Ice Nucleation Processes Research Articles

Physicochemical properties and their impact on ice nucleation efficiency of respiratory viral RNA and proteins.

Ice nucleation processes in the earth's atmosphere are critical for cloud formation, radiation, precipitation, and climate change. We investigated the physicochemical properties and ice nucleation potential of selected viral aerosols, including their RNA and proteins, using advanced techniques such as scanning-transmission electron microscopy (S/TEM), small angle X-ray scattering (SAXS), particle analyzers, and a peltier chamber. The experiments revealed that RNA particles obtained from MS2 bacteriophage had a mean freezing point of -13.9 ± 0.3 °C, comparable to the average ice nucleation temperature of global dust particles, which is approximatively -15 °C. RNA from MS2, Influenza, SARS-CoV-1 and SARS-CoV-2 demonstrated average ice nucleation temperatures of -13.9 ± 0.3 °C, -13.7 ± 0.3 °C, -13.7 ± 0.3 °C, and -15.9 ± 0.4 °C, respectively. SAXS analysis indicated a high local crystallinity value of 0.5 of MS2 RNA particles, hinting that high crystalline nature may contribute to their effectiveness as ice nuclei. Dilution experiments show that viral RNA consistently catalyzes ice nucleation. The addition of dust-containing particles, such as Fe2O3, CuO, and TiO2, to MS2 bacteriophage droplets enhanced ice nucleation, as did UV radiation. We herein discuss the implications of this work on ice nucleation and freezing processes.

Nucleation mechanism in the absence and presence of an electric field

The objective of this article is to investigate the mechanism of the ice nucleation process theoretically by assaying the thermodynamics of the ice nucleus as a small system, and experimentally verified by studying the effect of the static electric field on the ice nucleation process. Using the thermodynamic of small system related to Hill, we predicted that the equilibrium temperature of water and ice nucleus decreases with the reduction of nucleus size. By applying an electrostatic field, the Gibbs free energy of nucleus formation decreases due to this fact that the electrostatic work on the ice nucleus is lower than that for bulk water which it causes a decrease in the critical size of the ice nucleus. The experimental results showed that the nucleation temperature of water increases by applying the electric field with the voltage of 4.5 kV (internal strength of 2.2 × 104 V/m) while the size of ice crystals decreases in the frozen agar gel at the presence of electric field.

Aerosol‐Cloud Interactions From Aviation Soot Emissions

AbstractCurrent models estimate global aviation contributes approximately 5% to the total anthropogenic climate forcing, with aerosol‐cloud interactions having the greatest effect. However, radiative forcing estimates from aviation aerosol‐cloud interactions remain undetermined. There is an expected significant increase in aircraft emissions with aviation demand expected to rise by over 4% per year. Soot may play an important role in the ice nucleation of aircraft‐induced cirrus formation due to a high emission rate, but the ice nucleating properties are poorly constrained. Understanding the microphysical processes leading to atmospheric ice crystal formation is crucial for the reliable parameterization of aerosol‐cloud interactions in climate models due to their impact on precipitation and cloud radiative properties. Ice nucleation of aircraft‐emitted soot is potentially affected by particle morphology with condensation of supercooled water occurring in pores followed by ice nucleation. However, soot has heterogeneous properties and undergoes atmospheric aging and oxidation that could change surface properties and contribute to complex ice nucleation processes. This review synthesizes current knowledge of ice nucleation catalyzed by aviation in the cirrus regime and its effects on global radiative forcing. Further research is required to determine the ice nucleation and microphysical processes of cirrus cloud formation from aviation emissions in both controlled laboratory and field investigations to inform models for more accurate climate predictions and to provide efficient mitigation strategies.

