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FLASHFlux SSF Terra Version1 |
Investigation: FLASHFlux
Data Product: Single Scanner Footprint TOA/Surface Fluxes and Clouds (SSF)
| Data Sets: | Terra | (Instruments: CERES-FM1 or CERES-FM2, MODIS) |
| Aqua | (Instruments: CERES-FM3 or CERES-FM4, MODIS) |
Data Set Family: Version1
Data Set Versions: Version1A, Version1B, Version1C
The Fast Longwave and SHortwave Flux (FLASHFlux) project is based upon the algorithms developed for and data collected by the Clouds and the Earth's Radiant Energy Systems (CERES) project. CERES is currently producing world-class climate data products derived from measurements taken aboard NASA's Terra and Aqua spacecrafts. While of exceptional fidelity, these data products require a considerable amount of processing to assure quality and verify accuracy and precision. The result is that CERES data are typically released more than six months after acquisition of the initial measurements. For climate studies, such delays are of little consequence especially considering the improved quality of the released data products. There are, however, many uses for the CERES data products on a near real-time basis. These include CERES instrument calibration and subsystem quality checks, CloudSat operations, seasonal predictions, land and ocean assimilations, support of field campaigns, outreach programs such as S'COOL, and application projects for agriculture and energy industries.
The FLASHFlux project was envisioned as a conduit whereby CERES data could be provided to the community within a week of the initial measurements, with the trade-off that some degree of fidelity would be exacted to gain speed. Since the FLASHFlux project was created to provide retrievals for the entire globe, this document will focus on the Model B parameters (SSF-46 to SSF-49) that provide surface fluxes for cloudy and clear sky conditions.
The purpose of this document is to inform potential users of the FLASHFlux data of the differences between the FLASHFlux and CERES data product which have the same designation. This document also provides potential users with information concerning the difference between versions within the Version1 family. This document provides the data users with: cautions where they could possibly misinterpret the data; links to further information about the data product, algorithms, and accuracy; and information about planned changes. Even though the FLASHFlux endeavor intends to incorporate the latest input data sets and improvements into its algorithms, there are no plans to reprocess the FLASHFlux data products once these modifications are in place. Thus, in contrast to the CERES data products, the FLASHFlux data products are not to be considered of climate quality. Users seeking climate quality should instead use the CERES data products.
The FLASHFlux Version1 data sets refer to all files within the Version1 family. When changes are made that may noticeably affect one or more output parameters, the letter which follows the version number is changed (e.g., Version5D, Version5E, and Version5F would all belong to the Version5 family of SSF files). All files with the same number belong in the same version family, regardless of the letter that follows. Substantial changes will result in a version number change, which also changes the version family. By definition, adding or removing SSF parameters will always result in a version number/family change. Every SSF version family has its own Data Quality Summary. The Terra and Aqua data sets with the same Version number will usually be produced and made publicly available at nearly the same time. There is, however, no FLASHFlux requirement which stipulates that this must be done.
Please note, this document is a high-level summary and represents the minimum information for scientific users of this data product. We strongly urge authors, researchers, and reviewers of research papers to periodically re-check this URL for the latest status of this Data Set Version and particularly before publication of any scientific papers using the data.
The Terra and Aqua SSF data sets contain over 160 parameters which are associated with each field of view. These parameters contain information on time and position, viewing angles, surface maps, scene type, filtered and unfiltered radiance, top-of-atmosphere (TOA) and surface fluxes, footprint area (clear, cloudy and full), footprint imager radiance statistics, and MODIS land and ocean aerosols. The parameters of immediate concern for FLASHFlux are the TOA and surface fluxes associated with SW Model B (LPSA) and LW Model B (LPLA). The full, clear and cloudy footprint area parameters contain meteorological data that are also critical to calculating the surface fluxes.
