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The "Standardized Precipitation Index" (SPI) is used to describe extremely dry or wet climate situations. The World Meteorological Organization (WMO) recommends, that all national meteorological and hydrological services should use the SPI for monitoring of dry spells (Press report December 2009, WMO No. 872).
The advantages of SPI usage are:
Definition of SPI classes
The SPI, presented here, is different from the original SPI definition of McKee et al. 1993. An enhanced SPI is used, that significantly reduces errors resulting from the determination of the precipitation's distribution (Sienz et al. 2011). MC Kee et al. 1993 shifted the time series of the SPI one time step into the future, but this is not done for the calculation of the SPI presented here.
The SPI was calculated from two precipitation data sets:
Spatial and temporal resolution
SPI (ECA&D Precipitation), Europe
SPI (CRU Precipitation), Europe and USA
The quality of the SPI can not be better than the quality of the precipitation data used for its calculation. Inhomogeneities in the observed time series, the interpolation to the grid, as well as the irregular distribution of stations in space and time also affect the data presented here. On the quality of ECA & D data, see Haylock et al. (2008) and van den Besselaar et al. (2011) and for CRU data see Mitchell and Jones (2005).
Name: Frank Sienz
Institute: KlimaCampus / University of Hamburg
Email: frank.sienz@ zmaw.de
Name: Annika Jahnke-Bornemann
Institute: ICDC, KlimaCampus / University of Hamburg
In agreement with Frank Sienz