The impact of aerosols as ice nucleating particle on microphysics and electrification in the cumulus model

Here, a heterogeneous ice nucleation parameterization associated with aerosol acting as ice nucleating particle (INP) was implanted into a two-dimensional numerical cumulus model. To explore the impact of INP on microphysical and electrical processes, a comparison was conducted with the original approach, which employs an empirical formula. Simulation results indicate that INP greatly impacted microphysical evolution in the heterogeneous ice nucleation process, reducing total liquid precipitation amounts and causing a slight precipitation delay, as well as increasing the diameter and mixing ratio of ice crystals and the expansion of the vertical distribution of ice crystals. This led to a notable change in electrification in thunderstorms. The increase of ice crystal diameter was the dominant contributor to the enhancement of electrification using the new parameterization. Additionally, unlike the structure of thunderclouds in a mature stage, which always retains a normal dipole structure adopting the empirical formula, a tripole structure developed a lower positive charge, and the polarity inversion with upper negative and lower positive charges occurred when the new parameterization was adopted. This predominately was the result of abundant ice crystals present below the reversal temperature. The electrification characteristics of thunderstorms may have a close connection with lightning activity. It has been found that charge structure changed significantly in the two cases, with the tripolar charge structure facilitating the production of inverted intra-cloud (IC) flashes and negative Cloud-to-ground (CG) flashes simulated by the new parameterization; additionally, clouds simulated using the empirical formula may be able to develop normal IC lightning and positive CG flashes. Therefore, it will be very meaningful to obtain greater insight into the characteristics of thunderstorms electrification in cumulus model with aerosols.

On the Role of Starchy Grains in Ice Nucleation Processes.

Little is known about the role of starchy food on climate change processes like ice nucleation. Here, we investigate the ice nucleation efficiency (INE) of eight different starchy food materials, namely, corn (CO), potato (PO), barley (BA), brown rice (BR), white rice (WR), oats (OA), wheat (WH), and sweet potato (SP), in immersion freezing mode under mixed-phase cloud conditions. Notably, among all these food materials, PO and BA exhibit the highest ice nucleation efficiency with ice nucleation temperatures as high as -4.3 °C (T50 ∼ -7.0 ± 0.5 °C) and -6.5 °C (T50 ∼ -7.2 ± 0.2 °C), respectively. We also explore the effect of environmentally relevant physicochemical conditions on ice nucleation efficiency, including different pH, temperature, UV/O3/NOx exposure, and various cocontaminants. The change in shape, size, surface properties, hydrophobicity, and crystallinity of materials accounted for the altered INE. The increase in shape, size, and hydrophobicity of the sample generally reduces the INE, whereas an increase in crystallinity enhances the INE of the sample under our experimental conditions. The results suggest that environmentally relevant concentrations slightly alter INE, indicating their role as catalysts in environmental matrices. The outcome of studies on the ice nucleation properties of these food-containing aerosols might help in the physicochemical understanding of other biomolecule-induced ice nucleation, which is still an underdeveloped research area.

Structure Discovery in Atomic Force Microscopy Imaging of Ice.

The interaction of water with surfaces is crucially important in a wide range of natural and technological settings. In particular, at low temperatures, unveiling the atomistic structure of adsorbed water clusters would provide valuable data for understanding the ice nucleation process. Using high-resolution atomic force microscopy (AFM) and scanning tunneling microscopy, several studies have demonstrated the presence of water pentamers, hexamers, and heptamers (and of their combinations) on a variety of metallic surfaces, as well as the initial stages of 2D ice growth on an insulating surface. However, in all of these cases, the observed structures were completely flat, providing a relatively straightforward path to interpretation. Here, we present high-resolution AFM measurements of several water clusters on Au(111) and Cu(111), whose understanding presents significant challenges due to both their highly 3D configuration and their large size. For each of them, we use a combination of machine learning, atomistic modeling with neural network potentials, and statistical sampling to propose an underlying atomic structure, finally comparing its AFM simulated images to the experimental ones. These results provide insights into the early phases of ice formation, which is a ubiquitous phenomenon ranging from biology to astrophysics.

