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     All three dimensional fields are assumed to be (lon, lat, level)
      with the third index starting at the top of the model and
      ending at the bottom

     For tracers, it is the tendency of the mixing ratio (parts
        per unit mass) that is computed, not of the density (parts
        per unit volume) and analogously for all other fields

     If the dependence of the surface heat flux and evaporation
       on surface temperature is treated explicitly, then a single
       subrout