Earth Radiation Budget Experiment Regional, Zonal, and Global Gridded Nonscanner Averages Output Product (S-4GN) Langley ASDC Data Set Document
This document describes the Regional, Zonal, and Global Gridded Nonscanner Averages (S-4GN) Output Product and provides the user with the necessary information to use the Earth Radiation Budget Experiment (ERBE) data for scientific research studies.
The S-4GN product is produced by the ERBE Data Management Team and is archived at the EOSDIS Langley ASDC. It contains the same time and space averages of all the individual estimates of radiant flux at the top-of-the-atmosphere for one month and one spacecraft or combination of spacecraft as the S-4N product. The difference between the two products is that S-4N is arranged by region, with all parameters for a region grouped together, while S-4GN presents gridded data, with all regions for a given parameter grouped together.
The S-4GN data set consists of nonscanner data processed without scene identification information from the scanner and with the numerical filter cross track enhancement (see the Special Corrections/Adjustments Section).
Data are archived as the Regional, Zonal, and Global Gridded, Nonscanner Averages Product (S-4GN). There are five data sets which make up the S-4GN product.
The S-4GN product contains averages of flux and albedo on regional, zonal, and global scales for nonscanner data processed without scanner scene identification information.
The objectives of ERBE are:
The S-4GN product contains the same averages of flux and albedo on regional, zonal, and global scales for nonscanner data as the S-4N product. The difference in these products is the arrangement of the data; the S-4GN product is arranged by parameter values while the S-4N is arranged by region. The S-4GN is available for a single spacecraft and as a combination of all operational spacecraft for the nonscanner wide field-of-view (WFOV). The S-4GN consists of 5 Hierarchical Data Format (HDF) files per month. Each file contains a variable number of HDF Scientific Data Set Structures, depending on data type and resolution. Monthly (day), monthly (hour), daily, and monthly hourly averages are determined for each region. The data are represented as 32-bit floating point values. The values contained are as follows:
The goal of the ERBE is to produce monthly averages of longwave and shortwave radiation parameters on the Earth at regional to global scales. Preflight mission analysis lead to a three-spacecraft system to provide the geographic and temporal sampling required to meet this goal. Three nearly identical sets of instruments were built and launched on three separate spacecraft. These instruments differ principally in the spacecraft interface electronics and in the field-of-view limiters for the nonscanner instruments required because of differences in the spacecraft orbit altitudes.
The ERBS spacecraft was launched by Space Shuttle Challenger in October 1984 and was the first spacecraft to carry ERBE instruments into orbit. The ERBS was designed and built by Ball Aerospace Systems under contract to NASA Goddard Space Flight Center (GSFC), and ERBS was the first spacecraft dedicated to NASA science experiments to be launched by the Space Shuttle. The ERBS carries the Stratospheric Aerosol and Gas Experiment (SAGE II) in addition to the ERBE instruments. The Payload Operation and Control Center (POCC) at GSFC directs operations of the ERBS spacecraft and the ERBE and SAGE II instruments, employing both ground stations and the Tracking and Data Relay Satellite System (TDRSS) network. Spacecraft and instrument telemetry data are received at GSFC where the data are processed by the Information Processing Division that provides ERBE and SAGE II experiment data to the NASA Langley Research Center (LaRC).
The second and third spacecraft launched with ERBE instruments are Television Infrared Radiometer Orbiting Satellite (TIROS) N-class spacecraft, which are part of the NOAA operational meteorological satellite series. The NOAA-9 and NOAA-10 spacecraft were launched in December 1984 and September 1986, respectively. The NOAA spacecraft include other instruments, such as the Advanced Very High Resolution Radiometer (AVHRR) and the High-Resolution Infrared Radiometer Sounder (HIRS), which provide NOAA with data for near-real-time weather forecasting. Both spacecraft are in nearly sun-synchronous orbits. The equator-crossing times (at launch) of the orbital nodes for the NOAA-9 and NOAA-10 orbits were 1420 UT (ascending) and 1930 UT (descending), respectively, where UT denotes universal time. The Satellite Operations and Control Center (SOCC) at the National Environmental Satellite and Data Information Service (NESDIS) operates the NOAA spacecraft. NOAA also provides decommutation processing of the telemetry data and generates ERBE data for LaRC.
NASA tracks the ERBS spacecraft, and the North American Aerospace Defense Command (NORAD) tracks the NOAA spacecraft. The tracking data are provided to GSFC where orbit ephemeris data are calculated for all three spacecraft and provided to LaRC.
|SRB_Daily:||Surface Radiation Budget Daily Averages|
|SRB_Monthly:||Surface Radiation Budget Monthly Averages|
Earth Radiation Budget Experiment (ERBE)
The theory behind the measurements made to collect the ERBE data is non-trivial and well beyond the scope of this document. However, interested readers are referred to the following publications: NASA Reference Publication 1184, Angular radiation models for Earth-atmosphere system, Volume 1: Shortwave radiation, and Volume 2: Longwave radiation; NASA Technical Paper 2670, Calculation and accuracy of ERBE scanner measurement locations; and Smith (Reference 13).
All three sets of ERBE instruments were designed to collect data for one year but had a goal of two years. The nonscanner instruments continue to collect data for ERBS; however, the nonscanner instruments on-board NOAA-9 and NOAA-10 have been deactivated. Table 1 describes the nominal orbit parameters for each satellite at launch.
|Nominal Orbit Parameter||ERBS||NOAA-9||NOAA-10|
|Launch Date||October 5, 1984||December 12, 1984||September 17, 1986|
|Planned Duration||1 Year||1 Year||1 Year|
|Actual Duration Scanner||5-1/2 years
(February 28, 1990)
(January 20, 1987)
(May 22, 1989)
|Actual Duration Nonscanner||Operating||Over 12 years, deactivated April 3, 1997||Over 8 years, deactivated December, 1994|
|Semi-major Axis||6988 km||7248 km||7211 km|
|Mean Altitude||610 km||872 km||833 km|
|Inclination||57 deg||98 deg||98 deg|
|Nodal Period||98 minutes||102.08 minutes||101.2 minutes|
|Equator Crossing Time (at launch)||Variable||1430 Local Mean Solar Time, ascending||0730 Local Mean Solar Time, descending|
The ERBE instruments are on the ERBS, NOAA-9, and NOAA-10 satellites.
