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I Observations
II Assimilation
III Dynamics
IV Physics
V Ensemble
VI Technical
VII Waves

7.9 Code

The surface parameterization computations are shared between the vertical diffusion routine (VDFMAIN, see Chapter 3) and the main surface routine, SRFMAIN. In VDFMAIN, the tile fluxes and skin temperatures are computed: After the elimination part of the tridiagonal system of equations is computed, the energy budget for each tile is computed before back-substition.

At the start of the model integration, the following setup routine is called to initialize a module specific to the soicode:

• SUSOIL. Setup routine for soil/snow/ice constants.

The main subroutine of the surface code (SRFMAIN) is called from CALLPAR, with: (a) values of the surface prognostic equations at time step n, convective and large scale rainfall and snowfall, tile evaporation, sensible and latent heat fluxes, and temperatures, net surface longwave flux, tile net shortwave flux as inputs; and (b) tendencies for the surface prognostic variables, plus a comprehensive set of diagnostic arrays as outputs. SRFMAIN does a sequence of computations and subroutine calls:

• SRFSN. Solution of the snow energy and water budget and computation of the next time step density and albedo fields. Inputs: snow depth, temperature, density and albedo at the current time step, soil temperature, shortwave and longwave radiation fluxes, snowfall, and tile fluxes. Outputs: snow depth, temperature, density and albedo at the next time step, meltwater flux, and basal heat flux.
• SRFRCG. Computes apparent soil heat capacity, ie including effects of soil freezing. Inputs: soil temperature and vegetation covers. Output is volumetric heat capacity.
• SRFT. Solution of the soil heat budget. Inputs: Soil temperature, soil moisture, longwave radiative flux, snow basal heat flux, volumetric heat capacity, tile evaporation, sensible heat flux and shortwave radiative flux. Output: Soil temperature at the next time step. First the modified heat diffusivity, the soil energy per unit area and the right-hand sice of the system of equations are computed. The generalized surface tridiagonal solver, SRFWDIF, is called to solve for the semi-implicit variable, . The soil temperatures for the next time step are computed at the end.
• SRFI. Solution of the ice heat budget. Inputs: Ice temperature, longwave radiative flux, tile evaporation, sensible heat flux and shortwave radiative flux. Output: Ice temperature at the next time step. First the modified heat diffusivity, the ice energy per unit area and the right-hand sice of the system of equations are computed. The generalized surface tridiagonal solver, SRFWDIF, is called to solve for the semi-implicit variable, . The ice temperatures for the next time step are computed at the end.
• SRFWL. Solution of the interception layer water budget. Inputs: Interception layer contents, low and high vegetation water cover, maximum capacity of the interception layer, convective and large scale rainfall, snow evaporation of shaded snow tile, and tile evaporation. Outputs: Interception layer at next time step, convective and large scale throughfall and tile evaporation collected (or depleting) the interception layer.
• SRFWEXC. First part of the computation of the soil water budget, ie, computation of the coefficients of the tridiagonal system of equations for . This includes the partitioning of transpiration into root extraction at the different layers and soil hydraulic coefficients including the effect of frozen water. Inputs: Soil moisture and temperature, convective and large-scale throughfall, snowmelt, tile evaporation, tile evaporation collected (or depleting) the interception layer, and snow evaporation of the shaded snow tile. Outputs: Modified diffusivity for water, right-hand side of the tridiagonal system, and layer depths.
• SRFWDIF. Generalized surface tridiagonal solver. Inputs: Values of at the current time step, generalized modified diffusivities, soil energy (or water) per unit area, and right-hand side of equations. Output: . The routine computes the coefficients on the left-hand side of the equations and solves the equations using and LU-decomposition and back substitution in one downward scan and one upward scan.
• SRFWINC. Computation of next time step soil water. Inputs: and current time step soil water. Output: next time step soil water.
• SRFWNG. Bounded-value operator for intercepted water (limited to non-negative values and values below or equal the maximum contents of the interception layer) and soil water (limitted to non-negative values and values below or equal saturation). The "soil column" is scanned from top to bottom and the amount of water needed to satisfy physical limits in each layer are borrowed from the layer below. The water exchanged in this way is accounted for as runoff. Inputs: next time step intercepted water and soil water. Output: Bounded values of the same quantities.

Relevant routines from the vertical diffusion code, discussed in full detail in Chapter 3, include:

• SUVEG. Assignment of vegetation related constants.
• VDFBC. Definition of tile fractions and related characteristics.
• VDFSURF. Definition of bare soil resistance, low and high canopy resistances.
• VDFEXCS. Computation of aerodynamical part of exchange coefficients for heat and moisture, including stability computations.
• VDFEVAP. Computation of evapotranspiration for each tile.
• VDFSFLX. Surface fluxes for each tile, defined at time t.
• VDFTSK. Computation of the tile skin temperatures, as a the solution of the tile energy balance.
• VDFTFLX. Computation of the tile fluxes at time t + 1.