Snowfall

Over much of the Earth's extratropical continental area, a significant portion of precipitation falls as snow. The principal climatic roles of snow relate to its high reflectivity or albedo, low thermal conductivity, water-holding

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Fig. 4.3. Global annual mean snowcover. (NCEP Reanalysis data courtesy of NOAA/OAR/ ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

capacity, and thermal inertia (Sturm, 2003). Snow is stored on the surface for periods ranging from hours to months before melting and continuing through the land phase of the hydrologic cycle. It should be immediately apparent that snowmelt is effectively delayed precipitation. Moreover, the high albedo of fresh snow contributes to snow serving as a cold reserve that acts to depress temperatures in the spring when energy is required to warm the snow to 0 °C before melting occurs. In many areas, snowmelt is the main source of surface-water supply and the main cause of flooding. Snow and snowmelt play an important role in the hydrology of the middle latitudes and of rivers originating in high mountains. For mountainous regions, snowmelt contributes at least 50% of the annual runoff and exceeds 95% in some locations (Rango and Martinec, 1995).

Virtually all land areas above 40° N have a seasonal snowcover of significant duration (Fig. 4.3). This amounts to about 42% of the Northern Hemisphere. However, the perennial cryosphere covers only 8% of the Earth's surface. The annual cycle of snowcover shows the Northern Hemisphere cycle has a larger amplitude than the Southern Hemisphere (Peixoto and Oort, 1992). This is mainly due to the vast extent of snowcover during the northern winter. For the globe, the variation in snow plus ice cover is dominated by changes over the Northern Hemisphere continents. The fraction of average annual precipitation falling as snow shows a steep latitudinal increase and reaches 0.65 on the north coast of Alaska (Dingman, 1994). Other factors recognized as contributing to the portion of precipitation occurring as snow are location relative to oceans and elevation. Snow accumulation generally increases with elevation because of the combined effect of temperature and the increased frequency of precipitation due to orographic influences. Snow accumulation patterns can be complex due to topography, and patterns may change seasonally as temperature, precipitation, and the air flow change (Sturm, 2003).

4.5.1 Snowfall measurement

Snow depth, density, and water equivalent are of primary hydroclima-tological importance. Snow measurement must address both the depth and the density of snow. Snow depth expresses the accumulation of snow, and snow density quantifies the water content of the snow. The physical characteristics of snowflakes and snow on the ground underlie the changing relationship between snow depth and density and the need for repeated monitoring. Snow depth measurements are made in units of cm. The water content of the snow, or snow water equivalent (SWE), is the vertical depth of water obtained by melting the snowcover. The usual unit of measurement for SWE is mm (WMO, 1996).

Snowflakes are loose aggregates of ice crystals, most of which are branched. Much of the precipitation reaching the ground begins as snowflakes in clouds. Snowflakes commonly melt before reaching the surface and the precipitation assumes a different form as either rain or sleet. In winter the freezing level in the atmosphere may be close to the surface and falling snowflakes have a better chance of surviving to reach the surface. In relatively dry air, snowflakes may reach the ground even when the air temperature is considerably above freezing because the partial melting of the snowflake chills the remainder of the snow-flake and retards the rate of melting. Air temperature is a reasonable index of precipitation type, and chances are that snow will occur at temperatures below 0.6 °C (Linacre, 1992).

When snowflakes reach the surface they begin a process of metamorphism that continues until melting is complete. Accumulating snowflakes at the surface constitute the snowpack which builds layer by layer. The initial characteristics of each layer are determined by how much solid precipitation falls, whether the precipitation is accompanied by wind, and the temperature at the time of deposition. After deposition, each layer is subjected to thermal and mechanical metamorphic processes that alter the layer characteristics. New snow layers densify rapidly and settle as the snow crystals fragment and become more rounded (Sturm, 2003). The resulting snowpack is a granular porous medium consisting of ice and pore spaces. When the snow is cold, or its temperature is below the melting point of ice (0 °C), the pore spaces contain only air or water vapor. At the melting point, the pore spaces can contain liquid water as well as air, and the snowpack becomes a three-phase system.

Accurate snow measurement is difficult because falling snow is sensitive to turbulence and to wind depletion that produces large variations in snow depth and density over short distances. For locations that receive occasional snow during the winter, the common method for measuring snow depth is to use a ruler attached vertically to a board placed on the ground or on the previous snow surface. In areas where snow accumulation is substantial, a snow stake or pole with graduated markings is used. In remote areas, aerial snow markers that can be read from airplanes are used to determine snow depth and snow pillows equipped with pressure transducers measure the weight of accumulated snow (WMO, 1996).