ERBS was the first spacecraft dedicated to NASA science experiments to be launched by the Space Shuttle. ERBS carries SAGE II instruments in addition to the ERBE instruments. The NOAA spacecraft include other instruments, such as the Advanced Very High Resolution Radiometer (AVHRR) and the High-Resolution Infrared Radiometer Sounder (HIRS), which provide NOAA with data for near-real-time weather forecasting.
A complete list of the measured parameters is found in Table 2.
|Nonscanner Fixed Wide field of view||1||0.2-50.0||Total Radiance|
|2||0.2 - 5.0||Shortwave Reflected|
|Nonscanner Fixed Medium field-of-view||3||0.2 - 50.0||Total Radiance|
|4||0.2 - 5.0||Shortwave Reflected|
|Fixed Solar Monitor||5||0.2 - 50.0||Total Irradiance|
|Scanner Narrow field-of-view||1||0.2 - 50.0||Total Radiance|
|2||0.2 - 45.0||Shortwave Reflected|
|3||5.0 - 50.0||Longwave Emitted|
The ERBE is a multisatellite system designed to measure the Earth's radiation budget. The ERBE instruments fly on a mid-inclination NASA satellite, (ERBS), and two sun-synchronous NOAA satellites, (NOAA-9 and NOAA-10). Each satellite carries a nonscanner instrument package with characteristics listed in Table 2.
The nonscanner instrument package contains four Earth-viewing channels and a solar monitor. The Earth-viewing channels have two spatial resolutions: a horizon-to-horizon view of the Earth, and a field-of-view limited to about 1000 km in diameter. The former are called the wide field-of-view (WFOV) and the latter the medium field-of-view (MFOV) channels. For each of the two fields of view, there is a total spectral channel which is sensitive to all wavelengths and a shortwave channel which uses a high purity, fused silica filter dome to transmit only the shortwave radiation from 0.2 to 5 microns. The solar monitor is a direct descendant of the Solar Maximum Mission's Active Cavity Radiometer Irradiance Monitor detector. Because of the concern for spectral flatness and high accuracy, all five of the channels on the nonscanner package are active cavity radiometers.
The nonscanner elevation beams can be rotated to any of three positions: launch/stow/internal calibration position (180 degrees), solar calibration position (78 degrees), and Earth-viewing (nadir) position (0 degrees). The WFOV detectors view the Earth from limb-to-limb (plus a small ring of space). The MFOV detectors are designed to include approximately an Earth view of 10 geocentric degrees within the unencumbered field of view (FOV).
The ERBE instruments were developed by TRW, Inc.
The tolerance is 1 percent for the total channel and 2 percent for the shortwave channel.
In-flight calibrations of the nonscanners were normally performed on a bi-weekly basis.
The ERBE instruments were developed by TRW, Inc. Laboratory calibrations of the ERBE nonscanner and solar monitor instruments were completed in the TRW calibration facility at Redondo Beach, California in 1984. The fundamental standards used for the ERBE instruments were the International Pressure and Temperature Standard of 1968 (IPTS-68) and the World Radiation Reference (WRR). The TRW master reference blackbody (MRBB) was calibrated using these, and the MRBB was subsequently used to transfer the calibrations to the internal blackbody (IBB) and to the shortwave channels via an integrating sphere. The results of the calibrations were reported in detail in TRW calibration documents.
In-flight calibrations are performed in order to maintain the accuracy of radiometric measurements by accounting for internal instrument component parametric changes brought about by the spacecraft's environmental variables. In-flight calibrations of the nonscanners were normally performed on a bi-weekly basis. These included internal calibrations, space looks, and solar calibrations. Internal calibrations consist of cycling of IBB temperatures (total sensors) and shortwave internal calibration source (SWICS) voltages. Space looks consist of observations of "cold" space, both before and after solar calibrations. Solar calibrations consist of measurements made while the solar disc is within the instrument's field-fo-view.
On days when internal calibrations are performed, shortwave offsets are independently determined in four ways:
In cases where the first option is not viable, the second option is used, along with a linearly-fitted delta based upon the historical differences between method 1 and method 2. The offsets determined using options 3 and 4 have never been used in production processing.
The ERBE nonscanner instrument consists of four Earth-viewing detectors and one solar monitor detector located on the head assembly. The four Earth-viewing detectors are unchopped active cavity radiometers (ACR), whereas the solar monitor is an unfiltered chopped ACR designed to measure direct solar radiation for calibrating the Earth-viewing detectors. Two of these detectors have wide field-of-view (WFOV) apertures allowing the detectors to view the entire disk of the Earth; the other two detectors have medium field-of-view (MFOV) apertures allowing the detectors to view an area about 1000 km in diameter. Two of the Earth-viewing detectors, one WFOV and one MFOV, and the solar monitor detector measure total radiation, whereas the other two Earth-viewing detectors measure shortwave radiation. The total radiation detectors are unfiltered, and the shortwave spectral bands are achieved by use of fused silica dome filters placed over the detectors.
The nonscanner instrument microprocessor processes and executes ground-commanded and stored commands to direct and control the instrument operations. The instrument can operate in several modes so that radiation measurements can be made over a wide range of operational conditions. The instrument can operate at azimuth angles between 0 and 180 degrees, and at fixed elevation beam positions of 0(nadir), 78 (solar ports), and 180 (stow or internal calibration position) degrees. Normal Earth-viewing operation is at the nadir elevation position and at an azimuth position of 180 degrees for NOAA-10, 170 degrees for NOAA-9, and 0 degrees for ERBS. The ERBE nonscanner instrument output consists of a complete cycle of radiometric and housekeeping measurements every 16 seconds. There are 20 radiometric measurements every 16 seconds, while the frequency of housekeeping measurements is either 1, 2, or 4 measurements per 16 seconds, depending on the type of measurement.
Telemetry data from the ERBE instruments on the NOAA-9 and NOAA-10 spacecraft are transmitted to Control and Data Acquisition (CDA) ground stations at Gilmore Creek, Alaska, and Wallops Island, Virginia that relay the data through a geostationary communications satellite to the SOCC at NESDIS in Suitland, Maryland. NOAA provides decommutation processing of the telemetry data and provides the data to LaRC. During portions of the ERBE mission, telemetry data from the NOAA spacecraft were transmitted to GSFC for decommutation processing and delivery to LaRC. Telemetry and tracking data from the ERBE instrument on ERBS are transmitted to the NASA ground terminal at White Sands, New Mexico through the Tracking and Data Relay Satellite System (TDRSS). The data are transmitted from the ground terminal to the NASA communications center at GSFC, where the data are processed by the Information Processing Division (IPD) that provides ERBE data to LaRC.
