Transcript No Slide Title

Snow Hydrology
Don Cline
Presented at Hydromet 00-1
Monday, 25 October 1999
National Operational Hydrologic Remote Sensing Center
Office of Hydrology, National Weather Service, NOAA
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Why is Snow Important?
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Why is Snow Important?
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Snowmelt Flooding
• Snowmelt floods are a severe
problem:
– Red River of the North, April 1997
• $4 Billion in Damages
– Northeast Floods, January 1996
• Delaware R., Hudson R., Ohio R., Susquehanna R.,
Potomac R.
• 33 Deaths, $1.5 Billion in Damages
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Snow Hydrology
• Understanding and predicting the
physical processes of:
• Snow Accumulation
• Ablation
• Melt Water Runoff
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Snow Hydrology
• 4 Simultaneous Estimation Problems
– the quantity of water held in snow packs
– the magnitude and rate of water lost to the
atmosphere by sublimation
– the timing, rate, and magnitude of snow melt
– the fate of melt water
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Outline
•
•
•
•
•
•
•
•
•
Snowfall Formation
Snow Cover Distribution
Blowing Snow
Characteristics of Snow Packs
Snow Metamorphism
Water Flow through Snow
Snow Energy Exchanges
Snow Measurement/Remote Sensing
Snow Modeling
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Snowfall Formation
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Snowfall Formation
Water Vapor + Nucleus + T<0oC + Saturation
Nucleation
Ice Crystal
Sublimation Growth
Snow Crystal
Continued Growth
Sublimation
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Riming
Aggregation
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Snow Crystal Formation
A-Axis
Growth
60
C-Axis
Growth
50
Sector
Plates
40
Hollow
Dendrite
30
Prisms
Needle
20
Solid
Prisms
10
Sectored Plate
Very Thick Plates
0
0
-10
Solid Cups
Prisms
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-20
TEMPERATURE (oC)
-30
Prism
(Column)
-40
Dendritic
Sectored Plate
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Snow Cover Distribution
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Snow Cover Distribution
• Three Spatial Scales
– Macroscale
• Areas up to 106 km2
• Characteristic Distances of 10-1000 km
• Dynamic meteorologic effects are important
– Mesoscale
• Characteristic Distances of 100 m to 10 km
• Redistribution of snow along relief features due to wind
• Deposition and accumulation of snow may be related to terrain
variables and to vegetation cover
– Microscale
• Characteristic Distances of 10 to 100 m
• Differences in accumulation result from variations in air flow
patterns and transport
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Snow Cover Distribution
• Effect of Topography
– The depth of seasonal snow cover usually increases with
elevation if other influencing factors do not vary with
elevation
• This trend is generally due to:
– increase in the number of snowfall events
– decrease in evaporation and melt
• The rate of increase with elevation may vary widely from yearto-year
– However, elevation alone is not a causative factor in snow
cover distribution
• Many other factors must be considered:
– slope, aspect, vegetation, wind, temperature, and characteristics
of the parent weather systems
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Snow Cover Distribution
• Effect of Vegetation
– Snow falling into a vegetation canopy is
influenced by two phenomena:
• Turbulent air flow above and within the
canopy
– may lead to variable snow input rates and
microscale variation in snow loading on
the ground
• Direct interception of snow by the canopy
elements
– may either sublimate or fall to the ground
– Processes are related to vegetation type,
vegetation density, and the presence of
nearby open areas
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Snow Cover Distribution
• Forested Environments
– Differences in snow accumulation
between different species of conifers is
usually small compared to between
coniferous and deciduous stands
• coniferous stands are all relatively
efficient snow interceptors
• Once intercepted, cohesion between
snow particles helps keep snow in the
canopy for extended time periods
– snow is more susceptible to sublimation
losses in the canopy than on the forest
floor
» High surface area to mass ratio
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Snow Cover Distribution
• Forested Environments
– Most studies show greater snow
accumulation in clearings than in the
forest
– Most of the difference develops during
storms, not between storms
• redistribution of intercepted snow by
wind to clearings is not typically a
significant factor
– Interception and subsequent sublimation
are the major factors contributing to the
difference
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20-45%
Greater Snow
Accumulation
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Snow Cover Distribution
• Open Environments
– Over highly exposed terrain, the effects of meso- and
micro-scale differences in vegetation and terrain features
may produce wide variations in accumulation patterns.
