Chesapeake Lighthouse And Aircraft Measurements For Satellites Research Articles

Real-time mesoscale forecast support during the CLAMS field campaign

This paper reports the use of a specialized, mesoscale, numerical weather prediction (NWP) system and a satellite imaging and prediction system that were set up to support the CLAMS (Chesapeake Lighthouse and Aircraft Measurements for Satellites) field campaign during the summer of 2001. The primary objective of CLAMS was to validate satellite-based retrievals of aerosol properties and vertical profiles of the radiative flux, temperature and water vapor. Six research aircraft were deployed to make detailed coincident measurements of the atmosphere and ocean surface with the research satellites that orbited overhead. The mesoscale weather modeling system runs in real-time to provide high spatial and temporal resolution for forecasts that are delivered via the World Wide Web along with a variety of satellite imagery and satellite location predictions. This system is a multi-purpose modeling system capable of both data analysis/assimilation and multi-scale NWP ranging from cloud-scale to larger than regional scale. This is a three-dimensional, nonhydrostatic compressible model in a terrain-following coordinate. The model employs advanced numerical techniques and contains detailed interactive physical processes. The utility of the forecasting system is illustrated throughout the discussion on the impact of the surface-wind forecast on BRDF (Bidirectional Reflectance Distribution Function) and the description of the cloud/moisture forecast versus the aircraft measurement.

Aerosol Optical Properties and Particle Size Distributions on the East Coast of the United States Derived from Airborne In Situ and Remote Sensing Measurements

Abstract Airborne in situ measurements of vertical profiles and horizontal transects of aerosol optical and physical properties, obtained during the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) field campaign off the East Coast of the United States during the summer of 2001, are presented. Most of the measurements were obtained in relatively clean air dominated by airflows that had passed over Canada and the northern Atlantic Ocean. Results from the 17 July and 2 August 2001 flights are presented; on these days, the aerosol loading was relatively high. In the lower troposphere, ω0 values at a wavelength of 550 nm were consistently above 0.93 throughout the field experiment, indicating the dominance of weakly absorbing aerosol. Particle number size distributions are presented and discussed for transects at altitudes ∼0.05–3.5 km above mean sea level. Particles with diameters (Dp) <0.1 μm made up the majority of the aerosol number. Accumulation mode particles dominate the number size, surface area, and volume distributions. The variability of optical and physical aerosol parameters was analyzed on horizontal scales of ∼1–4 km. There was little horizontal variability in the single-scattering albedo (ω0), aerosol optical depth (AOD), and accumulation mode size, but greater variability in particle number concentration. Comparisons of the airborne measurements with remotely sensed aerosol parameters, such as ω0 and effective particle radius (reff), derived from the Multiangle Imaging Spectroradiometer (MISR) aboard the Terra satellite were generally in good agreement. Overall, the MISR retrievals captured both the similarities and the differences between the properties of the aerosols measured on 17 July and 2 August.

EOS Terra Aerosol and Radiative Flux Validation: An Overview of the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) Experiment

Abstract NASA developed an Earth Observing System (EOS) to study global change and reduce uncertainties associated with aerosols and other key parameters controlling climate. The first EOS satellite, Terra, was launched in December 1999. The Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) field campaign was conducted from 10 July to 2 August 2001 to validate several Terra data products, including aerosol properties and radiative flux profiles derived from three complementary Terra instruments: the Clouds and the Earth’s Radiant Energy System (CERES), the Multiangle Imaging Spectroradiometer (MISR), and the Moderate Resolution Imaging Spectroradiometer (MODIS). CERES, MISR, and MODIS are being used to investigate the critical role aerosols play in modulating the radiative heat budget of the earth–atmosphere system. CLAMS’ primary objectives are to improve understanding of atmospheric aerosols, to validate and improve the satellite data products, and to test new instruments and measurement concepts. A variety of in situ sampling devices and passive remote sensing instruments were flown on six aircraft to characterize the state of the atmosphere, the composition of atmospheric aerosols, and the associated surface and atmospheric radiation parameters over the U.S. eastern seaboard. Aerosol particulate matter was measured at two ground stations established at Wallops Island, Virginia, and the Chesapeake Lighthouse, the site of an ongoing CERES Ocean Validation Experiment (COVE) where well-calibrated radiative fluxes and Aerosol Robotic Network (AERONET) aerosol properties have been measured since 1999. Nine coordinated aircraft missions and numerous additional sorties were flown under a variety of atmospheric conditions and aerosol loadings. On one “golden day” (17 July 2001), under moderately polluted conditions with midvisible optical depths near 0.5, all six aircraft flew coordinated patterns vertically stacked between 100 and 65 000 ft over the COVE site as Terra flew overhead. This overview presents a description of CLAMS objectives, measurements, and sampling strategies. Key results, reported in greater detail in the collection of papers found in this special issue, are also summarized.

