ADVECTION TIME STEP DETERMINATION - 18 December 2009

 A method has been developed that modifies the calculation of advection time
 steps in CMAQ based on the horizontal divergence. This method supplements the
 existing scheme which considers the Courant-Fredrichson-Levy only. The method
 will fix occurances in which CMAQ runs have abnormally terminated due to
 creating negative mass situations which arise because of the operator splitting
 paradigm used for CMAQ.

 Where high horizontal divergence occurs in which the model would have
 abnormally terminated because the vertical advection algorithm cannot find a
 solution with the resulting negative transported densities, users have been
 reducing the maximum allowable advection step (through the CTM_MAXSYNC
 environment variable) to enable the model to run. It is expected that this also
 increases the model accuracy due to the smaller advection time steps, but at
 the expense of significantly increased run times. The new advstep will allow
 the model to run at a larger time step, reducing it only as necessary. However
 results may be less accurate than runs executed at smaller time steps for the
 whole scenario.