Secondary ice production processes in wintertime alpine mixed-phase clouds

Observations of orographic mixed-phase clouds (MPCs) have long shown that measured ice crystal number concentrations (ICNCs) can exceed the concentration of ice nucleating particles by orders of magnitude. Additionally, model simulations of alpine clouds are frequently found to underestimate the amount of ice compared with observations. Surface-based blowing snow, hoar frost and secondary ice production processes have been suggested as potential causes, but their relative importance and persistence remains highly uncertain. Here we study ice production mechanisms in wintertime orographic MPCs observed during the Cloud and Aerosol Characterization Experiment (CLACE) 2014 campaign at the Jungfraujoch site in the Swiss Alps with the Weather Research and Forecasting model (WRF). Simulations suggest that droplet shattering is not a significant source of ice crystals at this specific location – but break-up upon collisions between ice particles is quite active, elevating the predicted ICNCs by up to 3 orders of magnitude, which is consistent with observations. The initiation of the ice-ice collisional break-up mechanism is primarily associated with the occurrence of seeder-feeder events from higher precipitating cloud layers. The enhanced aggregation of snowflakes is found to drive secondary ice formation in the simulated clouds, the role of which is strengthened when the large hydrometeors interact with the primary ice crystals formed in the feeder cloud. Including a constant source of cloud ice crystals from blowing snow, through the action of the break-up mechanism, can episodically enhance ICNCs. Increases in secondary ice fragment generation can be counterbalanced by enhanced orographic precipitation, which seems to prevent explosive multiplication and cloud dissipation. These findings highlight the importance of secondary ice and "seeding" mechanisms – primarily falling ice from above and to a lesser degree blowing ice from the surface – which frequently enhance primary ice and determine the phase state and properties of MPCs.

Secondary Ice Production upon Freezing of Freely Falling Drizzle Droplets

Ice multiplication processes are known to be responsible for the higher concentration of ice particles versus ice nucleating particles in clouds, but the exact secondary ice formation mechanisms remain to be quantified. Recent in-cloud observations and modeling studies have suggested the importance of secondary ice production upon shattering of freezing drizzle droplets. In one of our previous studies, four categories of secondary ice formation during freezing of supercooled droplets have been identified: breakup, cracking, jetting, and bubble bursts. In this work, we extend the study to include pure water and an aqueous solution of analog sea salt drizzle droplets moving at terminal velocity with respect to the surrounding cold humid air. We observe an enhancement in the droplet shattering probability as compared to the stagnant air conditions used in the previous study. Under free-fall conditions, bubble bursts are the most common secondary ice production mode in sea salt drizzle droplets, while droplet fragmentation controls the secondary ice production in pure water droplets.

The impact of Secondary Ice Production on Arctic Stratocumulus

In-situ measurements of Arctic clouds frequently show that ice crystal number concentrations (ICNCs) are much higher than the available ice-nucleating particles (INPs), suggesting that Secondary Ice Production (SIP) may be active. Here we use a Lagrangian Parcel Model and a Large Eddy Simulation to investigate the impact of three SIP mechanisms (rime-splintering, break-up from ice-ice collisions and droplet-shattering) on a summer Arctic stratocumulus case observed during the Cloud Coupling And Climate Interactions in the Arctic (ACCACIA) campaign. Primary ice alone cannot explain the observed ICNCs, and droplet-shattering is an ineffective SIP mechanism for the conditions considered. Rime-splintering, a mechanism that usually dominates within the studied temperature range, is also weak owing to the lack of large droplets to initiate this process. In contrast, break-up enhances ICNCs by 1–1.5 orders of magnitude, bringing simulations in good agreement with observations. Combining both processes can further explain some of the largest ICNCs observed. The main conclusions of this study show low sensitivity to the assumed INP and Cloud Condensation Nuclei (CCN) conditions. Our results indicate that collisional break-up may be an important ice-multiplication mechanism that is currently not represented in large-scale models. Finally, we also show that a simplified treatment of SIP, using a LPM constrained by a LES and/or observations, provides a realistic yet computationally efficient description of SIP effects that can eventually serve as an efficient way to parameterize this process in large-scale models.

The effect of secondary ice production parameterization on the simulation of a cold frontal rainband

Secondary ice production via processes like rime splintering, frozen droplet shattering, and breakup upon ice hydrometeor collision have been proposed to explain discrepancies between in-cloud ice crystal and ice-nucleating particle numbers. To understand the impact of this additional ice crystal generation on surface precipitation, we present one of the first studies to implement frozen droplet shattering and ice–ice collisional breakup parameterizations in a mesoscale model. We simulate a cold frontal rainband from the Aerosol Properties, PRocesses, And InfluenceS on the Earth's Climate campaign and investigate the impact of the new parameterizations on the simulated ice crystal number concentrations (ICNC) and precipitation. Near the convective regions of the rainband, contributions to ICNC can be as large from secondary production as from primary nucleation, but ICNCs greater than 50 L−1 remain underestimated by the model. The addition of the secondary production parameterizations also clearly intensifies the differences in both accumulated precipitation and precipitation rate between the convective towers and non-convective gap regions. We suggest, then, that secondary ice production parameterizations be included in large-scale models on the basis of large hydrometeor concentration and convective activity criteria.