CERES defines SW (shortwave or solar) and LW (longwave or thermal infrared) in terms of physical origin, rather than wavelength. As with CERES, FLASHFlux refers to the solar radiation that enters or exits the Earth-atmosphere system as SW. LW is the thermal radiant energy emitted by the Earth-atmosphere system. Emitted radiation that is subsequently scattered is still regarded as LW. Roughly 1% of the incoming SW is at wavelengths greater than 4 µm. Less than 1 Wm-2 of the OLR is at wavelengths smaller than 4 µm. The unfiltered window (WN) radiance and flux represent emitted thermal radiation over the 8.1 to 11.8 µm wavelength interval.
The Terra and Aqua SSF data sets are unique products for studying the role of clouds, aerosols, and radiation. Each CERES footprint (nadir resolution 20-km equivalent diameter) on the SSF includes reflected shortwave (SW), emitted longwave (LW) and window (WN) radiances and TOA fluxes from the CERES instrument with temporally and spatially coincident imager-based radiances, cloud properties, aerosols, and meteorological information from a fixed 4-dimensional analysis provided by the Global Modeling and Assimilation Office (GMAO). Cloud properties are inferred from the Moderate-Resolution Imaging Spectroradiometer (MODIS) imager, which flies with CERES on both the Aqua and Terra spacecraft. MODIS is a 36-channel; 1-km, 500-m, and 250-m nadir resolution; narrowband scanner operating in crosstrack mode. To infer cloud properties, CERES uses a 1-km resolution MODIS radiance subset that has been subsampled to include only the data that corresponds to every fourth 1-km pixel and every second scanline. The SSF retains footprint imager radiance statistics for 5 of the 19 MODIS channels (SSF-115 through SSF-131). The SSF contains footprint aerosol parameters from both the 10-km spatial resolution MODIS aerosol product (SSF-132 through SSF-160) and the NOAA/NESDIS algorithm (SSF-73 through SSF-78). Surface fluxes derived from the CERES instrument using several different techniques (algorithms) are also provided. Sampling of the CERES footprints is performed to reduce processing time and data volume. When the viewing zenith is less than 63°, the SSF data sets contain only every other CERES footprint. All footprints with a viewing zenith (as defined in the CERES SSF Collection Guide (PDF)) greater than or equal to 63° are included in the SSF.
The SSF product combines the absolute calibration and stability strengths of the broadband CERES radiation data with the high spectral and spatial resolution MODIS imager-based cloud and aerosol properties. A major advantage of the SSF over the traditional ERBE-like ES-8 TOA flux data product is the angular models derived from CERES Rotating Azimuth Plane data that allow accurate radiative fluxes not only for monthly mean regional ensembles (ERBE-like capability) but also as a function of cloud type. Fluxes in the SSF are based on sets of global Terra and Aqua Angular Distribution Models (ADMs). Using these ADMs assures that accurate fluxes can be obtained for classes of both optically thin clouds and optically thick clouds. This is a result of empirical CERES angular models that classify clouds by optical depth, cloud fraction, and water/ice classes. ERBE-like TOA fluxes are only corrected for simple clear, partly-cloudy, mostly-cloudy, and overcast classes. In addition, clear-sky identification and clear-sky fluxes are expected to be much improved over the ERBE-like equivalent, because of the use of the imager cloud mask, as well as the angular models incorporating ocean wind speed and surface vegetation class.
CERES footprints containing one or more MODIS imager pixels are included on the SSF product. Since the MODIS imager can only scan to a maximum viewing zenith angle (VZA) of ~65°, this means that only CERES footprints with VZA < 67° are retained on the SSF when CERES is in the crosstrack scan mode. Sampling of the CERES footprints is performed to reduce processing time and data volume when the VZA is less than 63°. On March 30, 2005, the CERES Aqua FM4 instrument's SW channel stopped functioning, and therefore, FLASHFlux will process only Aqua FM3 data. On Terra, FLASHFlux has the choice to process either FM1 or FM2 data. FLASHFlux will typically choose to process the Terra instrument that is in the crosstrack scan mode. To determine operations on a given day from any previous month, refer to the CERES Operations in Orbit.
A full list of parameters on the SSF is contained in the SSF section of the CERES Data Products Catalog (PDF) and a definition of each parameter is contained in the SSF Collection Guide.