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Snow Cover Distribution
• Open Environments
– Relative accumulation on
various landscapes in an
open grassland
environment
• Normalized to snow
accumulation on level
plains under fallow
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Landscape
Level Plains
Fallow
Stubble
Pasture (grazed)
Gradual Hill and Valley Slopes
Fallow
Stubble, hayland
Pasture (ungrazed)
Steep Hill and Valley Slopes
Pasture (ungrazed)
Brush
Ridge and Hilltops
Fallow, ungrazed pasture
Stubble
Small Shallow Drainageways
Fallow, stubble, pasture (ungrazed)
Wide Valley Bottoms
Pasture (grazed)
Farm Yards
Mixed Trees
Relative
Accumulation
1.00
1.15
0.60
1.0 – 1.10
1.0 – 1.10
1.25
2.85
4.20
0.40 – 0.50
0.75
2.0 – 2.15
1.30
2.40
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Blowing Snow
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Blowing Snow
• Two major hydrological influences of
wind transport of snow:
Redistribution of Snow Water Equivalent
Loss of Water by Sublimation
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Blowing Snow
• Four Factors
1. Shear Velocity
2. Threshold Wind Speed
3. Types of Transport
4. Transport Rates
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Blowing Snow
• Shear Velocity
– Movement of snow particles occurs when
the drag force exerted on the snow surface
by the wind exceeds the surface shear
strength.
– The total atmospheric shear stress, J, is
equal to pau*2, where pa is the air density
and u* is the friction (shear) velocity.
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Blowing Snow
• Shear Velocity - Wind
– The friction velocity u* is usually calculated
from wind profiles, but can be estimated
from a single 10-m wind speed (u10):
u10 = 5 m/s
Antarctic Ice Sheet
u* =u10/26.5
u* = 0.19
Snow-covered Lake
u* =u10 1.18/41.7
u* = 0.16
Snow-covered
Fallow Field
u* =u10 1.30/44.2
u* = 0.18
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Blowing Snow
• Threshold Shear Velocity - Snow
– u*t is the friction velocity at which snow
transport begins
• depends on snow characteristics
1.2
Fresh, loose, dry snow,
and during snowfall:
u*t = 0.07 - 0.25 m/s
1
0.8
u*
Older, wind-hardened,
dense or wet-snow:
u*t = 0.25 - 1.0 m/s
0.6
0.4
0.2
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21
10-m Wind Speed
Antarctic
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Lake
Field
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Blowing Snow
• Three Types of Transport
TYPE
MOTION
HEIGHT
WINDSPEED
Creep
Roll
< 1 cm
<< 5 m/s
Saltation
Bounce
1 cm - 10 cm
5 - 10 m/s
Turbulent
Diffusion
Suspended
1 m - 100 m
> 10 m/s
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Blowing Snow
• Transport Rates
– Approximately proportional to u103
• Double the wind speed, ~8 times the transport rate
• 4 times the wind speed, ~64 times the transport rate
– Depends on snow surface conditions,
availability of erodible snow, wind
characteristics.
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Blowing Snow
• Sublimation Losses
– Snow particles are more exposed to
atmosphere during wind transport
– Sublimation losses can be very high as a
result
• depends on transport rate, transport distance,
temperature, humidity, wind speed, and solar
radiation
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Blowing Snow
• Sublimation Losses
Mean Annual Blowing Snow Sublimation
CANADA, 1970-1976
Loss in mm SWE over 1 km
22
16
20
25
30
22
50
25
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Blowing Snow
• Effect on Snow Characteristics
– Mechanical fragmentation and sublimation
losses result in small, rounded particles
– Windblown snow deposits are inherently
more dense
Snow crystal
collected during
snowfall under
calm winds
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Windblown snow
particle collected
during transport
2 mm
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Blowing Snow
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Snow Pack Characteristics
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Snow Pack Characteristics
• What is a Snow Pack?