Radiative Transfer Modeling for the CLAMS Experiment

Abstract Spectral and broadband radiances and irradiances (fluxes) were measured from surface, airborne, and spaceborne platforms in the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) campaign. The radiation data obtained on the 4 clear days over ocean during CLAMS are analyzed here with the Coupled Ocean–Atmosphere Radiative Transfer (COART) model. The model is successively compared with observations of broadband fluxes and albedos near the ocean surface from the Clouds and the Earth's Radiant Energy System (CERES) Ocean Validation Experiment (COVE) sea platform and a low-level OV-10 aircraft, of near-surface spectral albedos from COVE and OV-10, of broadband radiances at multiple angles and inferred top-of-atmosphere (TOA) fluxes from CERES, and of spectral radiances at multiple angles from Airborne Multiangle Imaging Spectroradiometer (MISR), or “AirMISR,” at 20-km altidude. The radiation measurements from different platforms are shown to be consistent with each other and with model results. The discrepancies between the model and observations at the surface are less than 10 W m−2 for downwelling and 2 W m−2 for upwelling fluxes. The model–observation discrepancies for shortwave ocean albedo are less than 8%; some discrepancies in spectral albedo are larger but less than 20%. The discrepancies between low-altitude aircraft and surface measurements are somewhat larger than those between the model and the surface measurements; the former are due to the effects of differences in height, aircraft pitch and roll, and the noise of spatial and temporal variations of atmospheric and oceanic properties. The discrepancy between the model and the CERES observations for the upwelling radiance is 5.9% for all angles; this is reduced to 4.9% if observations within 15° of the sun-glint angle are excluded. The measurements and model agree on the principal impacts that ocean optical properties have on upwelling radiation at low levels in the atmosphere. Wind-driven surface roughness significantly affects the upwelling radiances measured by aircraft and satellites at small sun-glint angles, especially in the near-infrared channel of MISR. Intercomparisons of various measurements and the model show that most of the radiation observations in CLAMS are robust, and that the coupled radiative transfer model used here accurately treats scattering and absorption processes in both the air and the water.

The NPOESS Airborne Sounding Testbed Interferometer—Remotely Sensed Surface and Atmospheric Conditions during CLAMS

Abstract During the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS), the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Airborne Sounder Testbed-Interferometer (NAST-I), flying aboard the high-altitude Proteus aircraft, observed the spatial distribution of infrared radiance across the 650–2700 cm−1 (3.7–15.4 μm) spectral region with a spectral resolution of 0.25 cm−1. NAST-I scans cross track with a moderate spatial resolution (a linear ground resolution equal to 13% of the aircraft altitude at nadir). The broad spectral coverage and high spectral resolution of this instrument provides abundant information about the surface and three-dimensional state of the atmosphere. In this paper, the NAST-I measurements and geophysical product retrieval methodology employed for CLAMS are described. Example results of surface properties and atmospheric temperature, water vapor, ozone, and carbon monoxide distributions are provided. The CLAMS NAST-I geophysical dataset is available for use by the scientific community.

Suborbital Measurements of Spectral Aerosol Optical Depth and Its Variability at Subsatellite Grid Scales in Support of CLAMS 2001

Abstract As part of the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) experiment, 10 July–2 August 2001, off the central East Coast of the United States, the 14-channel NASA Ames Airborne Tracking Sunphotometer (AATS-14) was operated aboard the University of Washington’s Convair 580 (CV-580) research aircraft during 10 flights (∼45 flight hours). One of the main research goals in CLAMS was the validation of satellite-based retrievals of aerosol properties. The goal of this study in particular was to perform true over-ocean validations (rather than over-ocean validation with ground-based, coastal sites) at finer spatial scales and extending to longer wavelengths than those considered in previous studies. Comparisons of aerosol optical depth (AOD) between the Aerosol Robotic Network (AERONET) Cimel instrument at the Chesapeake Lighthouse and airborne measurements by AATS-14 in its vicinity showed good agreement with the largest r-square correlation coefficients at wavelengths of 0.38 and 0.5 μm (>0.99). Coordinated low-level flight tracks of the CV-580 during Terra overpass times permitted validation of over-ocean Moderate Resolution Imaging Spectroradiometer (MODIS) level 2 (MOD04_L2) multiwavelength AOD data (10 km × 10 km, nadir) in 16 cases on three separate days. While the correlation between AATS-14- and MODIS-derived AOD was weak with an r square of 0.55, almost 75% of all MODIS AOD measurements fell within the prelaunch estimated uncertainty range Δτ = ±0.03 ± 0.05τ. This weak correlation may be due to the small AODs (generally less than 0.1 at 0.5 μm) encountered in these comparison cases. An analogous coordination exercise resulted in seven coincident over-ocean matchups between AATS-14 and Multiangle Imaging Spectroradiometer (MISR) measurements. The comparison between AATS-14 and the MISR standard algorithm regional mean AODs showed a stronger correlation with an r square of 0.94. However, MISR AODs were systematically larger than the corresponding AATS values, with an rms difference of ∼0.06. AATS data collected during nine extended low-level CV-580 flight tracks were used to assess the spatial variability in AOD at horizontal scales up to 100 km. At UV and midvisible wavelengths, the largest absolute gradients in AOD were 0.1–0.2 per 50-km horizontal distance. In the near-IR, analogous gradients rarely reached 0.05. On any given day, the relative gradients in AOD were remarkably similar for all wavelengths, with maximum values of 70% (50 km)−1 and more typical values of 25% (50 km)−1. The implications of these unique measurements of AOD spatial variability for common validation practices of satellite data products and for comparisons to large-scale aerosol models are discussed.

