Introducing CLEO: A New Superdroplet Model with Collisional Breakup

The uncertainty in cloud responses to climate change, both in regional LES and global storm-resolving models, accentuates the need to better understand cloud microphysics. The superdroplet model (SDM) has been well applied for this purpose in domains up-to O(10km) with decameter resolution, but it remains to be seen whether it can also be utilised in larger domains with hectometre resolution. Thus we introduce CLEO: a SDM for warm rain aiming to be computationally feasible in O(100km) LES.

Besides its computational implementation, a defining feature of CLEO is its representation of collisional breakup. The microphysics in CLEO is similar to pre-established SDMs, except that the collision algorithm from Shima et al. 2009 has been extended to account not just for coalescence, but also for rebound and breakup. The novel algorithm still conserves the number of superdroplets, but introduces one new free parameter which determines the expected number of fragments produced by collisional breakup. In comparison with the original it considerably alters raindrop formation. We show how the evolution of the droplet spectrum is sensitive to the chosen number of fragments. Counterintuitively, we find that for certain choices of fragments’ number, rebound and collisional breakup act to increase the mean size of the final droplet distribution.