– Porous Medium
• ice + air (+ liquid water)
– Generally composed of layers of different
types of snow
• more or less homogeneous within one layer
– Ice is in form of crystals and grains that
are usually bonded together
• forms a texture with some degree of strength
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Snow Pack Characteristics
• Primary physical characteristics of
deposited snow
Hardness
Water Equivalent
Depth
Density
Temperature
Impurities
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Albedo
Strength
Grain Shape
Grain Size
Liquid Water Content
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Snow Pack Characteristics
• Snow Water Equivalent (SWE)
– The height of water if a snow cover is
completely melted, on a corresponding
horizontal surface area.
• Snow Depth x (Snow Density/Water Density)
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Density of Snow Cover
Snow Type
Density (kg/m3)
Snow Depth for
One Inch Water
Wild Snow
10 to 30
98” to 33”
Ordinary new snow immediately
after falling in still air
50 to 65
20” to 15”
Settling Snow
70 to 90
14” to 11”
Average wind-toughened snow
280
3.5”
Hard wind slab
350
2.8”
New firn snow
400 to 550
2.5” to 1.8”
Advanced firn snow
550 to 650
1.8” to 1.5”
Thawing firn snow
600 to 700
1.6” to 1.4”
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Snow Pack Characteristics
• Grain Shape
– The “Smoking Gun”
– One of the most tell-tale characteristics
that allows inference of snow pack
evolution
– Morphological classification of snow grains
• several have been developed
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Snow Pack Characteristics
• General Attributes of Grain Shape
– Appearance:
• solid, hollow, broken, abraded, partly melted,
rounded, angular
– Surface:
• rounded facets, stepped or striated, rimed
– Interconnections:
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• bonded, unbonded, bond size, clustered,
number of bonds per grain, oriented texture,
arranged in columns
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Snow Grain Shapes
Rime on Plate Crystal
Wind-Blown Grains
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Melt-Freeze with
No Liquid Water
Early Rounding
Melt-Freeze with
Liquid Water
Faceted Growth
Early Sintering (Bonding)
Faceted Layer Growth
Hollow, Faceted Grain
(Depth Hoar)
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Electron Microscopy
of Snow Crystals
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Snow Pack Characteristics
• Grain Size
– The average size of the characteristic
grains within a mass of snow
• its greatest extension in mm
Term
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Very Fine
Fine
Medium
Coarse
Very Coarse
Extreme
Size (mm)
< 0.2
0.2 - 0.5
0.5 - 1.0
1.0 - 2.0
2.0 - 5.0
> 5.0
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Snow Pack Characteristics
• Liquid Water Content
– Wetness, Percentage by volume
Term
Dry
Moist
Remarks
Approximate Range
Usually T < 0oC, but can occur at any temperature up to 0oC.
Little tendency for snow grains to stick together.
T = 0oC. The water is not visible even at 10x magnification.
Has a distinct tendency to stick together.
0%
<3%
Wet
T = 0oC. The water can be seen at 10x magnification by its
miniscus between grains, but cannot be pressed out by
squeezing snow (pendular regime).
3-8%
Very Wet
T = 0oC. The water can be pressed out by squeezing snow,
but there is an appreciable amount of air (funicular regime).
8-15%
T = 0oC. The snow is flooded with water and contains a
relatively small amount of air.
