Implementation And Evaluation Of Generalized Radiative Transfer Scheme Within Canopy And Chlorophyll Fluorescence Scheme In The Land Surface Model

The vegetation has a profound impact on energy,water and carbon balances over the terrestrial surface.The vegetation impacts the surface energy distribution by radiative transfer process and has influence on global carbon cycling through terrestrial photosynthesis.Land surface models are important for the explornations of land surface peocesses.The radiative transfer within the canopy process and terrestrial photosynthesis are very important for the land surface modeling.The two-stream radiative transfer model,which is widely used in current land surface models,is unrealistic in their assumptions of the same optical properties of adaxial and abaxial leaf surfaces and uniform leaf angle distibution within the canopy.In this paper,to have a more accurate description of the radiative transfer process,a generalized radiative transfer scheme(GRTS)within canopy is implemented in the Simplified Simple Biosphere(SSiB)land surface model.Different from two-stream scheme,the GRTS can be used in more general cases:non-uniform optical properties of adaxial and abaxial leaf surfaces;the non-uniform canopy structure in the vertical direction.The assumptions of the new scheme are expected to be more general and closer to reality.This paper presents an approch to simplify the layered radiative transfer scheme within the canopy for the land surface modeling,which is suitable for both two-stream scheme and GRTS.Firstly,by using the basic solutions of radiative transfer scheme within the canopy under the condition of assumed zero soil reflectance,two sets of explicit analytical solutions with any soil reflectance magnitude are derived:one for incident diffuse,and the other for direct beam radiation.Using the explicit analytical solutions,the canopy reflectance,transmittance,absorlptance and other properties pertinent to the radiative transfer within the canopy can be estimated easily on the ground surface below the canopy(soil or snow surface)with any reflectance magnitudes.This method is proven to have a similar accuracy compared to the detailed model.For a vegetation species,the fitting of basic solutions for its different growth seasons and coupling with the derived analytical solution formulas can decrease the amount of computing required and meet the needs of the climate model.This development of decoupling of a vegetation species and the ground surface contributions is a simple and efficient computing way by allowing the canopy reflectance,transmittance,absorptance to be precomputed and stored in look-up tables.The meteorological measurements at half-hour intervals data from SACOL is used as the forcing data for the single point SSiB model.The resluts show that the surface albedo simulated in the SACOL/GRTS run is consistently closer to the observation than that from the SACOL/two-stream run.Subsequently,the upward shortwave radiation in SACOL/GRTS run is also more consistent with the observation data than that in the SACOL/two-stream run.In the offline global SSiB/GRTS simulation for the period of 2001—2012,the non-uniform leaf angle distribution within canopy layers is considered in SSiB/GRTS in the areas of evergreen broadleaf trees.Compared with the SSiB/two stream method,SSiB/GRTS produces lower canopy reflectance and higher transmittance,which leads to more realistic albedo simulation.The canopy absorbed radiation flux in SSiB/GRTS simulation is lower than that in SSiB/two-stream method simulation throughout the year in the areas of evergreen broadleaf trees.The largest difference of-18.4W/m2 occurs in the Amazon region in the autumn.The ground absorbed radiation flux increases in the SSiB/GRTS simulation,especially in the spring and autumn.The largest difference in the ground absorbed radiation flux between SSiB/GRTS simulation and SSiB/two-stream method simulation is 25.45W/m2.In the boreal winter season,compared with the two-stream method in the SSiB,the GRTS gives higher surface albedo in the areas with high snow cover fraction over leaf.Several studies have shown that satellite retrievals of solar-induced chlorophyll fluorescence(SIF)provide useful information on terrestrial photosynthesis or gross primary production(GPP).Here,we have incorporated equations coupling SIF to photosynthesis in SSiB land surface model.By comparing forward simulations of SIF,essentially as a byproduct of photosynthesis,in SSiB with observations of actual SIF,it 1s possible to check whether the model is accurately representing photosynthesis and the processes coupled to it.We provide some background on how SIF is coupled to photosynthesis,describe how SIF was incorporated into SSiB,and demonstrate that our simulated relationship between SIF and GPP values are reasonable when compared with satellite(Greenhouse gases Observing SATelhte;GOSAT).The terrestrial carbon cycle and water cycle are coupled through a plethora of connections of processes between soil,roots and leaves,and atmosphere.Yet,the strength and sensitivity of these couplings are not well known at the global scale,which contributes to the uncertainty in predicting the terrestrial water and carbon.budgets.For the first time,we now have synchronous,high fidelity,global-scale satellite observations of critical terrestrial carbon and water cycle components:solar-induced chlorophyll fluorescence(SIF)and soil moisture.We used these observations within the framework of SSiB to investigate carbon-water couplings processes.We tested sensitivity of model parameters to improve the simulation of both SIF and soil moisture with the ultimate objective of improving the first order terrestrial carbon component:gross primary production(GPP).After the soil moisture sensitivity B parameter and hydraulic conductivity at saturation is modified in SSiB model,it produces better simulated soil moisture matching against SMOS,but results in a worse GPP match against MPI-BGC GPP product,which shows the changes in soil moisture have large impacts on vegetation processes.Then our study shows that one parameter emerges as the key coupler between the carbon and water cycles,which is the wilting point.Calibration of the wilting point significantly improved simulations for both soil moisture and SIF,as well as GPP,especially in semi-arid regions.This study demonstrates the value of synchronous global measurements of terrestrial carbon and water cycle in improving understanding of the coupled carbon?water cycles.