Standardized Precipitation Index (SPI) Analyses of Multiple Global Precipitation Datasets at LDEO/IRI Climate Data Library

The Standardized Precipitation Index (SPI; McKee et al. 1993) is the number of
standard deviations that observed cumulative precipitation deviates from the
climatological average. The index is based entirely on monthly precipitation
accumulations and its values can be compared across different climatic and
geographic regions. These characteristics of the SPI have contributed to its
popularity for application towards drought and water resource monitoring.

SPI analyses were performed on the following climatological data sets:
NASA GPCP V2
NOAA NCEP CPC CAMS_OPI
NOAA NCEP CPC Merged_Analyis (CMAP)
UEA CRU New CRU05

Studies have suggested that a minimum of 50 years of precipitation data be used
to calculate SPI values. It should therefore be noted that the UEA New data set
is the only precipitation data set in the current set of analyses that meets
that recommendation. Extreme SPI values in the other three datasets (CAMS OPI,
CMAP, GPCP) may be suspect as they are based on roughly half of the recommended
amount of data.

The analyses in this data set are based on a Pearson Type III (i.e.,
3-parameter gamma) distribution as suggested by Guttman (1999). Fortran 77
source code made available in that reference was used to create the SPI
analyses.

The SPI analyses are available from:
http://ingrid.ldeo.columbia.edu/docfind/databrief/cat-atmos.html