Evaluation of the interpolation scheme and order of calculation for extreme value indices based on the updated GPCC Full Data Daily v2022

Precipitation is a globally fundamental parameter that has an influence in many fields and determines the hydro-meteorological cycle.

This year, the Global Precipitation Climatology Centre (GPCC) updates the daily in-situ precipitation product Full Data Daily (FDD). The FDD is a reliable raster product, based on global land-surface precipitation totals. The database includes data provided by national meteorological and hydrological services, regional and global data collections as well as WMO GTS-data. The FDD is characterized in particular by the detailed quality control of the input data.

The new FDD v2022 covers a period of 39 years from Jan 1982 - Dec 2020 with daily temporal resolution and 1° spatial resolution based on the modified SPHEREMAP interpolation scheme.

This paper introduces a test study on the methodological decisions for FDD and its effects on the extreme value indices (ETCCDI). For this purpose, we tested different methods for a selected time period in conjunction with the update of the GPCC FDD v2022. The study aims to analyze 1) the impact of the interpolation method on the ETCCDI. We recalculated a sub-period of FDD using modified Spheremap or krigging as interpolation scheme. Two two global data sets are generated each with a different interpolation method. 2) We examine the influence of the computing order on the ETCCDI (1°x1° resolution) by a) calculating the extreme value indices ETCCDI station based first and interpolating later and b) interpolating first and then calculating the ETCCDI. An existing difference is expected, as these are both answers to different questions. However, the quantification of this difference is intended to answer frequently asked questions regarding the data set.

This gives us the opportunity to estimate and quantify the influence of the methodical decisions on the ETCCDI and to improve the understanding of the FDD.