Ice Nucleation Under Climatically Changing Biotic and Abiotic Conditions in Himalayas: A Case Study of Naran, Mansehra District, Pakistan

Water has grave significance in human life and ice nucleation and rain are the two biggest sources of fresh water. In climate changing variables like temperature and other abiotic factors parallel to biotics factors paly vital role in natural process including ice nucleation. In order to have deep insight of ice nucleation process a data of abiotic factors like temperature, pH, wind direction was not only assessed but synchronized with biotics factors by taking example of two commonly spread species (chir and Diar common names) in the study area. In the study, it is revealed that temperature variation impacts ice nucleation process by lowering temperature from -1°C to -10°C. Analyzing temperature data between 2002 to 2022 reveals cycles impacting environmental phenomenon and other climate dynamics. Experimental data shoes inverse relationship with number of frozen drops and temperature. It is observed that bacterial presence in the atmosphere increases probability of ice nucleation. GIS reports indicate snow direct relation with vegetation signifying presence of biotic factors for ice nucleation.

Low-temperature ice nucleation of sea spray and secondary marine aerosols under cirrus cloud conditions

Abstract. Sea spray aerosols (SSAs) represent one of the most abundant aerosol types on a global scale and have been observed at all altitudes including the upper troposphere. SSA has been explored in recent years as a source of ice-nucleating particles (INPs) in cirrus clouds due to the ubiquity of cirrus clouds and the uncertainties in their radiative forcing. This study expands upon previous works on low-temperature ice nucleation of SSA by investigating the effects of atmospheric aging of SSA and the ice-nucleating activity of newly formed secondary marine aerosols (SMAs) using an oxidation flow reactor. Polydisperse aerosol distributions were generated from a marine aerosol reference tank (MART) filled with 120 L of real or artificial seawater and were dried to very low relative humidity to crystallize the salt constituents of SSA prior to their subsequent freezing, which was measured using a continuous flow diffusion chamber (CFDC). Results show that for primary SSA (pSSA), as well as aged SSA and SMA (aSSA+SMA) at temperatures >220 K, homogeneous conditions (92 %–97 % relative humidity with respect to water – RHw) were required to freeze 1 % of the particles. However, below 220 K, heterogeneous nucleation occurs for both pSSA and aSSA+SMA at much lower RHw, where up to 1 % of the aerosol population freezes between 75 % and 80 % RHw. Similarities between freezing behaviors of the pSSA and aSSA+SMA at all temperatures suggest that the contributions of condensed organics onto the pSSA or alteration of functional groups in pSSA via atmospheric aging did not hinder the major heterogeneous ice nucleation process at these cirrus temperatures, which have previously been shown to be dominated by the crystalline salts. Occurrence of a 1 % frozen fraction of SMA, generated in the absence of primary SSA, was observed at or near water saturation below 220 K, suggesting it is not an effective INP at cirrus temperatures, similar to findings in the literature on other organic aerosols. Thus, any SMA coatings on the pSSA would only decrease the ice nucleation behavior of pSSA if the organic components were able to significantly delay water uptake of the inorganic salts, and apparently this was not the case. Results from this study demonstrate the ability of lofted primary sea spray particles to remain an effective ice nucleator at cirrus temperatures, even after atmospheric aging has occurred over a period of days in the marine boundary layer prior to lofting. We were not able to address aging processes under upper-tropospheric conditions.

Large-scale assembly of isotropic nanofiber aerogels based on columnar-equiaxed crystal transition

Ice-templating technology holds great potential to construct industrial porous materials from nanometers to the macroscopic scale for tailoring thermal, electronic, or acoustic transport. Herein, we describe a general ice-templating technology through freezing the material on a rotating cryogenic drum surface, crushing it, and then re-casting the nanofiber slurry. Through decoupling the ice nucleation and growth processes, we achieved the columnar-equiaxed crystal transition in the freezing procedure. The highly random stacking and integrating of equiaxed ice crystals can organize nanofibers into thousands of repeating microscale units with a tortuous channel topology. Owing to the spatially well-defined isotropic structure, the obtained Al2O3·SiO2 nanofiber aerogels exhibit ultralow thermal conductivity, superelasticity, good damage tolerance, and fatigue resistance. These features, together with their natural stability up to 1200 °C, make them highly robust for thermal insulation under extreme thermomechanical environments. Cascading thermal runaway propagation in a high-capacity lithium-ion battery module consisting of LiNi0.8Co0.1Mn0.1O2 cathode, with ultrahigh thermal shock power of 215 kW, can be completely prevented by a thin nanofiber aerogel layer. These findings not only establish a general production route for nanomaterial assemblies that is conventionally challenging, but also demonstrate a high-energy-density battery module configuration with a high safety standard that is critical for practical applications.

