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, *p*_{s}_{
}is the surface pressure, *e*_{s}*(T _{s})*
is the saturated vapor pressure at the surface temperature

We can write the relationship of
*E* to *E** _{p}*
in terms of the evaporability b by combining equations A.1 and A.2
and factoring out

where *b** _{LSM}*
is

Land surface variability, given the
vegetative control over surface fluxes (contained in
*r _{surf}*) 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'

where *E* and
*E _{p}* 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