## 2 Datasets and Methods
### Intercalibration over the Amazon Rainforest  
The Amazon rainforest is often used as a natural land target for the calibration of scatterometer \(\sigma^{0}\) measurements, mainly because the azimuth dependence is negligible (less than 0.1 dB in magnitude) [19; 22; 23]. However, the annual, seasonal, and diurnal variations in \(\sigma^{0}\) over the Amazon should be further investigated. In addition, the incidence-angle dependence of \(\sigma^{0}\) over the Amazon rainforest is strong, but well-known and not a concern in this study [19]. The calibration target regions in the Amazon rainforest are defined by a spatial mask. In this study, a new Amazon mask, as shown in Figure 4, is defined based on HSCAT-B HH measurements in ascending passes in 2019. The \(\sigma^{0}\) data are firstly binned into a regular grid (\(0.25^{\circ}\times 0.25^{\circ}\)); then grid points with a standard deviation (SD) smaller than 0.5 dB are kept; lastly, some isolated grid points are manually removed (with less than three of eight kept as neighbor grid points). The new Amazon mask is different from the mask defined by the QuikSCAT data and used in [19], due to the rainforest declination, but the new one is fully contained in the older one. The average annual \(\sigma^{0}\) value of each grid point in the masked area in the Amazon is shown in Figure 4. Even though the annual variation in each grid point is less than 0.5 dB, the spatial distribution in the backscatter magnitude varies noticeably and ranges from \(-7.0\) dB to \(-8.0\) dB. Figure 4: Spatial mask defining the Amazon rainforest selected grid points. The color scale (in dB) depicts the average of the HSCAT-B HH measurements over 2019. To estimate the annual, seasonal, and diurnal variations in \(\sigma^{0}\) over the masked area in the Amazon rainforest, 9 years of QuikSCAT data, 33 months of HSCAT-B data, and 11 months of HSCAT-C data were used to generate Figures 5 and 6. Figure 5 provides the ten-day average of \(\sigma^{0}\) measured by QuikSCAT (solid lines) and HSCAT-B (dotted lines) for different years, in which the yearly average was subtracted separately. Figure 6 provides the full diurnal variation in \(\sigma^{0}\) for different seasons (winter (DJF), spring (MAM), summer (JJA), and autumn (SON)), which was measured by HSCAT-B, HSCAT-C, and RapidScat. As we can see from Figure 5, the patterns of inter-annual variation are different from polarizations (HH or VV) and satellite passing orientation (ascending or descending). For QuikSCAT, the inter-annual variation in descending passes (6:00) is smaller than that of ascending passes (18:00). While for HSCAT-B, variation in descending passes (18:00) is stronger than ascending passes (6:00). The \(\sigma^{0}\) differences between ascending and descending passes essentially come from the diurnal variations (as shown in Figure 6). Figure 6 shows that the diurnal variation reaches 0.5 dB, which is much stronger than the seasonal or annual variation. There is a clear peak near sunrise (local time between 6:00 and 7:00 in the morning), and this can be attributed to the dew drops forming on leaves [(24)]. The diurnal variations shown in Figure 6 are in line with some previous studies [(19; 24; 25; 26)]. Figure 5: Ten-day average \(\sigma^{0}\) for the Amazon as measured by QuikSCAT (solid lines) or HSCAT-B (dotted lines) for different years. (**a**) for ascending passes, HH polarization; (**b**) for ascending passes, VV polarization; (**c**) for descending passes, HH polarization; (**d**) for descending passed, VV polarization. It is clear from Figure 6 that the overall diurnal variations in ISS and HY-2 are quite similar, though the large vertical distances are caused by the incidence angle dependence and intercalibration, where QuikSCAT VV at 6:00 or 18:00 LST fits within the diurnal bounds. On the other hand, QuikSCAT HH and all HY-2 values are quite consistent. All HY-2 scatterometers provide measurements at 6:00 and 18:00, as dictated by the 6:00/18:00 LST orbit of HY-2B and HY-2A.