Ben Murphy, U.S. Environmental Protection Agency
The gravitational settling algorithm in the vertical dispersion module allows large particles (coarse mode) to transport to lower grid cells with the force of gravity. It had been noted in CMAQv5.2.1 and earlier that the algorithm did not conserve mass for certain aerosol size distribution parameter combinations. These conditions were observed to occur in the highest-altitude model layers, but could propagate to other parts of the domain if they were large enough.
Two updates are introduced in CMAQv5.3 to resolve these problems. First, the time step difference calculation has been corrected to give the remainder of the current sub-time-step and the master time step, which was the intended result when the algorithm was introduced. Second, a ceiling on the number of iterations used to calculate settling has been introduced, and is equal to 10 iterations. This way, if the algorithm becomes unstable, it will minimally disrupt the concentration field.
Resolves infrequent mass accumulation errors on the order of 10-50% of the total coarse mode particle mass. Particularly important for hemispheric CMAQ simulations where errors can accumulate over time.
CCTM/src/vdiff/acm2_m3dry/SEDIMENTATION.F
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