Thomas P. Charlock

Clouds and the Earth's Radiant Energy System (CERES): algorithm overview

The Clouds and the Earths Radiant Energy System (CERES) is part of NASAs Earth Observing System (EOS), CERES objectives include the following. (1) For climate change analysis, provide a continuation of the Earth Radiation Budget Experiment (ERBE) record of radiative fluxes at the top-of-the-atmosphere (TOA), analyzed using the same techniques as the existing ERBE data. (2) Double the accuracy of estimates of radiative fluxes at TOA and the Earths surface. (3) Provide the first long-term global estimates of the radiative fluxes within the Earths atmosphere. (4) Provide cloud property estimates collocated in space and time that are consistent with the radiative fluxes from surface to TOA. In order to accomplish these goals, CERES uses data from a combination of spaceborne instruments: CERES scanners, which are an improved version of the ERBE broadband radiometers, and collocated cloud spectral imager data on the same spacecraft. The CERES cloud and radiative flux data products should prove extremely useful in advancing the understanding of cloud-radiation interactions, particularly cloud feedback effects on the Earths radiation balance. For this reason, the CERES data should be fundamental to the ability to understand, detect, and predict global climate change. CERES results should also be very useful for studying regional climate changes associated with deforestation, desertification, anthropogenic aerosols, and ENSO events. This overview summarizes the Release 3 version of the planned CERES data products and data analysis algorithms. These algorithms are a prototype for the system that will produce the scientific data required for studying the role of clouds and radiation in the Earths climate system.

Multiple Scattering Parameterization in Thermal Infrared Radiative Transfer

Abstract A systematic formulation of various radiative transfer parameterizations is presented, including the absorption approximation (AA), δ-two-stream approximation (D2S), δ-four-stream approximation (D4S), and δ-two- and four-stream combination approximation (D2/4S), in a consistent manner for thermal infrared flux calculations. The D2/4S scheme uses a source function from the δ-two-stream approximation and evaluates intensities in the four-stream directions. A wide range of accuracy checks for monochromatic emissivity of a homogeneous layer and broadband heating rates and fluxes in nonhomogeneous atmospheres is performed with respect to the “exact” results computed from the δ-128-stream scheme for radiative transfer. The computer time required for the calculations using different radiative transfer parameterizations is compared. The results pertaining to the accuracy and efficiency of various radiative transfer approximations can be utilized to decide which approximate method is most appropriate for ...

A parameterization of ocean surface albedo

[1] Measurements at a sea platform show that the ocean surface albedo is highly variable and is sensitive to four physical parameters: solar zenith angle, wind speed, transmission by atmospheric cloud/aerosol, and ocean chlorophyll concentration. Using a validated coupled ocean-atmosphere radiative transfer model, an ocean albedo look up table is created in terms of these four important parameters. A code to read the table is also provided; it gives spectral albedos for a range of oceanic and atmospheric conditions specified by the user. The result is a fast and accurate parameterization of ocean surface albedo for radiative transfer and climate modeling. INDEX TERMS: 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 1620 Global Change: Climate dynamics (3309); 3339 Meteorology and Atmospheric Dynamics: Ocean/ atmosphere interactions (0312, 4504); 4552 Oceanography: Physical: Ocean optics; 1610 Global Change: Atmosphere (0315, 0325). Citation: Jin, Z., T. P. Charlock, W. L. Smith Jr., and K. Rutledge (2004), A parameterization of ocean surface albedo, Geophys. Res. Lett., 31, L22301, doi:10.1029/ 2004GL021180.

The CERES/ARM/GEWEX Experiment (CAGEX) for the Retrieval of Radiative Fluxes with Satellite Data

Abstract Results from a temporally intensive, limited area, radiative transfer model experiment are on-line for investigating the vertical profile of shortwave and longwave radiative fluxes from the surface to the top of the atmosphere (TOA). The CERES/ARM/GEWEX Experiment (CAGEX) Version 1 provides a record of fluxes that have been computed with a radiative transfer code; the atmospheric sounding, aerosol, and satellite-retrieved cloud data on which the computations have been based; and surface-based measurements of radiative fluxes and cloud properties from ARM for comparison. The computed broadband fluxes at TOA show considerable scatter when compared with fluxes that are inferred empirically from narrowband operational satellite data. At the surface, LW fluxes computed with an alternate sounding dataset compare well with pyrgeometer measurements. In agreement with earlier work, the authors find that the calculated SW surface insulation is larger than the measurements for clear-sky and total-sky condit...

Analytical solution of radiative transfer in the coupled atmosphere-ocean system with a rough surface

Using the computationally efficient discrete-ordinate method, we present an analytical solution for radiative transfer in the coupled atmosphere-ocean system with a rough air-water interface. The theoretical formulations of the radiative transfer equation and solution are described. The effects of surface roughness on the radiation field in the atmosphere and ocean are studied and compared with satellite and surface measurements. The results show that ocean surface roughness has significant effects on the upwelling radiation in the atmosphere and the downwelling radiation in the ocean. As wind speed increases, the angular domain of sunglint broadens, the surface albedo decreases, and the transmission to the ocean increases. The downward radiance field in the upper ocean is highly anisotropic, but this anisotropy decreases rapidly as surface wind increases and as ocean depth increases. The effects of surface roughness on radiation also depend greatly on both wavelength and angle of incidence (i.e., solar elevation); these effects are significantly smaller throughout the spectrum at high Sun. The model-observation discrepancies may indicate that the Cox-Munk surface roughness model is not sufficient for high wind conditions.

