Engineering topics
Publications and source records attributed to Wielicki, B. A..
Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, and sea level has risen.
Highly accurate measurements of Earth's thermal infrared and reflected solar radiation are required for detecting and predicting long-term climate change. We consider the concept of using the International Space Station to test instruments and techniques that would eventually be used on a dedicated mission such as the Climate Absolute Radiance and Refractivity Observatory. In particular, a quantitative investigation is performed to determine whether it is possible to use measurements obtained with a highly accurate reflected solar radiation spectrometer to calibrate similar, less accurate instruments in other low Earth orbits. Estimates of numbers of samples useful for intercalibration are made with the aid of year-long simulations of orbital motion. We conclude that the International Space Station orbit is ideally suited for the purpose of intercalibration.
The Clouds and Earth Radiant Energy System (CERES) project s objectives are to measure the reflected solar radiance (shortwave) and Earth-emitted (longwave) radiances and from these measurements to compute the shortwave and longwave radiation fluxes at the top of the atmosphere (TOA) and the surface and radiation divergence within the atmosphere. The fluxes at TOA are to be retrieved to an accuracy of 2%. Improved bidirectional reflectance distribution functions (BRDFs) have been developed to compute the fluxes at TOA from the measured radiances with errors reduced from ERBE by a factor of two or more. Instruments aboard the Terra and Aqua spacecraft provide sampling at four local times. In order to further reduce temporal sampling errors, data are used from the geostationary meteorological satellites to account for changes of scenes between observations by the CERES radiometers. A validation protocol including in-flight calibrations and comparisons of measurements has reduced the instrument errors to less than 1%. The data are processed through three editions. The first edition provides a timely flow of data to investigators and the third edition provides data products as accurate as possible with resources available. A suite of cloud properties retrieved from the MODerate-resolution Imaging Spectroradiometer (MODIS) by the CERES team is used to identify the cloud properties for each pixel in order to select the BRDF for each pixel so as to compute radiation fluxes from radiances. Also, the cloud information is used to compute radiation at the surface and through the atmosphere and to facilitate study of the relationship between clouds and the radiation budget. The data products from CERES include, in addition to the reflected solar radiation and Earth emitted radiation fluxes at TOA, the upward and downward shortwave and longwave radiation fluxes at the surface and at various levels in the atmosphere. Also at the surface the photosynthetically active radiation and ultraviolet radiation (total, UVA and UVB) are computed. The CERES instruments aboard the Terra and Aqua spacecraft have served well past their design life times. A CERES instrument has been integrated onto the NPP platform and is ready for launch in 2011. Another CERES instrument is being built for launch in 2014, and plans are being made for a series of follow-on missions.
Top-of-atmosphere (TOA) radiative fluxes from the Clouds and the Earth s Radiant Energy System (CERES) are estimated from empirical angular distribution models (ADMs) that convert instantaneous radiance measurements to TOA fluxes. This paper evaluates the accuracy of CERES TOA fluxes obtained from a new set of ADMs developed for the CERES instrument onboard the Tropical Rainfall Measuring Mission (TRMM). The uncertainty in regional monthly mean reflected shortwave (SW) and emitted longwave (LW) TOA fluxes is less than 0.5 W/sq m, based on comparisons with TOA fluxes evaluated by direct integration of the measured radiances. When stratified by viewing geometry, TOA fluxes from different angles are consistent to within 2% in the SW and 0.7% (or 2 W/sq m) in the LW. In contrast, TOA fluxes based on ADMs from the Earth Radiation Budget Experiment (ERBE) applied to the same CERES radiance measurements show a 10% relative increase with viewing zenith angle in the SW and a 3.5% (9 W/sq m) decrease with viewing zenith angle in the LW. Based on multiangle CERES radiance measurements, 18 regional instantaneous TOA flux errors from the new CERES ADMs are estimated to be 10 W/sq m in the SW and, 3.5 W/sq m in the LW. The errors show little or no dependence on cloud phase, cloud optical depth, and cloud infrared emissivity. An analysis of cloud radiative forcing (CRF) sensitivity to differences between ERBE and CERES TRMM ADMs, scene identification, and directional models of albedo as a function of solar zenith angle shows that ADM and clear-sky scene identification differences can lead to an 8 W/sq m root-mean-square (rms) difference in 18 daily mean SW CRF and a 4 W/sq m rms difference in LW CRF. In contrast, monthly mean SW and LW CRF differences reach 3 W/sq m. CRF is found to be relatively insensitive to differences between the ERBE and CERES TRMM directional models.
