Model emulation to understand the joint effects of ice-nucleating particles and secondary ice production on deep convective anvil cirrus

Abstract

Abstract. Ice crystal formation in the mixed-phase region of deep convective clouds can affect the properties of climatically important convectively generated anvil clouds. Small ice crystals in the mixed-phase cloud region can be formed by heterogeneous ice nucleation by ice-nucleating particles (INPs) and secondary ice production (SIP) by, for example, the Hallett–Mossop process. We quantify the effects of INP number concentration, the temperature dependence of the INP number concentration at mixed-phase temperatures, and the Hallett–Mossop splinter production efficiency on the anvil of an idealised deep convective cloud using a Latin hypercube sampling method, which allows optimal coverage of a multidimensional parameter space, and statistical emulation, which allows us to identify interdependencies between the three uncertain inputs. Our results show that anvil ice crystal number concentration (ICNC) is determined predominately by INP number concentration, with the temperature dependence of ice-nucleating aerosol activity having a secondary role. Conversely, anvil ice crystal size is determined predominately by the temperature dependence of ice-nucleating aerosol activity, with INP number concentration having a secondary role. This is because in our simulations ICNC is predominately controlled by the number concentration of cloud droplets reaching the homogeneous freezing level which is in turn determined by INP number concentrations at low temperatures. Ice crystal size, however, is more strongly affected by the amount of liquid available for riming and the time available for deposition growth which is determined by INP number concentrations at higher temperatures. This work indicates that the amount of ice particle production by the Hallett–Mossop process is determined jointly by the prescribed Hallett–Mossop splinter production efficiency and the temperature dependence of ice-nucleating aerosol activity. In particular, our sampling of the joint parameter space shows that high rates of SIP do not occur unless the INP parameterisation slope (the temperature dependence of the number concentration of particles which nucleate ice) is shallow, regardless of the prescribed Hallett–Mossop splinter production efficiency. A shallow INP parameterisation slope and consequently high ice particle production by the Hallett–Mossop process in our simulations leads to a sharp transition to a cloud with extensive glaciation at warm temperatures, higher cloud updraughts, enhanced vertical mass flux, and condensate divergence at the outflow level, all of which leads to a larger convectively generated anvil comprised of larger ice crystals. This work highlights the importance of quantifying the full spectrum of INP number concentrations across all mixed-phase altitudes and the ways in which INP and SIP interact to control anvil properties.

The impacts of secondary ice production on microphysics and dynamics in tropical convection

“…Over the last few years, there have been many new efforts based on systematic studies of the effect of SIP on cloud microphysics with the help of cloud simulations (e.g., Phillips et al, 2017Phillips et al, , 2018Sullivan et al, 2018;Hoarau et al, 2018;Fu et al, 2019;Sotiropoulou et al, 2020Sotiropoulou et al, , 2021Dedekind et al, 2021;Hawker et al, 2021;Huang et al, 2021Huang et al, , 2022and others). Most of these modelling efforts have been focused on matching simulated moments of particle size distributions (PSDs) with those observed in situ.…”

The ice-nucleating activity of African mineral dust in the Caribbean boundary layer

“…At lower concentrations and at temperatures above about −15 • C, the model suggests that marine organics become more important than mineral dust. This may mean that marine organics are important for primary ice production in Caribbean (and across the tropical Atlantic) deep convective clouds in the temperature regime where the influence of primary production is amplified by the Hallett-Mossop process (Hawker et al, 2021a;Heymsfield and Willis, 2014).…”

“…For example, shallow clouds can persist in a supercooled state over large parts of the world but can transition to a lower albedo state if INPs are present (Storelvmo et al, 2015;Tan et al, 2016;Vergara-Temprado et al, 2018;Murray et al, 2021). In deep convective clouds, the presence of INPs, ingested from the boundary layer or entrained from higher levels, can strongly alter the extent and lifetime of anvil cirrus through perturbation of the microphysics and dynamics in the mixed-phase region of the cloud (Hawker et al, 2021a;Rosenfeld et al, 2011;Gibbons et al, 2018;Hawker et al, 2021b). Despite the importance of INPs for clouds and climate, the global distribution, seasonal cycles, transport, sources, and sinks of INPs remain poorly characterized (Kanji et al, 2017;Vergara-Temprado et al, 2017).…”

Implementation of a double moment cloud microphysics scheme in the UK met office regional numerical weather prediction model

“…CASIM was first introduced by Shipway and Hill (2012) in the Kinematic Driver model and has since been further researched and developed in that model (Hill et al ., 2015; Miltenberger et al ., 2020), the Met Office–NERC cloud (MONC) model (e.g., Dearden et al ., 2018; Poku et al ., 2021; Hawker et al . 2021), and various bespoke configurations of the UM (e.g., Grosvenor et al ., 2017; Miltenberger et al ., 2018; Stevens et al ., 2018), which have aerosol–cloud interactions with and without in‐cloud mechanical processing. In this work, we present the first time that CASIM has been fully coupled to the UM for operational testing and implementation as part of the Met Office Regional Atmosphere and Land (RAL) process.…”