The spatial coverage differs with the channel and the spacecraft, as described below.
WFOV Instruments: these two fixed detectors continuously view the earth disc (plus a small ring of space). The measurements are continuous over the entire globe for NOAA-9 and NOAA-10, and between 57 degrees north and south latitudes for ERBS which precesses approximately 3.95 degrees west per day.
MFOV Instruments: these two fixed detectors continuously view an area about 1100 km in diameter (nominally, a 5 degree earth central angle at the top of the Earth atmosphere, TOA). The measurements are continuous over the entire globe for NOAA-9 and NOAA-10, and between 57 degrees north and south latitude for ERBS.
The ERBE instruments on board the NOAA-9 and NOAA-10 satellites provide global spatial coverage, while the scanner instruments on board the ERBS provides coverage between 67.5 degrees north and south latitude and the nonscanner instruments on board the ERBS provide coverage between 60 degrees north and south latitude.
Though a map is not available, the limits of coverage are discussed in the Spatial Coverage section above.
The spatial resolution differs with the four types of instruments and the two types of spacecraft (ERBS and NOAA). The WFOV instruments have 136 degree FOV on ERBS and 126 degree FOV on the NOAA satellites. The MFOV instruments have footprints of approximately 5 geocentric degree radius or 1000 km at the TOA. The solar instrument has an unencumbered FOV which observes the entire solar disk.
5 X 5 degree resolution and 10 X 10 degree nested grids are available for numerical filter nonscanner data, and 10 X 10 degree resolution is available for the shape factor nonscanner data on the S-4 output product.
Gridding is an equal-angle projection of 5.0 X 5.0 degree (MFOV, 2592 bins) and 10.0 X 10.0 degree (WFOV, 648 bins).
Binning of the data is based on an equal-angle grid of 5.0 X 5.0 degree (MFOV, 2592 bins) and 10.0 X 10.0 degree WFOV, 648 bins). In each resolution, the bin number 1 is found at 90 degree N, 0 degree W with the bin number increasing in an easterly direction.
Instruments on the three satellites (ERBS, NOAA-9, and NOAA-10) began acquiring Earth viewing data in November 1984, February 1985, and October 1986, respectively. All of the Earth-viewing nonscanner instruments collect measurements continuously except during calibrations. The solar instrument collects about 20 minutes of usable data during bi-weekly solar calibration periods.
This is an ongoing experiment, and the data are being processed out of sequence. Please consult the Langley ASDC IMS for available data granules.
Data records for the Level 2 products are instantaneous measurements and estimates. Gridded data (the S-10N, S-4N, and S-4GN products) are daily, monthly hour (hourly averages for a month), monthly day (daily averages for a month), and hourly.
For each satellite or combination of satellites, the data are stored in several different files based on the nonscanner processing; resolution (5.0 or 10.0 degrees); and type of average (regional, zonal, or global).
On the S-4GN, each item is a 32-bit floating point value; no scale factors are contained in the data.
Table 3 provides a list of all of the data items, their HDF names, data type, bits size, scale factors, and minimum and maximum values.
The scientific data items are defined by data type: monthly(day), monthly(hour), daily, or monthly hourly.
|Item Name||HDF Name||Data Type||Bits/Word||No. Values
|TSOLRD||TSOLRD MN (DAY)||MONTHLY (DAY)||32||1||100||0||500000|
|NET MN (DAY)||MONTHLY (DAY)||32||1||100||-200||200|
|NETCS MN (HOUR)||MONTHLY (HOUR)||32||1||100||-200||200|
|TSOLRH||TSOLRH MN (HOUR)||MONTHLY (HOUR)||32||1||100||0||500000|
|LW MN (DAY)||MONTHLY (DAY)||16||1||10||0||400|
|SW MN (DAY)||MONTHLY (DAY)||16||1||10||0||800|
|ALBEDO||ALBEDO MN (DAY)||MONTHLY (DAY)||16||1||1000||0||1|
|LW MN (HOUR)||MONTHLY (HOUR)||16||1||10||0||400|
|SW MN (HOUR)||MONTHLY (HOUR)||16||1||10||0||800|
|ALBEDO||ALBEDOCS MN (HOUR)||MONTHLY (HOUR)||16||1||1000||0||1|
Monthly (Day) Quantities: These are monthly means based on daily calculations of flux. For longwave (LW) quantities, the daily means are obtained from the extrapolation, interpolation, and diurnal modeling algorithms that operate on the existing longwave measurements. The extrapolation and interpolation algorithms will, in general, cross daily boundaries, but the longwave diurnal model applied to land scenes operates on a specific day.
The shortwave (SW) quantities are based on calculations for specific days. The days are defined to be symmetric about solar noon.
The monthly mean shortwave flux (SWF) based on daily SWF values, including "measurements" from the Inversion Subsystem (Reference 5) and modeled values, within this region (Wm-2).
where N = all days of month
The monthly mean longwave flux (LWF) based on all extrapolated, interpolated, and modeled LW values for the month in this region.
where N = all days of month.
ALBEDO: The monthly mean albedo from daily values, based on the sum of all SWFs calculated for days with at least one SW measurement (Dsw).
where S(d) = integrated solar radiance.
The solar incidence is integrated from sunrise to sunset for each day with SW data, assuming a sun position for the day that is fixed at its position for 0h0m0s UT. The summed SWF for each day is multiplied by the ratio of the integrated to summed solar incidence for that day to provide some corrections to the summation error.
where S'(d) and S(d) are the summed and integrated solar radiances, respectively.
Other equations used to calculate the albedo values in S-4GN may be found in the Calculates Variables Section of this document.
The monthly net flux defined from albedo in Monthly Time/Space Averaging, the sum of integrated solar incidence over the entire month, and monthly net LWF (Wm-2).
TSOLRD: The monthly total integrated solar incidence for all days of the month (W-hm-2).
Monthly (Hour) Quantities: These items are monthly means based on values averaged over the month at each local hour. In general, they result in different values for the same quantity, compared to the monthly (day) means.
