Abstract:

Based on eddy covariance observational data from November 11 in 2012 to January 20 in 2013 in Shanghai, diurnal variations of urban surface energy balance and carbon dioxide fluxes under different weather conditions were analyzed. The results show that under sunny and cloudy weather conditions, the maximum energy flux is storage heat, and then is convective transport of sensible heat, while the minimum fraction is latent heat used in evaporation and less than 50 w·m^-2. Daily storage heat flux has a peak at 11:00 a.m. which is earlier than that of net radiation flux, while it becomes negative before sunset. Diurnal curve of sensible heat flux is not symmetric around its peak, and its mean diurnal peak occurs in the early afternoon. Also, its value remains significantly positive after sunset. The Bowen ratio remains above 3 from noon to sunset. The effect of wind direction on sensible heat flux is the most significant. Under the dominant northwestly wind direction, daily peak value of sensible heat flux reduces from 175 w·m^-2 (under other wind directions) to 120 w·m^-2. Haze and cloud show a negative effect on shortwave radiation, and effect of cloud is bigger than that of haze. The existence of cloud results in obvious reduction of upward and net long wave radiation, while it leads to an increase of the atmospheric downward longwave radiation. Carbon dioxide flux keeps positive values throughout the day under sunny weather condition, which means a constant carbon emission. There are two peaks for daily carbon dioxide fluxes, and both occurrence times are corresponding with morning and evening rush hours. Peak value of daily carbon dioxide flux is larger in the evening than in the morning.

Key words: Surface energy balance, Radiation characteristic, CO₂ flux