Microphysical processes producing high ice water contents (HIWCs) in tropical convective clouds during the HAIC-HIWC field campaign: dominant role of secondary ice production

Abstract

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.

Predicted Particle Properties (P3) Microphysics Scheme Coupled With WRF‐Chem Model: Evaluation With Convective and Stratiform Cases

“…These were different from the underestimated ice number concentrations in convective clouds simulated by P3‐1CATEGORY and P3‐2CATEGORY with WRF model in Huang et al. (2021) due to the lack of secondary ice production (Huang et al., 2022). Overestimated number concentrations of ice particles with small size led to the smaller simulated effective diameters than that observed (Figure 14e).…”

“…As shown in Figure 14d, Chem53 (121,758 m −3 ) simulated the maximum averaged number concentration of ice particles at the size of 0.1-3 mm, followed by WRF53 (77,111 m −3 ), Chem52 (8,049 m −3 ), Chem51 (8,104 m −3 ), WRF52 (7,627 m −3 ), WRF51 (7,577 m −3 ), and observation (1,289.56 m −3 ). These were different from the underestimated ice number concentrations in convective clouds simulated by P3-1CATEGORY and P3-2CATEGORY with WRF model in Huang et al (2021) due to the lack of secondary ice production (Huang et al, 2022). Overestimated number concentrations of ice particles with small size led to the smaller simulated effective diameters than that observed (Figure 14e).…”

A bin-microphysics parcel model investigation of secondary ice formation in an idealised shallow convective cloud

“…These results are important as increasing the INP concentration does not always increase ICNC, especially given the un- Several studies have included M2 in their simulations, although usually combined with M1 (Phillips et al, 2018;Sotiropoulou et al, 2020;Qu et al, 2020;Georgakaki et al, 2022;Zhao and Liu, 2022;Huang et al, 2022;Karalis et al, 2022) which is equivalent to our M1+M2 simulations. For our idealised cloud conditions, the contribution from our M1+M2 simulations appears to be derived from the M2 aspect.…”

“…The largest contribution of M2 came from the transition simulations but this only contributed a small fraction compared to M1. The other studies which included M2 either did not provide a breakdown of the contribution from M1 and M2 Liu, 2021, 2022;Huang et al, 2022;Karalis et al, 2022) or was an ineffective ice enhancement mechanism (Sotiropoulou et al, 2020;Georgakaki et al, 2022).…”

“…Various models include these additional SIP mechanisms, sometimes alongside the rime-splintering mechanism (Phillips et al, 2018;Sotiropoulou et al, 2020;Georgakaki et al, 2022;Zhao and Liu, 2022;Huang et al, 2022). For example, Sotiropoulou et al (2021) included the ice-ice collision breakup mechanism in their Weather and Research Forecasting Model simulations of summer clouds over the Antarctic coast.…”