Assessment of the Global Monthly Mean Surface Insolation Estimated from Satellite Measurements Using Global Energy Balance Archive Data

Abstract Global datasets of surface radiation budget (SRB) have been obtained from satellite programs. These satellite-based estimates need validation with ground-truth observations. This study validates the estimates of monthly mean surface insolation contained in two satellite-based SRB datasets with the surface measurements made at worldwide radiation stations from the Global Energy Balance Archive (GEBA). One dataset was developed from the Earth Radiation Budget Experiment (ERBE) using the algorithm of Li et al. (ERBE/SRB), and the other from the International Satellite Cloud Climatology Project (ISCCP) using the algorithm of Pinker and Laszlo and that of Staylor (GEWEX/SRB). Since the ERBE/SRB data contain the surface net solar radiation only, the values of surface insolation were derived by making use of the surface albedo data contained in the GEWEX/SRB product. The resulting surface insolation has a bias error near zero and a root-mean-square error (RMSE) between 8 and 28 W m−2. The RMSE is mainly...

Analysis of Broadband Solar Radiation and Albedo over the Ocean Surface at COVE

Abstract A coupled atmosphere–ocean radiative transfer model has been applied to analyze a full year of broadband solar irradiances (up and down) measured over an ocean site 25 km east of the coast of Virginia in the Atlantic. The coupled model treats absorption and scattering by layers for both the atmosphere and the ocean explicitly and consistently. Key input parameters for the model (aerosol optical depth, wind speed, and total precipitable water) are also from in situ measurements. Having more observations to specify properties of the atmosphere than of the ocean, better model–observation agreement is obtained for the downwelling irradiance, which depends primarily on the atmospheric optical properties, than for the upwelling irradiance, which depends heavily on the ocean optical properties. The mean model–observation differences for the ocean surface albedo are generally less than 0.01. However, the modeled upwelling irradiances and albedo over the ocean surface are mostly less than the observations...

Computation of Domain-Averaged Irradiance Using Satellite-Derived Cloud Properties

Abstract The respective errors caused by the gamma-weighted two-stream approximation and the effective thickness approximation for computing the domain-averaged broadband shortwave irradiance are evaluated using cloud optical thicknesses derived from 1 h of radiance measurements by the Moderate Resolution Imaging Spectrometer (MODIS) over footprints of Clouds and the Earth’s Radiant Energy System (CERES) instruments. Domains are CERES footprints of which dimension varies approximately from 20 to 70 km, depending on the viewing zenith angle of the instruments. The average error in the top-of-atmosphere irradiance at a 30° solar zenith angle caused by the gamma-weighted two-stream approximation is 6.1 W m−2 (0.005 albedo bias) with a one-layer overcast cloud where a positive value indicates an overestimate by the approximation compared with the irradiance computed using the independent column approximation. Approximately one-half of the error is due to deviations of optical thickness distributions from a ga...

A comparison of the aerosol thickness derived from ground‐based and airborne measurements

The extinction optical thickness of particles obtained from scattering and absorption coefficients measured by an airborne integrating nephelometer and particle soot absorption photometer, respectively, is compared with the aerosol optical thickness derived from a ground-based multifilter rotating shadowband radiometer, a Sun photometer, and a Raman lidar for 9 days. These 9 days are selected from intensive operation periods of the Atmospheric Radiation Measurement in April 1997, September 1997, and August 1998 at the southern Great Plains. For April 1997 and September 1997 cases the difference between the extinction optical thickness of particles estimated from vertical profiles and the extinction optical thickness of aerosol derived from the multifilter rotating shadowband radiometer is not significant. For August 1998 cases when the boundary layer relative humidity is higher than April 1997 and September 1997 cases, the extinction optical thickness of particles is 0.03 to 0.07 less than the extinction optical thickness of aerosol. The difference corresponds to 25% to 31% of the extinction optical thickness of aerosol. Based on these comparisons, the upper and lower limits of the single-scattering albedo of particles present in the lower part of troposphere are 0.97 and 0.84, respectively.

On the radiative properties of contrail cirrus

Using the observed ice crystal size distribution in contrail cirrus from SUCCESS, we have carried out the scattering and absorption calculations based on a unified theory for light scattering by ice crystals covering all sizes and shapes. We illustrate the effects of ice crystal size and surface roughness on the scattering phase function features for remote sensing applications. The extinction coefficient and single-scattering albedo exhibit a minimum feature at 2.85 µm, referred to as the Christiansen effect, which is particularly pronounced for clouds consisting of a significant number of small ice crystals. Based on a line-by-line equivalent solar model, we show from spectral curves that cloud reflection increases as ice crystal sizes become smaller, but the cloud absorption increase is only evident for wavelengths longer than about 2.7 µm. The ice crystal shape has a substantial effect on the cloud reflection and absorption for a given size; more complex ice particles reflect more solar radiation. Finally, we propose a contrail cirrus cloud model consisting of a combination of bullet rosettes (50%), hollow columns (30%), and plates (20%), with sizes ranging from 1 to 90 µm in association with radiation perturbation studies.