Snow Formation Research Articles

Large-Scale Atmospheric Forcing of Recent Trends toward Early Snowmelt Runoff in California

Since the late 1940s, snowmelt and runoff have come increasingly early in the water year in many basins in northern and central California. This subtle trend is most pronounced in moderate-altitude basins, which are sensitive to changes in mean winter temperatures. Such basins have broad areas in which winter temperatures are near enough to freezing that small increases result initially in the formation of less snow and eventually in early snowmelt. In moderate-altitude basins of California, a declining fraction of the annual runoff has come in April–June. This decline has been compensated by increased fractions of runoff at other, mostly earlier, times in the water year. Weather stations in central California, including the central Sierra Nevada, have shown trends toward warmer winters since the 1940s. A series of regression analyses indicate that runoff timing responds equally to the observed decadal-scale trends in winter temperature and interannual temperature variations of the same magnitude, suggesting that the temperature trend is sufficient to explain the runoff-timing trends. The immediate cause of the trend toward warmer winters in California is a concurrent, long-term fluctuation in winter atmospheric circulations over the North Pacific Ocean and North America that is not immediately distinguishable from natural atmospheric variability. The fluctuation began to affect California in the 1940s, when the region of strongest low-frequency variation of winter circulations shifted to a part of the central North Pacific Ocean that is teleconnected to California temperatures. Since the late 1940s, winter wind fields have been displaced progressively southward over the central North Pacific and northward over the west coast of North America. These shifts in atmospheric circulations are associated with concurrent shifts in both West Coast air temperatures and North Pacific sea surface temperatures.

Improvements of an Ice-Phase Microphysics Parameterization for Use in Numerical Simulations of Tropical Convection

Abstract It is important to properly simulate the extent and ice water content of tropical anvil clouds in numerical models that explicitly include cloud formation because of the significant effects that these clouds have on the radiation budget. For this reason, a commonly used bulk ice-phase microphysics parameterization was modified to more realistically simulate some of the microphysical processes that occur in tropical anvil clouds. Cloud ice growth by the Bergeron process and the associated formation of snow were revised. The characteristics of graupel were also modified in accord with a previous study. Numerical simulations of a tropical squall line demonstrate that the amount of cloud ice and the extent of anvil clouds are increased to more realistic values by the first two changes.

TEP and coagulation during a mesocosm experiment

Transparent exo-polymeric particles (TEP) have been associated with the aggregation of diatoms. Comparisons of TEP concentrations to those of conventionally-determined particles during a mesocosm experiment show that particles larger than about 30 μm had concentrations very similar to, if not the same as, those of conventionally-determined particles. Smaller TEP were present mostly at concentrations less than those of non-TEP. Furthermore, their relative concentrations decreased over the course of the experiment. These comparisons suggest that the dominant interaction of TEP with algae in marine snow formation is to affect general particle stickiness rather than to aggregate by themselves.

A global radiative‐convective feedback

We have investigated the sensitivity of the intensity of convective activity and atmospheric radiative cooling to radiatively thick upper‐tropospheric clouds using a new version of the Colorado State University General Circulation Model. The model includes a bulk cloud microphysics scheme to predict the formation of cloud water, cloud ice, rain, and snow. The cloud optical properties are interactive and dependent upon the cloud water and cloud ice paths. We find that the formation of a persistent upper tropospheric cloud ice shield leads to decreased atmospheric radiative cooling and increased static stability. Convective activity is then strongly suppressed. In this way, upper‐tropospheric clouds act as regulators of the global hydrologic cycle, and provide a negative feedback between atmospheric radiative cooling and convective activity.

Deuterium and oxygen 18 in precipitation: Isotopic model, including mixed cloud processes

Modeling the isotopic ratios of precipitation in cold regions meets the problem of “switching” from the vapor‐liquid transition to the vapor‐ice transition at the onset of snow formation. The one‐dimensional model (mixed cloud isotopic model (MCIM)) described in this paper focuses on the fractionation of water isotopes in mixed clouds, where both liquid droplets and ice crystals can coexist for a given range of temperatures. This feature is linked to the existence of specific saturation conditions within the cloud, allowing droplets to evaporate while the water vapor condensates onto ice crystals. The isotopic composition of the different airborne phases and the precipitation is calculated throughout the condensation history of an isolated air mass moving over the Antarctic ice sheet. The results of the MCIM are compared to surface snow data both for the isotopic ratios and the deuterium excesses. The sensitivity of the model is compared to previous one‐dimensional models. Our main result is that accounting specifically for the microphysics of mixed stratiform clouds (Bergeron‐Findesein process) does not invalidate the results of earlier modeling studies.

