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MOSIAC aerosol size distribution with very high loading

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New member
Hello all,

I am running a WRF-Chem v4.1.5 simulation using the CBMZ-MOSAIC-8bin scheme. I have a strong volcanic source which emits about 10 kg/s of sulphate aerosol with a pre-defined size distribution into a single grid cell at 4000km altitude. I am nesting, which means I have a 1.1km x 1.1 km grid around my volcanic source. Overall, I am adding about 20 ug of sulphate per kg of dry air per second into the source grid cell. The total concentration of sulphate aerosol in this grid cell is typically around 1250 ug per kg of dry air during the simulation. The volcano is also a very strong emitter of water (4800 kg/s), and new particle nucleation would be expected due to the plume's sulfur chemistry.

I find that the aerosol immediately changes its size distribution in a curious way. In this source cell, all of the aerosol immediately partitions to bins 1, 3, 5, and 7, leaving the even numbered bins nearly "empty". However the overall aerosol mass appears to be correct. I have added debug code to the emissions part of the code, and I am confident that the emissions are being correctly assigned to the size bins I define. This redistribution is happening later in the code, and it seems to be occurring immediately, in the output I cannot identify a single grid cell "before" this redistribution occurs. This phenomenon only happens with the volcanic emissions, outside of the plume the aerosol size distributions seem normal.

My guess is that this very high aerosol loading is some part of the aerosol code to malfunction. I have tested disabling coagulation however this does not make any substantial difference.

I tried repeating this with the overall volcanic intensity reduced by a factor of 8 and the same phenomenon occurs.

Many thanks for any help with this.

Luke Surl