The relationship between MODIS LST and Ta, and The Influence of Land Surface

Figure 4.2. The average temperature of the four MODIS LST data at two stations, under two sky conditions:

all clear sky condition (CS_*) and only good condition (G_*).

As shown in Figure 4.2, the nighttime LST were consistent between the two stations, however, the daytime LST values have a large difference, approximately 4 ^oC for both Terra and Aqua daytime. This indicates that land surface characteristics have a strong impact on

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LST at daytime. Regarding the time-dependency, both stations have a close overpass time (the overpass time of Terra is approximately 11:20 a.m. and Aqua is approximately 12:50 p.m.), yet the difference between MYDD and MODD is almost 4 ^oC at both stations (Figure 4.2). It is suggested that the direction of solar radiation (at daytime) has a strong effect on LST, and this effect is quite similar at different land surface characteristics. However at nighttime, the difference of LST between MODN (overpass time approximately 10:15 p.m.) and MYDN (overpass time approximately 2:30 a.m.), lowered to about 1.5 ^oC (of all clear sky data) for both stations. This indicates that overpass time has some effects on the LST. It should be noted that the lowering of LST between MODN and MYDN were different between all clear sky data and good data. At Conoi station, the difference of all clear sky data (which lowered 1.5 ^oC) was consistent with good data (which lowered 1.6 ^oC); whereas at Muongte station, the lowering was only 1.5 ^oC and 0.06 ^oC with all clear sky data and good data, respectively. This can be explained by only 25.7% of all clear sky data at Muongte station having good quality, compared to 67.5% at Conoi station (Table 4.1).

Figure 4.2 also shows that the differences between daytime and nighttime of Aqua LST were larger than those of Terra LST. The larger differences between both Terra and Aqua LST were observed at Conoi station. This confirms that land surface characteristics have an impact on LST.

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Figure 4.3. The point density plots of the LST (all clear sky data/ good data) products and Ta (Ta_mean, Ta_max, and Ta_min) at MuongTe station. The point density from low to high is expressed by the colour ramp from blue to red.

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In general, Terra LST has a higher correlation with Ta_mean, Ta_max, and Ta_min than that of Aqua LST. This is true for both datasets; all clear sky LST (CS-data) and good LST (G-data).

Figure 4.3 also shows that both LST daytime and nighttime of both satellites were distributed above and below of 1:1 line of Ta_max and Ta_min, respectively. This is indicates that all most LST was lower than Ta_max and higher than Ta_min. However, with both CS-data and G-data, Terra LST was distributed closer to the 1:1 line than Aqua LST. This could be explained by the overpass time (local solar time) of Terra (11:20 a.m., 10:15 p.m.) being closer to Ta_max and Ta_min than Aqua.

Furthermore, looking at the Mean Difference (MD) between MODIS LST and Ta, it is clearly seen that MODD (11:20 a.m.) is closest to Ta_mean (MD = +2.16 ^oC) and MYDD (12:50 p.m.) is closest to Ta_max (MD = -2.43 ^oC). This is consistent with both data conditions; all clear sky and only good LST data. For Ta_min, there is a difference: the all clear sky data MODN (10:15 p.m.) is closest to Ta_min (MD = +0.3 ^oC), and the only good data MYDN (2:30 a.m.) is closest to Ta_min (MD = +0.26 ^oC). It is worth noting that the variation of the smallest MD from MODD (11:20 a.m.) to MYDD (12:50 p.m.) was larger (+2.16 to +6.45 with CS-data, and +2.94 to +7.85 ^oC with G-data) than that of the MD from MODN (10:15 p.m.) to MYDN (2:30 a.m.) (for all clear sky data, MD ranges from -0.3 ^oC to -1.04 ^oC; for only good data, from +1.17 ^oC to +0.26 ^oC). This indicates that the variation of LST at daytime is much higher than at nighttime (approximately 4 ^oC compared to 1 ^oC).

The G-dataset (of both Terra and Aqua) show a higher correlation with Ta than the CS-data (except MYDN with Ta_max). The exception of MYDN and Ta_max could be explained by the lowest percent (25.66%) of good data in the MYDN data (Table 4.1).

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Figure 4.4. The point density plots of the LST (clear sky data/good data) products and Ta (Ta_mean, Ta_max, and Ta_min) at Conoi station. The point density from low to high is expressed by the colour ramp from blue to red.

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At Conoi station, the rising absolute values of the MD for the two datasets (CS-data, G-data) were consistent with each other, and with Ta_mean, Ta_max, and Ta_min (Figure 4.4). This consistency could be explained by looking at Table 4.1, the percentage of good LST (over the total clear sky data) data ranged from 50 to 75.95%. The ascending order of Ta_mean, Ta_max, and Ta_min were MODN – MYDN – MODD – MYDD, MODD – MYDD – MODN – MYDN, and MYDN – MODN – MODD – MYDD.

Figure 4.3 and Figure 4.4 show that the difference between nighttime LST and Ta were consistent with respect to both stations. In general, all Ta (Ta_max, Ta_min, and Ta_mean) were higher than LST nighttime (i.e. MD > 0), while only MD of MYDN and Ta_min of the Muongte station was below zero. However, when comparing MYDN and Ta_min of CS-data and G-CS-data (Figure 4.3), it is clearly seen that the inconsistent results were because of thin cloud pixels. In contrast, the difference between LST daytime and Ta were larger. At Muongte station, the largest difference was observed between MYDD and Ta_min (MD = +10.68 and +12.45 for CS-data and G-data, respectively), while the smallest differences were observed between MODD and Ta_mean (MD = +2.16 for CS-data), and between MYDD and Ta_max (MD = -1.81 for G-data). At Conoi station, the largest difference was observed between MYDD and Ta_min (MD = +16.54, and +18.90 for CS-data and G-data, respectively). The smallest difference was observed between MODD and Ta_max (MD = -0.06 for CS-data, and MD = +0.6 for G-data). MYDD was closest to Ta_max with MD = +3.9 for CS-data, and +5.14 for G-data.

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