The monthly mean SWF based on summing SWF values over days with at least one SW measurement, and then over each local hour (Wm-2).
where N = all days of month.
The monthly mean LWF based on extrapolated, interpolated, and modeled LW values only for days during the month that had at least one actual LW measurement (Wm-2).
ALBEDO: The monthly mean albedo from monthly hourly values, based on the sum of all SWFs calculated. There is no correction for integrated solar incidence in the monthly hourly albedo calculations. The equations used to calculate the albedo values in S-4GN may be found in the Calculated Variables Section of this document.
where S(d) = integrated solar radiance, Dsw = days with at least one SW measurement.
The monthly net flux defined from albedo in Monthly Time/pace Averaging, the solar incidence summed (not integrated) over the entire month, and monthly net LWF defined from days with at least one LW measurement (Wm-2).
where mha = monthly hourly average.
TSOLRH: The monthly total solar incidence for all days of the month (W-hm-2).
Daily values: These quantities are calculated for each day in the month.
The daily shortwave flux; i.e., the sum of all measured and modeled SWFs for every day with at least one SW measurement, corrected by the ratio of integrated to summed solar incidence (Wm-2).
where S(d) and S'(d) are the integrated and summed solar radiances, respectively.
Daily LWF consisting of measurements and extrapolated, interpolated, and modeled values (Wm-2).
ALBEDO: The daily albedo is defined as the ratio of daily SWF to the integrated daily solar incidence. The equations used to calculate the albedo values in S-4GN may be found in the Calculated Variables Section of this document.
TSOLRD: The integrated solar incidence for a day that includes at least one SW measurement (W-hm-2).
NDSW: The number of hours with SW measurements for a day that includes at least one SW measurement.
NDLW: The number of hours with LW measurements for a day that includes at least one LW measurement.
Monthly hourly values: These values are calculated for the month at each local hour.
The monthly average SWF at this hour (Wm-2).
The monthly average LWF at this hour (Wm-2).
ALBEDO: Monthly hourly albedo. The equations used to calculate the albedo values in S-4GN may be found in the Calculated Variables Section.
SOLARH: The integrated solar incidence over those days with SW data for a given hour (W-hm-2).
NHSW: The number of days that contain SW measurements for a given hour.
NHLW: The number of days that contain LW measurements for a given hour.
The fraction of cloud-free (as determined by the Inversion Subsystem) land and desert geotype. If greater than 0.5, the half-sine model is applied in the calculation of LWF (Reference 1).
Units of measurement for the calculated and measured science variables for the S-4GN data product can be found in the Variable Description/Definition section above.
The purpose of the S-4GN Output Product is to provide averages of radiant flux values and albedos using data from the Monthly Time/Space Averaging Subsystem (Reference 3) on a regional, zonal, and global basis.
The S-4GN product contains data which have been averaged to 5.0 and 10.0 grid scales. The layout of a 2.5 system is given; the 5.0 and 10.0 systems are designed similarly. In this grid system, L = longitude and λ = latitude is replaced with colatitude, where λco = 90 - λ, so that 0° <= λco <= 180°.
The following list shows the number of regions for each resolution:
|Resolution||Total No. Regions|
Four nonscanner numerical filter 5.0 regions are nested to produce a 10.0 region. This nesting is pictured, and the weighting is described further on in this section.
The S-4GN product also contains averages over the latitudinal bands (zones). The following list shows the number of latitudinal bands for each resolution:
|Total No. Regions
in Each Band
The final type of average is on a global level. Each parameter is averaged over the entire globe with area weighting.
Equation 11 gives the formula for calculating averages:
= nested average flux value
N = number of regions included in nested average
Wi = area weighting factor
Mi = individual values
For each of the three ERBE spacecraft, (ERBS, NOAA-9, and NOAA-10), there is one set of measurements (nonscanner WFOV). For the nonscanner measurements, there are two data reduction techniques (shape factor and numerical filter). For each satellite, the on-line processing proceeds as follows:
Processing is controlled by the lower resolution region numbers. A simple set of calculations can be used to derive the four higher resolution region numbers which will be nested into the lower resolution region (see Nesting Figure).
The formulas for finding the four 5.0 degree region box numbers which are nested into a 10.0 degree region box are:
|B5(1) = 144 INT [(B10 - 1)/36] + 2 MOD [(B10 - 1),36] + 1|
|B5(2) = B5(1) + 1|
|B5(3) = B5(1) + 72|
|B5(4) = B5(1) + 73|
B10(N) = 10.0° region box number
The colatitude index is used to obtain the correct area weighting value and to distinguish between polar latitudinal bands and nonpolar latitudinal bands. It can be derived from the region number as follows:
COLAT = INT[B6 - 1)/N] + 1
B6 = higher resolution region number
N = number of regions in a latitudinal band
For this process, separate zonal and global products are produced for each resolution. For numerical filter data:
and for shape factor data:
So, for each satellite and combination of satellites, there are potentially three different global (monthly) averages of each calculated quantity.
Because the ERBE grid system divides the globe into regions which are defined by equal increments of latitude and longitude rather than equal areas, the nested averages must be normalized by weighting each region based upon its area. Since regions across a latitudinal band have the same area, only one weighting factor is needed for each latitudinal band. It also follows that since the area weighting factors across a latitudinal band are the same, the zonal averages do not have to be normalized. Equation 12 gives the formula for calculating the area weighting factors.(12)
Wi = area of by region in km2
= resolution in degrees (5.0, 10.0)
R = distance from the center of the Earth to the top-of-the atmosphere (km)
= colatitude in degrees of the center of the latitudinal band region
Polar day-night indicators are used to identify those regions within approximately 23.5 degrees of the poles that experience continuous darkness or continuous daylight at certain times of the year. These regions are treated differently from those which always experience day-night cycles.
In general, radiant flux values for regions not observed by the satellite are not accumulated as part of the averaging process. However, since it is known that the shortwave radiant flux is zero when there is no daylight, those unobserved regions which are in complete darkness for the entire month (i.e., near the polar regions at certain times of the year), will have the shortwave set to zero. The reason for doing this is to reduce the error in the zonal and global averaging process.
