Snow Formation Research Articles

Mercury’s inner core size and core-crystallization regime

Earth-based radar observation of Mercury’s rotation vector combined with gravity observation by the MESSENGER spacecraft yield a measure of Mercury’s moment of inertia and the amplitude of the 88-day libration of its silicate shell. These two geodetic constraints provide information on Mercury’s interior structure, including the presence of a fluid core, the radius of the core–mantle boundary and the bulk densities of the core and mantle. In this work, we show how they further provide information on the size of the solid inner core and on the crystallization regime of the fluid core. If Mercury’s fluid core is a Fe–FeS alloy with a sulfur concentration on the Fe-rich side of the eutectic, the largest inner core compatible with geodetic observations at the 1σ level is 1325±250km. Our results further suggest that the crystallization scenario that best fits the geodetic observations involves the formation of Fe-snow within the fluid core, and that this scenario is preferred for models with an iron-poor mantle composition. Consequently, Mercury’s dynamo most likely operates in concert with snow formation. For an inner core larger than ∼650km, snow formation extends to the inner core boundary. If a dynamo cannot be maintained by the dynamics of snow formation, or if such dynamo produces a magnetic field incompatible with observation, Mercury’s inner core must then be smaller than 650km.

Iron snow dynamo models for Ganymede

Ganymede’s internal magnetic field is dominated by the axial dipole. The measurements by the Galileo spacecraft only place an upper limit on the quadrupole moment. Ganymede’s magnetic field has the lowest ratio of quadrupole power to dipole power for all known planetary dynamos, not only at the planetary surface but possibly also at the top of the dynamo region. The dynamo operates in a fluid iron core that probably contains a significant amount of sulfur. Crystallization of the core will then proceed from the top by formation of iron snow in a layer that develops a stable compositional gradient. Remelting of the snow at the bottom of this layer enriches the underlying fluid in iron and drives compositional convection. Here we explore the consequences for the dynamo process of this scenario by numerical modeling. Convection is driven by an imposed buoyancy flux at the top of a convecting core region that is surrounded by a conducting fluid shell with a strongly stabilizing density gradient. Only horizontal flow is allowed in the outer shell. It is shown that this is a valid approximation in the case where the stabilizing density contrast in the upper shell exceeds by far the unstable density contrast in the convecting region. We vary the basic control parameters, concentrating on the regime where the magnetic field is dominantly dipolar. Compared to reference cases without an extra layer above the dynamo, we find that a stable fluid conducting layer with a thickness of 100km or larger reduces the ratio of quadrupole power R2 to dipole power R1 by a factor of at least four. With a stable outer layer R2/R1 is compatible with the Galileo observations for all tested dipolar models, whereas in the absence of such layer R2/R1 is too large or at best marginally compatible. For plausible values of the buoyancy flux the models reproduce Ganymede’s observed dipole moment. A stable layer that is comparable in thickness to the unstable region is found to promote a hemispherical type of dynamo whose field in incompatible with observations. This may indicate that the snow layer in Ganymede’s core has a moderate depth extent.

Formation of rapidly-sinking, oil-associated marine snow

Significant amounts of oil accumulated at the sea surface and in a subsurface plume during the Deepwater Horizon (DwH) spill in the Gulf of Mexico (GoM) in 2010. A substantial fraction of this oil was removed from the marine environment by mechanical recovery or burning, or it reached shorelines, whereas another fraction remained within the marine environment, where it dispersed (chemically or naturally), emulsified or sedimented. After the DwH accident the sedimentation of hydrocarbons to the seafloor via rapidly sinking, oil-associated marine snow has become a focus of attention, and it has been hypothesized that marine snow formation significantly impacted the distribution of the oil from the DwH spill.Here, roller table experiments are presented that investigated the conditions inducing the formation of oil-associated marine snow, focusing especially on the effects of oil type, photochemical aging of oil, and the presence of phytoplankton or dispersant. Large, mucus-rich marine snow, termed microbial marine snow, formed in treatments incubated with the oil that had accumulated at the sea surface. This bacteria-mediated formation of up to cm-sized marine snow in the absence of particles >1µm, represents a unique formation pathway different from that of the physical coagulation of particles. Microbial marine snow, albeit smaller, also formed in the presence of crude oil that had been aged for ≥3 weeks in sunlight, but no particles formed in the presence of unaltered crude. The dispersant Corexit 9500A (Corexit:oil ratio=1:100) impeded the formation of microbial marine snow, requiring a re-evaluation of the benefits and detriments of Corexit 9500A as a mediating measure. Phytoplankton aggregates also incorporated fossil carbon, providing an alternate pathway for the formation of oil-associated marine snow. The ubiquitous formation and rapid sedimentation of oil-rich marine snow can explain the high accumulation rate of flocculent material at the seafloor and on corals observed after the DwH spill. These results may raise awareness that oil spill response and assessment need to include sedimentation of hydrocarbons via marine snow as a significant distribution mechanism and may guide future modeling efforts and budget calculations.

