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Observational Analysis of Summer Atmospheric Downward Longwave Radiation at 4 Sites on the Tibetan Plateau
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摘要: 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↓ Ångström（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（CF）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（CF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CF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，CF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）。
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关键词:
- 大气向下长波辐射(L↓);
- 云份额数(CF);
- 云增强效应
Abstract: 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, CF) is presented by the solar radiation comparisons between the empirically calculation for cloud-free situation and the observed. CF 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 Ångström (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 CF. The decreasing cloud base height (no available cloud base height data in Lhasa) corresponding to increasing trends of CF 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 CF 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 CF. CF, significantly more than the zenith cloud base height, which affects the enhancement of L↓ on the Tibetan Plateau. -
图 2 那曲、拉萨、林芝和阿里晴天情况下太阳短波辐射计算值及与观测值对比
Fig. 2 Observed and calculated solar shortwave radiation at Naqu, Lhasa, Nyingchi and Ali under cloud-free condition in daytime
图 3 那曲、拉萨、林芝和阿里晴天情况下L↓计算值及与观测值对比
Fig. 3 Observed and calculated L↓ at Naqu, Lhasa, Nyingchi and Ali under
图 4 那曲、拉萨、林芝和阿里所有天气情况下L↓观测值与晴天计算值比较
Fig. 4 All observed and calculated cloud-free L↓ at Naqu, Lhasa, Nyingchi and Ali
图 5 那曲、拉萨、林芝和阿里L↓增强变化与人工观测云量
(总(低)云量所对应L↓增强的误差棒分别是下(上)方向)
Fig. 5 Enhancements of L↓ and the artificially observed cloud amount (error bars of L↓ enhancement corresponding to total(low) cloud amount are the downward(upward) direction) at Naqu, Lhasa, Nyingchi and Ali
图 6 那曲、拉萨、林芝和阿里L↓增强效应、CF及其对应天顶方向上平均云底高度变化
Fig. 6 L↓enhancement effect, CF and its corresponding variations of zenith averaged cloud basement height at Naqu, Lhasa, Nyingchi and Ali
图 7 那曲、林芝和阿里给定天顶方向云底高度白天CF与L↓增强效应变化
Fig. 7 Given the fixed zenith cloud base height, L↓ enhancement effects as functions of CF variations at Naqu, Nyingchi and Ali in daytime
表 1 那曲、拉萨、林芝和阿里4个观测站点基本信息
Table 1 General descriptions of 4 sites deploying L↓ observations
观测站 观测时段 观测时段日平均气象要素 辅助观测 气温/℃ 水汽压/hPa 云量/成 日照/h 那曲 2011-07—08 9.4±8.0 7.4±5.0 6.8 5.4 CM21，激光雷达 拉萨 2012-05—07 18.0±10.0 8.39±5.0 6.9 5.4 CM21 林芝 2014-06—07 16.8±10.0 13.4±4.0 9.0 2.6 CM21，激光雷达 阿里 2016-05—09 14.0±8.0 4.8±4.0 4.9 7.2 CM21，激光雷达 
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表 2 晴天比辐射率(ε)计算经验公式
Table 2 General descriptions of surface L↓ parameterization schemes under cloud-free condition
来源 晴天比辐射率ε a b c 文献[17] a-be-ce 0.83 0.18 0.067 文献[18] a+b(e)1/2 0.52 0.065 文献[19] aT2 9.365×10-6 文献[20] 1-ae-b(273-T)2 0.261 7.77×10-4 文献[21] a(e/T)1/b 1.24 7 文献[22] a(1-e-e(T/b)) 1.08 2016 文献[23] a+beec/T 0.7 5.95×10-5 1500 文献[24] 0.23+a(e/T)(1/b) 0.484 8 文献[25] 1-(1+a)e-(b+ca)0.5 46.5(e/T) 1.2 3 文献[26] 1-ae-be/T 0.43 11.5 注：e为近地面水汽压，单位为hPa; T为近地面气温，单位为K。
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表 3 白天云底高度频率分布及其对应的CF，L↓增强统计
Table 3 Statistics of day-time cloud base height frequency distribution and its corresponding CF and L↓ enhancements
台站 统计变量 (0 km，1 km] (1 km，2 km] (2 km，3 km] (3 km，4 km] (4 km，5 km] >5 km 那曲 出现频率/% 6.2 31.4 33.4 8.6 1.2 18.4 云底高度/km 0.8±0.15 1.5±0.28 2.5±0.28 3.3±0.26 4.4±0.28 10.6±2.7 CF/% 60±24 54±33 33±36 40±35 47±29 41±34 L↓增强/(W·m-2) 55±19 44±25 24±27 31±26 33±20 24±24 林芝 出现频率/% 1.3 15.9 50.5 16.7 6.2 9.1 云底高度/km 0.8±0.16 1.8±0.24 2.4±0.29 3.4±0.25 4.4±0.27 10.0±3.5 CF/% 76±13 68±17 55±25 41±29 36±32 27±34 L↓增强/(W·m-2) 57±5 52±5 45±9 35±11 26±13 18±15 阿里 出现频率/% 15.4 15.6 30.2 15.8 3.4 19.4 云底高度/km 0.5±0.21 1.6±0.3 2.5±0.28 3.3±0.24 4.5±0.28 7.77±1.4 CF/% 27±36 44±35 31±35 15±34 21±33 20±31 L↓增强/(W·m-2) 26±27 45±16 33±16 20±14 22±15 19±15
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