μm diameter), nonspherical and absorbing AOD have generally shown increasing trends over EUS and WEU regions, indicating that fine and light-scattering aerosol constituents have been more effectively reduced than coarse and light-absorbing constituents. These trends are consistent with the larger reduction ratios in SO2 and NOx emissions than in primary aerosols, including mineral dust and black carbon (BC). Over the ECC region, no significant trends are observed with respect to size distribution, morphology, or light absorption, which we attribute to a simultaneous increase in emissions of SO2, NOx, and primary aerosols including BC before 2006, and a simultaneous decrease after 2011. This study demonstrates the importance and usefulness of satellite-borne sensors, particularly MISR, in association with evaluating the effectiveness of air pollution control policies.

Direct Estimation of Land Surface Albedo From Simultaneous MISR Data

The availability of multiangular information from the NASA Multi-angle Imaging SpectroRadiometer (MISR) instrument provides an excellent opportunity for the characterization of surface anisotropy, which can be used for improving surface albedo estimation. However, the MISR data have been reported with large uncertainties and data gaps due to inaccurate aerosol estimation and/or cloud masking limiting its otherwise broader applications. To mitigate these issues, two approaches were proposed to estimate land surface albedo directly from surface reflectance (LSA_sfc) and Top-of-Atmosphere reflectance (LSA_toa), respectively. As a further development of the traditional albedo algorithms, this is the first attempt to simultaneously utilize multispectral and multiangular information in surface albedo estimation without any prior constraining information. Validations at AmeriFlux sites show that the proposed algorithms can achieve accuracies similar to that of the MISR product with respective bias and RMSE of 0.004 and 0.032 for LSA_sfc and 0.005 and 0.032 for LSA_toa algorithms. We found that the LSA_toa algorithm can significantly reduce data gaps and provide accurate surface albedo retrievals with two to three times more valid data than the current MISR product. In addition, these approaches can be easily applied to other optical sensors to produce accurate and gap-free clear-sky surface albedo estimations. The results of this paper also highlight the importance of having two to three simultaneous observations with sufficient angular sampling, which can improve albedo accuracy and reduce data gaps.

Altering rainfall patterns through aerosol dispersion

The possibility of recirculation mechanism on rainfall patterns is salient for sustenance of the human race through agricultural produce. The peculiarity of the lower atmosphere of south west region of Nigeria was explored using theoretical and experimental approach. In the theoretical approach, the reconstruction of 1D model as an extraction from the 3D aerosol dispersion model was used to examine the physics of the recirculation theory. The experimental approach which consists of obtaining dataset from ground instruments was used to provide on-site guide for developing the new recirculation theories. The data set was obtained from the Davis weather station, Nigeria Meteorological agency and Multi-angle Imaging Spectro-radiometer (MISR). We looked at the main drivers of recirculation and propounded that recirculation is a complex process which triggers a reordering of the mixing layer- a key factor for initiating the type of rainfall in this region.

Updated MISR dark water research aerosol retrieval algorithm – Part 1: Coupled 1.1 km ocean surface chlorophyll &amp;lt;i&amp;gt;a&amp;lt;/i&amp;gt; retrievals with empirical calibration corrections

Abstract. As aerosol amount and type are key factors in the atmospheric correction required for remote-sensing chlorophyll a concentration (Chl) retrievals, the Multi-angle Imaging SpectroRadiometer (MISR) can contribute to ocean color analysis despite a lack of spectral channels optimized for this application. Conversely, an improved ocean surface constraint should also improve MISR aerosol-type products, especially spectral single-scattering albedo (SSA) retrievals. We introduce a coupled, self-consistent retrieval of Chl together with aerosol over dark water. There are time-varying MISR radiometric calibration errors that significantly affect key spectral reflectance ratios used in the retrievals. Therefore, we also develop and apply new calibration corrections to the MISR top-of-atmosphere (TOA) reflectance data, based on comparisons with coincident MODIS (Moderate Resolution Imaging Spectroradiometer) observations and trend analysis of the MISR TOA bidirectional reflectance factors (BRFs) over three pseudo-invariant desert sites. We run the MISR research retrieval algorithm (RA) with the corrected MISR reflectances to generate MISR-retrieved Chl and compare the MISR Chl values to a set of 49 coincident SeaBASS (SeaWiFS Bio-optical Archive and Storage System) in situ observations. Where Chlin situ &lt; 1.5 mg m−3, the results from our Chl model are expected to be of highest quality, due to algorithmic assumption validity. Comparing MISR RA Chl to the 49 coincident SeaBASS observations, we report a correlation coefficient (r) of 0.86, a root-mean-square error (RMSE) of 0.25, and a median absolute error (MAE) of 0.10. Statistically, a two-sample Kolmogorov–Smirnov test indicates that it is not possible to distinguish between MISR Chl and available SeaBASS in situ Chl values (p &gt; 0.1). We also compare MODIS–Terra and MISR RA Chl statistically, over much broader regions. With about 1.5 million MISR–MODIS collocations having MODIS Chl &lt; 1.5 mg m−3, MISR and MODIS show very good agreement: r = 0. 96, MAE = 0.09, and RMSE = 0.15. The new dark water aerosol/Chl RA can retrieve Chl in low-Chl, case I waters, independent of other imagers such as MODIS, via a largely physical algorithm, compared to the commonly applied statistical ones. At a minimum, MISR's multi-angle data should help reduce uncertainties in the MODIS–Terra ocean color retrieval where coincident measurements are made, while also allowing for a more robust retrieval of particle properties such as spectral single-scattering albedo.