Zinc isotope systematics in snow and ice accretions in Central European mountains

Zinc (Zn) pollution negatively affects human and ecosystem health. We quantified atmospheric Zn inputs at six remote mountain-top locations in the Czech Republic (Central Europe), and used δ66Zn isotope ratios to identify Zn from different pollution sources. The study sites were located at an elevation of approximately 1000m near the state borders with Germany and Poland. During two winter seasons (2009–2010), over 400 samples of vertical deposition (snow) and horizontal deposition (ice accretions) were collected. Zinc pollution levels were generally low. Zinc concentrations in snow and ice accretions were less than twice as high in the east, compared to the west. Across the sites, over 90% of Zn was present in a weak-acid soluble form. Zinc concentrations were 5 times higher in ice accretions, which formed from small droplets originating in the basal cloud layer, rich in pollutants, than in snow. In contrast, droplets resulting in snow formation were larger and scavenged less pollution due to their smaller surface area. δ66Zn of Pribram sphalerite (west) and smelter-derived fly ash (west) were low, −0.23 and −0.47‰, respectively. Olkusz sphalerite (east) had a higher δ66Zn of 0.02‰. δ66Zn of snow ranged from −0.60 to 0.68‰. Ice accretions had δ66Zn between −0.67 and 0.14‰. At the three eastern sites, δ66Zn of ice accretions was lower than δ66Zn of snow, suggesting the presence of volatilized smelter-derived or coal-burning derived Zn. δ66Zn of ice accretions at two of the three western sites was higher than δ66Zn of snow. Different δ66Zn values of snow and ice accretions from the same site reflected different pollution sources, which may have been situated at different distances from the receptor site. δ66Zn of the soluble Zn fraction was higher than δ66Zn of the insoluble Zn fraction, possibly also indicating a different origin of these two Zn fractions. Zinc isotope heterogeneity in the atmosphere of remote areas indicates that δ66Zn can be a useful tool in pollution provenance studies.

Laser-induced supersaturation and snow formation in a sub-saturated cloud chamber

Calculation of the saturation ratio inside vortices formed below the filament in a sub-saturation zone in a cloud chamber was given. By mixing the air with a large temper- ature gradient, supersaturation was sustained inside the vor- tices. This led to precipitation and snow formation when strong filaments were created using short focal length lenses (f = 20 and 30 cm). However, when longer filaments were formed with the same laser pulse energy but longer focal length lenses (f = 50 and 80 cm), only condensation (mist) was observed. The lack of precipitation was attributed to the weaker air flow, which was not strong enough to form strong vortices below the filament to sustain precipitation.

