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.
Highlights
Homogeneous nucleation of supercooled droplets or heterogeneous nucleation on the surface of ice-nucleating particles (INPs) can effectively produce primary ice crystals at temperatures < −35 ◦C (Koop et al, 2000; DeMott et al, 2016)
As a companion paper of Huang et al (2021), the current study investigates the roles of different Secondary ice production (SIP) mechanisms, namely the H–M mechanism, shattering of freezing droplets, and fragmentation of ice–ice collisions, in the formation of numerous small crystals in High ice water content (HIWC) regions, through a series of sensitivity experiments with the P3 microphysics scheme
A previous study (Huang et al, 2021) used the Weather Research and Forecasting (WRF) model at 1 km horizontal grid spacing with four different bulk microphysics schemes to simulate tropical deep convective clouds observed during the High Altitude Ice Crystals (HAIC)-HIWC field campaign
Summary
Homogeneous nucleation of supercooled droplets or heterogeneous nucleation on the surface of ice-nucleating particles (INPs) can effectively produce primary ice crystals at temperatures < −35 ◦C (Koop et al, 2000; DeMott et al, 2016). At temperatures > −35 ◦C, heterogeneous nucleation on INPs dominates the ice nucleation process and primary ice production. Secondary ice production (SIP), known as “ice multiplication”, which produces new ice crystals involving preexisting ice particles, has been recognized as an important mechanism to explain the discrepancy between the concentrations of observed ice particles and INPs (Field et al, 2017; Korolev and Leisner, 2020). Korolev and Leisner (2020) summarized laboratory studies of six different SIP mechanisms, namely, (1) the rime-splintering or Hallett–Mossop (H–M) process, (2) ice–ice collision fragmentation, (3) shattering of freezing droplets, (4) fragmentation of sublimating ice particles, (5) ice particle fragmentation due to thermal shock, and (6) activation of INPs in transient supersaturation around freezing drops. The physical basis of these SIP processes remains poorly understood, and quantification of their production rates is not consistent among different studies
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