Organization of secondary ice production mechanisms among basic cloud-types

Description

Reasons for high concentrations of ice seen in clouds have provided an enigma, ever since it was appreciated decades ago that solid aerosol material in the air initiates the first ice.  This has led to the realisation that ‘secondary ice’ is formed somehow from pre-existing primary ice from this aerosol material.  Various types of fragmentation of pre-existing precipitation explain this secondary ice.

The present project began with lab observations of an overlooked type of fragmentation involving drops collding with more massive ice.  This was done at Manchester University.  Another type of SIP was represented for the first time in models, namely sublimational breakup (see weblink to paper).   More recently, a Masters student to analyse field observations from northern Sweden characterizing the fragmentation of snow in collisions with graupel. 

During the project, this work has enabled improved treatment of four pathways of SIP in AC:

• Rime-splintering (the Hallett-Mossop [HM] process);
• Breakup in ice-ice collisions;
• Fragmentation during freezing of rain and drizzle;
• Sublimational breakup.

Simulations of four contrasting cases of deep convection have been performed.  The cases are typified by some basic cloud-types: stratiform cloud with a slightly cold base ('ACAPEX'); and convective clouds with cold ('STEPS'), warm ('MC3E') and very warm ('GoAMAZON') bases.  

For every basic cloud-type, the tagged ice concentration from each SIP source is predicted in 3D.  Each SIP source is plotted in vertical profiles and and in the phase-space of vertical velocity and cloud-base temperature.  

Breakup in ice-ice collisions is predicted to prevail overall in all basic cloud-types. Cold-based convective clouds display less ice enhancement than warm or very warm convective clouds.  The other SIP processes make comparable contributions to the ice enhancement.  The ice enhancement from the HM process increases with cloud-base temperature and vertical velocity.  

In the very warm cloud-base case, sublimational breakup dominates the average ice concentrations in the upper half of the mixed-phase region.   Raindrop-freezing fragmentation makes only a weak contribution. 

Finally, a schematic conceptual diagram is provided of the relative roles of the four SIP processes in the phase-space of cloud-base temperature and ascent for all basic cloud-types.