The Clouds and the Earth s Radiant Energy System (CERES) experiment is one of the highest priority scientific satellite instruments developed for EOS. CERES products include both solar-reflected and Earth-emitted radiation from the top of the atmosphere to the Earth’s surface. Cloud properties are determined using simultaneous measurements by other EOS instruments such as the Moderate Resolution Imaging Spectroradiometer (MODIS). Analyses of the CERES data, which build upon the foundation laid by previous missions such as the Earth Radiation Budget Experiment (ERBE), will lead to a better understanding of the role of clouds and the energy cycle in global climate change.
The CERES instrument draws heavily on ERBE heritage, both in design and in the way the instruments are operated in flight.
The radiometer sensor system consists of three co-aligned broadband thermistor bolometer detectors, each with an active and a compensating flake. The three detectors are identical except for optical filters on two detectors (longwave and shortwave) which restrict their spectral ranges to a portion of the Earth’s radiation bandwidth. Smaller detector fields-of-view and a reduction in aliasing effects increase the resolution of the CERES instruments over that of ERBE. More importantly, the CERES instrument will have a significant reduction in the electronic noise output by the detectors.
CERES has compiled a comprehensive list of acronyms here.
The CERES collection is a global data set whose spatial coverage depends on the satellite orbit. The spatial coverage of CERES data over a 24hr period is shown in the following table.
The terms Field of View (FOV) and footprint are synonymous. The CERES FOV is determined by its PSF (point spread function) which is a two-dimensional, bell-shaped function that defines the CERES instrument response to the viewed radiation field.
The resolution of the CERES radiometers is usually referenced to the optical FOV which is 1.3° in the along-track direction and 2.6° in the cross-track direction. For example, on TRMM with a satellite altitude of 350 km, the optical FOV at nadir is 8 X 16 km which is frequently referred to as an equivalent circle with a 10 km diameter, or simply as 10 km resolution. On EOS-AM with a satellite altitude of 705 km, the optical FOV at nadir is 16 X 32 km or 20 km resolution.
The CERES FOV or footprint size is referenced to an oval area that represents approximately 95% of the PSF response. Since the PSF is defined in angular space at the instrument, the CERES FOV is a constant in angular space, but grows in surface area from a minimum at nadir to a larger area at shallow viewing angles. For TRMM, the length and width of this oval at nadir is 19 X 15 km and grows to 138 X 38 km at a viewing zenith angle of 70°. For EOS-AM/PM, the length and width at nadir is 38 X 31 km and grows to 253 X 70 km at a viewing zenith angle of 70°.
The TOA, Top-of-the-Atmosphere, is a surface approximately 20 km above the Earth surface. Specifically, the TOA is an ellipsoid x2/a2 + y2/a2 + z2/b2 = 1 ; where a = 6408.1370 km and b = 6386.6517 km.
Since not measured directly, the LW TOA radiance is determined using the TOT – SW measurements, each corrected for its spectral response. The LW TOA flux is determined by applying an empirical Angular Distribution Model (ADM) anisotropic correction factor to the LW radiance.
As the radiation from Sun reaches Earth it first interacts with top of the atmosphere (TOA) layers, then with various atmospheric layers containing gases, clouds, aerosols, and/or other constituents before reaching the surface. In each of these material layers solar radiation is being scattered and/or absorbed. Moreover, these atmospheric and surface constituents emit their own share of radiation. Due to the specifics of this complex interaction, one can separate its energetics it in two primary spectral parts: the shortwave (SW) and the longwave (LW). Depending on the specifics of the physical processes under investigation, this broad spectra can be further divided into finer and finer spectral intervals.
All-Sky – The all-sky (or total) scene is determined from all CERES footprints (20 km nominal resolution) within the given temporal or spatial domain.
Clear-Sky – The clear-sky scene has different algorithms depending on the product, as explained below:
The LW all-sky and clear-sky surface flux is calculated at all hourly increments during the month, regardless of cloud amount. The GEOS-4 profile is the same for both clear-sky and all-sky conditions. The all-sky condition includes the cloud properties in the LW flux parameterization. The SW clear-sky/all-sky surface flux is only calculated from hourly increments that have an associated observed or CERES-only flux temporally interpolated TOA clear-sky SW flux/TOA all-sky SW flux during the month. However, many cloudy (ITCZ, maritime stratus) regions may not have CERES clear-sky footprint observations for the entire month; the CERES SSF product makes no attempt to fill these regions. If there are no clear-sky footprints within the temporal or spatial domains the surface SW flux is set to a default value.
See CERES Instruments in Cross-Track Scan Mode.