Development and assessment of a higher-spatial-resolution (4.4 km) MISR aerosol optical depth product using AERONET-DRAGON data

Abstract. Since early 2000, the Multi-angle Imaging SpectroRadiometer (MISR) instrument on NASA's Terra satellite has been acquiring data that have been used to produce aerosol optical depth (AOD) and particle property retrievals at 17.6 km spatial resolution. Capitalizing on the capabilities provided by multi-angle viewing, the current operational (Version 22) MISR algorithm performs well, with about 75 % of MISR AOD retrievals globally falling within 0.05 or 20 % × AOD of paired validation data from the ground-based Aerosol Robotic Network (AERONET). This paper describes the development and assessment of a prototype version of a higher-spatial-resolution 4.4 km MISR aerosol optical depth product compared against multiple AERONET Distributed Regional Aerosol Gridded Observations Network (DRAGON) deployments around the globe. In comparisons with AERONET-DRAGON AODs, the 4.4 km resolution retrievals show improved correlation (r = 0. 9595), smaller RMSE (0.0768), reduced bias (−0.0208), and a larger fraction within the expected error envelope (80.92 %) relative to the Version 22 MISR retrievals.

Cloud heights measured by MISR from 2000 to 2015

AbstractDavies and Molloy (2012) reported a decrease in the global effective cloud height over the first 10 years of Multiangle Imaging Spectroradiometer (MISR) measurements on the Terra satellite. We have reexamined their time series for possible artefacts that might especially affect the initial portion of the record when the heights appeared anomalously high. While variations in sampling were shown to be inconsequential, an artefact due to the change in equator crossing time that affected the first 2 years was discovered, and this has now been corrected. That correction, together with the extension of the time series by five more years, yields no significant overall trend in global heights during the first 15 years of Terra operation. The time series is dominated by large interannual fluctuations associated with La Niña events that mask any overall trend on a global scale. On a regional basis, the cloud heights showed significant interannual variations of much larger amplitude, sometimes with fairly direct cancellation between regions. There were unexplained differences between the two hemispheres in the timing of height anomalies. These differences persisted over a large range of extratropical latitudes, suggestive of teleconnections. Within the tropics, there were very strong changes associated with the Central Pacific and Indonesian Maritime Continent regions that oscillated out of phase with each other, with interannual amplitudes that exceeded 1 km.

Satellite retrievals of dust aerosol over the Red Sea and the Persian Gulf (2005–2015)

Abstract. The inter-annual variability of the dust aerosol presence over the Red Sea and the Persian Gulf is analysed over the period 2005–2015. Particular attention is paid to the variation in loading across the Red Sea, which has previously been shown to have a strong, seasonally dependent latitudinal gradient. Over the 11 years considered, the July mean 630 nm aerosol optical depth (AOD) derived from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) varies between 0.48 and 1.45 in the southern half of the Red Sea. In the north, the equivalent variation is between 0.22 and 0.66. The temporal and spatial pattern of variability captured by SEVIRI is also seen in AOD retrievals from the MODerate Imaging Spectroradiometer (MODIS), but there is a systematic offset between the two records. Comparisons of both sets of retrievals with ship- and land-based AERONET measurements show a high degree of correlation with biases of &lt; 0.08. However, these comparisons typically only sample relatively low aerosol loadings. When both records are stratified by AOD retrievals from the Multi-angle Imaging SpectroRadiometer (MISR), opposing behaviour is revealed at high MISR AODs ( &gt; 1), with offsets of +0.19 for MODIS and −0.06 for SEVIRI. Similar behaviour is also seen over the Persian Gulf. Analysis of the scattering angles at which retrievals from the SEVIRI and MODIS measurements are typically performed in these regions suggests that assumptions concerning particle sphericity may be responsible for the differences seen.