阿尔卑斯山杉林冠层影响辐射传输的个例分析

利用瑞士Alptal观测站杉树林冠层上方、下方的辐射观测资料,分析了冠层对短波辐射的减弱及对长波辐射的增幅作用及其季节变化。结果表明,对比较密集的常绿针叶林,冠层对入射短波辐射的透过率随着太阳高度的降低而减小,春季以后趋于稳定;冠层对长波辐射的增幅作用随天气状况而变化,这种增幅作用在晴空条件下最显著,可达1.5倍。在冬季,因为太阳辐射较弱,冠层对长波辐射的增幅作用超过对短波辐射的减弱从而增加地面净辐射。在其它季节,太阳辐射比较强,冠层对短波辐射的减弱超过对长波辐射的增幅作用而减少地面净辐射。地面净辐射与冠层上方气温的变化趋势虽然在有些时段一致,但在伴随降雪过程的降温时段,地面净辐射与气温的变化趋势近乎反相,在积雪融化时段,地面净辐射的增加比气温升高更显著,尤其是在白天。

A Case Study of the Influence of Needle Leaf Forest Canopy on the Radiation Transfer over Alps Mountain

The impact of needle leaf forest canopy in reducing solar radiation and enhancing longwave radiation transfer through the canopy and the seasonal evolution of these two competitive influences were analyzed by using observations from the Alptal site in alpine Switzerland in this investigation.It was found that as for the dense needle leaf forest canopy under study,the part of solar radiation transmitted through the canopy decreases with the decline of solar angle and with the increase of canopy intercepted snow,which fluctuates around a relative high value from spring onwards.The ratio of below-canopy downward longwave radiation to that above the canopy is around 1.0 during cloudy days and can be as large as 1.5 under clear skies.During mid-winter when solar radiation is weak and surface albedo is high due to accumulated ground snow,the enhancement of daily mean longwave radiation outweighs the attenuation of solar radiation therefore the overall influence of canopy is to increase net all-wave radiation at the ground surface;during other periods when solar radiation is strong and surface albedo is low due to snow melting,the attenuation of solar radiation by the canopy dominates the enhancement of longwave radiation therefore decrease net all-wave radiation reaching the ground,especially during daytime.From winter to early spring,longwave radiation is the main component of the net all-wave radiation at the below-canopy ground surface,while solar radiation dominates the net all-wave radiation from mid-spring onwards.Although daily mean temperature above the canopy varies somewhat coincidently with ground surface net all-wave radiation sometimes during the snow season,their temporal variation are almost contrary to each other during snowfall events,when solar radiation is quite weak and downward longwave radiation from the relative warm canopy dominates the upward longwave radiation from the snow covered cold ground surface,resulting in stronger net longwave hence net all-wave radiation at the ground surface,on the contrary,the air temperature lowers significantly.During snowmelt periods,air temperature increases,both downward solar and longwave radiation intensify,but the upward longwave radiation from the ground surface is restricted because the temperature of melting surface is "locked" around the freezing point.Besides,the reflected solar is also weak because of the low albedo of the melting snow,both of the above two factors coincidently increase the surface net radiation,which is much significant than the warming trend of air temperature.The above mentioned contrary variation of above-canopy temperature and ground surface net radiation is more significant during daytime,while the synchronization of their variations is more obvious during nighttime when longwave radiation dominates the ground surface energy budget.