## 4 Retrieval of Atmospheric Water Vapor
Over land, a more than 12% degradation is observed. Table 3 shows the comparison over the Continental United States (CONUS) for the 5G RFI study. The degradation in TPW retrieval without channel 1 is more than 15% over land within the CONUS. The global results for 15 March and 26 April 2019 are very similar. So are the results over the Continental United States. We also checked the MiRS retrieval results using SNPP ATMS for 25 March 2019. The retrieved results for all three days are consistent. It needs to be pointed out that \"Sea\" in Table 3 only represents a small portion of the Atlantic and Pacific adjacent to land and water inland. The results above demonstrated the importance of ATMS ch. 1 at 23.8 GHz to TPW retrieval over land as well. The MiRS retrieval system utilizes the information from all channels. Figure 1 shows both the positive sensitivity (Jacobean) to water vapor in the boundary layer and the negative sensitivity (Jacobean) to water vapor above the boundary layer in the MiRS help TPW retrieval, although ATMS ch. 1 alone cannot be used to derive TPW when the total sensitivity is close to zero (see Equation (7)). The positive sensitivity means that the satellite-measured brightness temperature increases with water vapor because the boundary layer emission is higher than the surface emission. The surface emission is the product of the surface emissivity by Planck function for the surface temperature. Oppositely, the negative sensitivity indicates the atmospheric emission above the boundary layer is less than the surface emission. The underlying physics is the sensitivity of the ATMS ch. 1 to the boundary layer water vapor is canceled by the sensitivity to water vapor above the boundary layer over the land, which prevents one from using Equation (7) to calculate the water vapor increment mathematically. Together with other ATMS channels in MiRS, the ATMS channel 1 can provide good sensitivity for retrieving water vapor in the boundary layer. We notice that the impact values without using ATMS channel 1 on TPW are different over global (Table 2) and Continental United States (Table 3), in particular over land under cloudy conditions. The impact value depends on geographical regions because TPW and clouds depend on geographical regions as well. The global impact value (Table 2) is statistically representative. The conclusion is the same in that the TPW retrieval will be \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**All ATMS Channels**} & \multicolumn{3}{c}{**ATMS Without Channel 1**} \\ \hline  & \multicolumn{2}{c}{Clear} & \multicolumn{2}{c}{Cloudy} & \multicolumn{2}{c}{Clear} & \multicolumn{2}{c}{Cloudy} \\ \hline  & Land & Sea & Land & Sea & Land & Sea & Land & Sea \\ \hline Bias & 1.065 & 1.690 & 1.880 & 0.780 & 1.505 & 2.380 & 3.465 & 0.800 \\ \hline Standard deviation & 3.338 & 2.225 & 3.178 & 2.433 & \begin{tabular}{c} 3.840 \\ (15\%) \\ \end{tabular} & \begin{tabular}{c} 2.705 \\ (21.5\%) \\ \end{tabular} & \
\begin{tabular}{c} 4.288 \\ (34.9\%) \\ \end{tabular} &
\begin{tabular}{c} 3.655 \\ (50.2\%) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Same as Table 2, but for April 26, 2019 over the Continental United States. Figure 2: A case of a MiRS ATMS operational product. The black line is the ECMWF data (reference). The green line is the operational MiRS retrieval and the red line is the MiRS experimental test without ATMS channel 1.  
degraded without the ATMS channel 1 measurement. For comprehensive studies, more experiments to cover different seasons and geographies are definitely necessary.