IMPROVED MASS-CONSERVING ADVECTION - 18 December 2009

 The mass-conserving advection scheme in CMAQ uses an upwind donor cell method
 for the vertical component, which is known to be first order numerically
 diffusive. A higher order, less diffusive scheme has been developed, which
 adjusts the diagnosed mass fluxes using the Piecewise Parabolic Method (PPM).

 In the previous vertical advection scheme, the vertical velocities are
 calculated using mass fluxes derived from the differences between the
 tranported densities and those generated by the meteorological model, e.g. WRF
 (the "densities" are actually density coupled with the vertical Jacobian based
 on the generalized coordinates used).

 In the new version, the velocites are further adjusted by the ratio of the
 upwind fluxes to PPM-calculated fluxes based on the upwind velocities. This
 velocity adjustment is iterated, if necessary until the fluxes are closely
 matched.

 Testing has shown that the artifically high numerical diffusion arising from
 the low order, upstream method used in the yamo vertical advection has been
 significantly reduced. The tests indicate what seems to be a modest CPU time
 impact.