|FTP Site||CASPR User's Guide|
Retrieval results are available as twice daily images, monthly mean
twice daily and monthly mean values. The period covered by this data
set is from January 1, 1997 to December 31, 1998 for the local solar
times of 04:00 and 14:00.
The ftp site contains the following: 730 daily image result files and 12 monthly mean image result files, one land mask image, one latitude image, one longitude image, two daily image means files, and two monthly image means files for the local solar times of 04:00 and 14:00, respectively. In addition cloud mask images were also created. All files are stored in netCDF format. The daily image files including parameter and cloud mask image files all begin with app_ and end in .params.cdf and .cmask.cdf (but were gzipped so the actual extension is .params.cdf.gz and .cmask.cdf.gz); the monthly image files begin with mean_ and end in the same extension name as daily image files both for parameter and cloud mask image files. There are 4 means files, two for daily means results and other two for monthly means results for the whole study area. The file naming conventions will be described in data format section below. Images are 650 x 525 pixels in size, with contents and data types as described in the Image Parameter Files section below. The means files are described in the Mean Parameter Values section.
The land mask is a 650 x 525 pixel byte array, where a value of 254 indicates land and a value of 0 indicates non-land (ocean or lake). The latitude and longitude files are 650 x 525 pixel short integer (two byte) values in degrees times 100. Latitude are positive north of the equator and negative to the south. Longitudes are positive east of the Prime Meridian and negative to the west (0 to +-180 degrees times 100). These files are also stored in netCDF format.
Daily quick-look images in JPEG format are also available, but only for the period of Sept. 1, 1997 to Aug.3, 1998 corresponding to SHEBA field experiment period, and the names of the images begin with shb_ and end in .jpg. They are 3 channel composite images with channels 1, 2, and 4 used for daytime images, and channels 3, 4, and 5 for nighttime images. They are not gzipped.
The latitude, longitude, and landmask files are in the latlon directory; daily and monthly parameter results are in the params directory, mean parameter files are in the means directory, and quick-look images are in the quicklook directory.
The AVHRR sensor is a five-channel system. The scan mirror collects earth observation data during a discrete part of the scan cycle. The scan mirror observes the scene below the spacecraft in a continuous line from horizon to horizon as it rotates. Energy from the scene is collected by a telescope and separated according to wavelength by beam splitters. Signals are amplified, filtered, and applied to the 10-bit analog/digital converter, which samples all five channels simultaneously. The five channels, numbered sequentially from 1 to 5, cover the following spectral ranges: 0.58 - 0.68 mm (visible), 0.725 - 1.05 mm (reflected infrared), 3.55 - 3.92 mm (reflected/thermal infared), 10.3 - 11.3 mm (thermal infrared), 11.5 - 12.5 mm (thermal infrared).
The 10-bit resolution digital data is processed to create direct readout of High Resolution Picture Transmission (HRPT) data, Automatic Picture Transmission (APT) data, 4 km Global Area Coverage (GAC) data, and 1 km Local Area Coverage (LAC) data, to ground stations throughout the world. The AVHRR instrument scans in the across-track direction with a continuously rotating scan mirror, viewing a swath of over 100 degrees and up to 55 degrees off-nadir. Spatial resolution is approximately 1.1 km when the view is at nadir. Scanning to 55 degrees (68 degrees satellite zenith angle relative to the earth's surface) off nadir results in a ground resolution of over 2.4 km by 6.5 km at the maximum off-nadir position. The all of five channel signals received will be calibrated by calibration methods (Cracknell, 1997).
The APP data are twice-daily composites available at 5 km pixel size for June 1981-2000. The study period (January 1 1997 - December 31 998) and the area (Figure 1) correspond to the Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment, where an icebreaker drifted with the pack ice for one year . The APP standard products are clear sky surface temperature and broadband albedo, a cloud mask, sea ice motion, and the calibrated, geolocated channel data and viewing/illumination geometry (Maslanik et al., 2000; Maslanik et al., 1998; Meier 1997). We have extended this product set to include the all sky surface skin temperature and broadband albedo, cloud properties (particle phase, effective radius, optical depth, temperature, and pressure), and radiative fluxes using algorithms in the Cloud and Surface Parameter Retrieval (CASPR) system (Key, 2001). The extended APP data set is called APP-x data set. The calculation of cloudy sky surface skin temperature is based on an empirical relationship between the clear sky surface skin temperature, wind speed, and solar zenith angle (daytime). The cloudy sky broadband surface albedo is determined using the clear sky broadband albedo (interpolated from nearby pixels) adjusted by the cloud optical depth and the solar zenith angle. Radiative fluxes are computed in CASPR using FluxNet (Key and Schweiger, 1998). See Key (2001) and references therein for more information on the algorithms and their validation.
