The physical properties of snow define and shape its influence on climate and society. Temperatures in the middle and upper troposphere are below 0°C, so ice crystals can be found everywhere in the global atmosphere. As discussed in chapter 2, mixed clouds below this temperature consist of a blend of water vapor, ice crystals, and supercooled water droplets.
Once nucleated, ice crystals grow through deposition of water vapor. Saturation vapor pressure on the surface of rounded water droplets is greater than that of ice crystals, due to the effects of curvature on surface tension. This creates a vapor pressure gradient that drives diffusion of vapor to the ice-crystal surfaces. Ice crystals grow at the expense of water droplets in something known as the Bergeron process. As long as sufficient moisture is available in a cloud, ice crystals—snowflakes—will continue to grow until they become heavy enough to drift to the ground. Temperatures in the lower troposphere dictate whether precipitation will fall as rain, snow, or something in between (sleet).
The phase of precipitation is not always easy to predict; snowfall is commonly recorded at near-surface temperatures well above 0°C, and rain can occur at temperatures below 0°C. This depends on the temperature structure in the atmospheric boundary layer, the initial size of the precipitation particle, and the transit time (height, speed, and path) of the precipitation. In climate downscaling applications where one needs to estimate snowfall from the daily or monthly precipitation and temperature, it is recommended to estimate the fraction of precipitation to fall as snow as a statistical distribution about 0°c, rather than adopting a sharp transition at, for example, 0°c or 2°c, below which precipitation falls as snow. For instance, a cumulative distribution function over the range of approximately -6°C to +6°C represents observations well. Ideally, of course, atmospheric models are able to represent explicitly the precipitation processes and phase of precipitation on the temporal and spatial scales needed for modeling snow accumulation.
The structural forms of snowflakes are determined by the temperature, humidity, and wind conditions during crystal growth, although the specific processes that determine a snow crystal's form remain somewhat enigmatic. The many different crystal forms include pillars, disks, stellars, plates, and columns. The relation to cloud conditions is complex and nonintuitive, but some systematic tendencies are observed, with useful
generalizations given in figure 4.1. Faceted hexagonal plates are prevalent under dry conditions below -10°C and again at temperatures above -4°C, but columnar ice crystals are dominant between -4 and -10°C. When humidity is higher (i.e., under supersaturated, relatively mild conditions), plates evolve into dendrites and stellar crystals. Ice needles develop around -5°C, and columns prevail under cold, humid conditions, below ca. -20°C.
Ice crystals in the cold, dry upper troposphere tend to be hexagonal plates. These crystals align horizontally due to air resistance, and the hexagonal structure can reflect sunlight at specific angles (22°) to produce halos or sundogs. They also give us elegant cirrus clouds. Closer to the ground, more water vapor is commonly available, and supersaturated clouds produce the classical dendritic forms.
Snow crystals inherit their hexagonal macrostructure from the structure of the ice lattice, and humidity plays the primary role in shaping the growth of elaborately branched, stellar crystals from six-sided, faceted plates. Small plates develop when snow-crystal growth is moisture-limited and vapor is deposited equally on all six facets. With surplus moisture, vapor deposition occurs rapidly and near-symmetrically at the corners of the facets (the shortest distance for vapor diffusion) and a branching instability is excited, where dendritic branches grow at each corner. New branches are spawned at asperities and protrusions in the branches. The snowflake grows until it becomes heavy enough to float to the ground.
Snowfall is associated with two main mechanisms of precipitation: (i) orographic uplift of moist air masses and (ii) air mass mixing/frontal precipitation. The former is dominant along mountainous coastlines, where inland advection of maritime air masses leads to uplift, cooling, and a transition from rainfall at low altitudes to snowfall at higher elevations. Snow-capped peaks in New Zealand, Iceland, Norway, the Himalayas, and the American Cordillera all testify to this process. Orographic precipitation also delivers snowfall to the flanks of the Greenland and Antarctic ice sheets.
Frontal collisions produce much of the snowfall in interior continental regions, particularly in the mid-latitudes where extratropical cyclones usher a steady succession of cold, polar air masses into contact with lower-latitude (warmer, wetter) air masses. Mark Hel-prin expresses this snowfall mechanism sublimely in Winter's Tale:
Battalions of arctic clouds droned down from the north to bomb the state with snow, to bleach it as white as young ivory, to mortar it with frost that would last from September to May.
Although violent, this metaphor is consistent with the meteorological appropriation of the term front from its military origins. In the case of winter snow storms, conflicting air masses produce precipitation in several ways. Dense polar air in cold fronts plows under warmer air masses, forcing uplift, cooling, condensation, and precipitation. Developed extratropical cyclones are also accompanied by eastward or poleward movement of warm fronts, with warm, wet air masses buoyantly overriding cool air at the surface, again inducing cooling and precipitation over a region. In some cases, forced uplift along cold or warm fronts triggers free convection and even stronger storms (low pressures, high winds, and large amounts of moist air advection to a region). From fall to spring, these frontal interactions produce snowfall as the precipitation falls through cold air near the ground, although other forms of precipitation are also common.
Of course, orographic and frontal precipitation processes frequently act in concert. A common example is when moist Pacific air masses (often associated with extratropical cyclones) are forced upward by mountainous topography in New Zealand or western North America. other precipitation processes also give rise to snowfall, such as air mass modification (e.g., due to addition of moisture from open water bodies and/or isobaric cooling of air masses as they move poleward or inland).
There are many other forms of meteoric precipitation in the solid phase. Sleet is the name for liquid precipitation that has partially frozen during its descent through the lower atmosphere; it is amorphous, lacking the crystal structure of snowflakes. Freezing rain occurs when raindrops freeze upon contact with cold surfaces on the ground. Rime is an extreme form of this: supercooled water droplets that freeze on contact and can produce a thick, white coating over trees, power lines, or other objects that intercept humid, near-surface air flow. Grau-pel is a term for snowflakes that are partially melted and rounded into pellets and can be coated in rime. These are also called snow pellets, and you can recognize them because they often bounce upon impact. Hailstones, in contrast, do not begin life as snowflakes. Rather, hailstones are dense, amorphous pieces of solid ice, formed from riming and deposition onto ice nuclei in cumulonimbus clouds. Strong updrafts promote cycling of ice particles in these clouds. Large hailstones result from high humidity and long residence times and can sometimes be found with concentric growth layers.
Frost is a variation on solid precipitation. It is the solid-phase form of dew, associated with direct deposition of water vapor onto a surface when a near-surface air mass is cooled to saturation. The source of moisture in frost is usually atmospheric, but there are also splendid examples of surface hoar deposits or frost feathers, in which the vapor pressure gradient between a warm, wet surface and a cold, dry air mass drives vapor diffusion from the surface to the air. Upon contact with the cold air, this vapor freezes to form intricate frost patterns that rival those of snowflakes.
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