Snow Hydrology Instructor: Randy Julander Weather: a brief, non-technical overview introduction
Download 96.43 Kb.
|
- Navigate this page:
- Fundamental principal
- Processes that move and cool warm air parcels.
- Topography and snow accumulation
- Solar radiation and snowmelt
- A brief synopsis of snow formation
| Snow Hydrology
Instructor: Randy Julander Weather: a brief, non-technical overview INTRODUCTION In hydrology, we need to know some of the fundamentals of meteorology in order to construct mathematical models of hydrologic processes. In Snow hydrology, the most important weather principles we deal with are: 1) temperature and its behavior with respect to watershed characteristics, 2) precipitation and its behavior with respect to watershed characteristics, 3) solar radiation and ditto. Other meteorologic characteristics such as wind, relative humidity, etc are important but because we have very little data, and relatively poor procedures for calculating these variables in space and time, they are ignored. Solar and earth radiation Solar radiation is the Earth’s chief source of energy and it determines the world’s weather and climates. Both the earth and the sun radiate as blackbodies: i.e. they emit for every wavelength almost the theoretical maximum amount of radiation for their temperatures. Radiation from the sun is short wave and, from the earth it is long wave. The rate at which energy (solar radiation) reaches the upper limits of the earth’s atmosphere on a surface to the incident radiation and at earth’s mean distance from the sun is called the solar constant. It is about 1.94 Langleys/minute (l langley is 1 calorie/cm squared) a large part of solar radiation is reflected back to space by clouds and the earth’s surface or absorbed in the atmosphere. Radiation scattering is dependent on the wavelength: short-wave lengths scatter easily (0.45 microns) the blue range thus accounting for the blue sky, very little in the red range is scattered (0.65) 
Since the surface area of a sphere is 4 times greater than that of a circle, solar radiation intercepted by planet earth averages about 1/4 the solar constant or about 0.5 langleys/min. Various points on the earth’s surface receive different amounts of energy depending on their aspect and watershed orientation. 
In the above figure, solar radiation to the watershed surface has been calculated as an index for the Salt Lake city basins. In March and April, as the sun angle is still relatively low, there are wide differences in the amount of the radiation index. As the sun angle increases (we move closer to the summer months and the sun becomes more directly overhead), all watershed receive equal amounts of energy. Watershed orientation thus becomes a large factor in the timing of snowmelt and peak flow. The SLC watersheds are very similar and elevation and size become more dominant factors in the runoff scenario. The earth’s surface radiates as a blackbody at a mean temperature of about 15 degrees C - about 1.25 langleys /min. This is about 2.5 times the 0.5 langleys per minute of incoming radiation. Net loss of heat is prevented and a heat balance maintained because the atmosphere reflects back to the surface about 85% of the emitted long wave radiation. Were it not for this greenhouse effect, the mean temperature of the earth would be about -40 degrees C. Solar radiation is the engine that produces weather systems around the globe. This energy produces air masses of different temperatures, moisture content, elevation and travelling in different directions at different speeds. These air masses bring the conditions we call weather. Fundamental principal: Warm air holds more moisture than cold air. If you have warm air at 100% relative humidity and cool that parcel of air, the moisture must go somewhere thus it condenses and precipitates. Fundamental principal: Warm air is lighter than cold air and thus it rises. Adiabatic processes (very complex equations dependent on humidity, pressure, etc) cool warm air as it rises. Therefore, the main mechanism producing precipitation is warm, moist air forced to rise, cool, and dump moisture. Processes that move and cool warm air parcels. Frontal systems: a frontal surface is the boundary between two adjacent air masses of different temperature and moisture content. Frontal surfaces are actually layers or zones of transition. The line of intersection of a frontal surface with the earth is called a surface front. An upper air front is formed by the intersection of two air masses aloft. If air masses are moving so that warm air displaces cold air then it is called a warm front. Conversely if cold air is displacing warm air, then it is called a cold front. Cold fronts move faster than warm fronts and usually overtake them. The boundary layer between air masses is where most precipitation occurs 
CONVECTION: Warm air is capable of containing more moisture than cold air. Warm air rises. Through a series of complex processes, the warm air expands, losing heat... thereby becoming colder. This process: adiabatic loss rate or the lapse rate, under theoretical conditions, is 3 degrees per every 1000 feet of elevation gained. An air mass rising 10,000 feet is capable of 30 degrees of cooling without extenuating circumstances. Thus an air mass, full of moisture at 60 degrees, forced to rise 10,000 feet is cooled to 30 degrees and is capable of producing snow at that elevation. Most convective activity occurs in the summer as a result of the warming of the earth’s surface and the air mass next to it. As these air masses move and produce rain, at higher elevations, this may be snow.
