夏季西藏4个站点大气向下长波辐射观测分析

对2011—2016年部分夏季时段分别在西藏那曲、拉萨、林芝和阿里观测的大气向下长波辐射（L_↓）进行分析，结果显示:L_↓具有明显的日变化，最大值出现在北京时间15:00前后，而最低值出现在凌晨至10:00，日平均值林芝最高（368 W·m-2），其次是拉萨（319 W·m-2）、阿里（305 W·m-2）和那曲（299 W·m-2）。晴天L_↓ ?ngstr?m（1915）的经验公式最适合林芝，而Konzelmann（1994）的公式则适合那曲、拉萨和阿里；随着人工观测总云量的增加，L_↓增强趋势明显，满云（云量7~10成）情形4个站点云增强效应均从20 W·m-2上升至50 W·m-2以上，低云量对L_↓的增强效应明显高于总云量。云份额数（C_F）上升所对应天顶方向平均云底高度下降，但云增强效应上升。在晴天（C_F为-5%~5%、平均云底高度大于4 km）时，云增强效应仅为5 W·m-2左右（林芝接近20 W·m-2），但当C_F为90%以上（云底高度小于3.5 km）时，云增强效应则上升到60 W·m-2（林芝接近50 W·m-2）。固定云底高度，C_F与L_↓云增强效应呈显著相关（r2为0.91~0.97），远高于云底高度与L_↓云增强效应的相关（r2为0.32~0.58）。

Observational Analysis of Summer Atmospheric Downward Longwave Radiation at 4 Sites on the Tibetan Plateau

The summer downward longwave radiation (L_↓) observed in Naqu, Lhasa, Nyingchi and Ali is analyzed. The averaged L_↓ at 4 sites are 299, 319, 368 and 305 W·m-2, respectively. L_↓ is lower in the local morning and subsequently increases significantly in the afternoon. The mean diurnal variation at Naqu and Ali is about 30 W·m-2, while it's 9 and 19 W·m-2 in Lhasa and Nyingchi respectively. Based on solar shortwave radiation observations, a method to determine the daytime sky cloud-coverage index (cloud fraction, C_F) is presented by the solar radiation comparisons between the empirically calculation for cloud-free situation and the observed. C_F of -5% to 5% is assumed as daytime cloud-free situation to test the suitability of 10 empirical formula of L_↓ on the Plateau. It shows that the empirical formula of ?ngstr?m (1915) is most suitable for the Nyingchi where the vapor pressure is high, while the empirical formula of Konzelmann (1994) is most suitable for Naqu, Lhasa and Ali. Errors of the calculated daytime cloud-free L_↓ from the observed at Naqu, Lhasa, Nyingchi and Ali are 2.1%, -0.27%, -0.89% and 0.94%. The cloud-induced L_↓ enhancement effect (measured L_↓ minus the calculated cloud-free values given the surface temperature and humidity) clearly shows that the mean L_↓ enhancement effect at Naqu, Lhasa, Nyingchi and Ali are 30.8, 22.1, 38.8 and 15.6 W·m-2 with the median values of 24.4, 17.3, 42.7 and 6.8 W·m-2. With the increase of artificially visual total cloud amount, the increasing trend of L_↓ enhancement is obvious, especially when the cloud amount increase from less than 20% to 70% and above, the corresponding L_↓ enhancement effects rapidly increases from above 20 to more than 50 W·m-2 at all the 4 sites. Given the same visual cloud mount, the L_↓ enhancement effects induced by the low clouds in Lhasa and Ali are obviously higher than those induced by the total cloud. The effect from cloud coverage and height on the L_↓ enhancement is further confirmed by the aerosol lidar cloud base height at zenith direction and the C_F. The decreasing cloud base height (no available cloud base height data in Lhasa) corresponding to increasing trends of C_F and L_↓ enhancement is detected. The L_↓ enhancement effects are only about 5 W·m-2 with cloud-free condition, but they may rise to 60 W·m-2 when C_F is above 90% (the average cloud base height is less than 3.5 km). Given the fixed cloud base height, the L_↓ enhancement obviously increases with the growth of C_F. C_F, significantly more than the zenith cloud base height, which affects the enhancement of L_↓ on the Tibetan Plateau.