Chemical Characterization of Aerosols on the East Coast of the United States Using Aircraft and Ground-Based Stations during the CLAMS Experiment

Abstract The Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) experiment was carried out off the central East Coast of the United States in July 2001. During CLAMS, aerosol particle mass was measured at two ground stations and on the University of Washington’s Convair 580 research aircraft. Physical and chemical characteristics of the aerosols were identified and quantified. Three main aerosol regimes were identified in the region and are discussed in this work: local pollution/sea salt background, long-range transported dust, and long-range transported pollution. The major component measured in the fine mode of the aerosol on the ground at Wallops Island, Virginia, was sulfate, estimated as NH4HSO4, which accounted for 55% ± 9% on average of the fine particle mass (FPM) during the experiment period. Black carbon concentrations accounted for 3% ± 1% of FPM; soil dust was also present, representing on average 6% ± 8% of FPM. The difference between the sum of the masses of the measured compounds and the total fine particle mass was 36% ± 10% of FPM, which is attributed primarily to nitrates and organic carbon that were not measured. Aerosol chemical composition in the atmospheric column is also discussed and compared with ground-based measurements. Aerosol dust concentration reached 40% of FPM during an incursion of Saharan dust between 24 and 26 July. Sulfate aerosol reached 70% of FPM during the transport of regional pollution on 17 July. Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical thickness, coupled with air parcel back trajectories, supported the conclusion of episodes of long-range transport of dust from the Sahara Desert and pollutants from the continental United States.

Retrieval of Aerosol Scattering and Absorption Properties from Photopolarimetric Observations over the Ocean during the CLAMS Experiment

AbstractThe extensive set of measurements performed during the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) experiment provides a unique opportunity to evaluate aerosol retrievals over the ocean from multiangle, multispectral photometric, and polarimetric remote sensing observations by the airborne Research Scanning Polarimeter (RSP) instrument.Previous studies have shown the feasibility of retrieving particle size distributions and real refractive indices from such observations for visible wavelengths without prior knowledge of the ocean color. This work evaluates the fidelity of the aerosol retrievals using RSP measurements during the CLAMS experiment against aerosol properties derived from in situ measurements, sky radiance observations, and sun-photometer measurements, and further extends the scope of the RSP retrievals by using a priori information about the ocean color to constrain the aerosol absorption and vertical distribution.It is shown that the fine component of the aerosol observed on 17 July 2001 consisted predominantly of dirty sulfatelike particles with an extinction optical thickness of several tenths in the visible, an effective radius of 0.15 ± 0.025 μm and a single scattering albedo of 0.91 ± 0.03 at 550 nm. Analyses of the ocean color and sky radiance observations favor the lower boundary of aerosol single scattering albedo, while in situ measurements favor its upper boundary. Both analyses support the polarimetric retrievals of fine-aerosol effective radius and the consequent spectral variation in extinction optical depth. The estimated vertical distribution of this aerosol component depends on assumptions regarding the water-leaving radiances and is consistent with the top of the aerosol layer being close to the aircraft height (3500 m), with the bottom of the layer being between 2.7 km and the surface. The aerosol observed on 17 July 2001 also contained coarse-mode particles. Comparison of RSP data with sky radiance and in situ measurements suggests that this component consists of nonspherical particles with an effective radius in excess of 1 μm, and with the extinction optical depth being much less than one-tenth at 550 nm.

