To understand how the land surface variability is disabled, we write equations for evaporation, potential evaporation, and so-called 'evaporability' after Koster and Suarez (1992). In the LSM, evaporation E is defined as:
where r is the air density, e is the ratio of the molecular weight of water vapor to that of dry air, ps is the surface pressure, es(Ts) is the saturated vapor pressure at the surface temperature Ts, eref is the vapor pressure in the overlying air, rsurf is the surface resistance to evaporation of water from the ground (this is inversely proportional to the amount of water in the ground and the vegetation canopy) and ra is the aerodynamic resistance to water vapor transport provided by the atmosphere. The potential evaporation Ep is the maximum possible value for E given the atmosphere resistance to water vapor transport ra, obtained by setting rsurf to zero, defined as:
We can write the relationship of E to Ep in terms of the evaporability b by combining equations A.1 and A.2 and factoring out Ep to obtain:
where bLSM is
Land surface variability, given the vegetative control over surface fluxes (contained in rsurf) from land to atmosphere in the mosaic LSM being used, is inherent in land-atmosphere coupling. To disable this variability, a land surface parameterization climatology is determined. Mean monthly 'evaporabilities' bfixed, month, defined as:
where E and Ep with overbars are the mean monthly evaporation and potential evaporation, respectively, and are calculated over all mosaic tiles at each grid square from the 20-year simulation discussed in Chapter 3. The resulting values for bfixed,month are then linearly interpolated to the Julian date being simulated, and used to compute the fluxes at the land-atmosphere interface. In using interpolated mean monthly values, the diurnal, intraseasonal and interannual land surface variability is removed, and the mean annual cycle remains intact.