New Empirical Formulation for the Sublimational Breakup of Graupel and Dendritic Snow

Ice fragments are generated by sublimation of ice particles in subsaturated conditions in natural clouds. Conceivably, such sublimational breakup would be expected to cause ice multiplication in natural clouds. Any fragment that survives will grow to become ice precipitation that may sublimate and fragment further.As a first step towards assessing this overlooked process, a formulation is proposed for the number of ice fragments from sublimation of ice particles for an atmospheric model. This is done by amalgamating laboratory observations from previously published studies. The concept of a ‘sublimated mass activity spectrum’ for the breakup is applied to the dataset. The number of ice fragments is determined by the relative humidity over ice and the initial size of the parent ice particles. The new formulation applies to dendritic crystals and heavily rimed particles only.Finally, a thought experiment is performed for an idealized scenario of subsaturation with in-cloud descent. Scaling analysis yields an estimate of an ice enhancement ratio of about 5 (50) within a weak deep convective downdraft of about 2 m s-1, for an initial monodisperse population of dendritic snow (graupel) particles of 3 L-1 and 2 mm . During descent, there is a dynamic equilibrium between continual emission of fragments and their depletion by sublimation. A simplified bin microphysics parcel model exhibits this dynamical quasi-equilibrium, consistent with the thought experiment. The fragments have average lifetimes of around 90 and 240 seconds for dendrites and graupel respectively. Sublimational breakup is predicted to cause significant secondary ice production.

Ice multiplication from ice–ice collisions in the high Arctic: sensitivity to ice habit, rimed fraction, ice type and uncertainties in the numerical description of the process

Atmospheric models often fail to correctly reproduce the microphysical structure of Arctic mixed-phase clouds and underpredict ice water content even when the simulations are constrained by observed levels of ice nucleating particles. In this study we investigate whether ice multiplication from breakup upon ice–ice collisions, a process missing in most models, can account for the observed cloud ice in a stratocumulus cloud observed during the Arctic Summer Cloud Ocean Study (ASCOS) campaign. Our results indicate that the efficiency of this process in these conditions is weak; increases in fragment generation are compensated for by subsequent enhancement of precipitation and subcloud sublimation. Activation of collisional breakup improves the representation of cloud ice content, but cloud liquid remains overestimated. In most sensitivity simulations, variations in ice habit and prescribed rimed fraction have little effect on the results. A few simulations result in explosive multiplication and cloud dissipation; however, in most setups, the overall multiplication effects become substantially weaker if the precipitation sink is enhanced through cloud-ice-to-snow autoconversion. The largest uncertainty stems from the correction factor for ice enhancement due to sublimation included in the breakup parameterization; excluding this correction results in rapid glaciation, especially in simulations with plates. Our results indicate that the lack of a detailed treatment of ice habit and rimed fraction in most bulk microphysics schemes is not detrimental for the description of the collisional breakup process in the examined conditions as long as cloud-ice-to-snow autoconversion is considered.

Ice Multiplication by Fragmentation during Quasi-Spherical Freezing of Raindrops: A Theoretical Investigation

Ice multiplication by fragmentation during collision–freezing of supercooled rain or drizzle is investigated. A zero–dimensional dynamical system describes the time evolution of number densities of supercooled drops and ice crystals in a mixed–phase cloud. The characteristic time–scale for this collision–freezing ice fragmentation is controlled by the collision efficiency, the number of ice fragments per freezing event, and the available number concentration of supercooled drops. The rate of the process is proportional to the number of supercooled drops available. Thus, ice may multiply extensively, even when the fragmentation number per freezing event is relatively small. The ratio of total numbers of ice particles to those from the first ice, namely the ‘ice–enhancement factor’, is controlled both by the number of fragments per freezing event and the available number concentration of supercooled drops in a similar manner. Especially, when ice fragmentation by freezing of supercooled drops is considered in isolation, the number of originally–existing supercooled drops multiplied by the fragmentation number per freezing event yields the eventual number of ice crystals. When supercooled drops are continuously generated by coalescence, ice crystals from freezing fragmentation also continuously increase asymptotically at a rate equal to the generation rate of supercooled drops multiplied by the fragmentation number per freezing event. All these results are expressed by simple analytical forms, thanks to the simplicity of the theoretical model. These parameters can practically be used as a means for characterizing observed mixed–phase clouds.