This Quality summary is written for all files within the Version1 family. The FLASHFlux Terra and Aqua Version1 SSFs contain the same parameters as the CERES Terra and Aqua Edition2 SSFs and are written in an identical manner. Users are referred to the CERES SSF Collection Guide which functions as a user's guide.
In many ways, the FLASHFlux Single Scanner Footprint (SSF) Version1 family of data sets for Terra is very similar to the CERES-Terra Edition2B SSF data set. Therefore, users are referred to the Terra Edition2B SSF Data Quality Summary for discussion of the Terra product.
Likewise, the FLASHFlux Single Scanner Footprint (SSF) Version1 family of data sets for Aqua is very similar to the CERES-Aqua Edition2A SSF data set. Therefore, users are referred to the Aqua Edition2A SSF Data Quality Summary for discussion of the Aqua product.
When referring to a FLASHFlux data set, please include FLASHFlux, the satellite name (Terra or Aqua) and/or the CERES instrument designation (FM1, FM2, FM3 or FM4), the specific data set version or the data set version family, and the data product. Multiple files that are identical in all aspects of the filename except for the 6 digit configuration code (see CERES SSF Collection Guide) differ little scientifically. Thus, users may analyze FLASHFlux data from the same satellite/instrument, data set version, and data product without regard to configuration code. If all the files come from one data set version, refer to the data set using that specific data set version. For example, users working only with Terra Version1A files should refer to "FLASHFlux Terra Version1A SSF," and users working with the Aqua Version1B files should refer to "FLASHFlux Aqua Version1B SSF." If the files are from numerous data set versions of the same family, then refer to the data set as "FLASHFlux Terra Version1 SSF"or "FLASHFlux Aqua Version1 SSF."
Users must analyze
FLASHFlux and CERES data sets separately.
Users should analyze FLASHFlux
data sets from different version families separately.
Both FLASHFlux and CERES rely upon the same SW Model B and LW Model B algorithms to produce the surface fluxes. Hence, both FLASHFlux and CERES rely upon similar input data sets from the meteorological measurements and MODIS. The FLASHFlux SSF is sampled in the identical manner as the CERES SSF.
FLASHFlux and CERES SSF are very similar in many ways; however, there are important differences that users should consider. These are listed below.
FLASHFlux will provide high quality data sets to the community within a week of the initial measurements. Since the FLASHFlux data sets will not be reprocessed into consistent time series records, they should not be intermixed with the CERES climate quality data sets.
FLASHFlux input data sets and algorithms will change as improvements become available; however, no reprocessing will be implemented.
FLASHFlux uses GEOS-4 firstlook data as an input. CERES uses a late-look GEOS-4 data set that is being held constant.
FLASHFlux SSF processes very shortly after the data date, and therefore, users may find more data gaps than with the CERES SSF.
FLASHFlux uses a quicklook CERES input that is not of CERES Edition2 quality. To provide the best possible near-realtime CERES radiances and fluxes, a special set of correction coefficients that contain the latest gain, spectral correction, and Rev1 scaling factor adjustments are used to process the data. These correction coefficients are updated whenever a new set of adjustments are computed from the CERES Edition data.
CERES Rev1 corrections are included in the special set of correction coefficients used by FLASHFlux and should not be applied again by the user.
Data from only one Terra and one Aqua instrument are processed each day. The selected instrument is typically in the crosstrack mode of operation. When possible, the data from the same instrument are processed for the entire month. To determine the mode of operations for each instrument in previous months, users should refer to CERES Operations in Orbit.
The FLASHFlux Terra Version1A SSF data files for March 29, 2006 contain footprints that are affected by a solar eclipse. The Version1B processing code has been updated to exclude eclipse affected footprints. No other Terra or Aqua data files will contain observations made during full or partial solar eclipses. Similarly, CERES Aqua Edition2A and Terra Edition2B do not contain observations made during full or partial eclipses.
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Users are referred to the CERES Terra Edition2B SSF Data Quality Summary and the CERES Aqua Edition2A SSF Data Quality Summary for full lists of cautions and helpful hints that apply to that satellite's SSFs. Cautions and Hints that pertain exclusively to FLASHFlux are listed below.