The responses of early foliar litter humification to reduced snow cover during winter in an alpine forest

Ni, X., Yang, W., Li, H., Xu, L., He, J., Tan, B. and Wu, F. 2014. The responses of early foliar litter humification to reduced snow cover during winter in an alpine forest. Can. J. Soil Sci. 94: 453–461. Snow cover can be reduced by ongoing winter warming in alpine biomes, affecting foliar litter humification, but few reports are available. To quantitatively clarify how early foliar litter humification responds to reduced snow cover in winter, a field litterbag experiment was conducted in an alpine forest in southwestern China. Mass losses, ΔlogK, E4/E6, degrees of humification and humification rates of six typical local foliar litters were investigated at the snow formation, snow cover and snow melt stage under snowpack levels differing in depth (deep snowpack, medium snowpack, thin snowpack, no snowpack) from November 2012 to April 2013. The results indicated that 14–15% of willow (Salix paraplesia), 8–9% of fir (Abies faxoniana), 6–7% of birch (Betula albo-sinensis), 5–8% of cypress (Sabina saltuaria), larch (Larix mastersiana) and azalea (Rhododendron lapponicum) foliar litter was humified, which was about 50% of what decomposed during the first winter. Moreover, the early humification of foliar litter (except for fir and birch) responded positively to the reduced snow cover, but mass loss exhibited negative responses. Such results suggest that reduced snow cover in winter would increase soil carbon or other material sequestration in the scenario of climate change.

Triple water‐isotopologue record from WAIS Divide, Antarctica: Controls on glacial‐interglacial changes in 17Oexcess of precipitation

AbstractMeasurements of the 17Oexcess of H2O were obtained from ice cores in West and East Antarctica. Combined with previously published results from East Antarctica, the new data provide the most complete spatial and temporal view of Antarctic 17Oexcess to date. There is a steep spatial gradient of 17Oexcess in present‐day precipitation across Antarctica, with higher values in marine‐influenced regions and lower values in the East Antarctic interior. There is also a spatial pattern to the change in 17Oexcess between the Last Glacial Maximum (LGM) and Holocene periods. At coastal locations, there is no significant change in 17Oexcess. At both the West Antarctic Ice Sheet Divide site and at Vostok, East Antarctica, the LGM to Early Holocene change in 17Oexcess is about 20 per meg. Atmospheric general circulation model (GCM) experiments show that both the observed spatial gradient of 17Oexcess in modern precipitation, and the spatial pattern of LGM to Early Holocene change, can be explained by kinetic isotope effects during snow formation under supersaturated conditions, requiring a high sensitivity of supersaturation to temperature. The results suggest that fractionation during snow formation is the primary control on 17Oexcess in Antarctic precipitation. Variations in moisture source relative humidity play a negligible role in determining the glacial‐interglacial 17Oexcess changes observed in Antarctic ice cores. Additional GCM experiments show that sea ice expansion increases the area over which supersaturating conditions occur, amplifying the effect of colder temperatures. Temperature and sea ice changes alone are sufficient to explain the observed 17Oexcess glacial‐interglacial changes across Antarctica.

Natural oil slicks fuel surface water microbial activities in the northern Gulf of Mexico.

We conducted a series of roller tank incubations with surface seawater from the Green Canyon oil reservoir, northern Gulf of Mexico, amended with either a natural oil slick (GCS-oil) or pristine oil. The goal was to test whether bacterial activities of natural surface water communities facilitate the formation of oil-rich marine snow (oil snow). Although oil snow did not form during any of our experiments, we found specific bacterial metabolic responses to the addition of GCS-oil that profoundly affected carbon cycling within our 4-days incubations. Peptidase and β-glucosidase activities indicative of bacterial enzymatic hydrolysis of peptides and carbohydrates, respectively, were suppressed upon the addition of GCS-oil relative to the non-oil treatment, suggesting that ascending oil and gas initially inhibits bacterial metabolism in surface water. Biodegradation of physically dispersed GCS-oil components, indicated by the degradation of lower molecular weight n-alkanes as well as the rapid transformation of particulate oil-carbon (C: N >40) into the DOC pool, led to the production of carbohydrate- and peptide-rich degradation byproducts and bacterial metabolites such as transparent exopolymer particles (TEP). TEP formation was highest at day 4 in the presence of GCS-oil; in contrast, TEP levels in the non-oil treatment already peaked at day 2. Cell-specific enzymatic activities closely followed TEP concentrations in the presence and absence of GCS-oil. These results demonstrate that the formation of oil slicks and activities of oil-degrading bacteria result in a temporal offset of microbial cycling of organic matter, affecting food web interactions and carbon cycling in surface waters over cold seeps.

