## 5 Discussion  
This study provides an assessment of various well-established integrated water vapor products from several passive instruments on polar-orbiting satellites using measurements from a one-year time period, from June 2012 to May 2013, for a German domain. The instrument characteristics differ in spectral channels, spatial resolution and overpass times, while the corresponding IWV retrieval algorithms are subject to different types of limitations such as daytime and clear-sky situations. The quantification of the impact of the temporal sampling and retrieval-specific limitations on pixel-based IWV products as well as aggregated IWV data is performed using IWV measurements from the dense German GPS network as a reference dataset. The satellite IWV products under consideration are based on data from IASI, MIRS, and MODIS. The direct comparison of IWV values for each satellite instrument with the corresponding GPS IWV values reveal an overall bias of 1.77, 1.36, 1.11 and \(-\)0.31 kg/m\({}^{2}\) for IASI, MIRS, MODIS and MODIS-FUB, respectively. All instrument and GPS distributions show the most frequent occurrence of IWV around 8 kg/m\({}^{2}\), except MIRS. Both IASI and MIRS show narrower distributions, related to the large satellite pixel sizes of 12 km and 16 km, going up to about 40 km and 50 km at swath edge, respectively, which smooth out extreme IWV values. The distributions of MODIS and MODIS-FUB show the clearest bi-model shape. A direct comparison performed at each GPS station separately does not show a clear geographical distribution of mean and standard deviations of IWV differences, except for a tendency for somewhat lower standard deviations in Northern Germany, which is a flat \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline
**AIWV** & \(\overline{AI}\) & **10\%** & **50\%** & **90\%** & \(\overline{ITA}\) & \(\overline{SON}\) & \(\overline{DIF}\) & \(\overline{MAM}\) \\ \hline GPS with IASI sampling & \(-\)0.05 & 0.50 & 0.00 & \(-\)1.10 & \(-\)1.28 (28.8\%) & \(-\)0.90 (29.7\%) & \(-\)0.70 (23.1\%) & 0.45 (18.4\%) \\ IASI Bias & \(-\)1.83 & \(-\)0.28 & \(-\)1.48 & \(-\)3.79 & \(-\)3.66 & \(-\)2.62 & \(-\)2.25 & \(-\)0.74 \\ GPS with MIRS sampling & \(-\)0.25 & \(-\)0.10 & 0.30 & \(-\)1.11 & \(-\)0.83 (24.0\%) & \(-\)1.29 (24.6\%) & \(-\)1.07 (18.4\%) & 0.79 (33.0\%) \\ MIRS Bias & 1.12 & 1.90 & 1.70 & \(-\)0.81 & \(-\)0.26 & 1.01 & 0.45 & 2.05 \\ GPS with MODIS sampling & \(-\)0.05 & \(-\)1.20 & 0.30 & 0.49 & \(-\)0.41 (31.5\%) & \(-\)0.93 (32.3\%) & \(-\)3.44 (7.9\%) & \(-\)3.24 (28.3\%) \\ MODIS Bias & 1.06 & \(-\)1.35 & 1.20 & 3.85 & 1.53 & 0.07 & \(-\)4.23 & \(-\)2.40 \\ MODIS-FUB Bias & \(-\)0.36 & \(-\)1.46 & \(-\)0.11 & 0.49 & \(-\)1.23 & \(-\)1.11 & \(-\)3.36 & \(-\)3.24 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of difference between average GPS IWV obtained from satellite sampling (GPS with satellite sampling) as well as average satellite IWV (Bias) and average GPS IWV obtained from all GPS time steps and all GPS stations (\(\Delta\)IWV in kg/m\({}^{2}\)). Results are shown for the average computed from all IWV values (\(\overline{AI}\)), the 10 th, 50 th and 90 th percentiles and for the averages computed from all IWV values within the periods June-August (\(\overline{JIA}\)), September-November (\(\overline{SON}\)), December-February (\(\overline{DJF}\)) and March-May (\(\overline{MAM}\)). Numbers within brackets indicate the relative frequency of IWV values with respect to the complete time period of one year. Please note that the relative frequency of IWV values from all GPS time steps and all stations with respect to the complete time period of one year is about 25% for each season.