On the trophic fate of Phaeocystis pouchetii (Hariot): IV. The formation of marine snow by P. pouchetii

On the trophic fate of Phaeocystis pouchetii (Hariot): IV. The formation of marine snow by P. pouchetii

Assessment of ice particle growth processes at dye-3, Greenland

As part of the DGASP program on the Greenland Ice Sheet, an investigation was conducted into the nature of ice particle formation processes that result in the formation of snow. Ice particle habits were determined using Formvar replicas of falling snow crystals. From these measurements an assessment of the primary growth process and altitude of formation was made. Results indicate that the scavenging of cloud water by falling ice particles, growth by accretion or riming, rarely occurs. However, when riming does occur, it is usually associated with warmer air masses from the south. The occurrence of riming was also observed to be dependent on the season, with a greater frequency occurring during warmer months. It was estimated that ice particle riming contributes less than 5% of the average annual water mass, but up to 30% of the deposition of some chemical species, deposited to the Greenland Ice Sheet at Dye 3. Ice particle habits indicate that they originate at higher altitudes above the ice cap in summer than in winter. Variations in the magnitude of ice particle riming, the elevation of origin of ice particles, the meteorology and the season of the year are all essential when interpreting snow chemistry and comparing snow and aerosol chemistry at Dye 3.

The abundance and significance of a class of large, transparent organic particles in the ocean

Polysaccharide-specific staining techniques reveal the existence and high abundance of a class of large, discrete, transparent particles in seawater and diatom cultures formed from dissolved exopolymers exuded by phytoplankton and bacteria. Transparent exopolymer particles (TEP), ranged from 28 to 5000 particles ml −1 and 3 to 100s μm in longest dimension at five coastal stations off California. A high percentage of seemingly free-living bacteria (28–68%) were attached to these transparent sheets and films, suggesting that they may alter the distributions and microenvironments of marine microbes in nature. Preliminary coagulation experiments demonstrated that TEP are major agents in the aggregation of diatoms and in the formation of marine snow. The existence of microbial exudates acting as large, discrete particles, rather than as dissolved molecules or as coating on other particles, suggests that the transformation of dissolved organic matter into particulate form in the sea can occur via a rapid abiotic pathway as well as through conventional microbial uptake. The existence of these particles has far reaching implications for food web structure, microbial processes, carbon cycling and particulate flux in the ocean.

Icing on a small horizontal-axis wind turbine — Part 2: Three dimensional ice and wet snow formations

Icing on a small horizontal-axis wind turbine was monitored during winter 1990–1991 in Newfoundland. The three-dimensional form of incident glaze ice accretion on the rotor is described. Variations in ice accretion along the blade span resulted from centripetal forces, gravity, and crossflow over the blades. The dominant forces acting on impinging water droplets were centripetal forces, which caused a greater build up of ice at the blade tips and, in particular, at the leading edge towards the blade tips. Icicles formed beyond the blade tips. Wet snow also led to icing of the wind turbine rotor. The form of this accretion is described; wet snow was found to adhere only to the pressure side of the blades. The power output from the wind turbine was monitored. Icing affected the performance of the wind turbine at this site over winter 1990–1991, for a period equivalent to about 1.5% of a year.

The formation of large marine snow and its sustained residence in surface waters

The formation of large marine snow macroflocs and stringers was monitored at a fixed station in the southern North Sea. During an 18‐month investigation, large marine snow flocs occurred in the water column on nine occasions for periods lasting between 5 and 16 d. Their formation coincided with both high phytoplankton biomass and low windspeeds. Retention of marine snow in the surface layer was related to slow sinking, neutral buoyancy, and rising of the large flocs. Applying a specially developed instrument showed buoyancy to be caused by gas bubbles incorporated in the flocs. Marine snow flotation coincided with periods of oxygen supersaturation in the water column, suggesting that oxygen production could be responsible for gas bubble formation within the flocs. Disappearance of the large marine snow flocs from the surface layer coincided with drastic increases in windspeed, indicating that enhanced wind‐induced turbulent mixing in the surface layer could have resulted in the breakup of these flocs. Formation and disappearance of marine snow had a marked influence on the light regime in the water.