Orographics: Other influences cause air masses to rise, chiefly, orographics.
This is land topography that forces air masses to rise. As moist air masses over the pacific move along the jet stream or prevailing wind, they encounter land. The source of moisture is dramatically decreased, as is a great deal of energy. As these air masses are forced to rise, they cool and precipitate. The closer to the source of moisture along the prevailing winds, the more precipitation falls: i.e. the Oregon/Washington coast, the Sierras of California, etc. The farther from the source, without significant replenishment, the less moisture is available for precipitation.
In our case, the Sierras wring out most of the moisture, leaving the Great Basin high and dry. Snow water equivalents in the Sierras can easily get over 100 inches deep whereas in Nevada and Utah 20 to 40 inches is more the norm. 
The Wasatch Front is the next big rise for air masses coming across the Great Basin. Since the air mass is already quite dry, the snow that falls here tends to be very light in density and have very distinct layers (avalanche potential). The Sierras get very wet, heavy, dense snowpacks commonly referred to as Sierra cement. Utah has a cool continental climate, conducive to such snowpacks whereas the Sierras have a coastal influence. A further complicating factor is a significant moisture source in the form of the Great Salt Lake. Early season storms of cool air travel across the warm lake and gain moisture and energy. This effect is very short lived however and typically the Cottonwood Canyons are the beneficiaries. There is a great deal more snow in the Cottonwoods than in the canyons to the north or south, partly due to elevation and partly due to prevailing storm direction in the early season trending form the northwest to southeast. 
Topography and snow accumulation Topography: Orographics (or major land-forms- Mountainous areas are clearly distinguishable in the above Precipitation map of Utah, so much so that it is, in reality, a coarse topographical map) cause the air mass to rise and precipitate but micro-topography plays a huge role in the actual accumulation pattern. Canyons can funnel and trap moisture-laden air masses and get substantial accumulation. Wind patterns affect snow accumulation. As air masses compress and are forced up and over an obstacle, velocities increase near the summit, i.e. the Bernoulli effect, airplane wing, etc. in these areas; Snowpacks can be scrubbed clean constantly. 
Bald Mountain Pass, Uintahs – note the wind scrubbing. However, the leeward side becomes the beneficiary of this, because the wind densifies the snowflakes by breaking them apart and reforming in tighter patterns of plates and grains, etc. On the leeward side, there is a sudden decompression and wind velocities decrease tremendously, allowing the snow to fall to the surface. These areas often form giant snowdrifts of considerable size with great bonding strength. Cornices can be a hundred feet high and sometimes as long. 
Cornice formation on Farmington Peak. Even if conditions prevent the formation of a cornice, drifts can be very impressive such as the one that forms on the lee side of Bull pass in the Henry Mountains. It is often 300 to 500 feet in length and 20 to 40 feet deep. It blocks the road opening many times until mid July and sometimes even later. Being able to locate areas such as these allows one to strategically place roads and structures to avoid the maintenance costs of plowing, etc. One can also locate other structures to alter the shape and size of such drifts. Any topographical feature that produces a disturbance in the prevailing steady stream air flow is capable of producing anomalies in the snowpack, not only in actual precipitation amount but the other characteristics of the pack as well: such as grain size and distribution, bonding characteristics, etc. 
Solar radiation and snowmelt Topography has a great influence on the eventual melting of snowpacks. Aspect has perhaps the greatest impact. A north-facing slope receives little incoming radiation for much of the winter and early spring. What it does receive is at an oblique angle and mostly reflected. Not till later in the season does it start getting enough direct energy for melt and typically this is at a time that temperatures are quite warm for conductive and convective melt as well. 