>15%
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Slush
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Snow Characteristics
• Temperature
– Two basic situations:
• Variation in temperature between the top of the
snow pack and the ground
– Temperature Gradient
– Largely determined by thickness of snow pack and
the mean snow surface temperature
» Base of snow pack is usually near 0oC
• No temperature gradient
– Isothermal
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Snow Characteristics
• Diurnal Temperature Gradients
o
Temperature ( C)
0
-5
-10
Snow Surface
140
120
Day
Evening
100
Snow Pack
80
60
40
Temperature
Profile
20
Ground Surface
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0
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Snow Metamorphism
Why snow grains change...
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Snow Metamorphism
• Changes in snow morphology that take
place as a functions of temperature and
pressure
• Factors changed by metamorphism
– density
-- strength
– porosity
-- thermal conductivity
– reflectivity of radiant energy (albedo)
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Snow Metamorphism
• Why does snow undergo metamorphism?
– Close to melting temperature
– Thermodynamically unstable
• large surface to volume ratio, therefore large
surface free energy
– minimum surface to volume ratio is sphere
– Compaction due to overlying layers
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Snow Metamorphism
• Two types of snow metamorphism:
– DRY
• No liquid water present
• Temperature less than 0oC
• Solid state in equilibrium with vapor
– WET
• Liquid water present
• Temperature equal to 0oC (usually)
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Snow Metamorphism
• Dry Metamorphism:
– Driven by water vapor movement in pores
– Vapor movement is driven by vapor pressure
gradient, controlled by:
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• temperature: saturation vapor pressure depends
on temperature; warmer areas can hold more
vapor than colder areas
• radius of curvature: how curved a particular part
of a snow grain is; increased radius of curvature,
increased vapor density
• grain size: decreased grain size, increased radius
of curvature, therefore increased vapor density
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Snow Metamorphism
• Two Types of Dry Metamorphism:
– Equitemperature (ET)
• Destructive - destroys crystal structure
– Temperature Gradient (TG)
• Constructive - builds grains
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Snow Metamorphism
• ET Dry Metamorphism:
• reduces surface free energy to its stable state
• Depends mostly on radius of curvature
– Convex: positive; steeper convexity is higher radius, which
can hold a higher vapor density over it
– Hollows: negative
– Vapor flows along gradient - from points to hollows
• Reduces surface to volume ratio, therefore density
increases (fills pore spaces)
• Structural strength increases (builds bonds)
• Rounds the snow grains
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Snow Metamorphism
• TG Dry Metamorphism:
• Kinetic growth - rate of vapor transport very fast
• Builds angular, faceted grains, with poor bonding
• Resulting strength is poor, density decreases
• Must have temperature gradient of 10oC/m or
greater
• Must have snow density less than 350 kg/m3
– maintain sufficient vapor flow
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Snow Metamorphism
• Wet Snow Metamorphism:
• Liquid water in the snow pack
• Acts like supercharged Dry ET metamorphism
– rates are accelerated
– small grains are destroyed preferentially
– large grains become rounded (equilibrium forms)
• Melting and refreezing results in large, bonded grain
clusters
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Snow Energy Exchanges
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Snow Energy Exchanges
• Energy Transfer Methods
– Radiation
• transfer of energy by electromagnetic waves
– Conduction
• molecule to molecule contact
– Convection
• involves mixing
– Advection
• energy transfer by mass transport
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Snow Energy Exchanges
• Factors contributing to energy transfer
• Wind
– increase wind, increase mixing
– sensible heat exchange
• Water Vapor
– vapor pressure gradient between snow and air
– latent heat exchange
• Radiation (Net)
– shortwave and longwave
• Advected Heat (Rain)
• Soil Contact
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– convection
COMET
Snow Energy Exchanges
• (K\-K[) + (L\ - L[) + Qe + Qh + Qg + Qp = )Q
Atmosphere
Solar
K\
Solar
Incident/
Emitted
Longwave
L\
ENERGY
L[
MASS
Snow
Wind
Reflected
Solar
Qp
Rain
Canopy
Shortwave
Reduction
Canopy
Wind
Reduction
Canopy
Longwave
Emissions
K[
Temperature
Vapor
Humidity
Qe Qh
Turbulent
Exchange
Albedo
Snow
MELTING
Melt Flow
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Thermally Active Soil Layer
)Q
REFREEZING
Conduction
Qg
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Snow Energy Exchanges
• Radiation Energy Transfer
– Basic Principle
• All bodies radiate; as temperature increases,
the energy emitted increases, but the
wavelength at which the peak radiation is
emitted decreases.