The responses of early foliar litter humification to reduced snow cover during winter in an alpine forest

Ni, X., Yang, W., Li, H., Xu, L., He, J., Tan, B. and Wu, F. 2014. The responses of early foliar litter humification to reduced snow cover during winter in an alpine forest. Can. J. Soil Sci. 94: 453-461. Snow cover can be reduced by ongoing winter warming in alpine biomes, affecting foliar litter humification, but few reports are available. To quantitatively clarify how early foliar litter humification responds to reduced snow cover in winter, a field litterbag experiment was conducted in an alpine forest in southwestern China. Mass losses, ΔlogK, E4/E6, degrees of humification and humification rates of six typical local foliar litters were investigated at the snow formation, snow cover and snow melt stage under snowpack levels differing in depth (deep snowpack, medium snowpack, thin snowpack, no snowpack) from November 2012 to April 2013. The results indicated that 14-15% of willow (Salix paraplesia), 8-9% of fir (Abies faxoniana), 6-7% of birch (Betula albo-sinensis), 5-8% of cypress (Sabina saltuaria), larch (Larix mastersiana) and azalea (Rhododendron lapponicum) foliar litter was humified, which was about 50% of what decomposed during the first winter. Moreover, the early humification of foliar litter (except for fir and birch) responded positively to the reduced snow cover, but mass loss exhibited negative responses. Such results suggest that reduced snow cover in winter would increase soil carbon or other material sequestration in the scenario of climate change.

Aerosol impacts on California winter clouds and precipitation during CalWater 2011: local pollution versus long-range transported dust

Abstract. Mineral dust aerosols often observed over California in winter and spring, associated with long-range transport from Asia and the Sahara, have been linked to enhanced precipitation based on observations. Local anthropogenic pollution, on the other hand, was shown in previous observational and modeling studies to reduce precipitation. Here we incorporate recent developments in ice nucleation parameterizations to link aerosols with ice crystal formation in a spectral-bin cloud microphysical model coupled with the Weather Research and Forecasting (WRF) model in order to examine the relative and combined impacts of dust and local pollution particles on cloud properties and precipitation type and intensity. Simulations are carried out for two cloud cases (from the CalWater 2011 field campaign) with contrasting meteorology and cloud dynamics that occurred on 16 February (FEB16) and 2 March (MAR02). In both cases, observations show the presence of dust and biological particles in a relative pristine environment. The simulated cloud microphysical properties and precipitation show reasonable agreement with aircraft and surface measurements. Model sensitivity experiments indicate that in the pristine environment, the dust and biological aerosol layers increase the accumulated precipitation by 10–20% from the Central Valley to the Sierra Nevada for both FEB16 and MAR02 due to a ~40% increase in snow formation, validating the observational hypothesis. Model results show that local pollution increases precipitation over the windward slope of the mountains by a few percent due to increased snow formation when dust is present, but reduces precipitation by 5–8% if dust is removed on FEB16. The effects of local pollution on cloud microphysics and precipitation strongly depend on meteorology, including cloud dynamics and the strength of the Sierra Barrier Jet. This study further underscores the importance of the interactions between local pollution, dust, and environmental conditions for assessing aerosol effects on cold-season precipitation in California.

Effects of snowpack on early foliar litter humification during winter in a subalpine forest of western Sichuan