Clouds and the Earth s Radiant Energy System (CERES) investigates the critical role that clouds and aerosols play in modulating the radiative energy flow within the Earth-atmosphere system. CERES builds upon the foundation laid by previous missions, such as the Earth Radiation Budget Experiment, to provide highly accurate top-of-atmosphere (TOA) radiative fluxes together with coincident cloud and aerosol properties inferred from high-resolution imager measurements. This paper describes the method used to construct empirical angular distribution models (ADMs) for estimating shortwave, longwave, and window TOA radiative fluxes from CERES radiance measurements on board the Tropical Rainfall Measuring Mission satellite. To construct the ADMs, multiangle CERES measurements are combined with coincident high-resolution Visible Infrared Scanner measurements and meteorological parameters from the European Centre for Medium-Range Weather Forecasts data assimilation product. The ADMs are stratified by scene types defined by parameters that have a strong influence on the angular dependence of Earth's radiation field at the TOA. Examples of how the new CERES ADMs depend upon the imager-based parameters are provided together with comparisons with existing models.
The CERES Flight Model 1 and 2 instruments were launched aboard NASA's Earth Observing System (EOS) Terra Spacecraft on December 18, 1999 into a 705 Km sun-synchronous orbit with a 10:30 a.m. equatorial crossing time. These instruments supplement measurements made by the CERES Proto Flight Model (PFM) instrument launched aboard NASA's Tropical Rainfall Measuring Mission (TRMM) spacecraft on November 27, 1997 into a 350 Km, 38-degree mid-inclined orbit. An important aspect of the EOS program is the rapid archival and dissemination of datasets measured by EOS instruments to the scientific community. On September 22, 2000 the CERES Science Team voted to archive the Edition 1 CERES/Terra Level 1b and Level 2 and 3 ERBE-Like data products. These products consist of instantaneous filtered and unfiltered radiances through temporally and spatially averaged TOA fluxes. CERES filtered radiance measurements cover three spectral bands including shortwave (0.3 to 5 micron), total (0.3 to less than 100 microns) and an atmospheric window channel (8 to 12 microns). The current work summarizes both the philosophy and results of validation efforts undertaken to quantify the quality of the Terra data products as well as the level of agreement between the Terra and TRMM datasets.
To estimate the earth's radiation budget at the top of the atmosphere (TOA) from satellite-measured radiances, it is necessary to account for the finite geometry of the earth and recognize that the earth is a solid body surrounded by a translucent atmosphere of finite thickness that attenuates solar radiation differently at different heights. As a result, in order to account for all of the reflected solar and emitted thermal radiation from the planet by direct integration of satellite-measured radiances, the measurement viewing geometry must be defined at a reference level well above the earth s surface (e.g., 100 km). This ensures that all radiation contributions, including radiation escaping the planet along slant paths above the earth s tangent point, are accounted for. By using a field-of- view (FOV) reference level that is too low (such as the surface reference level), TOA fluxes for most scene types are systematically underestimated by 1-2 W/sq m. In addition, since TOA flux represents a flow of radiant energy per unit area, and varies with distance from the earth according to the inverse-square law, a reference level is also needed to define satellite-based TOA fluxes. From theoretical radiative transfer calculations using a model that accounts for spherical geometry, the optimal reference level for defining TOA fluxes in radiation budget studies for the earth is estimated to be approximately 20 km. At this reference level, there is no need to explicitly account for horizontal transmission of solar radiation through the atmosphere in the earth radiation budget calculation. In this context, therefore, the 20-km reference level corresponds to the effective radiative top of atmosphere for the planet. Although the optimal flux reference level depends slightly on scene type due to differences in effective transmission of solar radiation with cloud height, the difference in flux caused by neglecting the scene-type dependence is less than 0.1%. If an inappropriate TOA flux reference level is used to define satellite TOA fluxes, and horizontal transmission of solar radiation through the planet is not accounted for in the radiation budget equation, systematic errors in net flux of up to 8 W/sq m can result. Since climate models generally use a plane-parallel model approximation to estimate TOA fluxes and the earth radiation budget, they implicitly assume zero horizontal transmission of solar radiation in the radiation budget equation, and do not need to specify a flux reference level. By defining satellite-based TOA flux estimates at a 20-km flux reference level, comparisons with plane-parallel climate model calculations are simplified since there is no need to explicitly correct plane-parallel climate model fluxes for horizontal transmission of solar radiation through a finite earth.
