Modeling Secondary Ice Processes on a midlatitude squall line

Secondary ice processes (SIPs) can produce ice crystals with a number concentration much higher than that of ice nucleating particles (INPs) in mixed-phase clouds, and therefore influence cloud glaciation and precipitation. But the role of SIPs in midlatitude continental mesoscale convective systems (MCSs) such as squall lines is still unknown. This study investigates the relative importance of rime splintering, freezing drop shattering, and collision breakup in the mature stage of a squall line case in North China on 18 August 2020 using the WRF model. The simulations show that collision breakup has the most pronounced effect on ice production, and rime splintering plays a secondary role. It is because ice multiplication from SIPs can feedback to collision breakup and rime splintering in different ways. Collision breakup has a positive feedback because the numerous snow and graupel from SIPs in turn promote a higher collision breakup rate, while rime splintering is limited by itself and also limited by collision breakup because the weaker riming due to the two SIPs leads to a lower rime splintering rate. Freezing drop shattering has a negligible effect on ice production because there are few large droplets in the mature stage. Collision breakup can also redistribute surface precipitation in the squall line, which decreases in the convective region and increases in the stratiform region. The influence of aerosols as CCN and INPs on SIPs is further studied. Preliminary simulation results show that the effects of aerosol concentration on the rate of SIPs and anvil ice concentration are nonlinear. The mechanism remains to be analyzed.