There is a direct relationship between the daily solar declination and the colatitude of a region. This can be used to determine whether or not that region is experiencing total darkness or has some daylight and which part of the month is affected. Of course, none of this has any effect on regions in latitudinal bands which are not near the north or south poles.
The criteria for setting the polar day-night indicators for latitudes in the northern hemisphere are:
The criteria for setting the polar day-night indicators for latitudes in the southern hemisphere are:
In order to clarify this concept, some tables have been provided to illustrate the results of applying the logic described above. Table 4 shows which colatitudes are considered to be the northern and southern polar latitudes for each resolution.
Table 5 shows the solar declinations for 1985, which was chosen as the example year.
Table 6 shows the sunlit days for the northern and southern polar regions for the 2.5 degree resolution.
Table 7 gives a sample of the polar day-night indicator values for some latitudes at the 2.5 degree resolution.
|Year: 1985; Resolution: 2.5 deg.|
The S-4GN output product consists of a maximum of five nonscanner data sets for each data month, MFOV data are not being processed. The number of files per month may be less depending upon the instruments which are operational in a given month. Please refer to the Temporal Coverage Map section for the archival status of this product.
When the user is connected to the on-line Langley ASDC system, he will be able to select a particular data set pertaining to the type of ERBE S-4GN data in which he is interested. Column 1 of Table 8 lists the names of data sets which the user will see as he orders his data from the ASDC. Column 2 represents the names of the files of the data that the user will receive from the ASDC. Column 3 gives a description of each data set.
|Data Sets||Data Filename *||Description of Data Set|
|ERBE_S4GN_WFV_NF||s4gn_wnf5._yymm_s||Nonscanner wide field-of-view numerical filter 5.0° regional values|
|ERBE_S4GN_WFV_NF_N10||s4gn_wnf10_yymm_s||Nonscanner wide field-of-view numerical 5.0° nestd to 10.0° regional averages|
|ERBE_S4GN_WFV_NF_ZG||s4gn_wnfzg_yymm_s||Nonscanner wide field-of-view numerical 5.0° and 10.0° zonal averages and global averages|
|ERBE_S4GN_WFV_SF||s4gn_wsf10_yymm_s||Nonscanner wide field-of-view shape factor 10.0° regional averages|
|ERBE_S4GN_WFV_SF_ZG||s4gn_wsfzg_yymm_s||Nonscanner wide field-of-view shape factor 10.0° zonal and global averages|
* yy represents the year (e.g., 89 - 1989); mm represents the number value of a month (e.g., 01 = January, 12 = December)
s represents the satellite code:
ERBE data records are stored in a binary format called Hierarchical Data Format developed by National Center for Supercomputing Applications. When ordering this data set, the user has the option for receiving the sample read software for this data set. The "README" file for executing this software provides the necessary information to obtain data records from this data set.
A general description of data granularity as it applies to the IMS appears in the EOSDIS Glossary.
Each archived granule contains data of a defined spatial resolution, as described in Table 8, for one month.
There are two descriptive text items stored in each HDF file to generally describe the data.
File ID: Describes the data in the file with respect to instrument, resolution, and type of average. Also described are the year and month (in yymm form) in which the data were acquired by the instruments and the satellite(s) which collected the data. (Example: )
File Description: Detailed description about the data that indicates the following:
There are two types of missing data values, each based on the word size of the data. Missing regions are those regions which were not observed by the satellite. Missing data items are those regions which were observed by the satellite, but the data did not contain measurements for those data items.
The default values for all missing data items are listed in Table 9.
|No. of Bits
|Missing Data Items
For each parameter the following information is stored in the HDF file.
There are a number of steps in the processing of the ERBE data. The mathematics involved in each of these steps is beyond the scope of this document. However, interested readers are referred to the following: NASA Reference Publication 1184, Angular radiation models for Earth-atmosphere system, Volume 1: Shortwave radiation, and Volume 2: Longwave radiation; NASA Technical Paper 2670, Calculation and accuracy of ERBE scanner measurement locations; and Smith (Reference 13).
The S-4GN product is an alternative to the original ERBE S-4N product (Reference 12).
The input to the standard S-4N product is provided by the Monthly Time/Space Averaging Subsystem, which accumulates data for each region into a 32x25 matrix. There is a 31x24 submatrix within the larger matrix with each row representing one day of the month and each column representing a local hour for the region. The 25th column contains the daily averages for the month, referred to as the daily averages. The 32nd row contains the hourly averages for the month, referred to as the monthly hourly averages. The lower right-hand box contains the average of the daily averages, referred to as the monthly (day) average, and the average of the monthly hourly averages, referred to as the monthly (hour) average. The daily, monthly hourly, monthly (day), and monthly (hour) averages are calculated and stored in a regional average data base in the Monthly Time/Space Averaging Subsystem and passed to the S-4N Output Products Subsystem. These values are used to provide the nested, zonal, and global averages mentioned earlier.
Two separate types of data can be received from Monthly Time/Space Averaging (S-10N). One or more of these types of data may be missing without upsetting the processing. The two types of data are:
The Langley Research Center (LaRC) has the responsibility of processing and validating all science data from the ERBE mission and of distributing the resulting data products to the science community. The ERBE data processing system at LaRC uses a modular software subsystems approach to process the ERBE data, starting with the input telemetry and ephemeris data from Goddard Space Flight Center (GSFC) and NOAA and ending with the production of the required science data products.
The diagram in the Flowchart Figure shows the major steps in the science data processing, together with the primary input and output data products. The first step in this processing procedure is to ingest 24 hours of telemetry data from the ERBS, NOAA-9, or NOAA-10 spacecraft into the front-end processing subsystem of the Data Processing System. The processing at this step accounts for spacecraft differences and for differences in the data acquisition and handling systems of the ERBS and TIROS N satellites. The data are organized into a format that is common to data from GSFC and NOAA. Extensive data quality editing and evaluation are performed, including the checking of quality flags appended by the tracking networks and processing systems at GSFC and NOAA. The operational status of the instruments is determined, and all instrument housekeeping data and selected spacecraft housekeeping measurements are converted to engineering units and edited. Pointing vectors for the optical axes of the detectors are calculated in a local horizon coordinate system at the spacecraft.