Ecology of amorphous aggregations (marine snow) in the Northern Adriatic Sea. V. Role of fecal pellets in marine snow

Ecology of amorphous aggregations (marine snow) in the Northern Adriatic Sea. V. Role of fecal pellets in marine snow

Simulations of the HDO and H218O atmospheric cycles using the NASA GISS general circulation model: Sensitivity experiments for present‐day conditions

Incorporating the full geochemical cycles of stable water isotopes (HDO and H218O) into an atmospheric general circulation model (GCM) allows an improved understanding of global δD and δ18O distributions and might even allow an analysis of the GCM's hydrological cycle. A detailed sensitivity analysis using the NASA/Goddard Institute for Space Studies (GISS) Model II GCM is presented that examines the nature of isotope modeling. The tests indicate that δD and δ18O values in nonpolar regions are not strongly sensitive to details in the model precipitation parameterizations. This result, while implying that isotope modeling has limited potential use in the calibration of GCM convection schemes, also suggests that certain necessarily arbitrary aspects of these schemes are adequate for many isotope studies. Deuterium excess, a second‐order variable, does show some sensitivity to precipitation parameterization and thus may be more useful for GCM calibration. Due to strong sensitivity over polar regions, GCM isotope modelers must choose carefully the numerical scheme for isotope transport and the formulation of kinetic fractionation processes at snow formation. The GCM results support the assumption that isotope fractionation does not occur during evaporation over continental areas.

Deuterium excess in recent Antarctic snow

Deuterium excess (d = δD ‐ 8 * δ18O) values in surface snow are presented for central and east Antarctica. The samples are primarily from Soviet, French, and Australian traverses. The d values exhibit a large change going from coastal sites to high‐altitude sites on the ice sheet. The d values are relatively constant at 3 to 6‰ from the coast to an altitude of 2500 m, and at higher elevations d increases steadily to values of 16 to 18‰ at Vostok and Plateau Station. The data is modeled as d versus δD using the kinetic Rayleigh model for isotopes in precipitation developed by Jouzel and Merlivat. The model accounts for kinetic fractionation during evaporation into undersaturated air over the ocean and during snow formation in <−10°C clouds where vapor is supersaturated with respect to snow. The overall pattern of d versus δD can be fit well with a supersaturation function which increases linearly with decreasing temperature and which predicts reasonable values of the supersaturation. Vapor originating from 20° to 60°S was tested with different supersaturation functions. The data could only be fit with moisture originating from 30° to 40°S, indicating that these latitudes are the main source of vapor for snow falling in Antarctica. The conclusion of a mid‐latitude vapor source for polar snow agrees with the analysis of d and δ18O seasonal cycles in Greenland snow performed by Johnsen and coworkers. The model was also tested with moisture simultaneously originating from all latitudes from 30°S to the Antarctic coast. The addition of up to 20% of moisture evaporated from latitudes south of 50°, and 5% from latitudes south of 60°, is compatible with low d values occasionally observed in snow near the coast. The conclusion of a “local moisture” effect for coastal and near coastal (<2000 m elevation) snowfall supports a similar conclusion by Saigne and Legrand from their analysis of methanesulphonic acid in Antarctic snow. Finally, the effects of changes in the sea surface temperature and changes in oceanic humidity on the d values observed in Antartic snow are greatly modified during the precipitation process. Hence the interpretation of d values in ice cores should be done in the context of a precipitation model.

Mass transfer of supersaturated contaminants in cryogenic helium heat exchangers

A new experimental technique is developed to measure the distribution of frost accumulated inside the passage of a cryogenic helium heat exchanger. The frost is accumulated by condensation of a contaminant on the passage wall during steady-flow operation of the balanced counterflow heat exchanger. The distribution is measured by flushing the accumulated contaminant into a detector at the exit of the passage. A heat source is moved from the cold to the warm end to convert the spatial distribution of the contaminant to concentration as a function of time in the exiting helium stream. When the degree of supersaturation is small, the accumulation distribution agrees reasonably well with the analytical solution without snow formation. As the supersaturation increases, a clear secondary peak in the distribution is observed, which results from the formation of snow in the free stream and its subsequent deposition as frost on the wall. Based on the results, an onset condition is developed for the formation of snow of a supersaturated contaminant in helium.