Airborne Spectral Measurements of Ocean Directional Reflectance

Abstract The Cloud Absorption Radiometer (CAR) was flown aboard the University of Washington Convair 580 (CV-580) research aircraft during the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) field campaign and obtained measurements of bidirectional reflectance distribution function (BRDF) of the ocean in July and August 2001 under different illumination conditions with solar zenith angles ranging from 15° to 46°. The BRDF measurements were accompanied by concurrent measurements of atmospheric aerosol optical thickness and column water vapor above the airplane. The method of spherical harmonics with Cox–Munk wave-slope distribution is used in a new algorithm developed for this study to solve the atmosphere–ocean radiative transfer problem and to remove the effects of the atmosphere from airborne measurements. The algorithm retrieves simultaneously the wind speed and full ocean BRDF (sun’s glitter and water-leaving radiance) from CAR measurements and evaluates total albedo and equivalent albedo for the water-leaving radiance outside the glitter. Results show good overall agreement with other measurements and theoretical simulations, with the anisotropy of the water-leaving radiance clearly seen. However, the water-leaving radiance does not show a strong dependence on solar zenith angle as suggested by some theoretical studies. The spectral albedo was found to vary from 4.1%–5.1% at λ = 0.472 μm to 2.4%–3.5% for λ ≥ 0.682 μm. The equivalent water-leaving albedo ranges from 1.0%–2.4% at λ = 0.472 μm to 0.1%–0.6% for λ = 0.682 μm and 0.1%–0.3% for λ = 0.870 μm. Results of the validation of the Cox–Munk model under the conditions measured show that although the model reproduces the shape of sun’s glitter on average with an accuracy of better than 30%, it underestimates the center of the sun’s glitter reflectance by about 30% for low wind speeds (<2–3 m s−1). In cases of high wind speed, the model with Gram–Charlier expansion seems to provide the best fit.

MISR Calibration and Implications for Low-Light-Level Aerosol Retrieval over Dark Water

Abstract Studying aerosols over ocean is one goal of the Multiangle Imaging Spectroradiometer (MISR) and other spaceborne imaging systems. But top-of-atmosphere equivalent reflectance typically falls in the range of 0.03 to 0.12 at midvisible wavelengths and can be below 0.01 in the near-infrared, when an optically thin aerosol layer is viewed over a dark ocean surface. Special attention must be given to radiometric calibration if aerosol optical thickness, and any information about particle microphysical properties, are to be reliably retrieved from such observations. MISR low-light-level vicarious calibration is performed in the vicinity of remote islands hosting Aerosol Robotic Network (AERONET) sun- and sky-scanning radiometers, under low aerosol loading, low wind speed, relatively cloud free conditions. MISR equivalent reflectance is compared with values calculated from a radiative transfer model constrained by coincident, AERONET-retrieved aerosol spectral optical thickness, size distribution, and single scattering albedo, along with in situ wind measurements. Where the nadir view is not in sun glint, MISR equivalent reflectance is also compared with Moderate Resolution Imaging Spectroradiometer (MODIS) reflectance. The authors push the limits of the vicarious calibration method’s accuracy, aiming to assess absolute, camera-to-camera, and band-to-band radiometry. Patterns repeated over many well-constrained cases lend confidence to the results, at a few percent accuracy, as do additional vicarious calibration tests performed with multiplatform observations taken during the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) campaign. Conclusions are strongest in the red and green bands, but are too uncertain to accept for the near-infrared. MISR nadir-view and MODIS low-light-level absolute reflectances differ by about 4% in the blue and green bands, with MISR reporting higher values. In the red, MISR agrees with MODIS band 14 to better than 2%, whereas MODIS band 1 is significantly lower. Compared to the AERONET-constrained model, the MISR aft-viewing cameras report reflectances too high by several percent in the blue, green, and possibly the red. Better agreement is found in the nadir- and the forward-viewing cameras, especially in the blue and green. When implemented on a trial basis, calibration adjustments indicated by this work remove 40% of a 0.05 bias in retrieved midvisible aerosol optical depth over dark water scenes, produced by the early postlaunch MISR algorithm. A band-to-band correction has already been made to the MISR products, and the remaining calibration adjustments, totaling no more than a few percent, are planned.

Simultaneous retrieval of aerosol and thin cirrus optical depths using MODIS airborne simulator data during CRYSTAL‐FACE and CLAMS

We have developed a new methodology to simultaneously retrieve the optical depths of aerosols and thin cirrus clouds over the oceans by using the Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) data. This methodology takes advantage of the fact that the visible (or a near‐infrared window) reflectance from cirrus can be characterized by its correlation to the reflectance from a near‐infrared band located in a strong water vapor absorption region. Once the reflectance due to cirrus has been removed, the residual reflectance in these bands is used to retrieve aerosol optical depth. Cirrus cloud properties are subsequently obtained by minimizing the difference between observed and calculated reflectance values stored in look‐up tables that are constructed a priori and correlated to the retrieved aerosol optical depth. Examination of the data gathered from the recent Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL‐FACE) and Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) field campaigns demonstrates that our retrievals are consistent with both in‐situ and ground based measurements.