Detailed Assessment of Glaciated Clouds Aerosol Indirect Effects Using a Hybrid Bulk/Bin Microphysics Model

Clouds control the radiation and precipitation budgets of the Earth, but how they will respond in future to anthropogenic changes in aerosol chemistry and loading remains one of the greatest source of uncertainty in climate prediction according to the fourth assessment report of the inter-governmental panel on climate change (IPCC). Thus a state-of-the-art hybrid bulk/bin microphysics scheme was developed and coupled to the WRF model. The scheme was rigorously validated against observations from two contrasting cases (a tropical maritime and a mid-latitude continental). The aerosol-cloud model treats all the known modes of cloud droplet initiation and ice-crystal activation. It directly links these nucleation processes with aerosol chemistry and loading. This scheme was used to investigate salient microphysical mechanisms of aerosol indirect effects with focus on glaciated clouds - glaciated clouds aerosol indirect effects.

Results from the study showed that the contribution from glaciated clouds dominates the total aerosol indirect effect in both continental and maritime cases. This was predicted when only soluble aerosols were allowed to evolve from preindustrial to present-day aerosol loadings. This was principally because glaciated clouds are deeper and more extensive relative to water-only clouds. Secondly, because they exist above water clouds they therefore have the first interaction with solar radiation. The invigoration indirect effect is seen to have a cooling effect on the climate system. This is primarily due to an increase in cloud cover caused by the formation of secondary cells in the continental case while in the maritime case it has a warming effect. It was surprising to note that for the continental case, the glaciation effect was negative while the riming effect was positive.