The FLASHFlux Terra Version1A SSF data were inadvertently produced using the Aqua ADMs as input. Many of the TOA and Surface Flux parameters have been affected. In particular, users should exercise caution when using SSF-38 to SSF-46 and SSF-48. Unaffected by this error are Time and Position, Viewing Angles, Surface Map, Scene Type, Filtered Radiances, Unfiltered Radiances, Full Footprint Area, Clear Footprint Area, Cloudy Footprint Area, Footprint Imager Radiance Statistics, MODIS Land Aerosols, and MODIS Ocean Aerosols. Once identified, the error was corrected, and processing from Terra Version1B forward will use the correct Terra ADMs.
FLASHFlux only produces data sets for one crosstrack CERES instrument from each satellite. The instrument in crosstrack mode for a satellite may change over time. Instrument operation modes typically change at a monthly boundary and are seldom made in the middle of a month. When a failure or anomaly is detected, the instrument FLASHFlux processes may abruptly switch in the middle of a month.
Users should not apply the Rev1 user applied revisions discussed in the CERES Data Quality Summary to a FLASHFlux data set. As Rev1 scaling factors become available, they will be included in the FLASHFlux data sets via the set of correction coefficients used to unfilter the CERES radiances.
The FLASHFlux Terra Version1A SSF files for March 29, 2006 include data from areas experiencing a partial or total solar eclipse. Later FLASHFlux SSF files, both Terra and Aqua, do not contain any footprints observed during an eclipse.
The Model B surface flux parameters (SSF-46 to SSF-49) have been validated for the Version1 family of data sets. These surface fluxes are computed for all-sky, unlike the Model A surface flux parameters (SSF-41 to SSF-45) which are limited to footprints with a clear area coverage (SSF-81) of 99.9% or more. Users are encouraged to use the surface fluxes from Model B rather than those from Model A.
The CERES Terra Edition2B SSF accuracy and validation discussions, which are organized into sections, may be of interest to users of the FLASHFlux Terra Version1 SSF. The CERES Terra Edition2A and Edition2B SSF have identical Cloud properties and the discussion thereof is from Edition2A. For convenience, links to these sections are provided here. Please read those sections which correspond to the parameters of interest.
Likewise, the CERES Aqua Edition1B and Edition2A SSF accuracy and validation discussions, which are organized into sections, may be of interest to users of the FLASHFlux Aqua Version1 SSF. CERES Aqua Edition1B and Edition2A SSF have identical Cloud, Aerosol, and Spatial matching properties, and all discussion thereof are from Edition1B. TOA fluxes and Surface fluxes were updated for Edition2A; however, FLASHFlux has chosen to use the Edition1B Surface fluxes algorithms for consistency with the Terra Edition 2B Surface fluxes and larger than expected instantaneous errors associated with the Edition2A algorithms. For convenience, links to the most appropriate CERES Aqua sections are provided here. Please read those sections which correspond to the parameters of interest.
Validation of the FLASHFlux results is actively being pursued. The accuracy of FLASHFlux results will be documented in this work as they become available.
The Langley Parameterized Shortwave Algorithm (LPSA), as described in Gupta et al. (2001), was developed to provide a fast radiative transfer method to investigate processes involving the Earth's shortwave (SW) surface radiation budget. Selected by the GEWEX/SRB workshop in 1993 to monitor the performance of their primary SW algorithm, LPSA has also been used by the WCRP/SRB project to produce global insolation datasets. In addition, the CERES project has employed the LPSA to calculate both instantaneous Single Scanner Footprint (SSF) surface fluxes and Time Interpolation and Spatial Averaging (TISA) data products. The LPSA consists of physical parameterizations that account for the attenuation of solar radiation in simple terms separately for clear and cloudy atmospheres. Thus, LPSA is able to directly calculate the surface insolation using the TOA SW flux, the transmittance of the clear atmosphere, and the transmittance of the clouds (Darnell et al., 1988; 1992). The clear-sky transmittance is dependent upon the broadband extinction optical depth, which accounts for all absorption and scattering processes in the clear atmosphere, and the backscattering of surface reflected radiation by the atmosphere (gases and aerosols). The contribution of the aerosols is taken into account by using aerosol optical depths derived from a five year aerosol climatology based upon the Model of Atmospheric Transport and Chemistry (MATCH) aerosol product (Rasch et al., 1997 and Collins et al., 2001). In addition, the Optical Properties of Aerosols and Clouds (OPAC) products (Hess et al., 1998) are used to determine the single scattering albedo and asymmetry parameter values. The cloud transmittance is computed in terms of the overcast, clear, and measured reflectances within each CERES footprint using a standard threshold method similar to Moser and Raschke (1984).