The 8-day ephemeris data sets are processed and validated separately before combining them with the corresponding telemetry data. Orbital data are tested for consistency with data from the previous week, and tests are performed to verify the consistency of the orbit calculations between 1-minute data points. The tests include checks for in-plane and out-of-plane consistency and precision. The routine verification processing and other analyses performed to verify the accuracy of the ephemeris data have generally demonstrated accurate orbit determination for both the ERBS and NOAA spacecraft.
The next major processing stage begins with the merging of the output data from telemetry processing with data output from the ephemeris processing. The FOV locations on a surface at the TOA are determined for every radiometric measurement. The FOV locations are more critical for the scanner measurements than those of the nonscanner because of the small FOV of the scanner instrument. A FOV accuracy analysis has shown that the calculated locations of the scanner measurements are well within the FOV footprint of the instrument on the Earth.
At this processing stage, the raw measurements for each radiometric detector are also converted to incident radiances at the spacecraft. The conversion algorithms employ calibration coefficients that are based primarily on ground-based calibration data, but which are updated with results from in-flight calibrations.
In the inversion processing stage, the spacecraft altitude radiances are inverted or reduced to radiant fluxes at the TOA by both a numerical filter technique and a shape factor technique. The shortwave radiances are inverted by both techniques based on the mostly-cloudy over ocean angular distribution model. The longwave radiances are inverted based on precomputed inversion weights (numerical filter technique) and precomputed shape factors (shape factor technique). An archival product, called the S-7, is produced at this point to retain detailed time histories of estimates of the radiant fluxes at the TOA.
The time-ordered estimates of TOA fluxes are sorted into spatial sequences for the nonscanner measurements, grouping all estimates for a month together on a regional basis. A full calendar month of estimates is then retrieved for each region of the Earth. Hourly, daily, and monthly estimates of several different parameters are derived by interpolation using directional models that describe the temporal variation of the radiation budget components. Archival products of monthly averages of radiation components for the nonscanner are produced at this point.
Several archival products are produced at the final stage of data processing. The nested averages product gives values of the nonscanner fluxes from each instrument averaged over various spatial scales. The processing at this stage also combines data from all available spacecraft to produce a combined-satellite product of TOA fluxes averaged over the same spatial scales. An archival product for solar monitor measurements is also produced to provide time histories of solar calibration data. All archival data products are distributed first to the ERBE Science Team for review and validation and then to LaRC ASDC for archival.
Processing changes are described in the Special Corrections/Adjustments Section below.
Since the publication of Reference 12, several modifications have been made to Monthly Time/Space Averaging Algorithms which affected the S-4GN product in the following areas:
Monthly shortwave averages are calculated using the monthly mean albedo and the sum of the integrated daily solar incidence of all days (N) of the month:
( and S(d) are defined in Reference 12)
An alternate definition of monthly average values may be expressed in terms of monthly hourly averages. In this case, calculate the average for each of the 24 local hours using only the days with measurements and then take the mean of the local hour averages. The calculation of the monthly hourly average albedo and SWF are the same as Equation 12 (Reference 1) and Equation 6 respectively, whether one first sums through the days or the hours of the month. Obviously, shortwave interpolation cannot take place on a given day if there are no shortwave measurements for that day.
In general, is not equal to as defined by Equation 18 (Reference 1) and Equation 13 (Reference 1) respectively, differing by the usage of the longwave interpolated values on days for which there were no longwave measurements. This difference can be significant if several days of measurements are missing. If there are no shortwave measurements for a given month and the monthly total integrated solar incidence is greater than zero, the solar and net parameters of these regions are not used in the global averages. Some of these regions lie on the latitude belt where the solar terminator occurs with the seasonal movement of the solar declination. Naturally, if the monthly total integrated solar incidence equals zero, the shortwave portion of Equation 18 (Reference 11) and Equation 13 (Reference 1) is zero. The sampling problems outlined in this paragraph will ultimately have to be dealt with outside the context of ERBE operational software.
Normalized Directional Models (See Table 10)
|Solar Zenith Angle Bin Number|
|Scanner Index||0.95||0.85||0.75||0.65||0.55||0.43||0.35||0.25||0.15||0.05||Nonscanner Index|
|(Clear) 1||1.00000||1.07895||1.19737||1.32895||1.51316||1.75000||2.11842||2.67105||3.52632||4.39474||1 (Ocean)|
|(Partly Cloudy) 6||1.00000||1.12000||1.20000||1.36000||1.48000||1.72000||2.00000||2.40000||2.92000||3.56000||5 (Ocean)|
|(Mostly Cloudy) 11||1.00000||1.07843||1.13725||1.23529||1.29412||1.43137||1.56863||1.75686||1.96078||2.19608||7 (Ocean)|
Directional Model Index Selection for Scanner Measurements
Geotype(G) = 1 (Ocean) and fi = 1 (Clear) Then if fi = 1, INDEX = G
Geotype(G) = 2 (Land) and fi = 2 (Partly cloudy) Then if fi = 2, INDEX = G + 5
Geotype(G) = 3 (Snow) and fi = 3(Mostly cloudy) Then if fi = 3, INDEX = G + 10
Geotype(G) = 4 (Desert) and fi = 4 (Overcast) Then if fi = 4, INDEX = 16
Geotype(G) = 5 (Land/Ocean)+
* Storing separate but identical models for land, snow, desert, and
land/desert mix makes easier the generation of a scanner model index from cloud
and geotype information.
+ These are linear composite models (50-50 for each constituent), not independent models, which function as separate scene tyeps for scanner processing.
++Snow geotypes must be either clear or overcast.
ERBE Directional Albedo Models (See Table 11)
|Model No.||Solar Zenith Angle Bin Number|
ERBE Scene Types (See Table 12)
|Model No.||Scene||Cloud Cover (Percent)|
|1||Ocean||0 < C < 5|
|2||Land||0 < C < 5|
|3||Snow||0 < C < 5|
|4||Desert||0 < C < 5|
|5||Mixed, Land-Ocean||0 < C < 5|
|6||Partly cloudy over ocean||5 < C < 50|
|7||Partly cloudy over land or desert||5 < C < 50|
|8||Partly cloudy over land-ocean mix||5 < C < 50|
|9||Mostly cloudy over ocean||50 < C < 95|
|10||Mostly cloudy over land or desert||50 < C < 95|
|11||Mostly cloudy over land-ocean mix||50 < C < 95|
|12||Overcast||95 < C < 100|
Time/Space Averaging Deadscanner Option
The main difficulty for the time/space averaging algorithm is that there is no scene identification information when a scanner is inoperative. This condition is caused by the lack of scanner data which are necessary to apply the proper directional models when extrapolating shortwave data to hours with no observations. In order to process data retrieved during periods with no scanner, an alternative method for selecting scene identification information has been developed which is called the Deadscanner Option.
