## 3 Matching Satellite and GPS IWV Measurements  
The data sets were collected for a time period of one year, from May 2012 to June 2013 for a predefined German spatial domain (from 5\({}^{\circ}\) W to 17\({}^{\circ}\) W and from 47\({}^{\circ}\) N to 56\({}^{\circ}\) N). The satellite IWV products were all provided as orbital data on a pixel-basis and were cut to the German domain. Before matching the satellite measurements with the GPS measurements, a quality check was applied to the GPS data, also with the help of comparisons with reanalysis simulations from the model COSMO-REA2 [36], to exclude erroneous stations due to, e.g., wrong meteorological data or receiver problems [37]. Also, GPS stations for which the area of 20 km radius around the station consists of more than 10% water surfaces, e.g., lakes, were not used. Together, this resulted in about 160 GPS stations from which IWV data is used in this study. Additionally, the correspondingSEVIRI mean cloud fraction, taking the SEVIRI 15 min time step closest to each GPS measurement time, was stored. For each satellite overpass, one satellite pixel was attributed to each GPS station to create data pairs of corresponding satellite and GPS IWV values, taking into account several conditions. The pixel with a valid IWV retrieval that is closest to the GPS station is taken and must be located within an area with a radius of 20 km of the GPS station. The radius was chosen such that one can assume the measured IWV at the GPS station to be representative of the water vapor field within the surroundings of the station, while at the same time collecting a large number of satellite-GPS matches for proper statistical results. In this sense, it is a trade-off between exact coincidence in space and statistical significance of the match-up datasets. Furthermore, the time difference between the satellite measurement and the GPS measurement must be within 7.5 min, which basically means the closest GPS time step is taken. After collection of the satellite-GPS matches, additional filtering was performed. For MODIS, cases with cloud fraction of 50% or higher were filtered out, using the accompanying MODIS 1 km cloud mask, to decrease the probability of including the impact of clouds on, and therefore likely erroneous, retrieved IWV values. Also, cases with MODIS IWV values higher than 50 kg/m\({}^{2}\) were filtered out to prevent the use of, for this spatial domain, unrealistically high IWV values due to retrieval issues, e.g., miss-classified water surfaces. Moreover, all cases where the height difference between the GPS station and the height of the location of the satellite pixel is 100 m or larger, were filtered out. For this, the free Digital Elevation Model (DEM) from NASA's Shuttle Radar Topography Mission (SRTM) [38] with a spatial resolution of 83 m was used. The height of a satellite pixel was determined by computing the mean height within the pixel size. The mean height difference for all matched cases ranges between 3 m and 5 m for all satellite instruments, and therefore no impact of height difference on the statistical results, presented in the next section, is expected. The filtering removed about 21%, 35% and 19% of the matched cases for IASI, MIRS and MODIS, respectively. Figure 2 shows the temporal sampling of all matched IASI, MIRS and MODIS measurements with the selected GPS stations throughout the one-year time period. This means the overpass times of the corresponding satellites over the region of interest for which IWV retrievals can be performed and matched to GPS measurements. For all instruments, numerous matched cases can be found in the morning time between 9 h and 11 h UTC, while for IASI and MIRS there are also a large amount of cases in the evening and night time due to the type of measurements, i.e., microwave radiation, used in the algorithms allowing night time retrievals. The slightly higher amount of matched cases for MIRS towards the end of the time period is related to satellite Metop-B, with a morning orbit, becoming operational in April 2013.