Description
of Land surface model
(Click to see how the equations are calcuated in codes!) Main routine of Land
surfac model is LSMDRV,
is a driver for landsurface model. For the calculation of fluxes (e.g. momentum, heat) from land, subroutines SURPHY (energy fluxes and temperatures), VERH2O (surface and soil hydrology) and SURBGC (surface biogeochemical fluxes) are called here.
The land surface model works by gathering all the land points on a [lsmlon] by [lsmlat] grid into a vector of [lpt] land points. this is
then expanded into a "big" vector of [kpt] subgrid points, allowing for
up to [msub] subgrid points per land point. the [kpt] subgrid points
are processed as [numlv] "little" vectors of [numkpt] points for [numlv] calls to subroutine lsm. this subroutine:
o generates the calendar day (1.00 > 365.99), month (1 > 12),
and day (1 > 31), which are used to calculate the surface albedos and leaf and stem areas for the next time step.
o maps atmospheric fields from the [lsmlon] by [lsmlat] grid to subgrid vectors of length [kpt]
o calls the vector land surface model code [numlv] times in strips of [numkpt] points
o maps fields from the subgrid vectors of length [kpt] to the [lsmlon] by [lsmlat] grid
Surface temperature calculation starts from several calculations of variables.
Thermal conductivities of soil (frozen and unfrozen) are
1. Frozen soil
2. Unfrozen soil
where,
3. Soil layer
Heat capacities of soil (frozen and unfrozen) are
1. Frozen soil
2. Unfrozen soil
where,
3. Soil layer
where,
It is considered that when the surface is covered by snow,
Radiative fluxes at surface is determined in the subroutine SURRAD. And then temperatures at surface (SURTEM), soil (SOITEM) and lake (LAKTEM) are calculated.
SURRAD, Using the properties of ground and vegetation (e.g. seasonally varying leaf and stem area index), solar fluxes absorbed by vegetation and ground surface are calculated here.
: Solar fluxes absorbed by vegetation
: Solar fluxes absorbed by ground
These fluxes are used in the subroutine SURTEM
Surface (ground and vegetation) temperature is determined using energy balance. There is an iteration process to make this balance. For the temperature of vegetation, the ground temperature of previous time step is used (as a first guess). And this vegetation temperature is used for determination of ground temperature.
The energy balance for vegetation and ground temperature are
1. Vegetation temperature
2. Ground temperature
Here, longwave fluxes are
where,
: downward flux from atmosphere
: absorptivity of vegetation
: emissivity of vegetation
The flux terms which is used above balance equation is determined using below equations.
Sensible heat fluxes are calculated as following,
where,
: Leaf and stem area index
: average leaf boundary layer resistence
: aerodynamic resistence
Latent heat flux is,
where,
: vapor pressure (atmosphere)
: vapor pressure (vegetation)
: leaf area index (sunlit area)
: lear area index (shaded area)
: surface resistance
: fraction of wet part (vegetation)
The ground surface flux is defined as,
where,
: thermal conductivity
: thickness
The momentum fluxes are,
Aerodynamic resistances () have relationships with nondimentionalized exchange coefficients as follows,
And the resistances are defined using MoninObukhov similarity theory,
1. For momentum flux
2. For sensible heat flux
3. For latent heat flux
STOMATA
calculates leaf stomatal resistance
and photosynthesis. And The soil temperature is calculated by solving diffusion equation
at SOITEM,
Here, the heat capacity () and thermal conductivity () is calculated previously in the subroutine SURPHY.
The lake temperatures are calculated from a onedimensional thermal stratification model,
where,
: the molecular and eddy diffusion coefficients
: the thermal conductivity of water
: the heat capacity of water
: a solar radiation heat source term
This equation is solved numerically to calculate temperatures for sixlayer deep and shallow lakes with the boundary conditions of zero heat flux at the bottom and the net flux of energy at the surface.
Surface and soil hydrology
is calculated by VERH2O, surface hydrology
treat all water fluxes and pools are per unit ground area.
This processes simulated are interception,
throughfall/stemflow, snow accumulation and melt,
infiltration and surface runoff, soil water and drainage,
and irrigation.
The water vapor flux has three components,
vegetation evaporation, vegetation transpiration,
and soil/snow evaporation/sublimation.
Water balances are canopy water, snow water,
and soil water.Water conservation check
is computed for nonirrigated soil only. No soil hydrology,
for no soil , and some crops are irrigated.
Canopy water is a simple mass balance determined by gains from interception and dew and loss from evaporation.
Snow is a smiple mass balance determined by gains from the flux of snow at the ground surface and surface dew and losses from snow melt and sublimation.
The change of the soil moisture is represented as,
where,
: volumetric water content
: the change of soil moisture by evaporation at first layer and evapotranspiration at root layer
: Input and output of soil moisture
When we consider unsaturated soil, the vertical water flow is,
and in saturated soil that is,
The soil water is conserved (only in nonirrigated region),
The hydroulic conductivity and soil matrix potential is determined by the component of soil.
The liquid water at the soil surface (i.e. throughfall, snow melt, dew) either infiltrates into the soil column () or is lost as surface runoff (). This processes are considered using below balance,
Subroutine SURBGC
calculate the fluxes which are related with biogeochemical processes.
The net CO_{2} flux are,
where,
: foliage respiration at 25C
: stem respiration at 25C
: root biomass
: root respiration at 25C
: a temperature sensitivity parameter
Actually surface albedo
calculation is separated with land surface
model, since albedo calculation should be
prior to radiative transfer. LNDALB
is main subroutine of land surface albedo
calculation. The subroutine calculate the albedo at landsurface. By calling the subroutine ECODYN, the surface phenology is determined (e.g. seasonally varying leaf area index). And using these information, the albedo properties are determined at the subroutine SURALB.
The properties of vegetation which include leaf and stem area indices are calculated using previously determined values (monthly based).
Subroutine SURALB
calls the subroutines SNOALB and SOIALB to calculate the albedo at the ground.
Snow albedos are based on the work of Marshall (1989), For soot content ,
and for ,
where,
