Lecture11
Learning objectives
- Explain what happens to the density of a snowpack over the course of a snow season
- Discuss the phases of snowmelt and explain what it means for snow to be isothermal
- Compare the factors that influence snow accumulation in an open site vs. a forested site, and between different forest types
- Illustrate the direction (positive/negative) and relative magnitude of local energy exchanges on the snowpack under various weather conditions
- Compare the factors that influence snow ablation in an open site vs. a forested site, and between different forest types
- Explain which components of the local energy balance would be most adversely affected by forest harvesting
- Describe the radiative paradox and where it typically occurs
- Calculate SWE from field snow survey data
- Calculate and compare ablation rates for an open and forested site
- Define with your own words, and put into context, all the terms in the glossary at the end
Snow processes
Snow accumulation & ablation within water & energy balances
Processes at the vegetation layer (winter)
Inputs
- Precipitation (snowfall)
- Direct precipitation
- Throughfall
- Drip
- Stemflow
- Midwinter Melting
Outputs
- Canopy interception
- Transpiration
- Sublimation
- Wind redistribution
Processes at the vegetation layer (spring)
Inputs
- Drip
- Stemflow
- Snowmelt
Outputs
- Canopy interception
- Transpiration
- Sublimation
- Evaporation
Processes at the ground layer (spring)
- Rain on snow
- Snowmelt
- Infiltration
- Lateral flow
- Percolation
- Overland flow
- Subsurface flow
- Streamflow
- Groundwater Recharge
- Groundwater Discharge
Fundamentals of snow
Importance
- on average, 60% of Northern Hemisphere has snow in mid-winter
- Major source of drinking water in Canada
- Snow is typically in mountains and so are most of our forests
- BC -- flooding (rain on snow) resulting in increased erosion rates and structural damages
Definitions
- Snowpack: accumulated snow on the ground at a time of measurement
- Snowfall: precipitation that falls as snow
- Snow water equilavent (SWE) amount of liquid water present in a snowpack
- Peak SWE (maximum SWE recorded prior to melting)
- Snow depth (d) thickness of a snowpack [m]; perpendicular to mean sea level
- Snow density: mass of snow in a given volume
- Snow cover: fraction of area covered by snow [%]
- Snow melt: change of snow to liquid water resulting from energy inputs [mm / day]
- Snow ablation: loss of mass from a snowpack by the combination of wind transport, sublimation, and/or melt [mm / day]
General
- New snowfall typically has a density of around 100 kg / m³
- Winter snowpack's typically have mean densities between 200 and 300 kg m³
- The density of a snowpack reflects the characteristics of the various snowfall events and various processes such as snow compaction and snow melt and refreeze cycles
- SWE can be calculated by knowing snow density and depth
Phases of snowmelt
- warming phase: Steady temp increase until snowpack is isothermal at 0C
- Ripening phase: Melting occurs but water is retained in snowpack until Isothermal (0C) snowpack at the end of the phase cannot retain more liquid water.
- Output phase: further inputs of energy produce water output.
Processes might not be steady; snow melting in the surface releases water that percolates through the snowpack, refreezes, and raises temperature by releasing latent heat.
Snowpack temperature varies during the day.
Snowpack is ripe when it is primed to release liquid water: when snowpack temperature is 0C and the liquid-water-holding capacity of the snowpack has been reached. Any additional input of either energy or liquid water will then be released from the bottom of the pack.
Factors influencing snowpack
- Energy: Higher energy == higher melting
- Slope: There is a larger snowpack in the North, because the South slope has greater shortwave energy from the sun and a higher melt-rate
- Logging: There is more shortwave energy entering the snowpack
because of less cover from shortwave radiation from vegetation,
trees, etc. decreasing overall snowpack .
- When combined with slope, we can see that there will be even greater impact on the South slope.
- Vegetation: Canopy density & type also impacts snowpack. There is more energy under spruce than there is under pine. Likely to be more energy under deciduous than coniferous trees.
Ablation
- As forest cover decreases, interception decreases (more accuulation) and melt rates increase (faster snow dissapearance).
- Cases where greater melt rates occur under a denser forest have been
termed "radiative paradox", where increases in long-wave radiation
with increasing canopy cover are greater than corresponding
decreases in shortwave radiation.
- These conditions are likely to occur with low atmospheric emissivities, high snow albedos, and low solar elevations, are more common on shaded, poleward facing slopes.
R_(n) = K↓ - K↑ + L↓ - L↑
- Forest cover increases >>> K* decreases >>> L↑ loss decreases >>> L↓ gain from the forest canopy increases.
- R_(n) is minimum at around 20% canopy cover, but increases at dense forest canopy conditions because of the much higher L↓ radiation gains emitted by the forest canopy.
- R_(n) is still highest at 0% forest cover (open) with large input from K* radiation.
Snow measurements
- Lysimeters
- Ultrasonic snow depth sensors
- Snow pillows
- Remote sensing
- Satellite optical measurements -- snow cover
- Satellite passive microwave -- SWE
- Satellite active microwave -- SWE
- Hyperspectral radiometers -- snow grain size
- Airborne gamma -- SWE
- Airborne LiDAR -- Snow depth
- Satellite optical measurements -- snow albedo
- Limitations:
- Low spatial resolutions
- High cost
- Specializing processing (LiDAR or microwave)
- Low accuracy (gross empirical indirect estimations) E.g., SWE from microwave