The effect of secondary ice production parameterization on the simulation of a cold frontal rainband

Secondary ice production via processes like rime splintering, frozen droplet shattering, and breakup upon ice hydrometeor collision have been proposed to explain discrepancies between in-cloud ice crystal and ice-nucleating particle numbers. To understand the impact of this kind of additional ice number generation on surface precipitation, we present one of the first studies to implement frozen droplet shattering and ice-ice collisional breakup parameterizations in a larger-scale model. We simulate a cold frontal rainband from the Aerosol Properties, PRocesses, And InfluenceS on the Earth's Climate campaign and investigate the impact of the new parameterizations on the simulated ice crystal number concentrations (ICNC) and precipitation. Near the convective regions of the rainband, contributions to ICNC can be as large from secondary production as from primary nucleation, but ICNCs greater than 50 L−1 remain underestimated by the model. Addition of the secondary production parameterizations also clearly intensifies the differences in both accumulated precipitation and precipitation rate between the convective towers and non-convective gap regions. We suggest, then, that secondary ice production parameterizations be included in large-scale models on the basis of large hydrometeor concentration and convective activity criteria.

Initiation of secondary ice production in clouds

Disparities between the measured concentrations of ice-nucleating particles (INPs) and in-cloud ice crystal number concentrations (ICNCs) have led to the hypothesis that mechanisms other than primary nucleation form ice in the atmosphere. Here, we model three of these secondary production mechanisms – rime splintering, frozen droplet shattering, and ice–ice collisional breakup – with a six-hydrometeor-class parcel model. We perform three sets of simulations to understand temporal evolution of ice hydrometeor number (Nice), thermodynamic limitations, and the impact of parametric uncertainty when secondary production is active. Output is assessed in terms of the number of primarily nucleated ice crystals that must exist before secondary production initiates (NINP(lim)) as well as the ICNC enhancement from secondary production and the timing of a 100-fold enhancement. Nice evolution can be understood in terms of collision-based nonlinearity and the “phasedness” of the process, i.e., whether it involves ice hydrometeors, liquid ones, or both. Ice–ice collisional breakup is the only process for which a meaningful NINP(lim) exists (0.002 up to 0.15 L−1). For droplet shattering and rime splintering, a warm enough cloud base temperature and modest updraft are the more important criteria for initiation. The low values of NINP(lim) here suggest that, under appropriate thermodynamic conditions for secondary ice production, perturbations in cloud concentration nuclei concentrations are more influential in mixed-phase partitioning than those in INP concentrations.

Initiation of secondary ice production in clouds

Disparities between the measured concentrations of ice-nucleating particles (INP) and in-cloud ice crystal number concentrations (ICNC) have led to the hypothesis that mechanisms other than primary nucleation form ice in the atmosphere. Here, we model three of these secondary production mechanisms – rime splintering, frozen droplet shattering, and breakup upon collision – with a six-hydrometeor-class parcel model. We perform three sets of simulations to understand temporal evolution of ice hydrometeor number (Nice), thermodynamic limitations, and the impact of parametric uncertainty when secondary production is active. Output is assessed in terms of the number of primarily nucleated ice crystals that must exist before secondary production initiates (NINP(lim)), as well as the ICNC enhancement from secondary production and the timing of a 100-fold enhancement. Nice evolution can be understood in terms of collision-based non-linearity and the ``phasedness'' of the process, i.e., whether it involves ice hydrometeors, liquid ones, or both. Breakup is the only process for which a meaningful NINP(lim) exists (0.002 L−1 up to 0.07 L−1). For droplet shattering and rime splintering, a warm enough cloud base temperature and modest updraft are the more important criteria for initiation. The low values of NINP(lim) here suggest that, under appropriate thermodynamic conditions for secondary ice production, perturbations in CCN concentrations are more influential on mixed-phase partitioning than those in INP concentrations.

Residence Time and Heat Transfer When Water Droplets Hit a Scalding Surface

We study, using pump-probe optical methods with a time resolution of 1 ms, heat transfer when a series of water droplets impact a smooth surface whose temperature exceeds the boiling point. The volume of the individual water droplets is ≈10 nl, the time between droplets is ≈0.3 ms, and the number of water droplets in the series of droplets is 3, 20, or 100. In the temperature range 100 °C < T < 150 °C, our measurements of the heat transfer, and the residence time of water in contact with the surface, show that nearly all of the dispensed water vaporizes, but more rapidly, the higher the temperature. At higher temperatures, 150 °C < T < 220 °C, droplet shattering plays an increasingly important role in limiting heat transfer and, as a result, the volume of water evaporated and residence time decrease strongly with increasing temperature.