GlomapSedimentation

Notes on Aerosol sedimentation in UKCA GLOMAP

This work is primarily motivated by the desire to be able to run aerosol sedimentation with a shorter timestep than the rest of UKCA, and potentially to try alternative schemes for sedimentation. The code presently uses a flux-form advection scheme (FTBS) to sediment aerosols, with sedimentation velocity calculated by the routine UKCA_VGRAV_AV_K. In this scheme the sedimentation speed is limited by the model timestep, for stability reasons.

A quick hack has demonstrated that allowing aerosols to sediment more quickly (by substepping the existing scheme with a shorter timestep) makes a significant difference to the simulation of the coarse mode, with knock-on effects on the accumulation mode. However this hack is not suitable for inclusion in the UKCA trunk, and is inefficient as it substeps with the same timestep over all modes and all columns.