# Performance Evaluation and Noise Mitigation of the FY-3E Microwave Humidity Sounder
## 4 Bias Characteristics  
The \"O\(-\)B\" is assumed to be unbiased in satellite data assimilation and is widely employed to analyze the error characteristics of the instrument. To exclude the influence of clouds and surface emissivity, only clear-sky pixels on the Pacific Ocean were taken as the samples. Figure 4 shows the spatial distributions of clear-sky marine data counts within 2\({}^{\circ}\)\(\times\) 2\({}^{\circ}\) grid boxes in the 15-day sampling period on the Pacific Ocean. Most of the regions have more than 400 clear-sky FOVs, except for the regions with less than 100 clear-sky FOVs on the equatorial sides of the eastern Pacific. The area with the highest clear-sky data counts is off Northeast Australia, where the maximum is over 3300, while the areas with less clear-sky FOVs are roughly distributed in the Intertropical Convergence Zone (ITCZ), south of the equator in the eastern Pacific Ocean, northeast of Hawaii, and near the Solomon Islands. Figure 3: Spatial distribution of the: (**a**) observed; (**b**) simulated brightness temperature (unit: K); and (**c**) the difference (unit: K) between them (“O\(-\)B”) for Channel 12 (WF peaks at 500 hPa) on 2 October 2021. Using the clear-sky marine data mentioned above, the 15-day mean values and standard deviations of the \"O\(-\)B\" were calculated in this study (see Figure 5). The blue bars and the red dotted line in Figure 5 represent the mean values and the standard deviations of each channel. As displayed by Figure 5a, the bias of each channel is basically between \(-\)4 K and 2 K. Moreover, most channels show negative biases except for those in Channel 1, Channel 9 and Channels 11 to 12. The biases of the 118-GHz channels increase with decreasing sounding level heights. For the upper-air sounding Channel 2, the peak weighting function is located at approximately 70 hPa with a bias of \(-\)3 K, while Channel 9, whose detection level is close to the ground, is biased by 0.6 K. In contrast, the biases of the water vapor absorption channels decrease from 0.4 K (Channel 11) to \(-\)1 K (Channel 15) with decreasing detection heights. The standard deviations of the 15 channels are all less than 2 K, among which those of the two window channels are larger than the others. Channel 1 has the largest standard deviation of approximately 1.9 K, followed by Channel 10 which has 1.75 K. The standard deviations of the 118-GHz channels decrease first, from 1.2 K (Channel 2) to 0.5 K (Channel 6) and then increase with decreasing detection heights to 1.2 K for Channel 9. In contrast, the water vapor absorption channels have continuously deceasing standard deviations from 1.3 K (Channel 11) to 0.9 K (Channel 15). On average, the standard deviations of the water vapor absorption channels are slightly higher than those of the 118-GHz channels. To test the influence of background field on bias and observation error estimates, the same period of the Final (FNL) Operational Global Analysis data of the National Centers for Environmental Prediction (NCEP) was also used, and the results are shown in Figure 5b. The two evaluation results are very similar except for some slight differences. The bias calculated based on the NCEP FNL data were slightly larger than those of the ERA5 data for Channels 3-8 and Channels 10-12. Moreover, the standard deviations of \"O\(-\)B\" of water vapor absorption channels were slightly larger when using the NCEP FNL as the background field. In general, it can be considered that the influence of the background field on the bias and observation error estimation was very small and can be ignored. FY3E MWHS-2 is a cross-track detector whose optical path and FOVs change with scanning angles during detection, which will inevitably lead to changes in the biases of the observations. The scan bias of the \"O\(-\)B\" for each channel with FOVs (or scanning angles) is shown in Figure 6. The biases and standard deviations of the 118-GHz channels (left column) and water vapor channels (right column) are presented here for distinction.