Root Cause Analysis of An Inverse Relationship Between The Ice Nucleation Temperature, Process Efficiency And Quality of A Lyophilized Product

The aim of this study was to probe an unexpected relationship between the ice nucleation temperature (TIN), process efficiency and product attributes in a controlled ice nucleation (CIN) lyophilization process. An amorphous product was lyophilized with (CIN-5 °C, CIN-7 °C or CIN-10 °C) or without (NOCIN) control of ice nucleation. Process parameters and product attributes were monitored and compared using a series of advanced in-line and off-line process analytical technology (PAT) tools. Unexpectedly, an indirect relationship was observed between TIN and primary drying efficiency for the CIN processes. Further, the CIN-5 °C process was associated with higher product resistance to mass flow than corresponding CIN-7 °C and CIN-10 °C processes. Surprisingly, the air voids in some NOCIN products were larger than CIN-5 °C products but comparable to CIN-7 °C. Heat flux analysis revealed an indirect relationship between TIN and the minimum hold time required to complete solidification. The heat flux analysis also revealed all products underwent complete solidification prior to primary drying. The order of homogeneity in water activity of the products was CIN-5 °C ≥NOCIN>CIN-7 °C. The higher homogeneity in water activity of CIN-5 °C than corresponding CIN-7 °C processes indicated that the lower process efficiency of CIN-5 °C could not be attributed to unsuccessful induction of ice nucleation during CIN-5 °C. High resolution micro-CT imaging and Artificial Intelligence Image analysis revealed cake wall deformation in CIN-7 °C and NOCIN products but not in CIN-5 °C. In addition, NOCIN products had bimodal distribution in air voids with median size range of 4–5 µm and 151.9–309 µm, respectively, hence the lower process efficiency of NOCIN despite the higher D90. Thus, the observed relationship between TIN and process efficiency may be attributed to microstructural changes post freezing. This hypothesis was corroborated by visible macroscopic cake collapse in NOCIN products but not in CIN products after lyophilization at a higher shelf temperature. In conclusion, the advantages of controlling the ice nucleation temperature of a lyophilization process may only be attained through a robust process design that takes into consideration the primary and secondary drying process parameters. Further, combined use of advanced in-line and off-line PAT tools for process and product characterization may hasten the at scale adoption of advance techniques such as CIN.

Improvements of the Double-Moment Bulk Cloud Microphysics Scheme in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM)

Abstract This study revises the collisional growth, heterogeneous ice nucleation, and homogeneous ice nucleation processes in a double-moment bulk cloud microphysics scheme implemented in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM). The revised cloud microphysical processes are tested by 10-day global simulations with a horizontal resolution of 14 km. It is found that both the aggregation of cloud ice with smaller diameters and the graupel production by riming are overestimated in the current schemes. A new method that numerically integrates the collection kernel solves this issue, and consequently, the lifetime of cloud ice is reasonably extended in reference to satellite observations. In addition, the results indicate that a reduction in graupel modulates the convective intensity, particularly in intense rainfall systems. The revision of both heterogeneous and homogeneous ice nucleation significantly increases the production rate of cloud ice number concentration. With these revisions, the new version of the cloud microphysics scheme successfully improves outgoing longwave radiation, particularly over the intertropical convergence zone, in reference to satellite observations. Therefore, the revisions are beneficial for both long-term climate simulations and representing the structure of severe storms. Significance Statement Very high-resolution global atmospheric models have been developed to simultaneously address global climate and regional weather. In general, cloud microphysics schemes used in such global models are introduced from regional weather forecasting models to realistically represent mesoscale cloud systems. However, a cloud microphysics scheme that was originally developed with the aim of weather forecasting can cause unexpected errors in global climate simulations because such a cloud microphysics scheme is not designed for interdisciplinary usage across spatiotemporal scales. This study focuses on systematic model biases in evaluating the terminal velocity of ice cloud particles and proposes a method to accurately calculate the growth rate of ice cloud particles. Improvements in ice cloud modeling successfully reduce model biases in the global energy budget. In addition, the internal structure of intense rainfall systems is modified using the new cloud model. Therefore, improvements in ice cloud modeling could further increase the reliability of weather forecasting, seasonal prediction, and climate projection.

