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Abstract. Secondary ice production (SIP) is an important physicalphenomenon that results in an increase in the ice particle concentration and cantherefore have a significant impact on the evolution of clouds. In thisstudy, idealized simulations of a mesoscale convective system (MCS) wereconducted using a high-resolution (250 m horizontal grid spacing) mesoscalemodel and a detailed bulk microphysics scheme in order to examine theimpacts of SIP on the microphysics and dynamics of a simulated tropical MCS.The simulations were compared to airborne in situ and remote sensing observationscollected during the “High Altitude Ice Crystals – High Ice Water Content”(HAIC-HIWC) field campaign in 2015. It was found that the observed high icenumber concentration can only be simulated by models that include SIPprocesses. The inclusion of SIP processes in the microphysics scheme is crucialfor the production and maintenance of the high ice water content observed intropical convection. It was shown that SIP can enhance the strength of theexisting convective updrafts and result in the initiation of new updraftsabove the melting layer. Agreement between the simulations and observationshighlights the impacts of SIP on the maintenance of tropical MCSs in natureand the importance of including SIP parameterizations in models. 

Abstract. High ice water content (HIWC) regions in tropical deep convective clouds, composed of high concentrations of small ice crystals, were not reproduced by Weather Research and Forecasting (WRF) model simulations at 1 km horizontal grid spacing using four different bulk microphysics schemes (i.e., the WRF single‐moment 6‐class microphysics scheme (WSM6), the Morrison scheme and the Predicted Particle Properties (P3) scheme with one- and two-ice options) for conditions encountered during the High Altitude Ice Crystals (HAIC) and HIWC experiment. Instead, overestimates of radar reflectivity and underestimates of ice number concentrations were realized. To explore formation mechanisms for large numbers of small ice crystals in tropical convection, a series of quasi-idealized WRF simulations varying the model resolution, aerosol profile, and representation of secondary ice production (SIP) processes are conducted based on an observed radiosonde released at Cayenne during the HAIC-HIWC field campaign. The P3 two-ice category configuration, which has two “free” ice categories to represent all ice-phase hydrometeors, is used. Regardless of the horizontal grid spacing or aerosol profile used, without including SIP processes the model produces total ice number concentrations about 2 orders of magnitude less than observed at −10 ∘C and about an order of magnitude less than observed at −30 ∘C but slightly overestimates the total ice number concentrations at −45 ∘C. Three simulations including one of three SIP mechanisms separately (i.e., the Hallett–Mossop mechanism, fragmentation during ice–ice collisions, and shattering of freezing droplets) also do not replicate observed HIWCs, with the results of the simulation including shattering of freezing droplets most closely resembling the observations. The simulation including all three SIP processes produces HIWC regions at all temperature levels, remarkably consistent with the observations in terms of ice number concentrations and radar reflectivity, which is not replicated using the original P3 two-ice category configuration. This simulation shows that primary ice production plays a key role in generating HIWC regions at temperatures <-40 ∘C, shattering of freezing droplets dominates ice particle production in HIWC regions at temperatures between −15 and 0 ∘C during the early stage of convection, and fragmentation during ice–ice collisions dominates at temperatures between −15 and 0 ∘C during the later stage of convection and at temperatures between −40 and −20 ∘C over the whole convection period. This study confirms the dominant role of SIP processes in the formation of numerous small crystals in HIWC regions.