Characteristics of Solar Radiation and Radiative Transfer of a Forest Canopy in Huainan, Anhui Province
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摘要: 本文利用淮南森林观测站2018年7月1日至2019年6月30日冠层辐射观测，分析了淮南栎树森林下垫面冠层内外辐射变化特征。结果表明：（1）从春季到夏季，栎树冠层之上向下的太阳短波辐射增加，到冬季逐渐减少。从早春开始，由于叶片生长增多，冠层中间和冠层之下向下的太阳短波辐射下降，从秋季到冬季树叶凋落，其向下的太阳辐射增加，与冠层之上的变化趋势相反；对于向上的短波辐射，无论冠层之上、冠层中间还是冠层之下，随季节的变化都与向下的短波辐射相似，只是数值小很多。（2）冠层之上、冠层中间和冠层之下向下的长波辐射，随时间的变化从春季逐渐开始增大至夏季达到最大，随后逐渐减小并在冬季达到最小；就空间变化而言，冠层中间和冠层之下向下的长波辐射值比冠层之上的辐射值高，使得冠层对长波辐射的振幅增大，晴空条件最高可达1.3倍。（3）淮南森林区冠层之上（距地面25 m）年平均反照率为0.14，比中国北方地区（35°N）温带季风气候区（混交林为主）反照率的整体水平低0.01，表明淮南的森林茂密、灌丛更多些。（4）冠层上部分和整层的短波辐射透射率主要受叶片的影响。夏季，冠层的短波透射率平均为0.1。到了冬天，叶子凋落，透射率增加并趋于一个平稳的波动。冠层的短波辐射吸收率在夏季最高，秋季逐渐降低，随着叶子凋落在冬季迅速减小，趋于一常值。Abstract: The forest canopy, as an active interface between vegetation and the environment, transmits energy by reflecting, absorbing, and transmitting solar radiation through its leaves. The radiation levels above, within, and beneath the forest canopy are considerably important factors that affect the energy balance and water and carbon cycles. The variation of radiation with the seasons and the distribution of radiation among the forest canopies of the Huainan area have rarely been studied. Using total radiation data obtained by the Huainan forest observation station from July 1, 2018 to June 30, 2019, we investigated the temporal changes in solar radiation above the Sawtooth Oak canopy, analyzed the spatial distribution and transfer of solar radiation through the canopy, and determined the albedo, transmittance, and absorbance of the canopy. The results show the following. (1) The downward shortwave radiation above the Sawtooth Oak canopy increases from spring to summer and then decreases gradually toward winter. Unlike that above the canopy, the downward shortwave radiation within and under the canopy demonstrate a different trend with smaller values that decrease from early spring and increase from autumn to winter. Concerning the upward shortwave radiation, the seasonal variation pattern is the same as the downward pattern, whether above, within, or under the canopy, but the values are much smaller. (2) The downward longwave radiation above, within, and under the canopy gradually increases from spring to summer, then decreases gradually, and reaches a minimum in winter. In terms of spatial change, the radiation longwave values within and under the canopy are higher than that above the canopy, enhancing the longwave radiation by as much as 1.3 times under clear skies. (3) The annual average albedo above the canopy in the Huainan forest area is 0.14, which is 0.01 lower than that in the temperate monsoon climate area (mainly mixed forest) in northern China (35°N), which indicates that the forest is denser in Huainan. (4) The shortwave radiation transmittance values of the upper part and the whole canopy are mainly affected by the leaves. In summer, the average shortwave transmittance of the whole canopy is 0.1, whereas in winter, as the leaves fall, the transmittance increases and tends to a stable fluctuation. The absorbance of shortwave radiation in the canopy is highest in summer, decreases gradually in autumn, and then decreases rapidly in winter as the leaves fall, tending to a constant value. These results are useful for validating layered-radiative-transfer and photosynthesis models as well as for further investigations of the energy, water, and carbon cycles of forest ecosystems.
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Key words:
- Solar radiation /
- Shortwave radiation /
- Forest canopy /
- Radiative transfer /
- Albedo
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图 1 （a）淮南森林观测站地貌图及（b）108 m观测大塔
Figure 1. (a) Geomorphologic map of area around Huainan forest observation station and (b) the 108-m height observation tower
图 2 淮南森林观测站冠层顶（a）向下短波辐射、（b）向上短波辐射、（c）向下长波辐射和（d）向上长波辐射的月平均值（柱状）和标准差（短线）
Figure 2. Monthly means (bars) and standard deviations (short lines) of (a) downward shortwave radiation, (b) upward shortwave radiation, (c) downward longwave radiation, and (d) upward longwave radiation above the canopy at Huainan forest observation station
图 3 淮南森林观测站冠层顶净辐射月平均（柱状）和标准偏差（短线）
Figure 3. Monthly means (bars) and standard deviations (short lines) of net radiation above canopy at Huainan forest observation station
图 4 淮南森林观测站栎树植被冠层内（a）6 m高度和（b）3.5 m高度向下的短波辐射，以及（c）6 m和（d）3.5 m向上的短波辐射月平均日变化曲线
Figure 4. Diurnal variations of monthly mean downward shortwave radiation (a) within the canopy (6 m height) and (b) below the canopy (3.5 m height) and upward shortwave radiation (c) within the canopy (6 m) and (d) below the canopy (3.5 m) at Huainan forest observation station
图 5 淮南森林观测站栎树植被冠层内6 m和3.5 m高度（a）向下和（b）向上短波辐射月平均日变化
Figure 5. Diurnal variations of monthly mean (a) downward and (b) upward shortwave radiation within (6 m) and below (3.5 m) the canopy at Huainan forest observation station
图 6 冠层之上、冠层中间和冠层之下日平均太阳向下短波辐射
Figure 6. Daily mean downward shortwave radiation below, within, and above the canopy
图 9 冠层之下与冠层之上日平均入射长波辐射的比值（细实线）及日平均大气相对湿度（粗实线）
Figure 9. Ratio of downward longwave radiation below the canopy to that above the canopy (thin solid line) and daily mean atmospheric relative humidity (thick solid line)
图 7 冠层之上、冠层中间和冠层之下日平均太阳向上短波辐射
Figure 7. As in Fig. 6, but for upward shortwave radiation
图 8 冠层之上、冠层中间和冠层之下日平均太阳向下长波辐射
Figure 8. As in Fig. 6, but for downward longwave radiation
图 10 淮南森林站地表反照率的月平均（柱状）和标准偏差（短线）
Figure 10. Monthly means (bars) and standard deviations (short lines) of surface albedo at Huainan forest observation station
图 11 冠层之下、冠层中间和冠层之上的日平均反照率
Figure 11. Daily mean albedos below, within, and above the canopy
图 12 冠层上层、冠层下层和整个冠层的日平均短波辐射透射率
Figure 12. Daily mean transmittances of the upper layer, lower layer, and whole canopy
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