One netCDF output file is created that corresponds to each input image. First is a list of the parameters that the file contains. The values are: 1=the parameter is present, 0=not present, as a 18-element short integer array. The parameters are given in Table 1 below. In this data set, all parameters are included in the file. Next are two long integer values that specify the size (columns by rows) of the data arrays that follow. The image size in this data set is 650 x 525 (columns by rows). Then come the 18 parameters as short integer (2-byte) arrays, except for the cloud phase and the refined surface type mask which are short integer (2-byte) for daily images and float number for monthly mean images. Output images are band (parameter) sequential. Units for all values are listed in Table 1. After the image data comes the number of profile levels (short integer), followed by the temperature, humidity, and pressure profiles (float array) that are nearest the center of the image. Each profile has the specified number of levels.
Clear pixels in cloud parameter arrays and cloudy pixels in clear arrays are generally MISSING, but that is not guaranteed. MISSING is also used for any invalid pixel. MISSING values should, of course, be ignored. If no clear sky pixels are found for one or more surface types in the image, all results are set to MISSING. No cloud or flux calculations are done for solar zenith angles between DARKZEN (85 degrees) and 90 degrees, so the corresponding pixels values are MISSING. The value for MISSING is 9999.0.
Note: There is no cloud mask in the *.params.cdf files. However, one can be created directly from the cloud phase array or access the cloud mask data set to get them. Values of 0 and 1 indicate cloud; a value of 2 indicates either clear or invalid data for daily images. But for monthly mean cloud mask images the cloud phase array in *.params.cdf files are float-point number indicating the relative frequency of cloud phase occurrence of water or ice phase with the value ranged from 0 ~1 indicating more water phase or ice phase. The monthly cloud mask images are also the frequency of the cloud occurrence during that month, which can be obtained in this data set.
|17||Shortwave cloud forcing at the surface||W m-2||x10, intarr|
|18||Longwave cloud forcing at the surface||W m-2||x10, intarr|
Another file type contains the mean value of each parameter for each image. This is a text file with the date, parameter mean values, and parameter standard deviations for each of the standard parameters (Table 1) plus cloud amount. If more than one image was processed then data from all images is present. The file is structured as follows. The first record is the list of binary values corresponding to each parameter, 1 if computed, 0 if not. The second record is the number of cases (equals the number of images processed) that the file contains. The third record contains the date, which are year, month, day and hour. The fourth record contains the mean values of the parameters (19 total). The fifth record contains the standard deviations for all parameters. The records for the means and standard deviations will have values for every parameter, but those that were not computed will have values of MISSING. This includes the situation where no clear sky pixels were found for one or more of the surface types present. An example is:
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1992 6 29 21.88
272.999 0.379594 16.0942 4.33651 1.11329 262.619
652.282 1.84806 38.2547 613.432 277.001 220.099
316.748 715.730 314.146 227.239 -10.222 30.123 0.602507
8.72553 0.148731 16.9871 9.16805 0.922641 12.7292
171.225 0.257467 87.5301 135.422 34.8783 84.7163
42.6045 308.334 102.159 22.3348 5.123 6.456 0.00000
The daily quick-look image naming convention is shb_yyyymmddhhmmss.jpg, where shb means SHEBA data set, yyyy is 4-digit year, the first mm is 2-digit month, dd is 2-digit day, hh is 2-digit hour, the second mm is 2-digit minute, and ss is 2-digit second. The image means image data file, jpg indicates the JPEG image format . The values "99" in the file name indicate that the corresponding information is missing, e.g., minutes and seconds. For example, shb_19970901149999.jpg is the quick-look image for September 1, 1997 on the local time of 14:00, of which minute and second are not available.
The daily parameter image naming convention is app_n005_yyyyddd_tttt.params.cdf.gz, where app means the APP-x data set, yyyy is 4-digit year, ddd is a 3-digit julian day, tttt is 4-digit local solar time in 24-hour format with the last two digits representing the decimal hour. The params means parameter data file, cdf indicates netCDF format file, gz denotes it was compressed by "gzip" command. For example, app_n005_1997001_1400.params.cdf.gz is a parameter image in 5 km spatial resolution for the northern hemisphere for January 1, 1997 on the local solar time of 14:00, of which minute and second are not available. It was saved in netCDF file format, and compressed by "gzip".
