A Study of Enhanced Heterogeneous Ice Nucleation in Simulated Deep Convective Clouds Observed During DC3

Abstract

The impacts of enhanced heterogeneous ice nucleation (HET) on the properties of deep convective clouds (DCCs) have been investigated in cloud-resolving simulations with the WRF-CHEM model. The study focuses on a case observed during the Deep Convective Clouds and Chemistry (DC3) field campaign. For the simulated DCCs, which had cold cloud-base temperatures, an inverse relationship exists between ice crystal mass produced through HET and anvil ice crystal number concentrations. This seems to be due to the indirect competition between HET and subsequent homogeneous freezing (HOM) for liquid droplets. Furthermore, our simulations suggest that HET enhancements at warmer temperatures are more efficient in depleting liquid droplets below and hence have larger impacts on anvil properties than HET enhancements at colder temperatures do. This temperature dependence indicates that similar increases in the number of ice nucleating particles (INPs) may potentially have different impacts on DCCs, depending on the INP type and at which temperatures they can nucleate ice crystals. We also found that the reduced anvil ice number concentrations due to the enhanced HET may lead to optically thinner anvil clouds. The reduction in cloud optical depth comes from a decrease in ice crystal mass concentrations, and in some runs also from an increase in ice crystal sizes. These results suggest potentially large impacts of INPs on the properties of DCCs, especially if precipitation is predominantly produced through ice processes in the DCCs. The results underscore the importance of fully understanding the temperature-dependent ability of aerosol particles to nucleate ice crystals.

Disentangling the Microphysical Effects of Fire Particles on Convective Clouds Through A Case Study

“…Most of the droplet activation naturally takes place near cloud base where supersaturation maximizes, and hence additional particles at high altitudes only weakly affected droplet number concentrations (i.e., most of the cloud droplets must have been activated below and lifted upward). This finding suggests that the observed injection of the smoke plume into the DCCs likely had a small impact on droplets relative to its high particle number concentrations, although it might have had an influence on ice nucleation (e.g., Takeishi & Storelvmo, 2018). This effect of aerosols as INPs is beyond the scope of this study.…”

Introduction to the Deep Convective Clouds and Chemistry (DC3) 2012 Studies

“…Simulations of the 22 June 2012 Colorado severe storms using WRF‐Chem at Δx = 1 km found that storms with cold cloud bases and therefore small contributions of the warm‐rain processes to precipitation caused enhanced heterogeneous ice nucleation compared to the homogeneous ice nucleation process. When heterogeneous ice nucleation was enhanced, the storm anvils were optically thinner (Takeishi & Storelvmo, ).…”

The nature of ice-nucleating particles affects the radiative properties of tropical convective cloud systems

“…It is known from model simulations that changes in INP number concentration affect the microphysical properties and behaviour of deep convective clouds (Deng et al, 2018;Fan et al, 2010aFan et al, , 2010bGibbons et al, 2018;Takeishi and Storelvmo, 2018). However, in these model studies perturbations to INP number concentrations have predominantly involved uniform increases in aerosol or INP concentrations with all simulations using the same INP parameterisation (Carrió et al, 2007;Connolly et al, 2006;Deng et al, 2018;Ekman et al, 2007;Fan et al, 2010a;Gibbons et al, 2018;van den Heever et al, 2006;Phillips et al, 2005).…”

“…However, in these model studies perturbations to INP number concentrations have predominantly involved uniform increases in aerosol or INP concentrations with all simulations using the same INP parameterisation (Carrió et al, 2007;Connolly et al, 2006;Deng et al, 2018;Ekman et al, 2007;Fan et al, 2010a;Gibbons et al, 2018;van den Heever et al, 2006;Phillips et al, 2005). Where different INP parameterisations have been used (Eidhammer et al, 2009;Fan et al, 2010b;Liu et al, 2018;Takeishi and Storelvmo, 2018), the results have in most cases been interpreted in terms of the overall increase in INP number concentration (Fan et al, 2010b;Liu et al, 2018;Takeishi and Storelvmo, 2018). However, there are important structural differences between different INP parameterisations that have not yet been explored in detail.…”

“…We hypothesise that such large differences in ice production rates between INP parameterisations are likely to affect cloud properties. In simulations of deep convective clouds over North America (Takeishi and Storelvmo, 2018) there were differences in the magnitude and altitude of droplet depletion depending on INP parameterisation choice (Bigg, 1953;DeMott et al, 2010DeMott et al, , 2015.…”