Mass sedimentation of Phaeocystis pouchetii in the Barents Sea

Mass sedimentation of gelatinous rolonies of the prymensiophyte Phaeocystis pouchetii were observed In the upper !00 m of Atlantic v> ater in the central Barents Sea. Sedimentation rates of paniculate organic carbon and nitrogen as weCl as pigments were the highest recorded so far from oceanic environments of the North Atlantic or coastal areas of Norway. High relative concentrations of phytoplankton pigments found in the traps are interpreted as a combination of sinking of Intact phytoplankton cells and undegraded pigments present in macrozooplankton faecal pellets. Evidence presented in this study implies that the zooplankton community of the Barents Sea was not able to control this phytoplankton spring bloom. The suspended and sedimenting organic matter was rich in carbon and pigments, but poor in nitrogen. Tils U explained by the presence of large amounts of carbon-rich mucilage which P. pouchetii colonies develop during their development. In addition to diatoms, sedimentation of a gelatinous phytoplankton species like P. pouchetii may contribute significantly to the formation of Esrine snow and vertical flux from the euphotic lone. However, degradation of P. pouchetii derived derirus at depths less than 100 m greatly diminishes the likely significance of P. pouchetii blooms in processes such as the carbon flux to the deep ocean and sequestering of COj.

Effects of Solar Radiation on the Formation of Weak Wet Snow

Laboratory and field experiments on hardness of snow have shown that the free water contained in snow decreases its hardness, and that solar radiation further decreases hardness down to a value below the limit of that which would result only from the influence of the water content of snow. A quantitative relationship between the amount of solar radiation absorbed by snow and decrease in snow hardness was derived. Thin-section analyses of snow were used to reveal the mechanism of decrease in snow hardness which had been caused by solar radiation.

Effects of Solar Radiation on the Formation of Weak Wet Snow

Laboratory and field experiments on hardness of snow have shown that the free water contained in snow decreases its hardness, and that solar radiation further decreases hardness down to a value below the limit of that which would result only from the influence of the water content of snow. A quantitative relationship between the amount of solar radiation absorbed by snow and decrease in snow hardness was derived. Thin-section analyses of snow were used to reveal the mechanism of decrease in snow hardness which had been caused by solar radiation.

The Origin of Arctic Precipitation as Deduced from its Deuterium Excess (Abstract)

Continuous profiles of deuterium excess, d = δD − 8δ18O, in the snow-pack have been measured at continental, high-elevation stations: Dye 3/4B (2480 m a.s.l.) in southern Greenland, and Site G (3110 m a.s.l.; 70 km south-east of Crête) in central Greenland.Figure 1 shows the δ18O and the d profiles along the upper 4.3 m of firn at Site G. These profiles represent the snow accumulation from late 1974 to mid-1985, judging from the clear seasonality in the δ18O record. There is also a seasonality in the d values, which generally range from +5 to +12 ‰. In contrast to the findings at low-elevation stations, the two seasonal cycles are not in antiphase; the d curve lags the δ18O curve by only approximately 90°. These same features appear when plotting similar data from Dye 3.Modelling of the data proceeds in three steps. First, the isotopic composition of vapour evaporating from an oceanic source area is calculated on the basis of Merlivat and Jouzel’s (1979) model, which takes kinetic effects into account. Secondly, the vapour is advected to the ice sheet, without mixing with vapour from other sources. Any liquid precipitation is assumed to be formed by a Rayleigh condensation process down to −5°C. Thirdly, the snow formation at lower temperatures is modelled, taking account of kinetic effects due to supersaturation of vapour with respect to ice (Jouzel and Merlivat 1984).The result of this modelling is that the initial mixing ratio (wso; g H2O/kg dry air; data from IAEA/WMO) largely determines the change of d in the precipitate as δ18O decreases during cooling. Furthermore, the absolute magnitude of the deuterium excess is mainly determined by the sea-surface temperature (Ts) in the source area.The combined result of these effects is shown in Figure 2, the upper part of which applies to a high-latitude source region (Weathership A at 62 °N, 34 °W), and the lower part to a mid-latitude source region (Weathership E at 35 ° N, 48 ° W). dvo is the deuterium excess of the parent vapour as it leaves the source area; dpo is that of first-stage precipitation predicted by the model, and dp is the predicted deuterium excess in the precipitate at Site G.In the high-latitude case, dvo and dpo have pronounced summer minima and winter maxima. As the cooling proceeds (until the observed δ18O values at Site G are reached), this cycle is inverted so that at Site G the model predicts maximum dp in August and minimum dp in January, and deuterium excesses on the ice sheet which are far too low (cf. Fig. 1). This is true regardless of the choice of parameters for the model.In contrast, starting with vapour from a mid-latitude source (such as the Weathership E area), the model predicts for Site G a maximum in September and a minimum in March, with values of 14 and 4‰ respectively.The latter results are in good agreement with the observations at Site G (Fig. 1), and also with those at Site 4B, in respect of the annual mean as well as the seasonal amplitude and the phase.We conclude that, in contrast to polar low-elevation stations, the ice sheet receives precipitation from moisture which originates predominantly from much lower latitudes. The moisture has probably been carried northward at considerable altitudes by the cyclonic conveyer belts, and is thus more or less separated from the ocean surface at an early stage.A more complete description of the model will be submitted to Tellus in the near future.