The liquid water content is an important physical property of snow. Liquid water content (6) is defined as the ratio of the volume of liquid water in the snowpack to the total snow volume

where Vw is the volume of liquid water and Vs is the volume of snow. Snow density (ps) is the mass per unit volume of snow

where pi is the density of ice, Vi is the volume of ice, and pw is the density of water. The SWE refers to the amount of water the snow contains. SWE is expressed as the depth of water that would result from the complete melting of the snow in place. SWE is commonly estimated as

Pw where ps is snow density, pw is the density of water and hs is snow depth. In this form, snow density is understood to mean the relative density compared to water. A representative SWE for more than one snow sampling site is estimated by using the arithmetic mean of the snow depth measurements in Equation 4.3 (Seidel and Martinec, 2004).

The density of new-fallen snow is determined by the configuration of the snowflakes. Snowflake configuration is largely a function of air temperature, the degree of supersaturation in the precipitating cloud, and the wind speed at the surface of deposition. Observed densities of fresh snow range from 0.004 to 0.34. The lower values for powder snow occur under calm, very cold conditions, and the higher values for snow occur with higher winds and temperatures (Bras, 1990). It is difficult to measure the density of new snow and an average relative

50 0

Fig. 4.4. Snow course measurements of snow depth (solid line) and snow water equivalent (broken line) for the first five months of 2006 at Echo Summit, California (39% N, elevation 2258 m). (Data courtesy of the California Department of Water Resources from their website at http://cdec.water.ca.gov/.)

density of 0.1 is commonly assumed. This is the basis for the widely used "rule of thumb'' in mountainous areas of North America that 1 cm of snow equals 1 mm of water.

The standard method for measuring SWE is by gravimetric measurement using a tube to obtain a sample snow core. This method is the basis for snow survey procedures used in many countries. The snow sample from the tube is either melted to determine the liquid content or the frozen sample is weighted (WMO, 1996). Specific areas identified as snow courses are designated and visited around the first of each month from January through June. Sites selected as snow courses require the same protection from wind movement as needed by rainfall gauging sites to produce a representative measurement (Peck, 1997). Snow depth is determined and the snow density is measured by weighing an aluminum tube used to extract a core sample of snow. The Mount Rose snow sampler widely used in North America has an inside diameter of 38 mm so that a snow core weighing 28 g is equivalent to 25 mm of water (Bruce and Clark, 1966). Measurements for a snow course in the central Sierra Nevada in California during 2006 are shown in Figure 4.4. Repeat visits are necessary because as soon as snow accumulates on the surface it begins a process of metamorphism that continues until melting is complete. At the beginning of the melt season, the snowpack is typically vertically heterogeneous with several layers of markedly contrasting densities. During melt, density continues to increase and the vertical inhomogeneities tend to disappear due to the formation and drainage of meltwater. A snowpack at 0 °C and well drained tends to have a relative density near 0.35.

Fig. 4.4. Snow course measurements of snow depth (solid line) and snow water equivalent (broken line) for the first five months of 2006 at Echo Summit, California (39% N, elevation 2258 m). (Data courtesy of the California Department of Water Resources from their website at http://cdec.water.ca.gov/.)

1-Mar 15-Mar 29-Mar 12-Apr 26-Apr Date

1-Mar 15-Mar 29-Mar 12-Apr 26-Apr Date

Fig. 4.5. Snow sensor measurements of daily snow water equivalent for the 2005 maximum accumulation period at Meadow Lake, California (40° N, elevation 2182 m). (Data courtesy of the California Department of Water Resources from their website at cdec.water.ca.gov/.)

In remote mountain areas where access is difficult or restricted, SWE can be measured automatically by weighing the snow on a pressure-sensing snow pillow made of a metal plate or a flexible bag several meters in diameter filled with an antifreeze liquid. The overlying weight of snow is recorded as changes in pressure using a manometer or a pressure transducer (Ward and Robinson, 2000). Snow pillows are used in selected locations in Switzerland, the United Kingdom, and elsewhere in Europe, and they are widely used in the Western United States. The automated system in the United States called SNOTEL (SNOwpack TELemetry) consists of over 600 sites equipped with at least a pressure-sensing snow pillow, a storage precipitation gauge, and an air temperature sensor to produce daily SWE values. The daily SWE data in Figure 4.5 are from an automated snow sensor. SNOTEL uses VHF radio signals and meteor burst communication technology to collect and communicate data in near-realtime. The data are archived by the Natural Resource Conservation Service, National Water and Climate Center.