Aims Foliar litter humification is an important ecological process relating to soil carbon and nutrient budget in subalpine forest ecosystems, and the process of foliar litter humification can be affected by various snowpacks with different thicknesses in winter. However, little is known on the effects of snowpack on foliar litter humification. The objective of this study is to explore the effects of snowpack on early foliar litter humification during the first winter in subalpine forest. Methods A field litterbag experiment was conducted in a subalpine forest in southwestern China from November 2012 to April 2013. Air-dried foliar litter of fir (Abies faxoniana), cypress (Sabina saltuaria), larch (Larix mastersiana), birch (Betula albo-sinensis), willow (Salix paraplesia), and azalea (Rhododendron lapponicum) were incubated under snowpacks with varying thicknesses (deep snowpack, medium snowpack, thin snowpack, and no snowpack) due to variations in the canopy openness. Net accumulation of humus carbon, humification degrees, and humification rates were measured at snow formation, snow cover, and snow melt stage as foliar litter humification proceeded. Important findings Significant humification was observed in the six foliar litter types during the first winter regardless of the condition of snowpacks. The highest humification degree was observed in birch foliar litter (4.45%–5.67%), and the humification degrees were 1.91%–2.15%, 1.14%–2.03%, 1.06%–1.97%, 0.01%–1.25%, and 0.39%–1.21%, respectively, for fir, azalea, willow, larch, and cypress foliar litter under varying snowpacks. The net accumulation of humus carbon of all foliar litters increased at the snow formation, snow melt stage, and the whole winter, which exhibited an increasing tendency with the decrease of snow cover thickness. In contrast, net accumulation of humus carbon showed a declining trend at the snow cover stage, and significantly increased 倪祥银等: 雪被斑块对川西亚高山森林 6种凋落叶冬季腐殖化的影响 541 doi: 10.3724/SP.J.1258.2014.00050 with the decrease of snow cover thickness. In addition, correlation analysis results indicated that the humification degree was positively correlated with total nitrogen and acid-insoluble residues and negatively related to the organic carbon, total phosphorus, and waterand organic-soluble components. These results clearly suggest that foliar litter humification in subalpine forest can be enhanced by reduced snow cover in a scenario of climate warming, although the humification degree is controlled by snowpack and litter qualities at different stages of snow cover in winter.

Effects of snow patch on the dynamics of potassium and sodium during litter decomposition in winter in a subalpine forest of western Sichuan

Aims Snow patches of varying thickness could play an important role in potassium (K) and sodium (Na) dy- namics during litter decomposition in subalpine forests due to significant freeze-thaw events, but the detailed pro- cesses are unclear. Our objective was to understand how snow patches would affect the dynamics of K and Na during litter decomposition of six representative woody species in different snow cover periods. Methods A field experiment was conducted to investigate litter decomposition in a subalpine forest of western Sichuan by using litterbag method. Air-dried litter of Salix paraplesia, Larix mastersiana, Abies faxoniana, Rho- dodendron lapponicum, Betula albosinensis, and Sabina saltuaria were placed in nylon litterbags, and placed on the forest floor along a snow thickness gradient from forest gap to under the canopy cover. The samples were re- trieved at snow formation, during snow cover period, and at snow melt. Important findings Net Na immobilization during litter decomposition was observed over the entire snow cover season regardless of species, with the highest net immobilization rate occurring during the snow cover stage. In contrast, K was mainly released from litter during the snow cover period, showing rapid K release at the snow melt stage. Compared with the treatments with thin and absence of snow patches, the treatments with thick and moderate snow patches had higher rates of K release during litter decomposition. Although showing less effect on the release of Na compared with K, the snow cover significantly enhanced Na release from decomposing litter of Salix paraplesia, L. mastersiana, A. faxoniana, and R. lapponicum. Statistical analysis indicates that the dynamics of K and Na during litter decomposition are significantly influenced by species and snow cover at different snow cover stages. The rate of K release was positively related to daily mean temperature regardless of species, while the rate of Na release was positively related to daily mean temperature in litter of all species but B. albosinensis and Sabina saltuaria. Results in this study suggest that a decrease in snow cover under the scenario of winter

Laser-filament-induced snow formation in a subsaturated zone in a cloud chamber: Experimental and theoretical study

1 kHz, 2 mJ, 45 fs, 800 nm laser pulses were fired into a laboratory diffusion cloud chamber through a subsaturated zone (relative humidity ∼73%, T ∼ 4.3 °C). After 60 min of laser irradiation, an oval-shaped snow pile was observed right below the filament center and weighed ∼12.0 mg. The air current velocity at the edge of the vortices was estimated to be ∼16.5 cm/s. Scattering scenes recorded from the side show that filament-induced turbulence were formed inside the cloud chamber with two vortices below the filament. Two-dimensional simulations of the air flow motion in two cross sections of the cloud chamber confirm that the turbulent vortices exist below the filament. Based upon this simulation, we deduce that the vortices indeed have a three-dimensional elliptical shape. Hence, we propose that inside vortices where the humidity was supersaturated or saturated the condensation nuclei, namely, HNO(3), N(2)(+), O(2)(+) and other aerosols and impurities, were activated and grew in size. Large-sized particles would eventually be spun out along the fast moving direction towards the cold plate and formed an oval-shaped snow pile at the end.

