Influence of secondary ice production on cloud and rain properties: analysis of the HYMEX IOP7a heavy-precipitation event

Abstract. A significant part of precipitation originates from ice crystals; however, the representation of mixed-phase clouds by atmospheric models remains a challenging task. One well-known problem is the discrepancy between the concentration of ice-nucleating particles (INPs) and the ice crystal number concentration. This study explores the effect of secondary ice production (SIP) on the properties of the Intensive Observation Period 7a (IOP7a), an intense-precipitation event observed during the HYdrological Cycle in the Mediterranean EXperiment (HYMEX) campaign. The effect of SIP on cloud and rain properties is assessed by turning SIP mechanisms in the DEtailed SCAvenging and Microphysics (DESCAM) 3D bin microphysics scheme on or off. Our results indicate that including SIP gives better agreement with in situ aircraft observations in terms of ice crystal number concentration and supercooled drop number fraction. During the mature cloud stage and for temperatures warmer than −30 °C, 59 % of ice crystals are produced by fragmentation due to ice–ice collisions, 38 % by the Hallett–Mossop process, 2 % by fragmentation of freezing drops, and only 1 % by heterogeneous ice nucleation. Furthermore, our results show that the production of small ice crystals by SIP induces a redistribution of the condensed water mass toward particles smaller than 3 mm rather than toward larger ones. As ice crystals melt, this effect is also visible in the precipitating liquid phase. The shift toward smaller particles results in a reduced precipitation flux of both ice crystals and drops. Consequently, SIP induces a decrease in accumulated precipitation at the surface by 8 % and reduces heavy rainfall exceeding 40 mm by 20 %.