Theoretical and Implemental Study of a Four-Stream Radiative Transfer Parameterization Scheme within Canopy in Land Surface Process Model
|School||Nanjing University of Information Engineering|
|Keywords||Vegetation canopy Radiative transfer of the basic equation Four stream radiative transfer model Second-rate radiation in transmission mode Coupling test|
The lower boundary condition of atmosphere is provided by land surface process model. This paper develops a four-stream solar radiative transfer model within canopy in land surface process model. The model is use of four-stream approximation method of atmosphere to gain the analytic solutions of the basic equation of radiative transfer within canopy.The four-stream solar radiative transfer model within canopy in land surface process model is based on the two-stream short wave radiative transfer model. It can simulate the process of solar short wave radiative transfer within canopy.The radiative transfer theory within canopy is based on the radiative transfer theory in atmosphere. Each parameter of the basic equation of canopy have the special geometry and optical character of leaf or canopy. The paper proposes the analytic formula of phase function. The formula show that the distribution of diffuse energy within canopy is different with different transmission and reflectivity and leaf angle.The parameterized formulas of other parameters are also proposed in this paper. The upward or downward radiative fluxes are related to the diffuse phase function,G-function, leaf reflectivity and leaf transmission, leaf area index, solar angle of incident beam direction.The simulation is tested by the albedo of canopy on the condition of horizontal leaf compared to simulations of two stream radiative model within canopy. The model is proved to be successful by the same modelling results of canopy albedo of the any solar incident angle.The new four-stream radiative transfer scheme within canopy can be used to analyze the law of radiative transfer process within canopy in special cases. The results show that the new scheme can describe properly the solar radiative process within canopy.In order to compare and find the difference between the results predicted by the two models, a great deal of numerical experiments examine the effects of different setting of leaf area index, leave angle distribution, optical property of leaf and ground surface under canopy on the canopy albedo. It is shown from the parallel experiments that there are much difference in the canopy albedo predicted by the two models. They are as follows. For incident diffuse radiative transfer within canopy,the relativity difference is between from 10% to 49% provided leaf area index of canopy is less. Especially vertical distribution of leaf declining angle and less soil reflectivity, the proportion is 77% if the relativity difference is greater than 15%. The proportion is 46% if the difference is greater than 0.02. The relativity difference is greater than 10% in most cases provided leaf declining angle is vertical distribution. The results predicted by the two models also show that the difference is greater especially for the incident solar beam.The paper carry out two experiments to evaluate simulations of land surface model coupled with the two radiative transfer models. The tests indicate that the simulation of land surface process model coupled four-stream model is best than other radiative transfer models. The solar radiative absorbed by land surface is closest to observation.