Evaluating the Hydrological Cycle over Land Using the Newly-Corrected Precipitation Climatology from the Global Precipitation Climatology Centre (GPCC)
The 2015 release of the precipitation climatology from the Global Precipitation Climatology Centre (GPCC) for 1951-2000, based on climatological normals of about 75,100 rain gauges, allows for quantification of mean land surface precipitation as part of the global water cycle. In GPCC's 2011-release, a bulk climatological correction was applied to compensate for gauge undercatch. In this paper we derive an improved correction approach based on the synoptic weather reports for the period
... 5. The compared results show that the climatological approach tends to overestimate the correction for Central and Eastern Europe, especially in the northern winter, and in other regions throughout the year. Applying the mean weather-dependent correction to the GPCC's uncorrected precipitation climatology for 1951-2000 gives a value of 854.7 mm of precipitation per year (excluding Antarctica) or 790 mm for the global land surface. The warming of nearly 1 K relative to pre-industrial temperatures is expected to be accompanied by a 2%-3% increase in global (land and ocean) precipitation. However, a comparison of climatology for 30-year reference periods from 1931-1960 up to 1981-2010 reveals no significant trend for land surface precipitation. This may be caused by the large variability of precipitation, the varying data coverage over time and other issues related to the sampling of rain-gauge networks. The GPCC continues to enlarge and further improve the quality of its database, and will generate precipitation analyses with homogeneous data coverage over time. Another way to reduce the sampling issues is the combination of rain gauge-based analyses with remote sensing (i.e., satellite or radar) datasets. The most recent (2015) and prior release (2011) of the precipitation climatology dataset for the period 1951-2000 are compared to each other with regard to the global average in Section 3.2 and the spatial distribution of the differences is discussed. In Section 3.3, the variations of precipitation over time are studied by comparing the precipitation climatology data for the consecutive, partly overlapping, 30-year reference periods from 1931-1960 up to 1981-2010. Rain gauge measurements are affected by the systematic gauge-measuring error (Section 4), which is a general undercatch of the true precipitation caused by wind drift over the gauge orifice (most pronounced for snowfall and small droplets), evaporation from the gauge, and wetting losses [4, 5] . Besides the features of the instrument, the systematic gauge-measuring error is dependent on the meteorological conditions and the precipitation phase [6, 7] . In 1990, Legates and Willmott (hereafter referred to as L&W1990) evaluated a station-based correction for the systematic gauge measuring-error on a global scale for climatological conditions  . Some studies [6, 7] showed that L&W1990's bulk correction tended to overestimate the systematic gauge-measuring error, i.e., for the areas of the following three experiments of the GEWEX (Global Energy and Water Cycle Exchanges Project) Hydroclimatology Panel (GHP): the Baltic Sea Experiment (BALTEX), the GEWEX Asian Monsoon Experiment (GAME), and the Large-scale Biosphere-Atmosphere (LBA) Experiment in Amazonia. Therefore, only 85% of the correction from L&W1990 has been applied in  . Since the bulk climatological correction was identified as the largest uncertainty in the previous estimation of the global mean land surface precipitation from the GPCC's precipitation climatology, we replaced it by using an improved weather-dependent correction approach. The methodology to correct the systematic gauge-measuring error on the basis of weather information from synoptic stations, thereby taking the weather conditions in the observation period into account, was developed by [7, 9] . This method was then implemented at the GPCC  and allowed for the calculation of weather-dependent correction for the systematic gauge-measuring error by using the synoptic weather reports exchanged via the Global Telecommunication System (GTS) of the WMO (World Meteorological Organization) that are received at the Deutscher Wetterdienst (DWD, German Weather Service) in Offenbach. The GPCC started these evaluations in 2007, but so far, the use of this improved approach has been hampered for not being available retrospectively. The GPCC undertook the large effort of re-assessing the synoptic weather reports back to 1982 based on the methodology according to  . This now allows an improved assessment of the correction for the gauge undercatch, taking into account the weather conditions (inter alia), as well as the precipitation phase (liquid, mixed, solid) for each individual day in the entire period since 1982 (for details see Section 4). The improved undercatch correction reduces the uncertainty in the bias correction and thereby improves the quality of the GPCC's precipitation climatology, as well as of the other precipitation data products described in [10, 11] . After having presented the new GPCC undercatch correction for the period 1982-2015 (based on SYNOP weather reports) in Section 5 it will be discussed how the mean precipitation over land derived from GPCC's new precipitation climatology fits into the larger picture of the global water cycle as described in    and how the variations over the different 30-year periods compare to the expected changes in the global water cycle in the context of climate change  .