Roger Davies

Multi-angle Imaging SpectroRadiometer (MISR) instrument description and experiment overview

The Multi-angle Imaging SpectroRadiometer (MISR) instrument is scheduled for launch aboard the first of the Earth Observing System (EOS) spacecraft, EOS-AM1. MISR will provide global, radiometrically calibrated, georectified, and spatially coregistered imagery at nine discrete viewing angles and four visible/near-infrared spectral bands. Algorithms specifically developed to capitalize on this measurement strategy will be used to retrieve geophysical products for studies of clouds, aerosols, and surface radiation. This paper provides an overview of the as-built instrument characteristics and the application of MISR to remote sensing of the Earth.

New Directions in Earth Observing: Scientific Applications of Multiangle Remote Sensing

The physical interpretation of simultaneous multiangle observations represents a relatively new approach to remote sensing of terrestrial geophysical and biophysical parameters. Multiangle measurements enable retrieval of physical scene characteristics, such as aerosol type, cloud morphology and height, and land cover (e.g., vegetation canopy type), providing improved albedo accuracies as well as compositional, morphological, and structural information that facilitates addressing many key climate, environmental, and ecological issues. While multiangle data from wide field-of-view scanners have traditionally been used to build up directional “signatures” of terrestrial scenes through multitemporal compositing, these approaches either treat the multiangle variation as a problem requiring correction or normalization or invoke statistical assumptions that may not apply to specific scenes. With the advent of a new generation of global imaging spectroradiometers capable of acquiring simultaneous visible/near-IR...

Operational retrieval of cloud-top heights using MISR data

Due to its unique nine-angle configuration, the Multi-angle Imaging SpectroRadiometer (MISR) can retrieve cloud parameters such as cloud-motion vectors and cloud-top heights using a purely geometrical technique that involves locating the same cloud features at different viewing angles. The geometrical nature of this technique means that the retrievals are relatively insensitive to the absolute instrument calibration. Fast stereo-matching algorithms have been developed to perform this image matching automatically on an operational basis. Preliminary results are shown of the operational retrievals together with comparisons against other data. Cloud-top height is generally obtained on a 1.1-km grid with an accuracy of /spl plusmn/ 562 m, even over snow and ice. The limitations of the technique, resulting at times in height blunders, noisy retrievals, and discrete effects of wind correction, are discussed.

THE I3RC: Bringing Together the Most Advanced Radiative Transfer Tools for Cloudy Atmospheres

The interaction of clouds with solar and terrestrial radiation is one of the most important topics of climate research. In recent years it has been recognized that only a full three-dimensional (3D) treatment of this interaction can provide answers to many climate and remote sensing problems, leading to the worldwide development of numerous 3D radiative transfer (RT) codes. The international Intercomparison of 3D Radiation Codes (I3RC), described in this paper, sprung from the natural need to compare the performance of these 3D RT codes used in a variety of current scientific work in the atmospheric sciences. I3RC supports intercomparison and development of both exact and approximate 3D methods in its effort to 1) understand and document the errors/limits of 3D algorithms and their sources; 2) provide “baseline” cases for future code development for 3D radiation; 3) promote sharing and production of 3D radiative tools; 4) derive guidelines for 3D radiative tool selection; and 5) improve atmospheric science education in 3D RT. Results from the two completed phases of I3RC have been presented in two workshops and are expected to guide improvements in both remote sensing and radiative energy budget calculations in cloudy atmospheres.

MISR: A multiangle imaging spectroradiometer for geophysical and climatological research from Eos

The scientific objectives, instrument concept, and data plan for the multiangle imaging spectroradiometer (MISR), an experiment proposed for the Eos (Earth Observing System) mission, are described. MISR is a pushbroom imaging system designed to obtain continuous imagery of the sunlit Earth at four different view angles (25.8 degrees , 45.6 degrees , 60.0 degrees , and 72.5 degrees relative to the vertical at the Earths surface), in both the forward and aftward directions relative to nadir, using eight separate cameras. Observations will be acquired in four spectral bands, centered at 440, 550, 670, and 860 nm. Data analysis algorithms will be applied to MISR imagery to retrieve the optical, geometric, and radiative properties of complex, three-dimensional scenes, such as aerosol-laden atmospheres above a heterogeneously reflecting surface, nonstratified cloud systems, and vegetation canopies. The MISR investigation will address a number of scientific questions concerning the climatic and ecological consequences of many natural and anthropogenic processes, and will furnish the aerosol information necessary. >