The data needed to perform the Deadscanner Option are the observed total albedo and the percentage of each geotype (ocean, land, snow, and desert) for the region where the measurement was made. By combining the non-normalized directional albedo models proportionally by geotype percentage, a set of regionally specific directional models can be produced. The relationship of the observed albedo with the albedos predicted by these models is then used to establish which elements of the scene fraction array are filled. As an example, for a region that is 600cean, 30and, and 10 0esert, the region- specific directional models are shown in Graph 4. If the observed total albedo in this region is 21.4 at a cos (solar zenith angle) of 0.4 (represented by the X on Graph 4), it can be seen that this albedo lies halfway between the partly cloudy and clear directional models. The scene fraction array, sf, will be filled in the following manner:
|sf(1) = .30 = .5 * .6||(clear ocean)|
|sf(2) = .15 = .5 * .3||(clear land)|
|sf(3) = 0.||(clear snow)|
|sf(4) = .05 = .5 * .1||(clear desert)|
|sf(5) = .30 = .5 * .6||(partly cloudy ocean)|
|sf(6) = .20 = .5 * (.3 + .1)||(partly cloudy land/desert)|
|sf(7) = 0.||(mostly cloudy ocean)|
|sf(8) = 0.||(mostly cloudy land/desert)|
|sf(9) = 0.||(overcast)|
Half-sine Model for Nonscanner Longwave Flux
In nonscanner data, in some land regions like deserts and arid mountains, longwave flux exhibits a pronounced diurnal variation. A single diurnal fit to the monthly ensemble of all longwave data points based on a half-sine curve has been added to the nonscanner algorithm. Rather than daily fits, a fit is performed on monthly hourly averages. Given a month of data, there are five criteria which are applied to determine whether or not a good fit can be obtained:
If any of these criteria are not met, the fit will not be performed and the already calculated averages will be retained.
The daytime curve is a least squares sine fit weighted by the number of measurements at each local hour. The nighttime data are simply averaged and the constant value is used for all night hours. These monthly hourly values for day and night are then stored. The resulting averages of longwave are stored in the arrays formerly used for the Monthly Hourly Longwave Average. The Daily Longwave Average values are replaced with the Monthly Hourly Longwave average values over land and deserts, if a fit is made. These Daily Longwave Average values over land are then used to calculate net radiation for the land regions. The algorithm and data products for other scene types are unchanged.
A flag to indicate whether the half-sine fit was used in a given region was added to the first data record for each region.
A test to allow use or nonuse of the numerical filter cross-track expansion algorithm used by the Inversion Subsystem (Reference 5) has been added to Monthly Time/ Space Averaging nonscanner code.
Before the data are packed and written to the output file, albedos are calculated for monthly (day), monthly (hour), daily, and monthly hourly average quantities. The albedos are calculated on a regional, zonal, and global basis using the following equations:
for individual regions:
for nested regions, zones, and the globe:
= Monthly mean shortwave flux based on daily calculations
TSOLRD = Total of monthly integrated solar incidence for all days of the month (see Reference 10)
NDAYS = The total number of days in the month
This equation involves the assumption, previously made in calculating monthly regional net flux, that the regional albedo, calculated with (in general) some missing days, is representative of the entire month. The assumption is necessary because each region will have (in general) its flux defined for a different number of days.
for individual regions:
for nested regions, zones, and the globe:
= Monthly mean shortwave flux based on monthly hourly calculations
TSOLRH: = Total of monthly integrated solar incidence for all days of the month
NDAYS = The total number of days in the month
for individual regions:
for nested regions, zones, and the globe:
= Daily shortwave flux.
SOLARD = Daily integrated solar incidence
Given the hourly average shortwave flux and integrated solar incidence for a day, the albedo is defined as the total reflected energy divided by the total incident energy.
for individual regions:
for nested regions, zones, and the globe:
= Daily shortwave radiant flux for each hour of the month
SOLARH = Integrated solar incidence for the month
Dsw = Days with at least one shortwave measurement including those days of total darkness where shortwave is defined as 0.
A discussion of various factors that may lead to errors are discussed in the Confidence Level/Accuracy Judgement section below.
The measurement of radiation budget requires a massive data processing system. ERBE's system uses about 250,000 lines of FORTRAN code. This system also uses an additional 150,000 lines of off-line diagnostic work. The stringent requirements for accuracy in the budget dictate an acute attention to detail.
The ERBE data processing system uses about 25,000 coefficients. These coefficients are conveniently arranged in three groups. The first group is the set of "calibration coefficients" that appear in the algorithms converting telemetry counts to instrument irradiation. Ground- and in-flight-calibration sources provided these coefficients. The second group includes the angular distribution models (ADMs) and spectral unfiltering coefficients needed for inversion. A categorization of the Nimbus-7 ERB measurements forms the base for the ADM's. Missing bins were filled using the reciprocity principle. A combination of radiative transfer results and measurements of the instrument spectral responses provides the spectral correction coefficients. The third and final group of parameters consists of the coefficients needed for time averaging, mainly the directional models. These models describe the dependence of each scene type's albedo upon solar zenith angle. These directional models also came from the Nimbus-7 ERB, but have been suitably supplemented by Geostationary Operational Environmental Satellite (GOES) observations where needed. The majority of the coefficients are used the inversion process.
The earth's radiation budget is not easy to measure, even indirectly. The ERBE Science Team has relied on consistency and measurement intercomparisons for validation. Fortunately, ERBE data provides a number of these checks. These criteria provide a way of judging the consistency of the various parameters in the data processing system.
