This data set contains gridded precipitation anomalies calculated from the GHCN V2 monthly precipitation data set. 2064 homogeneity adjusted precipitation stations (from the U.S., Canada, and Former Soviet Union) were combined with a data set containing 20590 raw precipitation stations throughout the world to create these gridded fields. In grid boxes with no homogeneity adjusted data, GHCN raw data was used to provide the greatest possible global coverage. Each month of data consists of 2592 gridded data points produced on a 5 X 5 degree basis for the entire globe (72 longitude X 36 latitude grid boxes).
Gridded data for every month from January 1900 to the most recent month is available. The data are precipitation anomalies in millimeters. Each gridded value was multiplied by 100 and written to file as an integer. Missing values are represented by the value -32768.
The data are formatted by year, month, latitude and longitude. There are twelve longitude grid values per line, so there are 6 lines (72/12 = 6) for each of the 36 latitude bands. Longitude values are written from 180 W to 180 E, and latitude values from 90 N to 90 S. Data for each month is preceded by a label containing the month and year of the gridded data.
for year = begyr to endyr
for month = 1 to 12
format(2i7) month,year
for ylat = 1 to 36 (85-90N,80-85N,...,80-85S,85-90S)
format(12i7) 180-175W,175-170W,...,130-125W,125-120W
format(12i7) 120-115W,175-170W,...,70-65W,65-60W
format(12i7) 60-55W,55-50W,...,10-5W,5-0W
format(12i7) 0-5E,5-10E,...,50-55E,55-60E
format(12i7) 60-65E,65-70E,...,110-115E,115-120E
format(12i7) 120-125E,125-130E,...,170-175E,175-180E
Each file has been compressed using 'gzip'. They can be uncompressed with 'WinZip' for those using Windows 95 or with 'gzip' from most UNIX platforms. The FORTRAN utility program 'read_gridded_prcp.f' can be downloaded to assist in extracting data of interest. This program allows the user to extract non-missing values for selected months and write the data to an ascii output file. The latitude and longitude of the center of each corresponding grid box accompanies each gridded value in the output file.
We calculated these anomalies with respect to the period 1961 - 1990 using the traditional anomaly method. Anomalies were calculated on a monthly basis for all adjusted stations having at least 25 years of data in the 1961-1990 base period. Station anomalies were then averaged within each 5 X 5 degree grid box to obtain the gridded anomalies. For those grid boxes without adjusted data, anomalies were calculated from the raw station data using the same technique.
We developed this gridded data set to produce the most accurate time series possible. However, this required that we treat months and grid boxes independently through time. The use of this data is most appropriate for analyzing the change in precipitation within a particular grid box, or set of grid boxes, over a span of years. If one is more interested in analyzing temperature changes within individual years, e.g., the change in temperature between February and March, 1908, or between two regions in 1908, we recommend that the GHCN precipitation station data be used directly.
To obtain a copy of this dataset via your web browser follow these instructions:
To access anonymous FTP at NCDC use the following information:
Machine Address : ftp.ncdc.noaa.gov Login Name : anonymous Password : your email address Directory : /pub/data/ghcn/v2/grid/ Enter : bin Enter : get 'filename'
For questions about this report, please contact:
Jay Lawrimore
http://lwf.ncdc.noaa.gov/oa/climate/research/ghcn/ghcngrid-prcp.html