上海秋冬季地表能量平衡及CO2通量特征分析

利用2012年11月11日至2013年1月20日上海秋冬季涡动相关通量观测资料，对比分析地表能量平衡和CO₂通量在不同天气条件下的日变化特征。结果表明：2012-2013年上海晴天和多云天气条件下，最大能量通量为储热项，其次为感热项；用于蒸发的潜热通量项最小，低于50 w·m^-2。储热项日峰值出现在11时，出现时间早于净辐射通量，而在日落前转为负值。感热项日变化曲线并不以12时为中心呈对称分布，日落后感热项仍为明显正值。中午至日落时波文比值为3以上。感热通量受风向影响最大，在主导风向为西北风时，感热通量日峰值从其他风向的175 w·m^-2左右减小至120 w·m^-2左右。霾和云对短波辐射均表现为衰减作用，云的衰减作用明显大于霾。云使地表向上长波辐射和净长波辐射明显减少，而使大气逆辐射增加。晴天条件下，全天表现为CO₂排放源，且日变化呈双峰型，两个峰值出现时间正好对应上下班高峰时段，傍晚峰值大于早上峰值。

Characteristics of surface energy balance and CO 2 flux in autumn and winter in Shanghai

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.