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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
instrument was launched on 18 December 1999 on a morning 
platform called Terra, and the second was launched on 4 May 
2002 on an afternoon platform called Aqua. The Terra overpass 
time is around 10:30AM (local solar time) in its descending 
mode and 10:30PM in its ascending mode. The Aqua overpass 
time is around 1:30PM in its ascending mode and 1:30AM in its 
descending mode. 
The validations were carried out for five days during summer, 
2003. They are Julian days (day number of a year) 206, 209, 
217, 220, 221, covering a period from July to August. These 
days are selected because they are clear days for all the 
validation sites shown in Table 1. The retrievals of MODIS- 
based H and Ta (Air temperature) are averaged over 3*3 pixels 
enclosing the ground- based site. 
Three MODIS land products are used in this study: the 8-day 
leaf area indices product (MODI5), daily atmosphere profile 
product (MOD07) and the daily Ts product at 1-km resolution 
(MOD/MYD11). To examine the validity of using the different 
time pairs, the H retrieved using four combinations was 
compared: (1) Terra daytime and nighttime, (2) Aqua daytime 
and nighttime, (3) Terra daytime and Aqua daytime, (4) Aqua 
nighttime and Terra daytime. First, MODIS products cover 
Tongyu site were downloaded from website of EOS Data 
Gateway and Atmosphere Archive and Distribution System. 
Second, reprojecting and AOI-subset were done using MRT (for 
land product) and MRTSwath (for atmosphere product). Third, 
average situation of clear-sky cases was analyzed using all four 
combinations to find out which is optimal. Fourth, more clear- 
sky cases were computed to achieve a preliminary evaluation of 
the optimal time pair. 
The daytime average sensible heat flux is well correlated with 
the temporal change in surface radiometric temperature, which 
was illustrated in Fig2. The best choice of time interval will be 
that which maximizes both of these curves while large 
temperature difference could be observed as much as possible. 
The result shows that RMSE of 20-40 W m-2 for daily average 
sensible heat. In general, the range of errors reported by other 
authors in H flux is very variable. [9] Consider around 50 Wm-2 
an acceptable error for H. In the literature, errors in the best 
cases are around 22 Wm-2 and similar internal error (20Wm-2) 
exists in different measurement instruments (lysimeter, eddy- 
covariance and BREB). And converge computing procedure 
improves most error to less than 2 °C for Ta estimate at 10:30 
am. Although quantity of validation field data is small, the 
result is encouraging. 
Date 
DOY 
Site 
Period RMSE of H RMSE for Ta 
25-Jul 
206 
Crop 
0550-152022.69 
0.81 
28-Jul 
209 
Crop 
0550-152036.34 
1.88 
5-Aug 
217 
Crop 
0550-1520 23.10 
1.54 
8-Aug 
220 
Crop 
0550-152026.52 
1.04 
9-Aug 
221 
Crop 
0550-1520 21.41 
1.76 
25-Jul 
206* 
Grass 
0550-152031.34 
2.16 
28-Jul 
209 
Grass 
0550-1520 26.67 
1.25 
5-Aug 
217 
Grass 
0550-1520 39.14 
0.98 
8-Aug 
220 
Grass 
0550-1520 30.45 
1.48 
9-Aug 
221 
Grass 
0550-152037.18 
1.88 
*Fied Measurement from 1150 to 1250 is missing 
4.2 
Table I. Simulation result using CEOP Tongyu field data, 2003 
4.3 Comparison Bowen Ratio(BR) with field data 
The average energy balance closure ratio (EBCR) is about 0.75 
in the whole year of Tongyu, CEOP03, which means H+LE is 
smaller than expected, Rn-G, in energy balance theory (TABLE 
II). For it makes the direct comparison of estimated LE or ET 
and field data problematic, analysis is carried out on Bowen 
Ratio(H/LE) between estimation and measurement(Table III). 
Figure 2:The correlation between surface temperature A rad and 
daytime average sensible heat <H> vs the time interval between 
Month 
EBCR(Crop) 
Available 
Measurment 
EBCR(Grass) 
Available 
Measurment 
^ rad measurement 
1 
*missing 
*missing 
0.66 
5.85% 
2 
0.64 
59.90% 
0.61 
88.69% 
In that, combination of Terra daytime and Aqua daytime 
3 
0.66 
89.72% 
0.63 
95.70% 
couldn’t’ work out because too short interval (3hrs) and too 
4 
0.71 
24.72% 
0.64 
41.94% 
small change can be observed (2-3K in summer time) and it is 
5 
0.70 
63.51% 
0.65 
57.19% 
within the bias of surface temperature derivation. To satisfy the 
6 
0.74 
100.00% 
0.68 
100.00% 
aforementioned supposition, we use the nighttime measurement 
7 
0.74 
99.06% 
0.69 
100.00% 
in replacement of a virtual morning measurement. There are 
also about 3 hours or 3.5 hours between these two 
8 
0.74 
100.00% 
0.70 
98.59% 
measurements. Because sharp slope of radiometric temperature 
9 
0.73 
98.89% 
0.69 
98.89% 
and PBL development happened at this period, the precision of 
10 
0.73 
97.11% 
0.69 
100.00% 
sensible heat flux is better than Terra daytime - Aqua daytime. 
11 
0.74 
97.22% 
0.70 
100.00% 
Considering the bias in different thermal sensors’ calibration 
and cloud cover, we choose Terra nighttime and daytime as 
12 
0.69 
100.00% 
0.69 
100.00% 
optimal time pair, and assume that the temperature at Terra 
nighttime (10:30pm) is an approximation of these at 7:00am, 
which can be proven reasonable using field measurement. Table 
I is the model results using this time interval (7:00am and 
10:30am). 
4.4 Table II. monthly energy balance closure ratio in TOngyu, 
2003