Most studies about surface energy flux by using eddy covariance techniques have been initiated in European and North American cities. It is rarely done in Asian cities, especially in Chinese cities. However, Beijing city has its distinct architectural styles, building materials, climatic settings and energy use/emission patterns. The aim of this study was to understand the sensible heat, latent heat and momentum exchanges at various locations within Beijing City urban land-cover mosaic. In this paper we report the findings at Shunyi, a suburban location recently impacted by rapid urban sprawl.

This study was conducted in Shunyi observatory station (116°37'N, 40°08'E, elevation 28.6 m). Shunyi is part of greater Beijing city, which has experienced tremendous growth in recent years. The fractional land cover of the surface at this suburban site is approximately 52% of vegetated, 36.4% roofs, 11.6% impervious grounds such as roads, sidewalks and driveways.

The height of the meteorological tower at the observation site is 45 m. The CO2 fluxes, water vapor and sensible heat were quantified using tower-based eddy covariance. The flux measurement system consisted of a fast-response three-dimensional sonic anemometer-thermometer (CSAT3, CAMPBELL, USA) and a fast-response open-path infrared gas analyzer (LI-7500, LI-COR, Inc., Lincon, NE, USA) for measuring the mean and fluctuating quantities of wind speed and temperature, and CO2 and H2O vapor, respectively. The sensors of eddy fluxes were installed at 36 m above the ground level. The dominant diurnal and nocturnal wind directions above the canopy were North (NNE) both in summer and winter. So, the sensors of eddy fluxes were oriented in the direction of the mean wind on the upwind side of the tower in order to minimize the potential for flow distortion from the tower. In addition, one routine meteorological gradient system was installed on the tower with 5-level air temperature, and relative humidity sensors (HMP45C) with the heights of 4.5, 9, 18, 36, and 42 m. Net radiation was measured with a 4-component method (CM11, KIPP&ZONEN, the Netherlands) at 36 m above the ground. These factors were sampled at 0.5 Hz and the data were stored in the data loggers.

The CO2 fluxes, water vapor and sensible heat were recorded at 10 Hz in two CR5000 data loggers (Model CR5000 and Campbell Scientific) and 30 min mean values were calculated. The affiliated meteorological variables above the ground, including wind speed, air temperature, humidity and net radiation, are all measured simultaneously. The thirty-min average of each factor was also calculated by the data loggers and then stored. These measurements were started since June 2007, and the data from July 1 2007 to February 20 in 2008 were used in this paper.

The results showed that daily mean values of net radiation, latent heat flux, sensible heat flux and net heat storage flux for clear days in summer was 151.1 W/m2, 78.0 W/m2, 31.3 W/m2 and 42.2 W/m2 respectively; and in winter was 16.1 W/m2, 5.4 W/m2, 24.0 W/m2 and -13.3 W/m2 respectively. Energy partitioning was dominated by latent heat in summer, while by sensible heat in winter. Under clear condition, latent heat, sensible heat and net heat storage absorbed 50%, 18% and 32% of net radiation in summer, while 10%, 56% and 33% of net radiation in winter. In addition, the mean diurnal Bowen ratio value was 0.53 and 6.29 in summer and winter respectively. There existed great difference in surface energy partitioning of suburban between Beijing city and reported North American cities, which was considered as results due to study sites possessing distinct surface characteristics, such as building sizes, shapes and arrangements, and vegetation cover.

Keywords Energy balance; Net radiation; Latent heat flux; Sensible heat flux; Bowen ratio, Suburban; Beijing