# Performance Evaluation and Noise Mitigation of the FY-3E Microwave Humidity Sounder
## 3 RTM Simulation  
Assuming that the background field is an unbiased reference, the calculation of the \"O\(-\)B\" is widely used to estimate the biases and errors of the postlaunch instruments [6]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**Channel**} & \multicolumn{1}{c}{**Central**} & \multirow{2}{*}{**Polarization**} & \multirow{2}{*}{**Bandwidth**} & \multirow{2}{*}{**NEAT**} & \multicolumn{1}{c}{**Calibration**} \\ \multicolumn{1}{c}{**Number**} & & & & & **Accuracy** \\ \hline
1 & 89.0 & QH & 1500 & 0.4 & 1.0 \\
2 & 118.75 \(\pm\) 0.08 & QV & 20 & 2.2 & 2.4 \\
3 & 118.75 \(\pm\) 0.2 & QV & 100 & 1.0 & 1.2 \\
4 & 118.75 \(\pm\) 0.3 & QV & 165 & 0.8 & 1.2 \\
5 & 118.75 \(\pm\) 0.8 & QV & 200 & 0.8 & 1.2 \\
6 & 118.75 \(\pm\) 1.1 & QV & 200 & 0.8 & 1.0 \\
7 & 118.75 \(\pm\) 2.5 & QV & 200 & 0.8 & 1.0 \\
8 & 118.75 \(\pm\) 3.0 & QV & 1000 & 0.5 & 1.0 \\
9 & 118.75 \(\pm\) 5.0 & QV & 2000 & 0.5 & 1.0 \\
10 & 166.0 & QH & 1500 & 0.4 & 1.0 \\
11 & 183.31 \(\pm\) 1 & QV & 500 & 0.6 & 1.0 \\
12 & 183.31 \(\pm\) 1.8 & QV & 700 & 0.6 & 1.0 \\
13 & 183.31 \(\pm\) 3 & QV & 1000 & 0.5 & 1.0 \\
14 & 183.31 \(\pm\) 4.5 & QV & 2000 & 0.5 & 1.0 \\
15 & 183.31 \(\pm\) 7 & QV & 2000 & 0.5 & 1.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Channel number, central frequency (unit: GHz), polarization (QV and QH represent quasi-vertical and quasi-horizontal polarization, respectively), bandwidth (unit: MHz), noise equivalent differential temperature (NEAT, unit: K) and calibration accuracy (unit: K) of MWHS-2. The vertical profiles of the 15 channels of FY-3E MWHS-2 are shown in Figure 1. The 118-GHz channels (shown as dotted lines) have a wide vertical detection range, which extends from near the surface to approximately 20 hPa, allowing for a complete temperature profile of the atmosphere. The sounding levels of the water vapor absorption channels are evenly distributed vertically, with weighting functions peaking in the range of 900 hPa to 400 hPa. Therefore, this instrument can provide a detailed distribution of humidity in the middle and lower troposphere. As the channel numbering order increased, the sounding level heights of the 118-GHz channels and water vapor absorption channels all decreased. Fifteen days of FY-3E MWHS-2 observation data covering the period from 24 September to 8 October 2021, were employed to calculate the bias and errors. In this study, RTTOV Version 13 and ERA-5 data sets with 6-h intervals (UTC 00, 06, 12, 18) were adopted as the RTM and the background field for simulation, respectively. RTTOV is a fast one-dimensional RTM that allows rapid simulation of radiances for satellite visible, infrared or microwave scanning radiometers given atmospheric temperature profiles, variable gas concentrations, cloud and surface characteristics [29]. The RTTOV has been used extensively to evaluate satellite instruments [30; 31] since it was originally developed by the ECMWF in the 1990s to achieve direct assimilation of radiances in its variational system [32]. The ERA-5 data sets used in the clear-sky simulation in this study have a horizontal resolution of 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\), with 37 layers in the vertical direction. The top level in the ECMWF analysis model is at 1 hPa. The input variables of the RTTOV include latitude and longitude information, atmospheric state profiles, surface parameters and instrument scanning parameters. The atmospheric state profiles consist of the vertical profiles of pressure, temperature and specific humidity. The surface parameters are listed as follows: (I) surface type, height, temperature and air pressure; (II) sea surface salinity; (III) 2-m temperature, humidity and air pressure; and (IV) 10-m wind speed and direction. The scanning parameters include the zenith and azimuth angles of the satellite and the sun, respectively. Figure 1: Weighting functions (WFs) for the 15 channels of the MWHS-2 onboard the FY-3E.