The Origin of Arctic Precipitation as Deduced from its Deuterium Excess (Abstract)

Continuous profiles of deuterium excess, d = δD − 8δ18O, in the snow-pack have been measured at continental, high-elevation stations: Dye 3/4B (2480 m a.s.l.) in southern Greenland, and Site G (3110 m a.s.l.; 70 km south-east of Crête) in central Greenland. Figure 1 shows the δ18O and the d profiles along the upper 4.3 m of firn at Site G. These profiles represent the snow accumulation from late 1974 to mid-1985, judging from the clear seasonality in the δ18O record. There is also a seasonality in the d values, which generally range from +5 to +12 ‰. In contrast to the findings at low-elevation stations, the two seasonal cycles are not in antiphase; the d curve lags the δ18O curve by only approximately 90°. These same features appear when plotting similar data from Dye 3. Modelling of the data proceeds in three steps. First, the isotopic composition of vapour evaporating from an oceanic source area is calculated on the basis of Merlivat and Jouzel’s (1979) model, which takes kinetic effects into account. Secondly, the vapour is advected to the ice sheet, without mixing with vapour from other sources. Any liquid precipitation is assumed to be formed by a Rayleigh condensation process down to −5°C. Thirdly, the snow formation at lower temperatures is modelled, taking account of kinetic effects due to supersaturation of vapour with respect to ice (Jouzel and Merlivat 1984). The result of this modelling is that the initial mixing ratio (wso; g H2O/kg dry air; data from IAEA/WMO) largely determines the change of d in the precipitate as δ18O decreases during cooling. Furthermore, the absolute magnitude of the deuterium excess is mainly determined by the sea-surface temperature (Ts) in the source area. The combined result of these effects is shown in Figure 2, the upper part of which applies to a high-latitude source region (Weathership A at 62 °N, 34 °W), and the lower part to a mid-latitude source region (Weathership E at 35 ° N, 48 ° W). dvo is the deuterium excess of the parent vapour as it leaves the source area; dpo is that of first-stage precipitation predicted by the model, and dp is the predicted deuterium excess in the precipitate at Site G. In the high-latitude case, dvo and dpo have pronounced summer minima and winter maxima. As the cooling proceeds (until the observed δ18O values at Site G are reached), this cycle is inverted so that at Site G the model predicts maximum dp in August and minimum dp in January, and deuterium excesses on the ice sheet which are far too low (cf. Fig. 1). This is true regardless of the choice of parameters for the model. In contrast, starting with vapour from a mid-latitude source (such as the Weathership E area), the model predicts for Site G a maximum in September and a minimum in March, with values of 14 and 4‰ respectively. The latter results are in good agreement with the observations at Site G (Fig. 1), and also with those at Site 4B, in respect of the annual mean as well as the seasonal amplitude and the phase. We conclude that, in contrast to polar low-elevation stations, the ice sheet receives precipitation from moisture which originates predominantly from much lower latitudes. The moisture has probably been carried northward at considerable altitudes by the cyclonic conveyer belts, and is thus more or less separated from the ocean surface at an early stage. A more complete description of the model will be submitted to Tellus in the near future.

A Study of the Mechanisms of Acid Rain Formation

Abstract Samples of rain, snow, cloud water, aerosols and soil were collected in Colorado to study the mechanisms of acid rain formation. Chemical compositions of various types of samples were analyzed to investigate the stepwise incorporation of impurities into precipitation. Local soil was generally alkaline; atmospheric aerosols, which are mixtures of stirred-up soil particles and anthropogenic pollution, were slightly acidic; cloud condensation nuclei, which initiate clouds at condensation level, had an average pH of ∼6. However, local clouds were very acidic (pH ∼4), indicating that further acidification takes place in clouds by adsorption of acidic gases, e.g., CO2, SO2, and NOx. We found that summer showers formed by coalescence of cloud droplets are likely to be as acidic as cloud water. The chemistry of snow may differ from that of clouds, depending on the mechanisms of snow formation. If snow crystals are initiated by deposition nucleation and grown by diffusion of water vapor from surrounding e...