Lecture Panel 6

Question 1

Why is snowmelt important?

Answer 1

in many mountainous regions, can be 75% of water supply-

Question 2

What does snow cover due to sunlight?

Answer 2

reflects it, ie reflects heat

• fresh fallen snow albedo 0.8
• trees 0.3 or 0.2

Question 3

where is planetary albedo highest?

Answer 3

north and sahara (in winter)

avg 0.3

Question 4

how does pollution affect snow?

Answer 4

makes it less bright and thus easier to melt

Question 5

what happens to snow as it sits on the ground?

Answer 5

turns granular, albedo drops to 0.6

-gets easier to melt as albedo drops because more radiation is absorbed

Question 6

What disaster can snowmelt cause?

Answer 6

floods

Question 7

What can snowcover knowledge help predict?

Answer 7

• climate

- using correct initial conditions of snowpack increases predictive value of climate estimations

Question 8

Critical properties of snow

Answer 8

Low thermal conductivity, strong insulation

-keeps soil somewhat warm - snow can be modelled as a series of layeres, each with its own temp

Question 9

Result of snow as an insulator

Answer 9

diurnal fluctuation of temp doesn’t occur at depth

-keeps ground relatively unfrozen - helps prevent flood (unfrozen ground can absorb more water)

Question 10

Where does it snow globally?

Answer 10

virtually all land areas above 40 degrees latitude

Question 11

describe lake effect snow

Answer 11

• 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

Question 12

Where are snowbelts relative to great lakes?

Answer 12

eastern sides mostly, due to prevailing west winds (blowing to the east)

Question 13

What type of precipitation is more likely at altitude?

Answer 13

snow - more snow-water equivalent at altitude

Question 14

Canopy effects on snow

Answer 14

• snow can be intercepted by tress
• wind speed and surface roughness
• surface energy exchange
• albedo of nearby trees can affect snow pack

Question 15

where are the highest snow depth found?

Answer 15

clearings with diameters less than 20x surrounding tree height

Question 16

What is a good forest environment for snow accumulation?

Answer 16

-open deciduous forest

Question 17

How would snow accumulation differ around a deciduous vs coniferous tree?

Answer 17

conifers have large amount of snow in tress, less on ground, deciduous have a deep snow pack on ground with little interception

Question 18

Why would a clearing have more snow? How much more?

Answer 18

snow blows into clearing from surrounding trees, 20-45% greater accumulation.

Question 19

Wind effects on snow pack

Answer 19

-redistributes, changes nature and density of pack. Windy areas would have less accumulation in a field, for example.

Question 20

how do slope and aspect affect snow accumulation?

Answer 20

-on a steep slope nothing can accumulate. South facing slopes get sun to melt snow

Question 21

Snow water equivalent

Answer 21

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

Question 22

typical snow density

Answer 22

0.07 to 0.15 kg/m^3 for freshly fallen snow

Question 23

How does snow density change over season? What are density characteristics

Answer 23

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

Question 24

Snowpack characteristics over time

Answer 24

• 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

Question 25

What are problems with gauges and snow? Solutions?

Answer 25

• 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

Question 26

Methods of automated snowpack measurement

Answer 26

• 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.

Question 27

How can we estimate SWE of snowpack in an entire basin?

Answer 27

-could take single point measurement, but that’s not great
Snow survey - field measurements along snow course
-radar
-satellites (microwaves, MODIS)

Question 28

How to use a snow stake

Answer 28

Put tube into snow until it touches ground, keep snow in tube, and weigh it to find SWE

Question 29

How does radar sense SWE?

Answer 29

scattering of radar signal affected by pack

Question 30

How do microwave satellites measure SWE

Answer 30

• 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

Question 31

What are L-band microwaves sensitive to?

Answer 31

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

Question 32

Ablation

Answer 32

loss of water substance due to melt, evaporation, or sublimation

Question 33

Phases of the snow season

Answer 33

1. Accumulation phase
2. 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

Question 34

Energy Sources to a snowpack

Answer 34

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

Question 35

What are the Chinook winds?

Answer 35

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

Question 36

How does vapour pressure gradient affect snowpack?

Answer 36

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)

Question 37

Cold and warm rain effects on snowpack

Answer 37

• 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