The ERBE data products are complex assemblages of data and models. Thus, their uncertainties are difficult to compute. The following numbers represent estimates of the standard deviations about a given data point within which the true measurement might lie. They are not definitive confidence intervals, but are intuitively based on the observed discrepancies in the intercomparisons. It is also important to remember that different measurements have different uncertainties. First, for instantaneous radiances, we expect uncertainties of about 10f longwave observations of filtered radiance and 2 0.000000 or shortwave. Radiative transfer comparison and spectral consistency provide the basis for this uncertainty estimate. Second, on an instantaneous observation of flux from 2.5 X 2.5 degree geographic regions, the ERBS/NOAA-9 intercomparisons offer reasonable estimates of uncertainty. These are 5 Wm-2 in the longwave and 15 Wm-2 in the shortwave. Third, on a monthly average, regional basis, the uncertainties in the scanner data are about 5 Wm-2 for shortwave and 5 Wm-2 for longwave. These come from simulations with GOES data. This uncertainty represents no change from the preflight estimate. The nonscanner averages may be somewhat more uncertain because of sampling and diurnal averaging process. Fourth, the uncertainty in global, annual average net radiation is probably about 5 Wm-2. This estimate is based on the imbalance obtained using scanner data from the four validation months (April, July, and October 1985; January 1986).
Measurement errors are mentioned in the Confidence Level/Accuracy Judgement section above.
This data are being processed at the Langley ASDC on the and being archived in HDF. An inspection software has been developed by the ASDC to check to see if the data can be read. Because boundary ranges are provided in the data for every parameter, boundary checks are performed on every parameter. Finally, metadata information are extracted from the data for archive purposes.
There are no known limitations or unreliable aspects in the algorithms implemented to generate the ERBE science data.
There are no known problems or inconsistencies in the ERBE data.
The monthly hourly averaged results are a combination of measurements and models. The mean of these results represents the best estimate of the monthly hourly results. Also note that one should not just average the measurements alone to determine the monthly hourly means, because it will give a misleading diurnal cycle. The combination of measurements and models gives a more reasonable estimate when compared to full-time sampling of the GOES.
Measurements of the radiation budget provide one of the important tools for the validation of numerical models of the atmosphere. They also provide possibilities for "climate experiments" by allowing the sensitivity of the radiation budget to various forcings to be studied empirically.
The use of cloud discrimination has provided a significant new source of information on the influence of clouds and the characteristics of clear-sky fluxes. This information is particularly important in understanding cloud forcing. It is also important in describing the response of clouds to climate change: the climate cloud sensitivity.
The ERBE project plans to complete the reprocessing, which is currently in progress, of the nonscanner data using inversion and time/space averaging processes which do not use scanner scene identification information.
To continue the measurements of the radiation budget, a second project,the Clouds and the Earth's Radiant Energy System (CERES), is currently being developed. CERES is a key component of the Earth Observing System (EOS). The CERES instruments are improved models of the Earth Radiation Budget Experiment (ERBE) scanner instruments. The strategy of flying instruments on Sun-synchronous, polar orbiting satellites, such as NOAA-9 and NOAA-10, simultaneously with instruments on satellites that have precessing orbits in lower inclinations, such as ERBS, was successfully developed in ERBE to reduce time sampling errors. CERES will continue that strategy by flying instruments on the polar orbiting EOS platforms simultaneously with an instrument on the Tropical Rainfall Measuring Mission (TRMM) spacecraft, which has an orbital inclination of 35 degrees. In addition, to reduce the uncertainty in data interpretation and to improve the consistency between the cloud parameters and the radiation fields, CERES will include cloud imager data and other atmospheric parameters. The first CERES instrument is scheduled to be launched on the TRMM spacecraft in 1997. Additional CERES instruments will fly on the EOS-AM platforms, the first of which is scheduled for launch in 1998, and on the EOS-PM platforms, the first of which is scheduled for launch in 2000.
There is a program available from the EOSDIS Langley ASDC which reads the S-4GN HDF files. This program is available in both C and FORTRAN 77.
This program allows the user to read data and associated metadata to an ASCII file for a given data month, year, and satellite coverage. The programs allow you to select the type of average (regional, zonal, or global); the resolution (5.0- or 10.0-degrees as applicable to the data); and the field-of-view (medium or wide) and either numerical filter or shape factor. These selections will restrict the data to one of the 10 S-4GN HDF files. You can further subset the data selection by choosing a geographic region to read. This can be done by selecting a range of regions based on the equal-angle grid or by selecting latitude and longitude ranges. A menu of available parameters allows for the selection of particular data measurements (e.g., Longwave, Shortwave, etc.).
In order to run the read software, you will need to obtain the HDF libraries which are available in the public domain from the National Center for Supercomputing Applications (NCSA) via ftp (ftp.ncsa.uiuc.edu). These programs, along with a sample session, can be obtained through the Langley ASDC Information Management System interface or by contacting:
Langley ASDC User and Data Services Office
NASA Langley Research Center
Mail Stop 157D
Hampton, Virginia 23681-2199
Telephone: (757) 864-8656
FAX: (757) 864-8807
Comments or questions on the read software should also be sent to the Langley ASDC User and Data Services.
Typically, LaRC ASDC User Services will fill the data order and include any and all data manipulation software related to the data in the order. However, in order to compile and execute the HDF read software, you will need to obtain the HDF libraries. These libraries are in the public domain and are available from the National Center for Supercomputing Applications (NCSA) via ftp (ftp.ncsa.uiuc.edu or 126.96.36.199).
Data, programs for reading the data, and user's guides can be obtained through the EOSDIS Langley ASDC on-line system which will allow users to search through the data inventory and place orders on-line.
Langley ASDC User and Data Services Office
NASA Langley Research Center
Mail Stop 157D
Hampton, Virginia 23681-2199
Telephone: (757) 864-8656
FAX: (757) 864-8807
The Langley ASDC User and Data Services staff provides technical and operational support for users ordering data.
On a regular basis, individual ERBE data granules are reviewed by local members of the ERBE Science Team. Upon Science Team approval, the ERBE Data Management Team releases the data granule to the LaRC ASDC for archive.
There are no ERBE S-4GN output products available other than the data granules.
Brooks, D. R., E. F. Harrison, P. Minnis, J. T. Suttles, and R. S. Kandel, "Development of Algorithms for Understanding the Temporal and Spatial Variability of the Earth's Radiation Balance," Reviews of Geophysics and Space Physics, 24, 422-438, 1986.
Smith, G. L., R. N. Green, E. Raschke, L. M. Avis, B. A. Wielicki, and R. Davies, 1986. "Inversion Methods for Satellite Studies of the Earth's Radiation Budget: Development of Algorithms for the ERBE Missions." Rev. of Geophys., 24:407-421.