When Do Subpollen Particles Become Relevant for Ice Nucleation Processes in Clouds?

AbstractWhen exposed to sufficiently humid environments, pollen grains burst and release large quantities of small subpollen particles (SPPs) which carry ice nucleating macromolecules. In this study, for the first time we develop a physically based parameterization describing the bursting process of pollen by applying a turgor pressure parameterization and quantify the impact SPPs have on overall ice nucleation in clouds. SPPs are generated from simulated birch pollen emissions over Europe for a 10‐day case study in spring. We found SPP concentrations to surpass pollen grain concentrations by 4–6 orders of magnitude leading to an abundance of biological ice nuclei from SPPs in the range of 103−104 m−3. However, it is found that these concentrations lead to only small changes in hydrometeor number densities and precipitation. Addressing the question when SPPs become relevant for ice nucleation in clouds, we conducted a sensitivity investigation. We find that amplifying ice nucleation efficiency of biological particles by factors greater 100 increases the ice particle numbers by up to 25% (T ≈ 268 K). Strong reductions show in cloud droplet number concentration and water vapor at these temperatures while water vapor is increasing at 600 m. Overall, we found a net reduction of water in the atmosphere as liquid and particularly water vapor density is reduced, while frozen water mass density increases above 257 K. Findings indicate an alteration of mixed‐phase cloud composition and increased precipitation (up to 6.2%) when SPPs are considered as highly efficient biological ice nuclei.

Understanding the impact of ice nucleation on lightning and rainfall: A case study

A suitable parameterization of heterogeneous ice nucleation (HIN) is essential for the effective simulation of ice processes in climate models and comprehending their interactions with aerosols. In this study, the influence of HIN formulations in Morrison two-moment bulk cloud microphysical scheme of the Weather Research and Forecasting (WRF-ARW) model in simulating the thunderstorm event over Maharashtra, India, has been evaluated. The HIN parameterization is used to simulate the thunderstorm event is based on Fletcher (1962), Cooper (1986), and Mayers et al. (1992), along with the inclusion of measurements of ice nucleation from SPectrometer for Ice Nuclei at High Altitude Cloud Physics Laboratory, Indian Institute of Tropical Meteorology (IITM-SPIN). The results signify that different ice nucleation formulations are highly sensitive in simulating rainfall and lightning by the formation of cloud hydrometeors (e.g., cloud ice, snow, graupel, and rain). The HIN of IITM-SPIN and Fletcher (1962) simulate the better lightning flashes and rainfall compared to Cooper (1986) and Mayers et al. (1992). The results from a two-dimensional idealized simulation also corroborate the role of the HIN on cloud microphysical processes. Clouds and precipitation processes are influenced by the ice nucleation process, which modifies latent heat release and eventually modulates the dynamics. Therefore, the present work can provide a pathway in the selection of a better ice nucleation formulation for better portrayal of cloud dynamics.

ОЦЕНКА ПОТЕНЦИАЛЬНОГО УВЕЛИЧЕНИЯ КОЛИЧЕСТВА ОСАДКОВ ПРИ ЗАСЕВЕ ОБЛАКОВ ЛЬДООБРАЗУЮЩИМИ ВЕЩЕСТВАМИ НА ОСНОВЕ МОДЕЛИ WRF

The effects of airborne silver iodide (AgI) glaciogenic cloud seeding on the ice nucleation process and enhancement of orographic precipitation is evaluated using the aerosol-aware Thompson-Eidhammer microphysics scheme. The scheme is coupled to the GOCART aerosol model and implemented into the WRF model. Applying the scheme, which takes into account the aerosols serving as ice nuclei, enables the WRF to explicitly predict the effects of changes in aerosol number concentration on the precipitation formation. The selected seeding project was conducted in March 2015 over the Shirkouh Mountain in the center of Iran. The results show that the increased number of aerosols increased the updraft and downdraft, as well as the clouded volume. Therefore, depending on the relative humidity in the upper levels the airborne cloud-seeding operation is simulated to increase or decrease surface precipitation. Simulation data show that the 24-hour accumulated precipitation increased to 9.52% downwind of where AgI particles were released. However, the simulated seeding outcome for the southern regions is a 4.1% decrease in precipitation.

