NCEP/CPC 4km Global (60N - 60S) IR Dataset ---- --- --- ------ ---------- -- ------- I. BACKGROUND The Climate Prediction Center/NCEP/NWS is now making available via anonymous ftp (in digital form) globally-merged (60N-60S) pixel-resolution IR brightness temperature data (equivalent blackbody temps), merged from all available geostationary satellites (GOES-8/10, METEOSAT-7/5 & GMS). The availability of data from METEOSAT-7, which is located at 57E at the present time, yields a unique opportunity for total global (60N-60S) coverage. The global IR composites available are now available in digital form for every 1/2 hour via a weekly rotating file. The data have been corrected for "zenith angle dependence", i.e. IR temperatures for locations far from satellite nadir are erroneously cold due to a combination of geometric effects and radiometric path extinction effects. This correction allows for the merging of the IR data from the various geostationary satellites with greatly reduced discontinuities at their boundaries. Some residual differences among the data exist since the IR channels aboard the various spacecraft have slightly different characteristics and no intercalibration among the sensors has been performed. We are in the process of performing such an intercalibration, although this effect is considerably smaller than the zenith angle effects. The data availability are delayed by 2 days from real-time to abide by international data exchange agreements between NOAA and EUMETSAT (the METEOSAT data providers). We are working on a web page that will have an image browse capability (at reduced resolution) along with tools to make data downloading easier. The data are too voluminous (33 MB per 1/2 hour) to service requests for historical data or spatial subsets of the data. However, the National Climatic Data Center (NCDC) has agreed to be the official archive & distribution center for these data, although the infrastructure for this activity is not yet in place. Please note that, at present, we are using a makeshift assortment of existing in-house hardware to perform these tasks and, while reliable over the course of the past few months, is not optimal for the tasks at hand. We are in the process of procuring a dedicated system for this activity which should be installed within the next 2-3 months. ********************************************************************************* * * * This work is supported by funding from the NOAA Office of Global Programs * * for the Global Precipitation Climatology Project (GPCP and by NASA via the * * Tropical Rainfall Measuring Mission (TRMM). * * * ********************************************************************************* II. DATA STRUCTURE The "4 km" global (60N - 60S) merged IR data exist on a rectangular lat/lon grid. Each file contains 2 records: the 1st for the "on the hour" images (":00") and the 2nd for the "on the half hour" images (":30"). The data & time of each file is determined by the filename: merg_yyyymmddhh_4km-pixel, where yyyy = year mm = month dd = day hh = hour Each record is a 9896 x 3298 Fortran array of IR brightness temperatures that have been scaled to fit into 1-byte by subtracting "75" from each datum. Therefore it is necessary for the user to add a value of "75" to each data value when using the data. The orientation of the data in the array is EASTward from 0.0182E (center of gridbox) and the grid increment in the east-west direction is 0.036378335 degrees of longitude. The data proceed from North -> South beginning at 59.982N (center of gridbox) and the grid increment is 0.036383683 degrees of latitude in the north-sout direction. For GrADS users, the following ".ctl" file will be useful: DSET merg_1999042012_4km-pixel OPTIONS yrev little_endian template UNDEF 330 TITLE globally merged IR data XDEF 9896 LINEAR 0.0182 0.036378335 YDEF 3298 LINEAR -59.982 0.036383683 ZDEF 01 LEVELS 1 TDEF 99999 LINEAR 12z04Apr1999 30mn VARS 1 ch4 1 -1,40,1,-1 IR BT (add '75' to this value) ENDVARS The original ("raw") data for the GOES 12 & 11, MT-SAT, METEOSAT 9 & 7 were obtained via the McIDAS system, remapped to lat/lon gridded files & merged to form a global dataset. A zenith angle correction has been applied to the data to correct for erroneously cold IR temperatures for locations that are far from the satellite nadir position. At present no intercalibration correction has been applied to the data. The beginning scan times for each satellite, in terms of minutes after the hour, are as shown below. ":00" column refers to hour + 0 minutes; ":30" " " to hour + 30 minutes Satellite :00 :30 --------- ---- ---- GMS (centered @ 140E longitude) 57 30 **GOES-12 (east; 75W) 45 15 **GOES-11 (west; 135W) 00 30 Meteosat-9 (Greenwich) 00 30 Meteosat-7 (57E longitude) 00 30 ** GOES full-disc views are guaranteed only at 00Z, 03Z, ..., 21Z. For images NOT at these time, the GOES data may be assembled from various regioinal subsets of a full-disc view due to image scheduling of the GOES satellites. -------------------------------------------------------------------------------------- Note 1. Due to the voluminous nature of these data, historical data cannot be provided. Note 2. IR data from the various geostationary satellites are not scanned at exactly the same time. Note 3. While the nominal spatial resolution is 4 km, the data at locations far from nadir are considerably less resolved. Functionally, the data at such locations is essentially repeated among adjacent gridboxes. Note 4. At present, no intersatellite calibration has been performed on these data. Note 5. Full-disc images from GOES are guaranteed every 3 hrs only (00Z, 03Z, ... 21Z). At the non-3hr times, various subsets of GOES views are acquired and pieced together for the global product. This is the reason for relatively frequent missing data in the GOES domain. -------------------------------------------------------------------------------------- For more information contact: John Janowiak (jjanowia@umd.edu) or Bob Joyce (robert.joyce@noaa.gov)