## 2 Data and Methods
### Data Quality Check  
In our experiment, each atmPrf profile undergoes a two-step quality check. In the first step, the \"global attribute\" of the original atmPrf profile is checked to eliminate the error profiles which are marked as \"bad\" by CDAAC. In the next step, the corresponding GW Ep profile is further checked by its maximal and minimal values in the altitude range of  
Figure 1: Flow chart for calculating GW Ep.  
20-35 km. Specifically, if the maximal value in an Ep profile is higher than 50 J/kg or the minimal value is lower than 0 J/kg, this profile is considered to be non-physical, and is thus eliminated [44]. Table 1 presents the number of the original atmPrf profiles, the number of the profiles remained after the global attribute check, and the number of the qualified GW Ep profiles finally obtained from each of the RO missions during 2007 to 2020. A total of 12,247,106 dry temperature profiles from all GNSS RO missions are used in the experiment, among which 10,767,609 profiles remained after the global attribute check and are then used to calculate the GW Ep profiles. Finally, 10,736,341 qualified GW Ep profiles pass through the threshold check of Ep values. In the quality check step 1 and 2, 1,479,497 (12.08%) error profiles and 31,268 (0.26%) non-physical profiles are eliminated, respectively.  
The spatial distribution and the annual histogram of the number of the qualified GW Ep profiles derived from all the RO observations during 2007-2020 are shown in Figure 2. Figure 2 presents the spatial distribution of the number of the qualified GW Ep profiles derived from RO data after being binned into 15\({}^{\circ}\)\(\times\) 10\({}^{\circ}\) longitude--latitude grids. It can be seen that the number of the retrieved GW Ep profiles in the polar regions of 80\({}^{\circ}\) N/S-90\({}^{\circ}\) N/S is relatively small (<8000 in each grid), while other regions are generally covered by much more GW Ep profiles. The RO-derived GW Ep profiles in the mid-latitude regions of 20\({}^{\circ}\) N/S-60\({}^{\circ}\) N/S are particularly dense (>24,000 in each grid). Figure 2 shows the number of the qualified GW Ep profiles derived from each RO mission during different years. As we can see, during 2007 to 2013, the GW Ep profiles are mainly derived from the RO observations of the COSMIC mission, which provides more than 400,000 GW Ep profiles every year. However, after the year 2014, the amount of GW Ep profiles provided by the COSMIC RO observations gradually decreased, due to exceeding the service life of the mission. On the other hand, the METOP-B satellite was launched in the year 2013, and since then, large numbers of GW Ep profiles have been provided by METOP-A/B/C, other than the COSMIC mission. The total number of GW Ep profiles derived from all six RO missions in each year was more than 500,000 during 2007-2020.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**RO Mission**} & \multirow{2}{*}{**Number of the Temperature Profiles**} & **Number of the Profiles after** & **Number of the Qualified** \\  & & **Global Attribute Check** & **GW Ep Profiles** \\ \hline COSMIC & 6,669,523 & 5,521,060 & 5,507,018 \\ CHAMP & 114,161 & 106,567 & 106,242 \\ GRACE & 565,012 & 523,529 & 520,389 \\ METOP-A/B/C & 4,898,410 & 4,616,453 & 4,602,692 \\ Total & 12,247,106 & 10,767,609 & 10,736,341 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The statistics on the numbers of the original RO temperature profiles and the numbers of qualified GW Ep profiles.  
Figure 2: _Cont._