NOAA/NESDIS and CPC N.H. Snow Cover Data I. Operational 89X89 Northern Hemisphere Snow Cover Data 1973 - present A. Weekly Snow Cover Data File The operational snow cover product is output as an 89X89 grid that has been overlaid on a polar stereographic projection. Each yearly file consist of 52 or 53 weeks depending upon whether it's a leap year. Also, the snow data consists of ones and zeros, where one signifies that snow cover exists in a grid box and zero means no snow cover. The format of each row of data looks as follows: Where YYYY - year WK - week number (value range: 1-53 Week # of year, Note there are 53 weeks a year only during leap years.) RW - row number (There are 89 rows which corresponds with the 89X89 grid.) C - A -> columns 1-45 B -> columns 46-89 YYYYWKRWC snow data 1975 115A000000000000000000000000000000000011000000000 1975 115B00000000000000000000000000000000000000000000 1975 116A000000000000000000000000000001100011110000011 1975 116B10000000000000000000000000000000000000000000 The 89X89 snow cover data are archived in yearly files with the following names: wk1973, wk1974, ..., wk1996 Each file is approximately 500K bytes in size and the CPC plans to regularly update the last five years of data via ftp anonymous server. If the entire archive from 1973 to present is needed please send email to wd52dg@sun1.wwb.noaa.gov to arrange for that data to be made available for a limited time. (The reason is due to extremely limited storage space on our ftp anonymous server.) B. Weekly Snow Cover Calender File Next, you need to understand how each week of the year has been grouped. The calender file simply shows you what 7 days were included in each week number. The weekly snow cover calendar files are written with the following names: yr1973, y1974, ..., yr1996 Once again only the last 5 years of files have been listed on ftp anonymous, but all the files can be made available upon request. C. Land Sea Mask The water areas (grid boxes) over the Northern Hemisphere have been masked out in these data sets. If you are interested in seeing the land/sea masked used it is available via anonymous ftp with the following file name: land_sea_mask The land/sea mask is stored using the following schematic: Column Item Stored ------ ---------------------------------- 7-8 I row number 9-10 The number of patches of geography 11-78 The J range of each geography patch Example: 23011313 ie. Row 23 - 1 land patch at point 13 or land at (23,13) 240209236870 ie. Row 24 - 2 land patches 1) from point 9 to 23 2) from point 68 to 70 or land at (24,9) (24,10) ...., (24,23) and land at (24,68) (24,69) (24,70) D. Northern Hemisphere Grid This file contains the latitude, longitude, and area in SQ. KM. of each grid box. This file is useful if you need to calculate the area of snow over different regions. The CPC calculates the monthly snow cover area over North America, Eurasia, and the Northern Hemisphere. This file is available via ftp anonymous upon request and is named: grid II. Simple Program to Read 89X89 Snow Cover Data Use the program to simply read and write the 89X89 grid snow data. This file is available on ftp anonymous. A program to calculate the monthly snow cover area for different regions is available upon request. III. Program to Convert 89X89 Snow Data to 2X2 Degree Grid The program takes the original 89X89 snow data and writes it out as an unformatted 2x2 degree file. If you are using a SUN workstation then this program is probably not necessary to run because the 2x2 degree files located on ftp anonymous are compatible with your platform. The grdyyyy (where yyyy - year) files are available on anonymous ftp (Remember that the grdyyyy files on the ftp site were created using a SUN workstation and are SUN compatible ONLY!). The last 5 years of grdyyyy files can be found online and previous years are available upon request. If you are not using a SUN workstation and want to use 2x2 degree files then you must run the program convert_89to2.f to create files with a compatible format for the platform you are using. Note that this program shows you how to read the 89X89 grid and the output 2x2 degree files named: grd1973, grd1974, ..., grd1996 The program is available on anonymous ftp. Also, this program requires the file , which masks out any grid boxes over water that may be mistakenly reporting snow cover after the conversion to 2x2. IV. View 2x2 Degree Snow Cover Using GrADS Once the one year 2x2 degree grid file has been created or copied from ftp then you're just about ready to display the data using GrADS. First, copy the files ,, and plt_snw.ctl> from ftp anonymous. Compile and execute the file using the one year 2x2 file as input. The output file of this program needs to be used in the file where it says DSET. Also, change the TDEF to the week date you are looking at to get the proper title on the map. Lastly, in the file change the 'open /?/?/' statement in the file to reflect the path of the file plt_snw.ctl' . Then, 1. RUN THE GRADS PROGRAM BY TYPING: grads 2. HIT FOR LANDSCAPE MODE 3. TYPE: run snwscr 4. THIS WILL STARTUP THE SCRIPT AND THE WEEK CHOOSEN SHOULD BE DISPLAYED. If more sophisticated GrADS scripts to process more than one week of snow data is desired please contact: Don Garrett email: wd52dg@sun1.wwb.noaa.gov