Users of CERES data should be aware that the CERES instrument has three primary scan modes. The Crosstrack scan mode is the same as that used in ERBE. The Rotating Azimuth Plane (RAP or biaxial) scan mode is used by CERES to provide angular coverage for Angular Distribution Models construction. The Alongtrack scan mode is used for validation of CERES instantaneous fluxes. To determine the scan mode a CERES instrument is in on any given day, just click on the scan mode character (e.g. M, X, R) in the month of interest and it will link you to daily information. Given that multiple scan modes can occur on the same day, users of instantaneous data are advised to also check the “azimuth scan plane” flag for each data record when analyzing the data to properly identify the scan mode for each CERES footprint.
Selection of CERES Instrument Data Depends on Science Goals
The tables below only represent the instrument operation modes and do not reflect the availability of data.
X = Crosstrack Scanning ( 90% or more of the data for the month is in Crosstrack scan mode )
R = Rotating Azimuth Plane (RAPS) Scanning ( 75% or more of the data for the month is in RAPS scan mode)
M = Mixed Scanning ( There is less than 90% Crosstrack data and less than 75% RAPS data )
L = Limited Days Available ( Less than 75% of CERES data available for the month )
” – ” = No Data Available
Use data from the same CERES instrument through the interval of interest. This will minimize aliasing small instrument absolute calibration differences (~ 0.5 to 1.0 W m -2) between CERES scanners into the record. The scanner calibrations are not “tuned” into agreement. In this case, instrument stability is much better than absolute accuracy. Estimated instrument stability is to within 0.25 W m -2. Note that if regional trends are being examined, reducing spatial sampling noise to less than 0.3 W m -2 requires averaging at least 9 monthly mean 1 degree grid boxes for crosstrack data, and at least 36 grid boxes for rotating azimuth plane data. The averaging can be in time and/or space as the spatial sampling error is random and unbiased.
Use crosstrack data from whichever CERES instrument was operating in crosstrack mode in the days or months of interest. This will maximize contiguous spatial coverage in the field of view and gridded data. Rotating Azimuth Plane data has large spatial sampling gaps caused by its optimization of angular sampling coverage over spatial coverage. For 100km scale regions, spatial gridding sampling noise is largest for SW fluxes. For daytime average 200 W m -2 SW reflected flux, spatial gridding noise 10 W m -2 1 sigma for crosstrack data and is 20 W m -2 for rotating azimuth plane data.
Use rotating azimuth plane scanning data to maximize angular sampling over the entire hemisphere of emitted and reflected broadband radiation.
Use along-track scanning to maximize the number of viewing angles of a particular region. This mode is used primarily for validation purposes and is activated typically every 15 days on the rotating azimuth plane scanner.