Figure 6. Statistics of land surface temperature on July 6, 2004 
and December 3, 2006 
4.3 Seasonal change in net radiation 
As a primary parameter in land surface processes, the seasonal 
variation of albedo was an important factor for causing the 
difference in the radiation budgets. From summer to winter, 
there were prominent decreases in albedo over all surfaces. 
While water decreased about 4.14%, other surfaces reduced by 
18.40%~36.47%. Decrease of albedo enhanced the ability to 
absorb the total incoming radiation. On the other hand, the 
amount of the incoming irradiance reaching the ground 
decreased dramatically in the winter, which was determined by 
solar elevation angle (SEA). When Landsat-5 passed on July 6, 
2004, SEA was 62.96°, and the measured total incoming 
radiation was 872.22W/m 2 ; while SEA decreased to 25.79° and 
the incoming radiation to 375.00 W/m 2 on December 3, 2006. It 
can be concluded that the changes in albedo and solar 
irradiance led to the change in radiation budgets, but the latter 
played a more important role. 
Longwave emission is controlled by land surface temperature. 
In the winter, because the energy absorbed by land surfaces 
decreased significantly, land surface temperature decreased 
accordingly. The longwave emission in the winter was only 
about 33.22%~ 40.76% of that in the summer. However, the 
different between the shortwave net radiation flux and the 
longwave emission decrease largely in winter compared with 
that in summer. It should be noted that, the heat island and cool 
land caused by the thermal inertia (Wang et al, 2007), play an 
importance role in the spatial patterns of net radiation fluxes in 
summer and winter. As a result, the city turned into a “plateau” 
in the net radiation map of the winter from a “basin” of the 
summer. 
It is important to investigate the radiation budgets over urban 
and suburban surfaces for urban climate research. In this paper, 
the applicability and feasibility of Landsat-5 TM images in 
conjunction with meteorological data were investigated to 
estimate the radiation budgets in Beijing, China. Validation 
using in situ measurements suggests that the estimation 
accuracy for the net radiation was acceptable. It is concluded 
that satellite remote sensing can be used as an effective 
technique to improve modeling and analysis of the spatial 
patterns of surface energy balance for a mega-city such as 
Beijing. 
It is found that the net radiation in Beijing has unique features 
with spatial distribution. In the summer, urban area was the 
“basin” in the distribution map of the net radiation flux, while 
transferred to the “plateau” in the winter. Concentric patterns 
can be seen in the map of the radiation flux in both the summer 
and the winter. This interesting phenomenon can be explained 
by the distribution of albedo and land surface temperature in 
urban and suburban areas. The seasonal variations of radiation 
budgets were also investigated. From summer to winter, the 
decrease in the shortwave net radiation and the net radiation 
were mainly controlled by the change in solar elevation angle. 
REFERENCES 
Bisht, G., Venturini, V., Islam, S., et al., 2005. Estimation of 
the net radiation using MODIS (Moderate Resolution Imaging 
Spectroradiometer) data for clear sky days. Remote Sensing of 
Environment, 97, pp. 52-67. 
Chander, G., Markham, B., 2003. Revised Landsat-5 TM 
radiometric calibration procedures and postcalibration dynamic 
ranges. IEEE Transactions on Geoscience and Remote Sensing, 
41, pp. 2674-2677. 
Chrysoulakis, N., 2003. Estimation of the all-wave urban 
surface radiation balance by use of ASTER multispectral 
imagery and in situ spatial data. Journal of Geophysical 
Research, 108, 4582, doi: 10.1029/2003JD003396. 
Frey, C., Rigo, G., Parlow, E., 2007. Urban radiation balance of 
two coastal cities in a hot and dry environment. International 
Journal of Remote Sensing, 28, pp. 2695-2712. 
Kato, S., Yamaguchi, Y., 2005. Analysis of urban heat-island 
effect using ASTER and ETM+ data: separation of 
anthropogenic heat discharge and natural heat radiation from 
sensible heat flux. Remote Sensing of Environment, 99, pp. 44- 
54. 
204