# Performance Evaluation and Noise Mitigation of the FY-3E Microwave Humidity Sounder
## 4 Bias Characteristics
For the 118-GHz channels, the absolute biases in the high-latitude zones are obviously smaller than those in the tropical regions, and for the water vapor absorption channels, biases (including the sign) decrease with the increasing latitude. Moreover, the standard deviations for the water vapor absorption channels are more latitudinally dependent compared to the 118-GHz channels, with large values appearing at near 50\({}^{\circ}\) S/N and decreasing toward the tropics. In addition, the observation errors continuously increase with the detection heights, corresponding to the distribution characteristics of atmospheric water vapor. In comparison, the standard deviations of the 118-GHz channels are relatively stable, but the observation errors also increase with decreasing latitude, indicating that the data assimilation needs a bias correction scheme and adjustment to the observation weight according to the latitudinal characteristics. In order to prove the suitability of the background field, ERA-5, the spatial distribution of the \"O\(-\)B\" standard deviation for Channels 6 and 13 are presented, as well as the spatial distribution of the variability of ERA-5's atmospheric temperature and specific humidity at the heights where the peak weighting functions of the two channels located. To calculate the standard deviation and variability, we first convert the data to grid data. The mesh division is consistent with Figure 4 in the paper. Standard deviations are calculated based on the clear-sky marine pixels located in each 2\({}^{\circ}\)\(\times\) 2\({}^{\circ}\) grid box within 15 days. Here, the results of Channels 6 and 13 are displayed to represent the 118-GHz channel and the water vapor absorption channel, respectively. As can be seen from Figure 9, the standard deviation of \"O\(-\)B\" for Channel 13 is within 2.0 K, and the large values are mainly located in the Sea of Japan, the ocean south  
Figure 8: Latitudinal dependence values of the: (**a**) biases (unit: K); and (**b**) standard deviations (unit: K) of the “O\(-\)B” calculated in each 5\({}^{\circ}\) latitudinal band between 50\({}^{\circ}\) S and 50\({}^{\circ}\) N. The black box represents the 118-GHz channels. of Australia, and the northeast Pacific. The standard deviation of \"O\(-\)B\" increases significantly with the increasing latitude. Although the large atmospheric temperature variability at 600 hPa also appears in the high latitudes, the comparison between Figures 1b and 9a shows that the standard deviation of \"O\(-\)B\" has little relationship with atmospheric temperature variability since there is no large temperature variability center in the Sea of Japan and the ocean south of Australia and the location of the large value center of temperature variability in the northeast Pacific is significantly different from that of the \"O\(-\)B\" standard deviation. Figure 9c shows the spatial distribution of the variability of specific humidity at 600 hPa. The large specific humidity variability is mainly located in the tropics where moisture is abundant and small- and meso-scale weather systems prevail. This is obviously different from the spatial distribution characteristics of \"O\(-\)B\" standard deviation. Figure 9: Spatial distribution of: (**a**) “O\(-\)B” standard deviation (unit: K) for Channel 13 (WF peaks at 600 hPa); (**b**) variability of atmospheric temperature (unit: K); and (**c**) specific humidity (unit: kg/kg) at 600 hPa of ERA-5 from 24 September to 8 October 2021. The blank boxes in the Eastern Pacific are caused by quality control. Channel 6 is one of the 118-GHz channels, and its peak weighting function is at approximately 350 hPa. As can be seen from Figure 10a, the spatial variability of \"O\(-\)B\" standard deviation for Channel 6 is relatively small, basically between 0.4-0.6 K. There are no complex weather signals, and the large \"O\(-\)B\" standard deviation is mainly located in the tropical region.