Effects of Cloud Heterogeneities on Shortwave Radiation: Comparison of Cloud-Top Variability and Internal Heterogeneity

This paper examines the processes through which cloud heterogeneities influence solar reflection. This question is important since present methods give numerical results only for the overall radiative effect of cloud heterogeneities but cannot determine the degree to which various mechanisms are responsible for it. This study establishes a theoretical framework that defines these mechanisms and also provides a procedure to calculate their magnitude. In deriving the framework, the authors introduce a one-dimensional radiative transfer approximation, called the tilted independent pixel approximation (TIPA). TIPA uses the horizontal distribution of slant optical thicknesses along the direct solar beam to describe the radiative influence of cloud heterogeneities when horizontal transport between neighbors is not considered. The effects for horizontal transport are then attributed to two basic mechanisms: trapping and escape of radiation, when it moves to thicker and thinner cloud elements, respectively. Using the proposed framework, the study examines the shortwave radiative effects of cloud-top height and cloud volume extinction coefficient variations. It is shown and explained that identical variations in cloud optical thickness can cause much stronger heterogeneity effects if they are due to variations in geometrical cloud thickness rather than in volume extinction coefficient. The differences in albedo can exceed 0.05, and the relative differences in reflectance toward the zenith can be greater than 25% for overhead sun and 50% for oblique sun. The paper also explains a previously observed phenomenon: it shows that the trapping of upwelling radiation causes the zenith reflectance of heterogeneous clouds to increase with decreasing solar elevation.

Feasibility and Error Analysis of Cloud Motion Wind Extraction from Near-Simultaneous Multiangle MISR Measurements

Satellite wind measurements represent an invaluable contribution to the description of the flow field over the oceans. Conventional cloud-tracking techniques suffer from the inability to simultaneously determine wind speed and height. Currently, the uncertainty in the independently calculated heights is the major factor limiting the accuracy of cloud motion winds. Near-simultaneous multiangle imagery from the multiangle imaging spectroradiometer (MISR) forms the basis of a unique method able to simultaneously retrieve cloud motion and height. The coupled motion and height parallaxes can be unscrambled from three properly selected multiangle views through a purely geometric, stereoscopic approach. Results based on simulated data indicate that for a mesoscale domain the average along-track and cross-track horizontal wind components may be obtained with an accuracy as good as 3‐4 m s21, and 1‐2 m s21, respectively, and with a corresponding height error of 300‐400 m. The technique also possesses a limited capability to distinguish between low and high features moving at different velocities in a multilayer cloud field.

Effect of cloud inhomogeneities on the solar zenith angle dependence of nadir reflectance

A significant discrepancy has been noted between satellite measurements of shortwave reflectance at nadir and the results of plane-parallel model calculations: For moderate to large solar zenith angles, observed nadir reflectances increase with solar zenith angle, whereas plane-parallel values decrease. Consequently, cloud optical depths retrieved using one-dimensional (1-D) theory have a bias which increases systematically with solar zenith angle. Using Monte Carlo model simulations of photon transport through stochastic, isotropic, scale-invariant cloud fields with variable cloud top heights and volume extinction coefficients, we show that nadir reflectances from three-dimensional cloud fields increase with solar zenith angle, consistent with the observations. The difference from the 1-D case is shown to be explainable by cloudside illumination as well as by the presence of structured (i.e., non-flat) cloud tops. Cloud sides enhance the amount of incident solar radiation intercepted by cloud, allowing more radiation to be scattered upward in the nadir direction. Structured cloud tops change the slope of illuminated cloud top surfaces, such that nadir reflectance at low solar elevations increases with the slope of the illuminated surface. For simple cloud geometries the two effects make equivalent contributions to the increase in nadir reflectance with solar zenith angle. While this increase is most pronounced for vertically extensive broken cloud fields, it also affects reflectances from overcast cloud fields with inhomogeneous (bumpy) cloud tops. Thus the observed solar zenith angle bias in cloud optical depth for the general cloud scene likely also occurs for extensive overcast cloud fields. Internal inhomogeneities due to small-scale liquid water content variations within clouds are shown to cause no changes at low Sun and only slight decreases in nadir reflectance for high solar elevations.