Northern Hemispheric cryosphere response to volcanic eruptions in the Paleoclimate Modeling Intercomparison Project 3 last millennium simulations

AbstractWe analyzed last millennium simulations (circa 850–1850 Common Era) from the Paleoclimate Modeling Intercomparison Project 3 (PMIP3) project to determine whether current state‐of‐the‐art models produce sudden changes and persistence of cold conditions after large volcanic eruptions as inferred from geological records and previous climate modeling. Snow cover over Baffin Island in the eastern Canadian Arctic shows large‐scale expansion (as seen in proxy records) in two of the five models with snow cover information available, although it is not sustained beyond a decade. Sea ice expansion in the North Atlantic is seen in some PMIP3 models after large eruptions, although none of these models produce significant centennial‐scale effects. Warm Baffin Island summer climates stunt snow expansion in some models completely, and model topography tends to miss the critical plateau elevations that could sustain snow on the island. Northern Hemisphere sea ice extent is lower in six of the eight models than in reconstructions over the past millennium. Annual average Northern Hemisphere mean climates have a range of 3 K across models, while Arctic summer land‐only climates span more than 6 K. This has critical consequences on ice and snow formation and persistence in regions such as the Arctic where temperatures are near the freezing point and small temperature changes affect ice and snow feedback that could induce further climate changes. Thus, it is critical that models accurately represent absolute temperature.

On conditions of snow avalanching and landslides

The problem of elastic equilibrium of a wedge-shaped ground or snow massive on a rigid inclined plane under the action of gravity force and a constant force on its outer surface is considered in the three-dimensional statement. An exact solution that allows one to determine the displacements, strains, and stresses at each point of the massive is obtained in final form. An analysis shows that there are several critical relations between the problem input parameters at which the massive equilibrium is impossible. In this case, the earth or snow tears off from the slope and becomes a dangerous avalanche. In particular, this result can be used to predict formation of landslides and snow avalanching in mountains.

Impacts of nitrogen limitation on the sinking rate of the coccolithophorid Emiliania huxleyi (Prymnesiophyceae)

Pantorno A., Holland D.P., Stojkovic S. and Beardall B. 2013. Impacts of nitrogen limitation on the sinking rate of the coccolithophorid Emiliania huxleyi (Prymnesiophyceae). Phycologia 52: 288–294. DOI: 10.2216/12–064.1The biological carbon pump in the ocean plays an important role in controlling atmospheric CO2 levels. Approximately 1% of the yearly 50–60 Pg C of marine primary production settles in the deep ocean, where it is effectively sequestered for centuries to millennia. Central to the strength of the pump is the sinking velocity of phytoplankton cells and other organic debris. Stokes' Law indicates that the sinking velocity of a spherical cell will depend on its size and density, where larger, heavier cells will sink at a faster rate. Given that growth conditions can result in changes in cell size and macromolecular composition of phytoplankton, it might be expected that such changes could cause alterations in sinking velocity and carbon drawdown via the biological carbon pump. In the future, phytoplankton cells in the open ocean are predicted to be more subject to nutrient limitation. In this study we examined the effects of nitrogen limitation on the sinking velocity of Emiliania huxleyi, a coccolithophore responsible for significant phytoplankton blooms and biological drawdown of carbon. Nitrogen limitation caused changes in macromolecular composition, especially lipid content and coccosphere thickness. However, the overall density of the cells remained similar, and, as a consequence, cell size was the major determinant of sinking rate with N-limited cells in exponential phase sinking more slowly than N-replete cells. Cells in stationary phase showed the reverse trend with N-limited cells sinking faster, although not as fast as N-replete cells in exponential phase. N-limited cells produced more transparent exopolymers, suggesting an increased capacity for marine snow formation. These observations have implications for the efficiency of the biological carbon pump in areas that have increased nutrient stress caused by global climate change.

