Comparative Performance of SST Algorithms In the Tropical Ocean Using OCTS Data
6.0 Computation of Sea Surface Temperature
Equation (3) can be used for atmospheric correction at any satellite angle or on a pixel-by-pixel basis. Cloud free seawater pixels for bands 10, 11 and 12 were extracted for SST retrieval. The digital numbers were converted to brightness temperatures and their corresponding zenith angles were calculated. The emissivity was assumed to be 0.96 (the value given in the MODTRAN manual for 12
mm) in both bands. Appropriate transmittance function derived earlier was used for each image date. Having determined the transmittance as a function zenith angle and the emissivity for seawater for each band, equation (3) can be applied to measure SST
7.0 Comparison With NASDA MCSST Algorithm
The Multichannel Sea Surface Temperature algorithm developed by NASDA (Sakaida et al. 1998) is expressed as
MCSST = co+c1T11+c2(T11-T12)+c3(T11-T10)+c4(1/cosq-1)(T11-T12)+c5(1/cosq-1)(T11-T12)
(5)
The coefficients c
o to c
5 were taken from the table provided in the report.
The SST values computed using the proposed algorithm were then compared with the values retrieved using the MCSST algorithm. The outputs from these algorithms were correlated. The RMS deviations of the computed values using the proposed algorithm from the MCSST outputs were estimated. Table 1 and the set of graphs in Figure 1 show the results of the analysis. With these data sets the present algorithm has shown comparable performance with the established algorithm produced by NASDA.
Table 1 Comparison of proposed algorithm with MCSST
|
R |
RMS(°C) |
| 22 March 1977 |
0.91 |
0.89 |
| 21 April 1977 |
0.98 |
0.48 |
| 13 June 1977 |
0.99 |
0.26 |
(a)
(b)
(c)
Figure 1 Comparison of the proposed algorithm with the MCSST algorithm (a) 22 March1997, (b) 21 April1997and (c) 13 June 1997
8.0 Validation of Results
The in-situ data collected within the region of interest were used for verification of the retrieved SST values. From the analysis, unfortunately, we discovered that all the in-situ points were located on cloudy pixels. Measured SST values were interpolated spatially by plotting contour maps of the in-situ data. These maps were used to verify the computed SST values. The locations of the cloud free pixels were determined and the corresponding in-situ values were read from the map. The 22 March 1997 and 21 April 1997 cloud free pixels areas occurred within the generated contour maps. The results of the verification using the proposed algorithm and the MCSST algorithm are listed in Table 2. Figure 2 shows their scatter plots.
Table 2 Verification results from the present study
|
Proposed algorithm (RMS,°C) |
MCSST (RMS,°C) |
| 22 March 1977 |
1.27 |
1.10 |
| 21 April 1977 |
1.24 |
1.18 |
(a)
(b)
(c)
(d)
Figure 2 Comparison of computed SST values with the measured values (a) the proposed algorithm (22/3/1999), (b) MCSST algorithm (22/3/1999), (c) proposed algorithm (21/4/1999) and (d) MCSST algorithm (21/4/1999)
