  The contribution to the potential caused by the land water 
storage vriation is computed using the surface pressure field at 
an equi-angular latitude/longitude grid with resolution 2048x4096 
computed from the output of the hydrology numerical weather model.

  The surface pressure of equivalent water is multiplied by the land-sea 
mass function of dimension 2048x4096 over latitude/longitude (~10 km) 
which is the fraction of land in a grid cell. Then the spherical harmonic 
transform of degree/order 1023 is performed. The spherical harmonics 
expansion coefficients S(k,n) are scaled by factors 

  Sv(k,n) = 3/(4*pi)*K_n/(2*n+1)/(mean_dens*mean_grav*Earth_rad) * S(k,n)

where K_n is the loading Love numbers, k is order, n is degree, 
mean_dens is the mean density of the Earth, mean_grav is the mean 
gravity acceleration at the sea level, and Earth_rad is the Earth's
equatorial radius.

  The expansion is truncated to degree/order 64.

  NB: The harmonic model that contains 20 terms, annual, semi-annual, 
diurnal, semi-diurnal etc, was evaluated with least squared are removed 
from the surface pressure. Therefore, these Stokes coefficients do not
contain harmonic variations at these frequencies.

  The loading Love numbers are defined for the total Earth
system (solid Earth + here atmosphere). They were 
computed by P. Gegout (private communication, 2005). 
In accordance with this definition the Stokes coefficients,
(0,0), (1,0) and (1,1) are zero.

  Normalization: the Stokes coefficients are so-called "4-pi 
fully normalized" according to Heikassen and Moritz, 1979.

This mean for any m,n, the integral over the entire sphere

\int Y_m^n(\phi,\lambda) \cdot Y_m^n(\phi,\lambda) \cdot
\cos ( \phi ) d \phi d \lambda \cdot = 4 \pi


References:

1) L. Petrov, J.-P. Boy, "Memo on computing Stokes coefficients
   of the expansion of the atmosphere contribution to the
   geopotential into a series of spherical harmonics", 2005,
   Unpublished, 
   http://gemini.gsfc.nasa.gov/agra/agra_memo_01.ps

2) W. Heikassen, H. Moritz, Physical geodesy, Graz, 1979, p. 31.

