# Performance Evaluation and Noise Mitigation of the FY-3E Microwave Humidity Sounder
## 4 Bias Characteristics
For the two window channels, the biases of Channel 1 are positive and vary little with the FOVs, ranging from 1.2 K to 1.6 K. However, the biases of Channel 10 significantly change with the scanning angles, which increased from approximately \(-\)1.3 K for the first FOV to approximately 0.4 K for the central FOV and then decreased to approximately \(-\)2.4 K for the last FOV. In the 118-Ghz channel, the deviation of Channel 2 hardly changes with the field of view. The absolute values of the biases of Channels 3 and 4 at both ends of the scanlines are approximately 0.4 K smaller than those in the middle. For Channels 5 to 9,  
Figure 4: Spatial distributions (color shading) of MWHS-2 clear-sky marine data counts in \(2^{\circ}\times 2^{\circ}\) grid boxes from 24 September to 8 October 2021.  
the absolute bias values generally increase with increasing scanning angles (except for the FOVs near the endpoint). The maximum bias difference between Channels 5 and 6 is approximately 0.4 K, and that between Channels 7 and 9 is approximately 1.3 K, which may result from the vertical gradient of temperature. For all the water vapor absorption channels, the average bias of the first 49 FOVs is slightly larger than that of the last 49 FOVs, which may be due to the polarization misalignment in the radiometer receiver [33]. Figure 6a,b show a jagged change in the bias along the scanning direction, which is recognized as the high-frequency oscillating noise related to the scanning positions and will be further discussed in Section 4. The same calculation is performed using the NCEP FNL data as the background field. It can be seen from Figure 7 that the high-frequency oscillating noises still exist in the biases of each channel, indicating that the noise exists in the instrument observation and is independent of the background field. According to Figure 6c,d, the standard deviations for the window channels (i.e., Channel 1 and Channel 10) and the 118-GHz channels are basically symmetrical with respect to the magnitude of scanning angles. The standard deviations in the 12 FOVs at both ends of the scanlines for the two window channels significantly decrease with increasing scanning angles. The decreasing gradient of Channel 1 is approximately 0.03 K/FOV and that of Channel 10 is approximately 0.06 K/FOV, while the standard deviations in the central 74 FOVs show no dependence on the scanning angles. Among the 118-GHz channels, the standard deviations of Channels 7 to 9 gradually decrease from the center to the two ends of the scanline, with decreases of approximately 0.2 K (Channel 7), 0.3 K (Channel 8), and 0.5 K (Channel 9), respectively. The other 118-GHz and water vapor absorption channels do not appear to have a scanning angle dependence in their standard deviations. Figure 5: Biases (blue bars, unit: K) and standard deviations (red dotted curves, unit: K) of the “O–B” calculated from clear-sky marine pixels based on the background field of: (**a**) ERA-5; and (**b**) NCEP FNL, respectively. The calculation covers the period from 24 September to 8 October 2021. Figure 6: Variations of the biases ((**a,b**), unit: K) and standard deviations ((**c,d**), unit: K) with the 98 FOVs for the 15 channels. The left column indicates the results of the 118-GHz channels, and the right column indicates the other channels. Figure 7: Variations of the biases for (**a**) the 118-GHz channels and (**b**) the other channels with FOVs using the NCEP FNL data as the background field. Figure 8a,b show latitudinal dependencies of the biases and standard deviations of the \"O\(-\)B\" brightness temperature for 15 channels. The most obvious negative bias is in Channels 2 near 118.75 GHz, whose magnitude reaches a maximum of approximately 3.6 K in the latitudinal bands of \(-\)15\({}^{\circ}\)S to 15\({}^{\circ}\)N. In contrast, the bias absolute values of the low-level sounding channels are smaller, basically within \(\pm\)1.0 K. Notably, except for the window channels, all the other channels show a certain latitude dependence.