## 4 Results
### Characteristics of Sea Surface Temperature Errors
#### 4.4.2 Effect of Satellite Zenith Angle  
The spatial coverage of geostationary satellite images is extensive, with latitudinal and longitudinal limits of about \(\pm\)60\({}^{\circ}\), and the satellite zenith angle of each pixel of COMS/MI varies over a wide range. Therefore, it is necessary to investigate if there is any influence of the SZA on the accuracy of derived SSTs. In the regression procedure, the matchup points at a high range of SZA (\(>\)60\({}^{\circ}\)) were removed to avoid unpredictable errors related to poor angles. Investigation of the number density of the SZA showed that the most frequent SZAs of the matchup database were mainly distributed in the range of 20\({}^{\circ}\)-50\({}^{\circ}\). At this range, there was no significant tendency in the distribution of SST biases with respect to the SZA for both MCSSTs and NLSSTs by negligible mean bias errors close to 0 \({}^{\circ}\)C. In contrast to this, negative biases (\(<\)0.3 \({}^{\circ}\)C) were still detected at small SZA values, i.e., less than 10\({}^{\circ}\), for both MCSSTs and NLSSTs. Considering such a small number of matchup points near the nadir, however, the overall contribution of the SZA effect on the SST errors seemed to be trivial. The NLSST equation, containing three terms related to SZA, as shown in [20] of Table 3, did not give any limit of the SZA values in the derivation of the SST coefficients [20]. In order to examine the SZA effect on the SST accuracy of COMS/MI, the coefficients of MCSST and NLSST were preliminarily derived the entire range of SZA to estimate SSTs. Figure 8 shows the SST errors of the MCSST and the NLSST algorithms as a function of SZA. As shown in Figure 8a-d, the differences of estimated SSTs from in situ SSTs were very small at a range of 20\({}^{\circ}\)-60\({}^{\circ}\) for both nighttime and daytime data. However, the mean differences tended to be negative by up to about \(-\)1 \({}^{\circ}\)C at high SZA near the outer boundary of the full-disk region [66]. Figure 7: Distributions of SST residuals (satellite SST—in situ SST) of MCSST during the (**a**) daytime and (**b**) nighttime and NLSST during the (**c**) daytime and (**d**) nighttime as a function of wind speed (m s\({}^{-1}\)). The color represents the percentage of the data to the total number of matchup points in each bin; error bars represent the standard deviation of SST errors of each interval. To investigate the SZA effect on the SST errors, we included all the matchups for an SZA range below than 80 in the regression procedure. Figure 8 shows the SST errors as a function of the SZA values from 20\({}^{\circ}\) to 80\({}^{\circ}\) for MCSST and NLSST as marked in green and blue. In the daytime, the mean errors of MCSSTs were larger than those of NLSSTs, as indicated in Figure 8e. However, the reduction effect of NLSST errors disappeared when the same errors for a range of SZA less than 60\({}^{\circ}\) in the nighttime was shown. This implies that the present algorithms might still have SZA influences unless high SZA values, both in the matchup procedure and in the regression procedure, are removed. Therefore, it is noted that the present algorithms could be applied only for a small range of SZA (<60\({}^{\circ}\)) by eliminating the undesirable pixels with high SZA values to maintain SST accuracy (Table 2). It is hypothesized that the tendency of significant underestimation at high angle limits (>60\({}^{\circ}\)) can be reduced if the SZA term (\(S_{\theta}=\sec(\theta)-1\)), as shown the last term of [20] in Table 3, was included in the regression procedure. As expected, such significant underestimation of the estimated SSTs disappeared by showing much smaller errors for MCSST and NLSST algorithms, as indicated in red lines in Figure 8e,f. However, even though the angle term was included in the SST retrieval formulations, the overestimation of SSTs still remained at high angle limits (>60\({}^{\circ}\)), particularly at a range of SSTs below 7 \({}^{\circ}\)C, as pointed out in Figure 4a,b.