Effect of upstream fencing on shelter zone behind solid models simulating sand formations and dunes

This paper presents and discusses flow patterns around solid models constructed to represent sand (or snow) formations and dunes with and without upstream fencing. These flow patterns were obtained experimentally (by visualization) as well as computationally. The objective here is to give recommendations regarding protection against dangers posed by drift and/or movement of dunes on nearby roads and structures. The effects of mounting upwind fences on the flow pattern and the location of reattachment relative to dune base (shelter zone length) were examined. The effect of fencing was graphically plotted with fence position and height for given dune form. Computation and experiment showed acceptable agreement.Moreover, results indicate that some dune/fence combinations may cause shifting of the dune upwind (instead of downwind in the absence of fence). This effect means that, with such combinations, a dune would eventually disappear. The distance between the model downwind base line and the location of reattachment (length of shelter zone) was plotted against the distance of fence from upwind base line of model to determine the best possible dune/fence combination. Solid fencing (constructed from masonry bricks or stones) to shelter isolated sand humps and dunes is effective in alleviating dangers on nearby structures (dune shifting upwind and to less sand drift and saltation downwind). Also, the results indicated that, it is recommended to start by dune fencing and give enough time for the project zone to widen and be effectively protected before starting the construction.

Effects of superheat on characteristics of flashing spray and snow particles produced by expanding liquid carbon dioxide

This research experimentally investigates influences of superheat on flashing spray characteristics and on the snow formation of liquid CO2. Results show that liquid, two-phase flow or even three-phase flow can be found upon the release of liquid CO2 from high pressure to atmospheric pressure. This is due to complicated phase transition processes that involve hydrodynamic instabilities and thermal non-equilibrium conditions. Results also show that the spray pattern transfers from jet spray to cone spray, and then to a bowl spray configuration with the increase of superheat. The drastic changes in spray angle and mass flow rate indicate onset conditions from external-flashing to internal-flashing atomization mode. This is due to bursts of bubbles inside the nozzle chamber that result in the choking of the two-phase flow. Moreover, a few microns of CO2 snow size are measured in this research, which is consistent with records from literature on this topic. The narrower size distribution of snow particles and a critical spray angle resulting from two competing phenomena under internal-flashing atomization mode are also discussed in this research.

Mechanism and Performance of a CO2 Snow Jet in Co-axial Type Injection Systems

This paper describes the characteristics of CO2 snow formation with a co-axial type injection system. Flow field visualization and spray characteristics measurements are carried out using particle image velocimetry (PIV) and a real time particle analyzer. The result shows that the CO2 snow particles collide with each other and generate larger particles in the recirculation zone. The particle size distribution is influenced by the geometry of the injection device as well as the length of the formation chamber. In the region after the reattachment zone of the recirculation flow, fine particles deposit on the chamber wall and form a deposition layer. Particles in the main flow further collide onto the deposition layer and result in a snowball. This leads the mean particle size to become larger as the length of the chamber increased. The results also show that the CO2 snow jet becomes more focused and has greater velocity when nitrogen auxiliary gas is used. The mist produced outside the spray is also eliminated in this process.

Effects of initial humidity and temperature on laser-filamentation-induced condensation and snow formation

We investigated the effects of initial humidity and temperature on the formation of water condensation and various snowflake patterns by firing high-repetition filaments of femtosecond Ti-sapphire laser pulses at 9 mJ/50 fs per pulse into a cloud chamber. While adjusting initial temperature and humidity distributions near the filaments, snowflakes and ice particles with various shapes and sizes were observed on the bottom cold plate. In addition, significant differences were found in the weights of the laser-induced snow below the filament center. From those data, initial conditions of low temperature ( (90 ± 11) % and to ice RHi > (120 ± 15) %) were found being important to assist an efficient laser-filamentation-induced condensation and snow formation.