Looking north at south facing slopes, west is left and east is right: note the difference in snow covered aspects. A brief synopsis of snow formation Moisture in an air mass is forced to lift, cooling it, causing condensation and forming water vapor. In order for this to happen, some form of nuclei must be present. Water in an air mass may become super cooled before condensing if these particles are not present. Water vapor condenses on a particle, then more and more vapor condenses. If it is rain, the drop gets larger and larger finally overcoming the lifting and turbulent airflow and settles to the ground. With a snowflake, the same process occurs, but snowflakes have a much greater surface area, lighter density and can take more time. Water vapor also forms ice crystals directly on the particle, which grow in what appears to be perfect 6 sided symmetry. This flake continues to grow until it overcomes the internal storm forces, submits to gravity and falls to the earth. During this time, collision with other flakes can occur and sometimes 2 flakes become 1 or flakes become damaged/broken and continue to grow. Once on the ground, the flake continues to undergo changes. The wind can pick it up again, break its little fragile arms and rearrange its face. Snowflake size and pattern are dependent on the conditions of its birth. Calm cold conditions produce large fluffy flakes. Wet, warm conditions produce more granular crystals. Once on the ground, the flake bonds to one degree or another with the snow already present. The metamorphosis continues as other flakes pile up on top. Vapor pressure within the interstices or voids determines water migration from one crystal to another. Crystals become less dendritic, more faceted. During this time the crystal becomes more and more subject to the increasing weight of snowpack above. Compressive forces mold the crystals even further. Crystals on or near the surface are exposed to other forces such as sun action, wind, and temperature extremes. Depth hoar layers may form which are crystals that grow long fine and branched dendritic patterns. The light fluffy stuff of skiers dreams. These layers can form on or near the surface. These layers constitute weak bonding surfaces in the snowpack and have the potential for large-scale avalanche activity later in the season as the weakness persists for long periods of time. Later in the season, as snowpacks receive more energy, they start to process of becoming isothermal. This is a condition that must occur preceding melt. This is simply that the pack is at or very near 32 degrees from top to bottom and that subsequent energy may convert solid to liquid. In this condition, free water may start migrating through the pack, which in turn may give some energy to lower layers. Water and energy continue to change the snow crystals right up till the final melting phase. Temperature Temperature is the most widely used driving process in snowmelt modeling. Why? - it has been the only variable that is widely available. There are some problems - as always. Temperature is only an index to the major process that drives snowmelt: solar radiation. Temperature is yet another variable for which we need some kind of spatial interpolation. Temperature - max, min and average are typically collected by the NWS through many avenues but almost always in cities and towns- not necessarily at the top or even middle of a watershed where we need it. Temperature, as a general rule, is much more stable over similar elevations than is precipitation. If you have the temperature in SLC, you can with 98% confidence, predict the temperature in Ogden and usually, with reasonable accuracy, even predict the higher elevations such as Alta or Brighton. Temperature is elevationally dependent. It gets colder as elevation increases USUALLY. The adiabatic process defines how much colder. As air compresses without any other energy input, the air warms. How much is dependent on relative humidity, pressure, etc. As air is lifted, it decompresses and cools. The range is anywhere from 1 degree to almost 6 degrees F per 1000 feet of elevation gain or loss. The most typically used values are 3 degrees f and 3.8 degrees f per 1000 feet. Thus a parcel of air lifted 10,000 feet will cool (on average) 30 degrees. It may cool only 10 degrees or it could cool as much as 60 degrees. You get the idea real fast that using an average value may, at times, really produce bad results in hydrologic snowmelt modeling. So why is it used in this way? Again, the only available temp data when these models were developed and calibrated were the lower elevation locations. (remember, you typically use what you have and try to derive the best possible application) Inversions: an inversion is where cool air, which is very heavy, pools into the lower elevations and without any wind or thermal activity stays in place: typically associated with fog which takes enormous energy to burn off, leaving little for heating. The infamous Great Basin High pressure systems can generate fairly long lasting inversions in Utah. High elevations can have temperatures of 40 to 55 degrees and the valleys may remain in the 30's. Fortunately, these typically occur in January and February with minimal snowmelt. At the peak of snowmelt, the adiabatic lapse rate may display a wide range of values thus when modeling, it is always best to have temperatures from several elevation zones and calculate linear interpolations between stations based on elevation and aspect. Download 96.43 Kb. Share with your friends:
|
Notice southern California and how dry it is... this is a function of the prevailing winds, storm tracks (jet stream), location relative to warm and cool water bodies and the Coriollis effect. The higher the landmass, the more moisture is wrung out. 