A new method for determining unfiltered shortwave (SW), longwave (LW) and window (W) radiances from filtered radiances measured by the Clouds and the Earth's Radiant Energy System (CERES) satellite instrument is presented. The method uses theoretically derived regression coefficients between filtered and unfiltered radiances that are a function of viewing geometry, geotype and whether or not cloud is present. Relative errors in insta.ntaneous unfiltered radiances from this method are generally well below 1% for SW radiances (approx. 0.4% 1(sigma) or approx.l W/sq m equivalent flux), < 0.2% for LW radiances (approx. 0.1% 1(sigma) or approx.0.3 W/sq m equivalent flux) and < 0.2% (approx. 0.1% 1(sigma) for window channel radiances.
The theoretical bases for the Release 1 algorithms that will be used to process satellite data for investigation of the Clouds and Earth's Radiant Energy System (CERES) are described. The architecture for software implementation of the methodologies is outlined. Volume 2 details the techniques used to geolocate and calibrate the CERES scanning radiometer measurements of shortwave and longwave radiance to invert the radiances to top-of-the-atmosphere (TOA) and surface fluxes following the Earth Radiation Budget Experiment (ERBE) approach, and to average the fluxes over various time and spatial scales to produce an ERBE-like product. Spacecraft ephemeris and sensor telemetry are used with calibration coefficients to produce a chronologically ordered data product called bidirectional scan (BDS) radiances. A spatially organized instrument Earth scan product is developed for the cloud-processing subsystem. The ERBE-like inversion subsystem converts BDS radiances to unfiltered instantaneous TOA and surface fluxes. The TOA fluxes are determined by using established ERBE techniques. Hourly TOA fluxes are computed from the instantaneous values by using ERBE methods. Hourly surface fluxes are estimated from TOA fluxes by using simple parameterizations based on recent research. The averaging process produces daily, monthly-hourly, and monthly means of TOA and surface fluxes at various scales. This product provides a continuation of the ERBE record.
Brightness temperature difference (BTD) values are calculated for selected Geostationary Operational Environmental Satellite (GOES-6) channels (3.9, 12.7 micrometer) and Advanced Very High Resolution Radiometer channels (3.7, 12.0 micrometer). Daytime and nighttime discrimination of particle size information is possible given the infrared cloud extinction optical depth and the BTD value. BTD values are presented and compared for cirrus clouds composed of equivalent ice spheres (volume, surface area) versus randomly oriented hexagonal ice crystals. The effect of the hexagonal ice crystals is to increase the magnitude of the BTD values calculated relative to equivalent ice sphere (volume, surface area) BTDs. Equivalent spheres (volume or surface area) do not do a very good job of modeling hexagonal ice crystal effects on BTDs; however, the use of composite spheres improves the simulation and offers interesting prospects. Careful consideration of the number of Legendre polynomial coefficients used to fit the scattering phase functions is crucial to realistic modeling of cirrus BTDs. Surface and view-angle effects are incorporated to provide more realistic simulation.
The Earth Radiation Budget Experiment (ERBE) consisted of a scanning radiometer and non-scanning radiometers on each of three spacecraft. These instruments began flying in October 1984. The nonscanning radiometers continue to operate, providing broadband radiation measurements of the Earth's outgoing longwave radiation and reflected solar radiation, in addition to measurements of the solar output. The Clouds and Earth Radiant Energy System (CERES) features a scanning radiometer, which is an improved version of the ERBE scanning radiometer, and will fly on the Tropical Rainfall Measurement Mission and Earth Observation System platforms in the late nineties. The CERES project will provide not only radiant fluxes at the 'top of the atmosphere' (TOA), but also at the surface and will compute radiant flux divergence through the atmosphere.
Satellite measurements from the Earth Radiation Budget Experiment (ERBE) are providing important quantitative data on the diurnal variability of broadband shortwave and longwave radiation. The results derived from the combination of the Earth Radiation Budget Satellite (ERBS) and NOAA-9 indicate that the largest diurnal variations in longwave radiation occur typically over deserts and over land areas which experience intense convective activity. Maximum values of the albedo diurnal amplitude factor are over oceans. Seasonal and cloud cover variations have important effects on the diurnal cycles of earth's radiation budget. ERBE results derived for individual regions are in substantial agreement with the diurnal results derived from the Geostationary Operational Environmental Satellite (GOES) measurements.