Characteristics of Precipitation Particles and Microphysical Processes during the 11–12 January 2020 Ice Pellet Storm in the Montréal Area, Québec, Canada

Abstract Freezing rain and ice pellets are particularly difficult to forecast when solid precipitation is completely melted aloft. This study addresses this issue by investigating the processes that led to a long-duration ice pellet event in Montreal, Québec, Canada, on 11–12 January 2020. To do so, a benchmark model initialized with ERA5 data is used to show that solid precipitation was completely melted below the melting layer, which discards partial melting from the possible ice pellet formation processes. Macro photography of precipitation reveals that small columnar crystals (∼200 μm) and ice pellets occurred simultaneously for more than 10 h. The estimation of ice crystal number concentration using macro photographs and laser-optical disdrometer data suggests that all supercooled drops could have refrozen by contact freezing with ice crystals. Rimed ice pellets also indicate ice supersaturation in the subfreezing layer. Given these observations, the formation of ice pellets and ice crystals was probably promoted by secondary ice production and the horizontal advection of ice crystals below the melting layer, as we illustrate using a conceptual model. Overall, these findings demonstrate how ice nucleation processes at temperatures near 0°C can drastically change the precipitation phase and the impact of a storm. Significance Statement Ice pellets are generally formed when snow particles partially melt while falling through a warm layer aloft before completely refreezing in a cold layer closer to the surface. Ice pellets can also be formed when snow particles completely melt aloft, but freezing rain is often produced in such conditions. On 11–12 January 2020, ice pellets were produced during more than 10 h in Montreal, Quebec, Canada. Macro photographs of the precipitation particles show that ice pellets occurred simultaneously with small ice crystals. Most of the ice pellets were produced while snow particles were completely melted aloft. The supercooled drops probably refroze due to collisions with the ice crystals that could have been advected by the northeasterly winds near the surface.

Aerosol parameterisation in a three-moment microphysical scheme: Numerical simulation of submicron-sized aerosol scavenging

This paper explores the scavenging of submicron aerosol particles (APs) collected by cloud water and rainwater and by cloud ice nucleation on APs. The scavenging coefficients for different collection processes were calculated and then implemented in a three-moment microphysical scheme of a three-dimensional numerical model. The concentration and mixing ratio of the APs in air and in all hydrometeors were calculated explicitly in the model. We performed several numerical experiments to test the influence of individual processes of AP collection and nucleation of cloud ice on AP scavenging, as well as the redistribution of APs among hydrometeors. We estimated AP scavenging based on the change in the total number and mass of APs in air over time or on the basis of the total number or mass of APs deposited on the ground by precipitation. We found that the initial number of APs in air is reduced mostly due to collection by cloud water. In the absence of ice nucleation on APs, their number in air during 90 min of integration is reduced by 49.73%, which is significantly larger than the reduction in their total mass in air, namely 7.74%. The decrease in the total number of APs in air is largely due to collection by Brownian diffusion, and the decrease in total mass is influenced equally by turbulent and Brownian diffusion. In addition to collection, when all ice nucleation processes are active, the total number of APs in air is reduced by 53.97%, and their total mass in air is reduced by 67.47%. Therefore, ice nucleation on APs is much more important for reducing their total mass in air than collection with liquid hydrometeors. The dominant process is depositional nucleation. The total number (total mass) of APs deposited by precipitation on the ground during 3 h divided by the initial total number (total mass) of APs in air (RAPN and RAPM, respectively) are 29.65% and 7.25%, respectively, in the case of collection without ice nucleation, and 27.63% and 14.88%, respectively, in the case of collection with ice nucleation. In the case of collection of APs by cloud water and rainwater, the RAPN decreases from 29.65% to 27.63% by including ice nucleation, and the RAPM increases from 7.25% to 14.88%.

