Deutscher Wetterdienst, World Meteorological Organization
The Global Precipitation Climatology Centre drought index is a combination of the Standardized Precipitation Index (SPI; McKee et al., 1993) with the DWD modification according to Pietzsch et al. (2011) and the Standardized Precipitation Evapotranspiration Index (SPEI) according to Vicento-Serrano et al. (2009).
The 'Full Data Monthly, Version 7' DOI:10.5676/DWD_GPCC/FD_M_V7_100 is used as precipitation input data. For the monthly mean temperature data, NOAA's Climate Prediction Center (CPC) Monthly Global Surface Air Temperature Data Set is utilized (Fan and van den Dool, 2008). The temperature data are applied to calculate the potential evapotranspiration (PET) according to Thornthwaite (1948). Parameters to calculate SPI and SPEI were referenced to 1952-2013. Due to the non-applicability of the gamma distribution for precipitation in arid regions the SPI can not be calculated there. On the other hand, the PET can not be computed in cold regions applying the formula from Thornthwaite. Hence, the GPCC Drought Index is calculated as the mean of the SPI and SPEI. If one of these indices can not be computed, the one available is used, yielding an almost global coverage of the GPCC Drought Index.
The GPCC Drought Index is computed with aggregation periods of 1, 3, 6, 9, 12, 24 and 48 months. Each aggregation period is stored in a separate netCDF file. The GPCC Drought Index has a spatial resolution of 1° latitude by longitude using gridded input data fields at the same resolution.