ERBE scanner data from the Earth Radiation Budget Satellite and NOAA-9 satellites obtained from February 1985 through January 1986 are used to investigate the diurnal cycles of both LW radiation and albedo for each month of the year. Seasonal variations of the diurnal cycles are examined for the deserts, vegetated land, and oceans over the globe. Comparisons are made between clear-sky and total-scene conditions. ERBE satellite data showed that many areas of the earth exhibit significant diurnal variations in both LW flux and albedo. The LW diurnal range was found to be greatest for deserts and smallest for oceans, whereas the albedo diurnal amplitude factor is a maximum over the tropical oceans and a minimum over land. Cloud cover and seasonal variations have a major effect on the diurnal cycles. Generally, maximum diurnal ranges were found in the summer hemisphere and minimum values in the winter hemisphere.
To quantify the diurnal radiative heating and cooling cycles of the earth and the atmosphere, data from instruments on the Earth Radiation Budget Satellite (ERBS) spacecraft and NOAA-9 satellites obtained from February 1985 through January 1986 were used to investigate longwave (LW) and shortwave (SW) flux as well as albedo for each month of the year. Seasonal variations of radiative parameters and their diurnal cycles are examined for the deserts, vegetated land, and oceans over the globe. The results show significant seasonal variations in both the outgoing LW and the absorbed SW flux, and a pronounced difference was found between oceanic and continental surfaces. Over much of the globe, LW warming is balanced by SW cooling, and clouds have a net cooling effect on the earth. Many areas of the earth were found to exhibit significant diurnal variations in both the LW flux and albedo.
The longwave angular radiation models that are required for analysis of satellite measurements of Earth radiation, such as those from the Earth Radiation Budget Experiment (ERBE) are presented. The models contain limb-darkening characteristics and mean fluxes. Limb-darkening characteristics are the longwave anisotropic factor and the standard deviation of the longwave radiance. Derivation of these models from the Nimbus 7 ERB (Earth Radiation Budget) data set is described. Tabulated values and computer-generated plots are included for the limb-darkening and mean-flux models.
Presented are shortwave angular radiation models which are required for analysis of satellite measurements of Earth radiation, such as those fro the Earth Radiation Budget Experiment (ERBE). The models consist of both bidirectional and directional parameters. The bidirectional parameters are anisotropic function, standard deviation of mean radiance, and shortwave-longwave radiance correlation coefficient. The directional parameters are mean albedo as a function of Sun zenith angle and mean albedo normalized to overhead Sun. Derivation of these models from the Nimbus 7 ERB (Earth Radiation Budget) and Geostationary Operational Environmental Satellite (GOES) data sets is described. Tabulated values and computer-generated plots are included for the bidirectional and directional modes.
The structural characteristics of stratocumulus cloud fields off the coast of southern California are investigated using Landsat Multispectral Scanner imagery. Twelve scenes in this area are examined along with three other stratocumulus scenes near San Francisco, over central Oregon, and in the Gulf of Mexico. Results from this initial study of stratocumulus clouds indicate that cloud-background threshold selection techniques based upon edge detection gradient assumptions are not appropriate for cloud segmentation and classification algorithms, cloud size distributions obey a power law, and cell horizontal aspect ratio increases with cell diameter. It was also found that stratocumulus clouds are bifractal in nature with fractal dimension of d of about 1.2 for cells with diameter D smaller than 0.5 km and d of about 1.5 for cells with D greater than 0.5 km; stratocumulus cloud fields appear to be homogeneous over regions of about 100 km x 100 km, a much smaller region than the 2.5-deg x 2.5-deg boxes to be used in the ISCCP regional averaging algorithms; and that structural properties of stratocumulus clouds observed off the coast of southern California are similar to those observed for stratocumulus clouds at three other locations.
The Earth Radiation Budget Experiment (ERBE) involves the use of a system of three spacecraft with instrument packages for measuring the earth's radiation balance to an accuracy which has been unattainable in previous experiments. The experiment is to determine the monthly averaged radiant exitances at te 'top of the atmosphere' (TOA) over the earth with a resolution of 250 km. The realization of this goal requires the conduction of high accuracy measurements, and the computation of the instantaneous radiant exitances from these measurements. The operation involved in this computation is called an 'inversion'. A third requirement is related to the computation of the daily and monthly averaged regional values from the instantaneous values. A brief description is provided of the algorithms for the inversion of the measurements to TOA radiant exitances, taking into account also the techniques which are used to validate the methods and results.