## 5 Discussion and Conclusions  
ATMS channel 1 at 23.8 GHz has high sensitivity to TPW over oceans. Together with ATMS channel 2 at 31.4 GHz, the dual channels can be used to quickly estimate TPW and column cloud liquid water over oceans. The estimate is applied for quality control to satellite radiance assimilation at numerical prediction centers around the world. Over land, the ATMS channel 1 sensitivity is weak and, in some cases, close to zero, which prevents it from acquiring TPW in a single-channel retrieval. Thus, a comprehensive retrieval system like NOAA Microwave Integrated Retrieval System is used. This study found that the zero sensitivity at 23.8 GHz on TPW over land is due to the cancellation of negative sensitivity in the upper atmosphere and positive sensitivity from the low atmosphere. The sensitivity itself in each atmospheric layer is meaningful and valuable. The MiRS can generate useful precipitation and water vapor profiles by utilizing information from ATMS channel 1 and other ATMS channels in which ATMS water vapor channels 18 to 22 have the weighting peaks between 300 and 800 hPa [18]. The approach may be equivalent to water vapor slicing that is adopted from the concept [19; 20] of CO\({}_{2}\) slicing for retrieval of the temperature profile. ATMS channel 18 provides good sensitivity for high atmosphere while ATMS channel 1 contains good sensitivity for low atmosphere. The cutoff of high vs. low is where the channel 1 layer sensitivity is zero, which mainly depends on surface emissivity. The state-of-the-art NOAA Microwave Integrated Retrieval System has been operationally acquiring geophysical parameters, including TPW. MiRS uses all ATMS channels to retrieve geophysical parameters simultaneously. Global statistics of the TPW retrievals shows that the performance over lands is degraded more than 12% without using channel 1. The statistics over CONUS show that the degradation is more than 15% without using channel 1. Therefore, ATMS channel 1 is useful for acquiring TPW over oceans as well as over land.  
Passive microwave signal is very weak and can be easily contaminated by radio frequency interference. The radio frequency interference from wireless communication, such as 5G, could affect the passive microwave remote sensing data used for weather forecasts and other applications [21]. The RFI will cause a positive bias and increase the noise. At present, it is unknown how many 5G stations there are and the operating frequency. It also depends on the antenna design because RFI to satellite depends on the antenna pointing direction and side lobes. In this study, the assessment of the impact on TPW is simply without ATMS channel 1 in comparison to with the channel 1. For a typical clear-sky case of U.S. standard atmosphere over land, the simulated radiance in the unit of \(mW/\left(m^{2}.sr.cm^{-1}\right)\) for NOAA-20 ATMS channel 1 at 23.8 GHz is 0.151925E\(-\)02, more than four orders of magnitude lower than 0.102587E+03 for the infrared channel M15 at 10.7 \(\upmu\)m of the NOAA-20 Visible Infrared Imaging Radiometer Suite (VIIRS).  
Conceptualization, Q.L. and C.C.; methodology, Q.L., C.G., Y.-K.L.; software, Y.-K.L.; validation, Y.-K.L. and C.G.; formal analysis, all authors; investigation, all authors; writing--original draft preparation, Q.L.; writing--review and editing, C.C., C.G., Y.-K.L.; All authors have read and agreed to the published version of the manuscript.  
This work was supported by the NOAA under NA19 NES4320002 at the Cooperative Institute for Satellite and Earth System Studies (CISESS) at the University of Maryland/Earth System Science Interdisciplinary Center (ESSIC).  
Not applicable.  
The manuscript contents are solely the opinions of the authors and do not constitute a statement of policy, decision, or position on behalf of NOAA or the U.S. government.