Lecture Panel 6 Flashcards
Why is snowmelt important?
in many mountainous regions, can be 75% of water supply-
What does snow cover due to sunlight?
reflects it, ie reflects heat
- fresh fallen snow albedo 0.8
- trees 0.3 or 0.2
where is planetary albedo highest?
north and sahara (in winter)
avg 0.3
how does pollution affect snow?
makes it less bright and thus easier to melt
what happens to snow as it sits on the ground?
turns granular, albedo drops to 0.6
-gets easier to melt as albedo drops because more radiation is absorbed
What disaster can snowmelt cause?
floods
What can snowcover knowledge help predict?
- climate
- using correct initial conditions of snowpack increases predictive value of climate estimations
Critical properties of snow
Low thermal conductivity, strong insulation
-keeps soil somewhat warm - snow can be modelled as a series of layeres, each with its own temp
Result of snow as an insulator
diurnal fluctuation of temp doesn’t occur at depth
-keeps ground relatively unfrozen - helps prevent flood (unfrozen ground can absorb more water)
Where does it snow globally?
virtually all land areas above 40 degrees latitude
describe lake effect snow
- cold, dry air (low saturation pressure) blows over warm water
- air warms up and picks up moisture
- hits cold land again (possibly with lift) and saturation pressure reduces, causing snowfall
Where are snowbelts relative to great lakes?
eastern sides mostly, due to prevailing west winds (blowing to the east)
What type of precipitation is more likely at altitude?
snow - more snow-water equivalent at altitude
Canopy effects on snow
- snow can be intercepted by tress
- wind speed and surface roughness
- surface energy exchange
- albedo of nearby trees can affect snow pack
where are the highest snow depth found?
clearings with diameters less than 20x surrounding tree height
What is a good forest environment for snow accumulation?
-open deciduous forest
How would snow accumulation differ around a deciduous vs coniferous tree?
conifers have large amount of snow in tress, less on ground, deciduous have a deep snow pack on ground with little interception
Why would a clearing have more snow? How much more?
snow blows into clearing from surrounding trees, 20-45% greater accumulation.
Wind effects on snow pack
-redistributes, changes nature and density of pack. Windy areas would have less accumulation in a field, for example.
how do slope and aspect affect snow accumulation?
-on a steep slope nothing can accumulate. South facing slopes get sun to melt snow
Snow water equivalent
depth of water equivalent due to the complete melting of a unit area of snow
-measured as SWE (m) = snow densitysnow depth/density of water
or
SWE (m^2) = snow densitysnow depth
typical snow density
0.07 to 0.15 kg/m^3 for freshly fallen snow
How does snow density change over season? What are density characteristics
density changes due to metamorphism
- 0.32-35 will support weight
- 0.35-0.38 foot only leaves slight impression
- above 0.4 no marks are left on surface
Snowpack characteristics over time
- fresh snow contains more air and is less dense
- drifted/settled snow is more dense.
- sharp edged flakes change to stackable cubes
- density increases as winter progresses
What are problems with gauges and snow? Solutions?
- snow is hard to get into gauge due to wind eddies
- nipher shield attempts to stop eddies, better than the Alter shield. quite effective at lower wind speeds
Methods of automated snowpack measurement
- Snow pillows: steel panels plumbed together, underlain with antifreeze solution. Weight of the water in the snow sends a signal to a pressure transducer
- Radioactive: small amount of radioactive salt on ground, and detector somewhere above snow. Snowpack attenuates release of gamma radiation as a function of SWE.
How can we estimate SWE of snowpack in an entire basin?
-could take single point measurement, but that’s not great
Snow survey - field measurements along snow course
-radar
-satellites (microwaves, MODIS)
How to use a snow stake
Put tube into snow until it touches ground, keep snow in tube, and weigh it to find SWE
How does radar sense SWE?
scattering of radar signal affected by pack
How do microwave satellites measure SWE
- intensity of microwave radiation emitted from pack is impacted by snow water content, crystal size, snow depth, temperature, stratification, etc.
- not great for complex topography
What are L-band microwaves sensitive to?
wet snow - snow is invisible to these waves, they try to see water in soil - can get info about whether ground is frozen or not
Ablation
loss of water substance due to melt, evaporation, or sublimation
Phases of the snow season
- Accumulation phase
- Melt phase
- warming phase: energy required to warm snowpack to 0
- ripening phase: energy required to fill pore spaces with liquid water
- output phase: additional energy results in water output
Energy Sources to a snowpack
Shortwave - most important. Cloud cover and melt are highly correlated
Longwave - mostly negative, but can be important in warm temperature with could cover or dense canopy
Sensible heat flux - from earth surface to atmosphere. important over patchy surfaces, early in melt season, eg over a parking lot
Latent Heat flux - (flux energy from Earth surface that carries water) condensation and sublimation effect energy balance
Heat input by Rain- typically not important
soil heat flux - important in early season, but less so after accumulation and during melt
What are the Chinook winds?
Western Canada, come inland. Low humidity, high speed, pick up snow by evaporation and sublimation
-air that heats up from adiabatic compression as it comes off the mountains
How does vapour pressure gradient affect snowpack?
Latent heat exchange
- If low in atmosphere and near ground, snow sublimates and cools
- If high in atmosphere and low on ground, atmospheric vapour deposits onto snowpack and warms (requires turbulent transfer)
Cold and warm rain effects on snowpack
- Warm (10C) rain percolates through snowpack and does not freeze, warms snow slightly
- Cold (0C) rain percolates through pack and freezes, releasing latent heat and warming snowpack
