Meteorological Reference Period Summary of Replies Jürgen Vogt http://desert.jrc.ec.europa.eu/ http://edo.jrc.ec.europa.eu/

Meteorological Reference Period (SPI)

Meteorological Reference Period (SPI)

Meteorological Reference Period (SPI)

Meteorological Reference Period (SPI)

Meteorological Reference Period - SUMMARY

Key Issues Spatial comparability of SPI results (i.e., inconsistent conclusions could be obtained if different time lengths of precipitation records are involved in SPI calculation)  use of the same Reference Period is preferable! Enough stations with consistent records should be available for a good estimate of the spatial patterns.  a compromise between length of Reference Period and station density! The length of precipitation records determines the stability of the parameter estimation of the underlying distribution (e.g., Gamma, Pearson III), especially in the tails that are important for the SPI analysis.  the longer the Reference Period, the more reliable the SPI values! The current and widely used Standard Normal Period (WMO) is 1961 to 1990. Using more current periods (e.g., 1971-2000) results in slight improvement of predictive accuracy.  prediction is not the key issue for SPI! To capture recent changes in climate, the Reference Period should include recent data.  if possible, the Reference Period should include recent data!

Provisional Conclusions 30 years minimum (1961-1990, 1971-2000, 1976-2005) Include recent years to set a baseline, including recent climate trends (e.g., include the 2003 event) How far to go back without loosing too many stations? Test the impact of using two Reference Periods? 1961 – 2000 against 1971 – 2005, for example At European level use alternative data sources? GPCC? ECMWF re-analysis data?

Further Reading LLOYD-HUGHES B, SAUNDERS MA (2002): A drought climatology for Europe. Int. J. Clim., 22:1571-1592 McKEE TB, DOESKEN NJ, KLEIST J (1993): The relationship of drought frequency and duration to time scales. Proceedings of the 9th Conference on Applied Climatology, Am. Met. Soc., Boston, 179-184. WMO (2010): Guide to Climatological Practices. 3rd Edition. WU H, HAYES M, WILHITE D and SVOBODA M (2005): The effect o f the Length of Record on the SPI Calculation. Int. J. Clim., 25:505-520.

Thank You! juergen.vogt@jrc.ec.europa.eu http://desert.jrc.ec.europa.eu http://edo.jrc.ec.europa.eu