Long-term characterisation of the vertical structure of the Saharan Air Layer over the Canary Islands using lidar and radiosonde profiles: implications for radiative and cloud processes over the subtropical Atlantic Ocean

Abstract. Every year, large-scale African dust outbreaks frequently pass over the Canary Islands (Spain). Here we describe the seasonal evolution of atmospheric aerosol extinction and meteorological vertical profiles on Tenerife over the period 2007–2018 using long-term micropulse lidar (MPL-3) and radiosonde observations. These measurements are used to categorise the different patterns of dust transport over the subtropical North Atlantic and, for the first time, to robustly describe the dust vertical distribution in the Saharan Air Layer (SAL) over this region. Three atmospheric scenarios dominate the aerosol climatology: dust-free (clean) conditions, the Saharan summer scenario (summer-SAL) and the Saharan winter scenario (winter-SAL). A relatively well-mixed marine boundary layer (MBL) was observed in the case of clean (dust-free) conditions; it was associated with rather constant lidar extinction coefficients (α) below 0.036 km−1 with minimum α (< 0.022 km−1) in the free troposphere (FT). The summer-SAL has been characterised as a dust-laden layer strongly affecting both the MBL (Δα = +48 % relative to clean conditions) and the FT. The summer-SAL appears as a well-stratified layer, relatively dry at lower levels (Δr∼-44 % at the SAL’s base, where r is the water vapour mixing ratio) but more humid at higher levels compared with clean FT conditions (Δr∼+332 % at 5.3 km), with a peak of α> 0.066 km−1 at ∼ 2.5 km. Desert dust is present up to ∼ 6.0 km, the SAL top based on the altitude of SAL's temperature inversion. In the winter-SAL scenario, the dust layer is confined to lower levels below 2 km altitude. This layer is characterised by a dry anomaly at lower levels (Δr∼ −38 % in comparison to the clean scenario) and a dust peak at ∼ 1.3 km height. Clean FT conditions were found above 2.3 km. Our results reveal the important role that both dust and water vapour play in the radiative balance within the summer-SAL and winter-SAL. The dominant dust-induced shortwave (SW) radiative warming in summer (heating rates up to +0.7 K d−1) is found slightly below the dust maximum. However, the dominant contribution of water vapour was observed as a net SW warming observed within the SAL (from 2.1 to 5.7 km) and as a strong cold anomaly near the SAL's top (−0.6 K d−1). The higher water vapour content found to be carried on the summer-SAL, despite being very low, represents a high relative variation in comparison to the very dry clean free troposphere in the subtropics. This relevant aspect should be properly taken into account in atmospheric modelling processes. In the case of the winter-SAL, we observed a dust-induced radiative effect dominated by SW heating (maximum heating of +0.7 K d−1 at 1.5 km, near the dust peak); both dust and atmospheric water vapour impact heating in the atmospheric column. This is the case of the SW heating within the SAL (maximum near the r peak), the dry anomaly at lower levels (Δr∼ −38 % at 1 km) and the thermal cooling (∼ 0.3 K d−1) from the temperature inversion upwards. Finally, we hypothesise that the SAL can impact heterogeneous ice nucleation processes through the frequent occurrence of mid-level clouds observed near the SAL top at relatively warm temperatures. A dust event that affected Tenerife on August 2015 is simulated using the regional DREAM model to assess the role of dust and water vapour carried within SAL in the ice nucleation processes. The modelling results reproduce the arrival of the dust plume and its extension over the island and confirm the observed relationship between the summer-SAL conditions and the formation of mid- and high-level clouds.

New investigations on homogeneous ice nucleation: the effects of water activity and water saturation formulations