310 K (98.6oF)
Total Energy Emitted: 525 Wm-2
Peak Wavelength: 9.28 :m
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273 K (32oF)
Total Energy Emitted: 315 Wm-2
Peak Wavelength: 10.5 :m
COMET
Snow Energy Exchanges
• Radiation Energy Transfer
– Equations and Terms
• Stefan-Boltzmann Law
– Total Energy Emitted = gFT4
»
»
»
»
where g = emissivity,
if g= 1, referred to as a blackbody
where F = Stefan-Boltzmann constant, and
where T = Temperature (Kelvin)
• Absorption = Emissivity
• Reflectance = 1 - g
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Snow Energy Exchanges
• Radiation Energy Transfer
– Shortwave Radiation
• Radiation from the sun - wavelength 0-4 :m
• Visible Range 0.4 - 0.7 :m
– < 0.4 ultraviolet, > 0.7 infrared
• Peak Intensity ~ 0.5 :m
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Snow Energy Exchanges
• Radiation Energy Transfer
– Longwave Radiation
• Radiation from the earth and atmosphere
• Wavelength 4 - 100 :m
• Peak Intensity (300 K) ~ 10 - 12 :m
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COMET
Snow Energy Exchanges
• Reflective Properties of Snow
1.0
Snow Grain Radius (r)
r = 0.05 mm
r = 0.2 mm
r = 0.5 mm
r = 1.0 mm
0.8
0.6
0.4
0.2
0.0
0.0
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0.5
1.0
1.5
2.0
2.5
3.0
WAVELENGTH (microns)
COMET
Snow Energy Exchanges
• Shortwave Radiation Properties of Snow
100
100
80
80
Accumulation
Season
60
60
Melt
Season
40
40
0
5
10
15
Time since last snow fall (days)
20
0
5
10
15
20
Summation Tmax since last snow fall (days)
Why does snow albedo decrease over time?
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COMET
Snow Energy Exchanges
• Atmospheric (Longwave) Radiation
Total Energy Emitted = gFT4
CLOUD, T = 0oC
CLEAR DRY AIR, T = 0oC
Net Energy Loss
From Snow Pack
No Net Energy Loss
From Snow Pack
SNOW, T = 0oC
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Emissivity
Air
Water, Ice, Snow
0.60 - 0.70
0.92 - 0.97
COMET
Snow Energy Exchanges
• Atmospheric (Longwave) Radiation
500
400
Overcast
Sky Radiation
300
100
30 0
Snow Radiation
Relative Humidity
at Surface
200
Clear Sky Radiation
-10 -5
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0
5
10 15 20 25 30
Surface Air Temperature (oC)
COMET
Snow Energy Exchanges
• Turbulent Energy Exchange
– Dominates energy transfer on cloudy and
rainy days
• small shortwave radiation exchanges
• longwave exchanges tend to cancel each other
– A very intense snowmelt usually requires a
large turbulent transfer
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COMET
Snow Energy Exchanges
• Turbulent Energy Exchange
– Sensible and Latent Heat Fluxes
– Boundary layer
– Function of wind, temperature, humidity
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COMET
Snow Energy Exchanges
• Latent Heat (Qe) (condensation or
sublimation)
• function of:
– latent heat of vaporization (Lv)
– vapor pressure gradient
– turbulence
• If the vapor pressure increases with height:
– water vapor is condensed on the snow
– the Lv is released to the snow
• If the vapor pressure decreases with height:
– water vapor is sublimated from the snow
– the Lv is lost from the snow
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• In both cases, there must be mechanical turbulence
to maintain the vapor pressure gradient.
