# Assessment of ZTD Derived from COSMIC Occultation Data with ECWMF, Radiosondes, and GNSS
## 3 Results and Discussions  
Between 1 January and 31 December 2008, more than 710,500 effective RO events occurred, with an average of about 2000 RO events per day having accurate inversion results. The atmospheric physical parameters corresponding to each event in this experiment were obtained by interpolation of the ECH model, so the number of events counted during this time period is consistent with the number of successfully retrieved atmospheric profiles. Due to the high credibility of the ECH model results, we use the 3 sigmas principle to remove events with occZTD deviating greatly from the corresponding echZTD value. The problematic data are less than 1%. There were 710,477 data points in the final comparison between occZTD and echZTD. Figure 1 shows the statistical diagram of the difference \(\delta\)ZTD\({}_{1}\) between the matched occZTD and echZTD data. It can be seen from Figure 1 that \(\delta\)ZTD\({}_{1}\) is normally distributed with a bias of \(-\)2.3 mm, indicating that the refractive index of the occultation data is smaller than that of the ECMWF model; the standard deviation is 9.0 mm. Figure 1: ZTD differences between matched atmPrf and echPrf in 2008. Figure 2 shows the global distribution of the matching results of occZTD and sonZTD, where the color indicates the lowest point of the common height of matched events in HSL, which is consistent with the distribution of the surface elevation. Figure 3 shows the statistical diagram of the difference \(\delta\)ZTD\({}_{2}\) between the matched occZTD and sonZTD data. Similar to the data preprocessing of occZTD and echZTD, we removed the unreliable occZTD and sonZTD data pairs by the 3 sigmas principle. The final number of statistical points is 208,536, which is one-third of the \(\delta\)ZTD\({}_{1}^{\prime}\)s number. \(\delta\)ZTD\({}_{2}\) is normally distributed (the bias is 0.8 mm; the standard deviation is 17.6 mm, which is greatly higher than that of \(\delta\)ZTD\({}_{1}\)). This phenomenon is mainly due to the temporal and spatial inconsistency between the occZTD and sonZTD data. The global distribution map of generated matched dataset introduced in Section 2 is shown in Figure 4, where the color indicates the horizontal distance (\(\delta\)r, km) between the lowest point of each occultation profile and the GPS observation station. To verify the consistency of ZTDs from occultation data and the ground based GNSS, a filter of a  
Figure 3: ZTD differences between matched atmPrf and sonPrf in 2008. Figure 2: Bottom height distribution of sonPrf matched with atmPrf in 2008. The color indicates the lowest point of the common height of matched events in HSL. horizontal distance of 300 km and a vertical distance of 1 km was reused for the statistics of the differences between GNSS-ZTD and RO ZTD. Figure 5 shows the statistical diagram of the difference \(\delta\)ZTD\({}_{3}\) between the matched data of occZTD and gnsZTD. Similar to the data preprocessing of occZTD and sonZTD, we removed the unreliable occZTD and gnsZTD data pairs by the 3 sigma principle. \(\delta\)ZTD\({}_{3}\) includes 24,278 statistical points normally distributed (bias is \(-4.6\) mm; standard deviation is 20.5 mm), but the bias and standard deviation are higher than those of \(\delta\)ZTD\({}_{1}\) or \(\delta\)ZTD\({}_{2}\). Although the ZTD of the solution from the ground based GNSS station has been greatly improved after correction, the ZTD is still very sensitive to the vertical distance (\(\delta\)h, km) between the lowest point of each occultation profile and the GPS observation station, which is discussed in the following subsection. From the comparison of the ZTDs of 2008 and 2012 in Table 1, we can find that the accuracy of the data in these two years is similar. The standard deviation (STD) of \(\delta\)ZTD\({}_{1}\), \(\delta\)ZTD\({}_{2}\), and \(\delta\)ZTD\({}_{3}\) are within 10 mm, 20 mm, and 30 mm, respectively. Compared with 2008, the number of matches in 2012 is smaller, and its STD is slightly higher.