# Assessment of ZTD Derived from COSMIC Occultation Data with ECWMF, Radiosondes, and GNSS
## 3 Results and Discussions
### Residual Variations in Matched ZTDs between occZTD and echZTD  
The ECMWF model has better accuracy in the Northern Hemisphere than in the Southern Hemisphere [29; 30]. The ZTD is affected by temperature, which has a strong relationship with latitude. To this end, we calculated the accuracy of \(\delta\)ZTD\({}_{1}\) in different latitude regions. Figure 6a shows the total amount of data in each region and the amount of data involved in the statistics after deducing the gross error (3 sigmas principle). The occultation data are evenly distributed in the Northern and Southern Hemispheres and are mainly concentrated in the middle and low latitudes (approximately 80%). Figure 6b shows the trend of \(\delta\)ZTD\({}_{1}\) with latitude. In the northern and southern high-latitude areas (latitudes greater than 60\({}^{\circ}\)), due to the low temperature and low water vapor content, the bias and standard deviation of \(\delta\)ZTD\({}_{1}\) are extremely small, and their absolute values are all less than 5 mm. As the latitude decreases, the temperature and water vapor content increase, causing the ZTD error to increase. In particular, the standard deviation near the equator reaches more than 12 mm. In addition, the bias of the ZTD in the northern equatorial region is slightly smaller than that in the southern equatorial region, which verifies that the Northern Hemisphere ECMWF model can match the occultation observation data better than the Southern Hemisphere model.  
To verify whether this phenomenon is related to the distribution of land and sea, Figure 7 showed the different accuracies of \(\delta\)ZTD\({}_{1}\) in the land and sea regions of the Northern and Southern Hemispheres. Figure 7a shows that in the Northern Hemisphere, the \(\delta\)ZTD\({}_{1}\) difference on the land has a normal distribution with an average value of \(-\)1.6 mm and a standard deviation of 6.5 mm, which is more consistent than that in the sea. Figure 7b shows that the Southern Hemisphere land precision with standard deviation of 8.1 mm is slightly better than the Southern Hemisphere marine precision with standard deviation of 8.4 mm, but the land precision is still worse in the Southern Hemisphere than in the Northern Hemisphere. Compared with land, the ocean area has fewer atmospheric instruments to obtain surface reflectivity, temperature, humidity, and pressure, etc., so that the accuracy of the ECH model in the ocean area is not as high as that of the land area, which reduces the precision of \(\delta\)ZTD\({}_{1}\).  
\begin{table}
\begin{tabular}{c c c c c} \hline  & **Year** & **echZTD** & **sonZTD** & **gnsZTD** \\ \hline Bias/mm & 2008 & \(-\)2.3 & 0.8 & \(-\)4.6 \\ STD/mm & 9.0 & 17.6 & 20.5 \\ \hline Bias/mm & 2012 & \(-\)1.2 & 0.3 & \(-\)11.9 \\ STD/mm & 2012 & 10 & 18.7 & 25.0 \\ \hline \end{tabular}
\end{table}
Table 1: Statistics of differences between occZTD and echZTD, sonZTD and gnsZTD.  
Figure 6: Latitudinal variations in 2008 of (**a**) the number of occultation events; and (**b**) residual ZTDs of matched atmPrf and echPrf.  
Figure 7: _Cont_.