The Langley Parameterized Longwave Algorithm (LPLA) is a fast parameterization derived from an accurate narrowband radiative transfer model (Gupta 1989). Because of its accuracy and global applicability, the LPLA was chosen to be incorporated into the processing of both the GEWEX/SRB and CERES Single Scanner Footprint (SSF) datasets. The LPLA computes the downward longwave (LW) flux (DLF) in terms of an effective emitting temperature of the atmosphere, the column water vapor, the fractional cloud amount, and the cloud-base height for each footprint (Gupta et al., 1992). The effective emitting temperature is a weighted-average of the surface skin temperature and temperatures of the lower tropospheric layers. The effective temperature of the surface is dependent upon the surface emissivity (Wilber et al., 1999). The effective emitting temperature and column water vapor are computed from the temperature and humidity profiles available from the MOA (Meteorology, Ozone, and Aerosols) database which is maintained for all CERES processing (Gupta et al., 1997). Fractional cloud amount and cloud-base height are available for the flux calculation from the CERES cloud subsystem processing (Minnis et al., 1997). The LPLA, which inherently assumes that the LW TOA and surface fluxes are decoupled, can be used to calculate the surface LW fluxes for both clear and cloudy conditions.
In addition to the meteorological data available through the CERES SSF product, the LPSA also requires total column ozone amounts, which are only available through the FLASH MOA product. As currently implemented, both the LPSA and LPLA obtain all the required meteorological data to process the CERES data directly through FLASH MOA. The FLASH MOA data product, and hence the FLASH SSF meteorological parameters, currently rely upon the first-look GMAO products. This allows for the assimilation of the necessary meteorological parameters within the timeframe of a few days rather than a month.
The LPSA algorithm incorporates monthly climatological aerosol maps based upon external MATCH data to account for aerosol attenuation.
To improve the accuracy of the derived LW fluxes, the LPLA makes use of external surface emissivity maps which were produced at LaRC (Wilber et al., 1999).
FLASHFlux will not be able to hold the SSF processing constant. As inputs and algorithms change, the quality of the data product will also change. Minor changes that do not impact the science will be denoted by an increase in the 6 digit configuration code that appears just before the data date and hour. Changes that impact the science enough to be noted will result in a letter change within the data set version. Major changes will result in a change to the data set family.
The following are expected to have an impact on the FLASHFlux SSF:
The FLASHFlux and CERES Teams have gone to considerable trouble to remove major errors and to verify the quality and accuracy of this data. Please provide a reference to the following paper when you publish scientific results with the CERES data:
Wielicki, B. A., B. R. Barkstrom, E. F. Harrison, R. B. Lee III, G. L. Smith, and J. E. Cooper, 1996: Clouds and the Earth's Radiant Energy System (CERES): An Earth Observing System Experiment, Bull. Amer. Meteor. Soc., 77, 853-868.
When Langley DAAC data are used in a publication, we request the following acknowledgment be included:
"These data were obtained from the NASA Langley Research Center EOSDIS Distributed Active Archive Center."
The Langley DAAC requests two reprints of any published papers or reports which cite the use of data that we have distributed. This will help us determine the use of data that we distribute, which is helpful in optimizing product development. This also helps us to keep the product related references current.
For questions or comments on the FLASHFlux Quality Summary, contact the User and Data Services staff at the Atmospheric Science Data Center.
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