Contrasting ice formation in Arctic clouds: surface coupled vs decoupled clouds

In the Arctic summer of 2017 (June, 1st to July, 16th) measurements with the multiwavelength polarization lidar PollyXT-OCEANET, 35-GHz cloud radar of the OCEANET platform, and radiosonde measurements were conducted during cruise PS106 of the research vessel Polarstern around Svalbard. In the scope of the presented study, the influence of cloud height and surface coupling on the probability of clouds to contain and form ice is investigated. The analyzed data set shows a significant impact of the surface-coupling state on the probability of ice formation. Surface-coupled clouds, identified by a quasi-constant potential temperature profile from the surface up to liquid layer base, in the same cloud-top temperature range contain ice more frequent than decoupled clouds by a factor of up to 5 for cloud-top intervals between −7.5 and −5 °C (169 vs. 31 profiles). These findings provide evidence that heterogeneous ice formation in Arctic mixed-phase clouds occurs by a factor of 2–5 more likely when the cloud layer is coupled to the surface. In turn, for cloud-top temperatures below −15 °C, the frequency of ice-containing cloud profiles for coupled and decoupled conditions approached the respective curve for the Central-European site of Leipzig, Germany (51° N, 12° E). This provides further evidence that the free-tropospheric ice nucleating particles (INP) reservoir over the Arctic is controlled by continental aerosol. One possible explanation for the observation is that turbulent mixing of the air below surface-coupled clouds allows ice particles, acting as seeds for ice multiplication, or marine aerosols, acting as INP, to be transported into the cloud layer more efficiently than in the case of decoupled conditions. This hypothesis is corroborated by recent in-situ measurements of INP in the Arctic, of which much higher concentrations were found in the surface-coupled atmosphere in close vicinity to the ice shore. Using lidar measurements we also found evidence for enhanced INP number concentrations (INPC) within surface-coupled cloud-free air masses. The INPC have been estimated based on particle backscatter profiles, published freezing spectra of biogenic INP and existing parameterizations.

Ice multiplication from ice-ice collisions in the high Arctic: sensitivity to ice habit, rimed fraction and the spectral representation of the colliding particles

Atmospheric models often fail to correctly reproduce the microphysical structure of Arctic mixed-phase clouds and underpredict ice water content, even when simulations are constrained by the observed levels of ice nucleating particles. In this study we investigate whether ice multiplication from ice-ice collisions, a process missing in most models, can account for the observed cloud ice in a stratocumulus cloud observed during the Arctic Summer Cloud Study campaign. Our results indicate that including ice-ice collisions can improve the modeled cloud water properties, but the degree of influence depends on other poorly constrained microphysical aspects that include ice habit, rimed fraction and cloud ice-to-snow autoconversion rate. Simulations with dendrites are less sensitive to variations in the assumed rimed fraction of the particle that undergoes break-up, compared to those with planar ice. Activating cloud ice-to-snow autoconversion decreases the sensitivity of the break-up process to both the assumed ice habit and rimed fraction. Finally, adapting a relatively small value for the threshold diameter at which cloud ice is converted to snow enhances break-up efficiency and improves the macrophysical representation of the cloud.

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.

Empirical Formulation for number of ice crystal fragments by sublimational breakup

<p>There is much uncertainty about high concentrations of ice observed in clouds and their origins. In the literature, there have been previous experimental studies reported about the sublimation process of an ice crystal causes emission of fragments by breakup.   Such sublimational breakup is a type of secondary ice production, which in natural clouds can cause ice multiplication. </p><p>To represent this process of sublimation breakup in any cloud model, the present study proposes a numerical formulation of the number of ice fragments generated by sublimation of pristine ice crystal. This is done by amalgamating laboratory observations from previous published studies. The number of ice fragments determined by relative humidity (RH) and initial size of the ice particle were measured in the published experiments, and by simulating them we are able to infer parameters of a sublimation breakup scheme.   At small initial sizes, the dependency on size prevails, whereas at larger sizes both dependencies are comparable. This formulation is compared with observations to see the behaviour of it.</p>