COMET
Snow Energy Exchanges
• Latent Heat (condensation or sublimation)
– Vapor Pressure Gradients over Snow
Vapor Pressure at the snow surface is
generally at or very near the
saturation level.
20
Saturation vapor pressure of a melting
snow cover at 0oC is about 6 mb.
15
Most of the time the atmosphere is not
saturated, and air samples would plot
to the right side of the curve (e.g. “A”).
y
x
If we hold the temperature at point A constant and
increase the water vapor by amount “y”, the air will
saturate (vapor pressure deficit: “drying power relative to
saturated surface”).
If we hold the water vapor at point A constant and
decrease the temperature by amount “x”, the air will
saturate (dew point).
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10
A
Melting
Snow Surface
-20
-10
0
10
5
0
20
Temperature (oC)
COMET
Snow Energy Exchanges
• Latent Heat (condensation or sublimation)
– Are water losses due to sublimation important
to snow hydrology?
20
Any time the vapor pressure of the air falls
within the dark blue area, a vapor pressure
deficit exists and sublimation is possible.
15
In the western U.S., large water losses from
high mountain snow packs due to
sublimation are common.
• Dry Air (large vapor pressure deficits)
• High Winds (lots of turbulence)
y
x
10
A
Melting
Snow Surface
-20
-10
0
10
5
0
20
Temperature (oC)
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COMET
Snow Energy Exchanges
• Sensible Heat (Qh) (convection)
• function of:
– specific heat of the air (Cp)
– air temperature gradient
– turbulence
• If the air temperature increases with height:
– heat is convected to the snow
• If the vapor pressure decreases with height:
– heat is lost from the snow
• In both cases, there must be mechanical turbulence to
maintain the vapor pressure gradient.
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COMET
Snow Energy Exchanges
• Heat Advected by Rain on Snow (Qp)
– First Case
– Rainfall on a melting snow pack, where the rain does not
freeze
• Qp = 4.2TrPr (kJ/m2.d)
– where Tr is the temperature of the rain (oC)
– and Pr is the depth of rain (mm/day)
• If Tr = 2oC and Pr = 2 mm, then Qp = 16.8 kJ/m2.d or 0.19 Wm2
– Very small compared to 800 Wm-2 Incident Solar Radiation!
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COMET
Snow Energy Exchanges
• Heat Advected by Rain on Snow (Qp)
– Second Case
– Rainfall on a cold snow pack (<0oC) where the water freezes
and releases its latent heat of fusion (Lf)
• Freezing exerts a considerable influence on the thermal regime
of the snow pack
– Lf of Water = 335 kJ/kg
– Specific Heat of Snow = 2.09 kJ/(kg.oC)
• For example:
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– 10 mm of rain at 0oC uniformly distributed in a 1-m depth of snow
cover having a density of 340 kg/m3
– Upon refreezing, would raise the average temperature of the snow
pack from -5oC to 0oC.
» Distribution of heat released by refreezing is strongly affected
by the way the water moves through the pack.
COMET
Snow Energy Exchanges
• Internal Energy Exchanges and Snowmelt ()Q)
– Includes changes in phase (melting/refreezing) and
temperature
– Snowmelt typically occurs at the snow surface during the day
when the snow surface temperature reaches 0oC.
If the snow temperature below the surface is less than
0oC, refreezing will occur.
0oC
When the snow pack becomes isothermal at
(“ripens”), snowmelt can occur as long as energy is
supplied and the snow pack does not cool.
o
Snow Surface
Isothermal
Snow Pack
Temperature ( C)
-5
-10
140
120
Day
Evening
100
Snow Pack
Nightime refreezing of melt water is common due to
cooling of the snow pack - results in complex changes
to internal energy of snow pack.