The daily cloud mask image naming convention is app_n005_yyyyddd_tttt.cmask.cdf.gz, where app means the APP-x data set, yyyy is 4-digit year, ddd is a 3-digit day number between 1 and 365 in a year starting from the first day of January, tttt is 4-digit local solar time in 24-hour format with the last two digits representing the decimal hour.The cmask means cloud mask data file, cdf indicates netCDF format file, gz denotes it was compressed by "gzip" command. For example, app_n005_1997001_1400.cmask.cdf.gz is a cloud mask image in 5 km spatial resolution for the northern hemisphere for January 1, 1997 on the local solar time of 14:00, of which minute and second are not available. It was saved in netCDF file format, and compressed by "gzip".
The monthly mean parameter
image naming convention is mean_yyyymmdd_tttt.params.cdf.gz, where
mean means it is monthly mean APP-x data set, yyyy is
4-digit year, mm is 2-digit month, dd is
2-digit day, and tttt
is 4-digit local solar time in 24-hour format with the last two digits
representing the decimal hour. The params
means parameter data file, cdf indicates netCDF format file, gz
denotes it was compressed by "gzip" command.
For example, mean_19970999_1400.params.cdf.gz is a monthly mean
image for September 1997. It was saved in netCDF file format, and
compressed by gzip. The 99 in
the dd position means that day
information is not available since it is a monthly mean image.
The monthly mean cloud mask image naming convention is mean_yyyymmdd_tttt.cmask.cdf.gz, where mean means it is monthly mean APP-x data set, yyyy is 4-digit year, mm is 2-digit month, dd is 2-digit day, and tttt is 4-digit local solar time in 24-hour format with the last two digits representing the decimal hour. The cmask means cloud mask data file, cdf indicates netCDF format file, gz denotes it was compressed by "gzip" command. For example, mean_19970999.cmask.cdf.gz is a monthly mean cloud mask image for September 1997. It was saved in the netCDF file format, and compressed by "gzip". The 99 in the dd position means that day information is not available since it is a monthly mean image.
The daily mean parameter statistical file naming convention is app_n005_1997001_tttt.means.cdf.gz, where tttt could be 1400 or 0400 standing for different local solar times. These two files contain mean value of each parameter for each of the days on the two local solar times during the SHEBA period over the entire study area, i.e., entire area average results. They are in netCDF file format and compressed by"gzip" command.
The monthly mean parameter statistical file naming convention is mean_19970199_tttt.means.cdf.gz, , where tttt could be 1400 or 0400 standing for different local solar times. These two files contain mean value of each parameter for each of the months on the two local solar times during the SHEBA period over the entire study area, i.e., entire area average results. They are in netCDF file format and compressed by "gzip" command.
The file nlandmask5.img.cdf.gz contains the land mask type value of each pixel in the entire study area. It is netCDF file format and compressed by "gzip": command. The files nlat5.img.cdf.gz and nlon5.img.cdf.gz contains latitude and longitude values, respectively, of each pixel in the entire study area. They are in the netCDF file format and compressed by "gzip".
The routines provided for reading the netCDF files were written in IDL (Interactive Data Language from Research Systems, Inc., Boulder, Colorado). Of course, any computer programming language that has access to netCDF libraries can be used. The IDL procedures are in the file read_sheba_caspr.pro in the read directory.
|read_netcdf_landmask||Read land mask netCDF file.|
|read_netcdf_latlon||Read latitude and longitude netCDF files.|
|read_netcdf_cloudmask||Read cloud mask netCDF file.|
|read_netcdf_params||Read parameter image netCDF file.|
|read_netcdf_means||Read parameter statistical mean and standard deviation file.|
Additional documentation is provided in the individual procedures. NOTE: The netCDF files are compressed, and must be uncompressed before reading.
Key, J., 2001, The Cloud and Surface Parameter Retrieval (CASPR) System for Polar AVHRR Data User's Guide. Space Science and Engineering Center, University of Wisconsin, Madison, WI, 62 pp.
Key, J. and A.J. Schweiger, 1998, Tools for atmospheric radiative transfer: Streamer and FluxNet, Computers and Geosciences, 24(5), 443-451.
Maslanik, J.A., J. Key, C. Fowler, T. Nguyen, 2000. AVHRR-derived regional cloud and surface conditions during SHEBA and FIRE-ACE. J. Geophys.l Res., in press.
Maslanik, J., C. Fowler, J. Key, T. Scambos, T. Hutchinson, and W. Emery, 1998. AVHRR-based Polar Pathfinder products for modeling applications. Annals of Glaciology 25:388-392.
Meier, W.N., J.A. Maslanik, J.R. Key, and C.W. W. Fowler. 1997.
AVHRR-derived products for Arctic climate studies, Earth
Interactions, Vol. 1.