A CERES Dateset name is formedCER_‹ProductID›_‹ Sampling-Strategy›_‹ Production-Strategy›Eg. Dataset Name: CER_SSF_Terra-FM1-MODIS_Edition2B
A CERES file name is formed using the dataset name with additional information to make each file name unique.CER_‹ProductID›_‹ Sampling-Strategy›_‹ Production-Strategy›_‹ Config-Code›.‹ date›Eg. File Name: CER_SSF_Terra-FM1-MODIS_Edition2B_120145.2001052812
The ERBE-like Instantaneous TOA Estimates (ES-8) product contains 24 hours of instantaneous Clouds and the Earth’s Radiant Energy System(CERES) data for a single scanner instrument. The ES-8 contains filtered radiances recorded every 0.01-second for the total (TOT), shortwave (SW), and window (WN) channels and the unfiltered SW, longwave (LW), and WN radiances. The SW and LW radiances at spacecraft altitude are converted to Top-of-the-Atmosphere (TOA) fluxes with a scene identification algorithm and Angular Distribution Models (ADMs) which are “like” those used for the Earth Radiation Budget Experiment (ERBE). The TOA fluxes, scene identification, and angular geometry are included on the ES-8. Complete listings of metadata and science parameters are listed in Tables 2.2-1 through 2.2-4.
The ERBE-like Monthly Geographical Averages (ES-4) product contains a month of space and time averaged Clouds and the Earth’s Radiant Energy System (CERES) data for a single scanner instrument. The ES-4 is also produced for combinations of scanner instruments. For each observed 2.5° spatial region, the daily average, the hourly average over the month, and the overall monthly average of shortwave and longwave fluxes at the Top-of-the-Atmosphere (TOA) from the CERES ES-9 product are spatially nested up from 2.5° regions to 5° and 10° regions, to 2.5°, 5°, and 10° zonal averages, and to global monthly averages. For each nested area, the albedo and net flux are given. For each region, the daily average flux is estimated from an algorithm that uses the available hourly data, scene identification data, and diurnal models. This algorithm is “like” the algorithm used for the Earth Radiation Budget Experiment (ERBE).
The ES-4 archival data product is created as an HDF file which contains nine HDF Vgroups corresponding to regional, nested regional, zonal, and global averages (see Table 2.4-3). There are 10,368 2.5° regions for the ERBE-like data; therefore, there is a maximum of 10,368 records in the 2.5° regional data set. The second set of data is the 2.5° nested to 5° regional data, which constitutes a maximum of 2,592 records. The third set of data is the 5° nested to 10° regional data, which constitutes up to 648 records. The fourth, fifth, and sixth sets of data are the 2.5°, 5°, and 10° zonally averaged data which constitute 72, 36, and 18 records, respectively. The seventh, eighth, and ninth sets of data are the 2.5°, 5°, and 10° globally averaged data which constitutes 1 record each. A summary of the contents of this data product can be found in Table 2.4-1.
The Single Scanner Footprint TOA/Surface Fluxes and Clouds (SSF) product contains one hour of instantaneous Clouds and the Earth’s Radiant Energy System (CERES) data for a single scanner instrument. The SSF combines instantaneous CERES data with scene information from a higher-resolution imager such as Visible/Infrared Scanner (VIRS) on TRMM or Moderate-Resolution Imaging Spectroradiometer (MODIS) on Terra and Aqua. Scene identification and cloud properties are defined at the higher imager resolution and these data are averaged over the larger CERES footprint. For each CERES footprint, the SSF contains the number of cloud layers and for each layer the cloud amount, height, temperature, pressure, optical depth, emissivity, ice and liquid water path, and water particle size. The SSF also contains the CERES filtered radiances for the total, shortwave (SW), and window (WN) channels and the unfiltered SW, longwave (LW), and WN radiances. The SW, LW, and WN radiances at spacecraft altitude are converted to Top-of-the-Atmosphere (TOA) fluxes based on the imager defined scene. These TOA fluxes are used to estimate surface fluxes.
Only foot prints with imager coverage are included on the SSF which is much less than the full set of footprints on the CERES ES-8 product. The number of possible footprints on an SSF depends on the elevation scan mode, azimuth scan mode, and height of the satellite. Since elevation and azimuth scan modes are programmable, the range on the number of footprints in an SSF product has been set to the largest possible range, namely 0..360000 as shown in Table2.5-2. A smaller number of footprints is used in SSF sizing estimates, namely the estimated maximum number of TRMM full Earth-view footprints per hour given a normal elevation scan and an along-track azimuth scan. Accounting for the need for imager coverage, the actual number of footprints is expected to be even smaller. This reduction of footprints due to lack of imager coverage is very evident when CERES is operating in a cross-track azimuth scan mode. A complete listing of parameters for this data product can be found in Tables 2.5-3 to Table 2.5-15.