4.5.2 Estimating point snowfall

Snowfall displays high variability over short distances in areas of uneven terrain and for areas with diverse vegetation cover. High spatial variability decreases the correlation between adjacent points and reduces confidence in estimating point snowfall from existing point observations. Consequently, it is common to use a snow stake described in the previous section as a representative measurement rather than attempting to develop an estimating equation for other points (Seidel and Martinec, 2004). Snowfall rates are estimated using radar, and snowcover is estimated using other remote sensing techniques described in Chapter 5. Although these techniques do not produce direct estimates of point snowfall depth, advances in snow simulation models provide promising results in estimating snow accumulations (Clark and Vrugt, 2006).

4.6 Wind

Local-scale atmospheric motion is the variable of interest to be measured in climate of the second kind and the terrestrial branch of the hydrologic cycle. Local surface winds are generated by synoptic-scale atmospheric motion, horizontal temperature differences in the planetary boundary layer, topography, and atmospheric instability. These winds advect heat and moisture that can account for spatial and temporal heterogeneities in energy and mass fluxes. Sea breeze, land breeze, and katabatic winds are examples of local surface winds that are embedded in the larger-scale synoptic and global winds of climate of the first kind and the atmospheric branch of the hydrologic cycle.

4.6.1 Wind measurement

In general meteorological and climatological applications, surface wind is considered to be a horizontal, two-dimensional flow represented by its velocity, or rate of movement, and direction. Windfield refers to the combination of air speed and direction in a space-time continuum (DeFelice, 1998). Separate instruments are used to measure these wind characteristics. Anemometers measure wind velocity, and vanes indicate wind direction (Fig. 4.6). Since wind speed and direction are almost always reported as averages, a processing and receiving system is required as part of the equipment installation.

Fig. 4.6. A cup anemometer and wind vane. (Photo by author.)
1-Jan 7-Jan 13-Jan 19-Jan 25-Jan

Date

Fig. 4.7. Daily average wind speed for January 2007 at Orland, California (41° N). (Data courtesy of the California Department of Water Resources from their website at http://wwwcimis.water.ca.gov/.)

Anemometers routinely used at climatological stations respond to the kinetic energy of the air. Other types of anemometers (e.g. pressure and sonic) are used for specialized applications. The widely used anemometers have a rotating sensor, and a transmitter that transforms the sensor's rotation into a measurable quantity. The rotating component is three or four small, hollow hemispherical cups or conical cups each attached to arms extending horizontally from a vertical axis. The number of cup revolutions per minute is registered electronically and converted to wind velocity by the transmitter. Wind velocity is usually expressed in ms^and is often reported as the average for hourly or daily periods (Fig. 4.7).

An alternative design common in the United States is a propeller anemometer resembling a model airplane without wings. The tail swings the propeller into the wind, and the propeller's rotation generates a measurable voltage (Linacre, 1992). Propeller anemometers respond more quickly than cup anemometers to accelerating and decelerating wind. Also, propeller anemometers are more sensitive to low wind velocities (Guyot, 1998).

Wind direction is measured with a vane balanced on a vertical axis to prevent favoring a particular direction. The preferred design is one or two rectangular metal plates attached vertically to the end of a horizontal rod that pivots around a vertical axis. The construction must be heavy enough to smooth out flutter resulting from the smallest wind changes. Vanes will not usually turn when the wind speed is less than 1.5 m s_1 (Linacre, 1992). The vane direction is observed over a few minutes and recorded as a compass direction. For automated stations, several types of transmitters are available to send vane information electrically in either analog or numerical form (Guyot, 1998).

Appropriate exposure for an anemometer and wind vane is important for reliable measurements. These instruments are typically mounted in an open area at a height of 10 m. Ideally, the land surrounding the instruments should be uniform for a distance equal to 100 times the height of any obstruction (WMO, 1996).

4.6.2 Estimating wind

Local surface winds in open, exposed areas display high correlations between two adjacent points when no major terrain features exist between the locations. However, correlations between hourly and daily wind movements are very low for adjacent sites when one site is protected (Peck, 1997). Hills, mountains, valleys, and proximity to the ocean all induce wind variations within short distances.

The standard method for estimating the wind at a point between widely spaced anemometers involves determining a distance-weighted average of the anemometer measurements. The weighting is chosen to make the nearest measurement the most influential in the estimate calculation (Linacre, 1992).

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