Models - Land Model
In comparison to BCC_AVIM1.0, two improvements were implemented in BCC_AVIM1.5: environment dependent phenology of vegetation, soil character and soil moisture dependent threshold of temperature for soil freezing/melting.
In BCC_AVIM1.0, dates for leaf onset in spring and leaf senescence in autumn for deciduous vegetation and the associated loss rate are prescribed according to current climatology of each plant functional type (Ji, 1995). Similar to Canadian Terrestrial Ecosystem Model (CTEM) (Arora, 2005), the life cycle of deciduous vegetation in BCC_AVIM1.1 consists of 4 stages: maximum growth in early spring, normal growth in spring and summer, senescence of leaves in late autumn, and dormancy in winter. A carbon-gain-based scheme initiates leaf onset when it is beneficial for the plant, in carbon terms, to produce new leaves. Leaf offset is initiated by unfavorable environmental conditions that incur carbon losses. During the maximum growth period, all net primary production (NPP) is allocated to leaves during leaf-out for trees and crops. During normal growth period, photosynthetic assimilated carbon is shared between leaves, stems and roots, the allocation is determined by the solar radiation and soil moisture conditions. During leaf-fall/crop harvest period, allocation to leaves ceases but continues for stem and roots. During dormant/no leaf period, no allocation in the absence of CO2 uptake. Shorter day length, cooler temperature, and (or) dry soil moisture will initiate fall of vegetation canopy in addition to normal senescence.
When ice exists in soil layers, soil matric potential is in balance with vapor pressure of ice surface, combining the relationship of soil matric potential-soil temperature (based on Clausius-Clapeyron equation) and soil matric potential-volumetric soil moisture, one can derive the threshold of temperature for soil freezing/melting which is always lower than the freezing point of pure water (zero degree Celsius) (Xia, 2011), different from the fixed zero degree Celsius in BCC_AVIM1.0.
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 new scheme proposes the analytic formula of phase function. The formula shows that the distribution of diffuse energy within canopy is different with different transmission and reflectivity and leaf angle. 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.
We carry out lots of experiments to evaluate simulations of land surface model coupled with the two-stream and four-stream 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.
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