PARAGON: An Integrated Approach for Characterizing Aerosol Climate Impacts and Environmental Interactions

Aerosols exert myriad influences on the earths environment and climate, and on human health. The complexity of aerosol-related processes requires that information gathered to improve our understanding of climate change must originate from multiple sources, and that effective strategies for data integration need to be established. While a vast array of observed and modeled data are becoming available, the aerosol research community currently lacks the necessary tools and infrastructure to reap maximum scientific benefit from these data. Spatial and temporal sampling differences among a diverse set of sensors, nonuniform data qualities, aerosol mesoscale variabilities, and difficulties in separating cloud effects are some of the challenges that need to be addressed. Maximizing the long-term benefit from these data also requires maintaining consistently well-understood accuracies as measurement approaches evolve and improve. Achieving a comprehensive understanding of how aerosol physical, chemical, and radiative processes impact the earth system can be achieved only through a multidisciplinary, inter-agency, and international initiative capable of dealing with these issues. A systematic approach, capitalizing on modern measurement and modeling techniques, geospatial statistics methodologies, and high-performance information technologies, can provide the necessary machinery to support this objective. We outline a framework for integrating and interpreting observations and models, and establishing an accurate, consistent, and cohesive long-term record, following a strategy whereby information and tools of progressively greater sophistication are incorporated as problems of increasing complexity are tackled. This concept is named the Progressive Aerosol Retrieval and Assimilation Global Observing Network (PARAGON). To encompass the breadth of the effort required, we present a set of recommendations dealing with data interoperability; measurement and model integration; multisensor synergy; data summarization and mining; model evaluation; calibration and validation; augmentation of surface and in situ measurements; advances in passive and active remote sensing; and design of satellite missions. Without an initiative of this nature, the scientific and policy communities will continue to struggle with understanding the quantitative impact of complex aerosol processes on regional and global climate change and air quality.

Comparison of microwave and optical cloud water path estimates from TMI, MODIS, and MISR

[1] This study investigated the consistency between microwave and optical water path estimates of oceanic clouds from Tropical Rainfall Measurement Mission (TRMM) Microwave Imager (TMI), Moderate Resolution Imaging Spectroradiometer (MODIS), and Multiangle Imaging Spectroradiometer (MISR). We used microwave estimates from the Wentz algorithm for warm, nonprecipitating clouds and both Wentz retrievals and standard TMI profiles for cold, precipitating clouds. Optical estimates were derived from cloud optical thickness and particle effective radius. For warm, nonprecipitating clouds the two methods showed good agreement at the 25-km resolution of the microwave measurements, with liquid water path means being within 5–10%, an overall correlation of 0.85, and RMS difference of � 25 g m � 2 . Multiangle optical retrievals showed only weak variations with view zenith angle, building further confidence in the results. An error analysis suggested that optical estimates were more certain than microwave ones, primarily because 1-D plane-parallel radiative transfer seemed to apply well at the coarser comparison scales. However, there appeared to be a slight but systematic dependence on cloud amount as microwave retrievals increasingly overestimated optical ones at cloud fractions below � 65%. For cold, precipitating clouds we tested three common interpretations of optical retrievals: liquid water path, ice water path, and total water path. The relationship between microwave and optical estimates was weak in all cases, with correlations no more than 0.5, and RMS differences at least an order of magnitude larger than for warm clouds. The weakest correlation (0.37) was found when optical retrievals were interpreted as ice water paths. If anything, optical estimates appeared best correlated with microwave cloud liquid water paths.