Abstract. Laboratory measurements at the AIDA cloud chamber and airborne in situ observations suggest that the homogeneous freezing thresholds at low temperatures are possibly higher than expected from the so-called “Koop line”. This finding is of importance, because the ice onset relative humidity affects the cirrus cloud coverage and, at the very low temperatures of the tropical tropopause layer, together with the number of ice crystals also the transport of water vapor into the stratosphere. Both the appearance of cirrus clouds and the amount of stratospheric water feed back to the radiative budget of the atmosphere. In order to explore the enhanced ice onset humidities, we re-examine the entire homogeneous ice nucleation process, ice onset, and nucleated crystal numbers, by means of a two-moment microphysics scheme embedded in the trajectory-based model (CLaMS-Ice) as follows: the well-understood and described theoretical framework of homogeneous ice nucleation includes certain formulations of the water activity of the freezing aerosol particles and the saturation vapor pressure of water with respect to liquid water. However, different formulations are available for both parameters. Here, we present extensive sensitivity simulations testing the influence of three different formulations for the water activity and four for the water saturation on homogeneous ice nucleation. We found that the number of nucleated ice crystals is almost independent of these formulations but is instead sensitive to the size distribution of the freezing aerosol particles. The ice onset humidities, also depending on the particle size, are however significantly affected by the choices of the water activity and water saturation, in particular at cold temperatures ≲205 K. From the CLaMS-Ice sensitivity simulations, we here provide combinations of water saturation and water activity formulations suitable to reproduce the new, enhanced freezing line.

Lipid bilayers as potential ice nucleating agents.

Cellular damage is a key issue in the context of cryopreservation. Much of this damage is believed to be caused by extracellular ice formation at temperatures well above the homogeneous freezing point of pure water. Hence the question: what initiates ice nucleation during cryopreservation? In this paper, we assess whether cellular membranes could be responsible for facilitating the ice nucleation process, and what characteristics would make them good or bad ice nucleating agents. By means of molecular dynamics simulations, we investigate a number of phospholipids and lipopolysaccharide bilayers at the interface with supercooled liquid water. While these systems certainly appear to act as ice nucleating agents, it is likely that other impurities might also play a role in initiating extracellular ice nucleation. Furthermore, we elucidate the factors which affect a bilayer's ability to act as an ice nucleating agent; these are complex, with specific reference to both chemical and structural factors. These findings represent a first attempt to pinpoint the origin of extracellular ice nucleation, with important implications for the cryopreservation process.

Characteristics of clouds, precipitation, and latent heat in midlatitude frontal system mixed with dust storm from GPM satellite observations and WRF simulations

A heavy dust storm originating in Mongolia and Inner Mongolia traveled to Northeast China and met a midlatitude frontal system on May 3, 2017. The potential ice nuclei (IN) effects of mineral dust aerosols on the vertical structure of clouds, precipitation, and latent heat (LH) were studied using Global Precipitation Mission (GPM) satellite observations and Weather Research and Forecasting (WRF) model simulations. The WRF simulations correctly captured the main features of the system, and the surface rain rate distribution was positively correlated with data retrieved from the GPM Microwave Imager. Moreover, the correlation coefficient increased from 0.31 to 0.54 with increasing moving average window size. The WRF-simulated rainfall vertical profiles are generally comparable to the GPM Dual-Frequency Precipitation Radar (DPR) observations, particularly in low layers. The joint probability distribution functions of the rain rate at different altitudes from the WRF simulation and GPM observations show high positive correlation coefficients of ~0.80, indicating that the assumptions regarding the raindrop size distribution in the WRF model and DPR retrieval were consistent. Atmospheric circulation analysis and aerosol optical depth observations from the Himawari-8 satellite indicated that the dust storm entered only a narrow strip of the northwest edge of the frontal precipitation system. The WRF simulations showed that in carefully selected areas of heavy dust, dust can enhance the heterogeneous ice nucleation process and increase the cloud ice, snowfall, high-altitude precipitation rate, and LH rate in the upper layers. This effect is significant at temperatures of −15 °C to −38 °C and requires dust number concentrations exceeding 10<sup>6</sup> m<sup>−</sup><sup>3</sup>. It is important to accurately classify the dusty region in this type of case study. In the selected vertical cross section, the WRF-simulated and DPR-retrieved LH have comparable vertical shapes and amplitudes. Both results reflect the structure of the tilted frontal surface, with positive LH above it and negative LH below it. The simulated area-averaged LH profiles show positive heating in the entire column, which is a convective-dominated region, and this feature is not significantly affected by dust. DPR-based LH profiles show stratiform-dominated or convective-dominated shapes, depending on the DPR retrieval product.