# Performance Evaluation and Noise Mitigation of the FY-3E Microwave Humidity Sounder
## 3 RTM Simulation
To rule out the uncertainty of the RTM on the cloud simulation and surface emissivity, clear pixels over the Pacific Ocean between 50\({}^{\circ}\)S and 50\({}^{\circ}\)N are considered to calculate the biases and standard deviations of the \"O\(-\)B\", whose dependence on the scanning angles and latitude bands will also be analyzed later. The criteria for judging clear sky pixels are as follows: (I) the total cloud liquid water path is less than 0.02 kg\(\cdot\)m\({}^{-2}\); (II) the absolute value of the \"O\(-\)B\" for any channel is less than or equal to 5 K.  
Channel 3 and Channel 12 are one of the 118-GHz channels and water vapor absorption channels, with weighting functions peaking at 70 hPa and 500 hPa, respectively. Figure 2 shows the global distributions of the observed brightness temperature (Figure 2a), the model simulated brightness temperature (Figure 2b) and the \"O\(-\)B\" (Figure 2c) (only the ascending orbit) of MWHS-2 Channel 3 on 2 October 2021. Figure 2: Spatial distribution of the: (**a**) observed; (**b**) simulated brightness temperature (unit: K); and (**c**) the difference (unit: K) between them (“O\(-\)B”) for MWHS-2 Channel 3 (WF peaks at 70 hPa) on 2 October 2021. The observed brightness temperature mainly represents the temperature characteristics of the stratosphere, where the temperature in the tropics is lower than that in the middle and high latitudes due to the ozone distribution. The sun shines more directly on the Northern Hemisphere in summer, and the whole North Pole is in the high temperature region. In the Southern Hemisphere, the middle latitudes are characterized by high temperatures, while the Antarctic is characterized by low temperatures. Due to cross-track scanning, the brightness temperature cannot properly reveal the delicate temperature structure in the tropical region, only showing a decrease in the temperature on both sides of the scanlines. Simulations of the brightness temperature reproduce the spatial characteristics of the observation, including the high temperature zones located at the middle and high latitudes in the Northern Hemisphere and the middle latitudes in the Southern Hemisphere as well as the low temperature zone at the South Pole. The RTM also exhibits the limb darkening effect of the Earth sense in the tropical region. However, the brightness temperature simulated by the model is approximately 3.5 K higher than that observed, making the \"O\(-\)B\" distribution uniformly negative. Figure 2c shows that there are obvious striping noises along the tracks in the \"O\(-\)B\" distribution, which will be further discussed in Section 4. Similar to Figure 2, Figure 3 shows the observation, simulation and \"O\(-\)B\" distribution for Channel 12, which is a water vapor absorption channel whose weighting function peaks at approximately 500 hPa. As displayed in Figure 3a, water vapor in the tropics is obviously higher than that in the middle and high latitudes at 500 hPa, especially in the tropical Southern Hemisphere, where there are several high bright temperature centers. In the Pacific Ocean, the high brightness temperature region is closer to the tropics at approximately 5\({}^{\circ}\)S, while that in the Indian Ocean and Atlantic Ocean is close to 20\({}^{\circ}\)S. Due to the intense variation in water vapor in the middle troposphere, cross-track scanning has relatively little impact on obtaining the spatial structures of the brightness temperature distribution, reproducing almost all the global extreme centers. Figure 3c shows that the \"O\(-\)B\" in the marine area within 50\({}^{\circ}\)S/N mostly exhibits a positive bias. Although the observed and simulated brightness temperature distributions are very similar to each other, many details in the weather systems are not well reproduced in the \"O\(-\)B\" distribution. There are obvious local variations, especially at the edges of the extreme centers where the distribution of the positive and negative values alternates, indicating that there may be a shift in the position of the weather system.