陆面模式中不同植被辐射模式的对比

Comparative studies of different radiation schemes within vegetation in land model

针对目前在陆面过程数值模拟研究中对于地表植被冠层辐射传输参数化方案精确性不足的问题,在植被二流辐射传输模式基础上新发展了一个描述太阳短波辐射在植被中传输的冠层四流辐射传输模式.为了更好地评估新发展的四流模式在模式应用中的模拟性能,依据陆面模式中定义的16种植被参数给定四流模式和二流模式独立试验中需要的参数,开展多组离线的独立理想实验,计算16种植被冠层在不同太阳高度角以及不同叶面积指数时的反照率、透射率以及吸收率.独立试验结果表明,太阳高度角对于短波在植被内的辐射传输过程影响比较显著,两个模式对于反照率、透射率和吸收率的模拟差别随不同种类植被呈现不同特征.将四流模式耦合到陆面模式中进行数值实验,结果表明,采用四流模式后,能够改善陆面模式在全球多数区域对陆面地表反照率、叶面积指数和净初级生产力的模拟结果.

The lower boundary condition in the atmosphere is provided by a land surface process model.One of the directions of research of the land surface model is to perfect land surface model in existence.Since the accurate estimation of the albedo is required in climate modeling and there are some shortages in either the accuracy or the application area for radiative transfer scheme within a canopy,it is necessary to develop an accurate and simple four-stream radiation transfer model within a canopy based on the second vegetation radiative transfer model.The four-stream model can simulate the process of short-wave solar radiative transfer within a canopy.The four-stream model utilizes the four-stream approximation method in the atmosphere to provide analytical solutions to the basic equation of radiative transfer within a canopy.The four-stream solar radiative transfer model within a canopy in the land surface process model is based on the two-stream short-wave radiative transfer model.The two-stream model developed by Dickinson is computational efficiency and can gain accurate analytical solution.Sellers ever calculate the canopy hemisphere albedo in visible band and near infrared band by use of the two-stream model and can gain analytical solution.Many land surface models still use the two-stream model.But the two-stream model is not very accurate,thus it is necessary to develop more accurate four-stream radiative model.The radiative transfer theory within a canopy is based on the radiative transfer theory in the atmosphere.Each parameter in the basic canopy equation canopy accounts for the special geometry and optical character of the leaf or canopy.The upward or downward radiative fluxes are related to the diffuse phase function,G-function,leaf reflectivity and leaf transmission,leaf area index,and the solar angle of incident beam direction.The four-stream model and the two-stream model can simulate the reflectance,transmittance and absorption of the canopy.The newly developed the four-stream model can reduce the calculate difference of land surface radiative in theory.To compare and determine differences between the results predicted by the two models,several experiments were carried out to examine the effects on the canopy albedo by varying the leaf area index,leave angle distribution and the optical properties of the leaf and ground surface under the canopy.To better assess the simulation performance of the new four-steam model in the application for a land surface model,we needed to determine the parameters for the four-steam scheme and the second-stream scheme to utilize in an off-line test.These parameters also included calculating the reflectance,absorbance and transmission at different sun angles and different leaf area indexes for the 16 kinds of vegetation canopy.The offline test results showed that the sun angle had a significant impact on the solar radiative transfer process within vegetation and on the simulations of two schemes for reflectance and transmittance and absorption varies with different vegetation types.The offline test results show that the sun angle has significantly impact on the solar radiative transfer process within vegetation,and the simulations of two schemes for reflectance and transmittance and absorption varies with different vegetation types.To evaluate the simulations of the four-stream model for solar radiative flux reflected by the land surface in the land surface model,the four-stream model and the two-stream models were coupled into a land surface process model.These tests indicated that the simulation of the land surface process model coupled to the four-stream scheme outperformed two-stream scheme radiative transfer models regarding the global surface albedo,leaf area index and net primary productivity.