80
60
40
Temperature
Profile
20
Ground Surface
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0
0
COMET
Energy Flux Partitioning
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COMET
Energy Flux Partitioning
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COMET
Water Flow Through Snow
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COMET
Water Flow through Snow
• Wide Range of Flow Velocities
– 2 - 60 cm/min
– Depends on several factors
• internal snow pack structure
• condition of the snow pack prior to introduction of
water
• amount of water available at the snow surface
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COMET
Water Flow Through Snow
• Flow through
Homogeneous Snow
– At melting temperature, a thin
film of water surrounds each
snow grain
• Much of the water can flow
through this film
– Once pores are filled, laminar
flow can occur
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• Very efficient mechanism for
draining the snow pack
COMET
Water Flow through Snow
• Four Liquid Water Regimes
• Capillary: < 1% free water
– water doesn’t drain due to capillary tension
• Unsaturated: 1-14% free water
– water drains by gravity, but air spaces are continuous
– Pendular Regime
• Saturated: > 14% free water
– water drains by gravity, but air spaces are discontinuous
– Funicular Regime
• Melt/Freeze
– water melts and refreezes, possible several times, before it
drains from the snow pack
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COMET
Water Flow Through Snow
• Flow through
Heterogeneous Snow
– Preferential Flow Paths
Ice Lens
with Ponding
• Dye studies reveal vertical
channels or macropores in
most natural snowpacks
– Ice Layers
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Preferential Flow Paths
• Develop from surface melt or
refreezing
• Relatively impermeable
• Forces ponding of water and
lateral flow
Water Flow
Ice Lens
COMET
Water Flow Through Snow
• Liquid Water Transmission
Niwot Ridge, Colorado
May 2-30, 1995
Melt and rain water are
lagged and attenuated
as they move through
the snow cover.
Function of depth,
density, ice layers, grain
size, and refreezing.
NOHRSC
6
4
2
0
122
6
4
2
0
130
6
4
2
0
138
6
4
2
0
146
Snow Melt at Surface
Outflow from Base
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
Day of Year
151
152
153
154
Rain
COMET
Fate of Snowmelt
NOHRSC
COMET
Fate of Snowmelt
• Depends on slope, snow, and soil
conditions
Snowmelt encountering thawed,
permeable soil at the base of the
snow pack, at a rate less than
the infiltration rate, will enter the
soil.
Surface Melt
Snowmelt in this case behaves
much like rainfall would.
Thawed Soil
NOHRSC
COMET
Fate of Snowmelt
• Depends on slope, snow, and soil
conditions
Surface Melt
Snowmelt encountering frozen
soil at the base of the snow pack,
or other impediments to
infiltration, may pond at the
snow/soil interface.
Ponding
Frozen Soil
NOHRSC
COMET
Fate of Snowmelt
• Basal Ice Development
On shallow slopes, ponded
meltwater may refreeze at the
base of the pack, forming ice
layers that may impede further
meltwater infiltration.
NOHRSC
COMET
Fate of Snowmelt
• Subnivean Flow on a Slope
Lateral flow of basal ponded
water may develop, depending
on slope. If snow is still present,
lateral flow is still through a
porous medium. Presence of
liquid water in base of snow pack
causes rapid destruction of small
snow grains, leaving larger
grains, and allowing more rapid
flow.
NOHRSC
Surface Melt
Thickening of Basal Flow Layer
COMET
Snow Measurement
NOHRSC
COMET
Snow Measurement
• Ground Observations
– Snow Water Equivalent (SWE)
• Snow Pillows
– SNOTEL Sites (Western U.S.)
• Snow Courses
– Transects with snow depth and density
• Snow Tubes/Cutters
– measure volume and mass of snow cores
• Snow Pits
– Measure vertical profiles of SWE, and other snow
pack variables.
NOHRSC
COMET
Snow Measurement
Grain Size
Hardness
Temperature
Stratigraphy
Depth
Density
Chemistry
NOHRSC
COMET
Snow Measurement
• Airborne Snow Survey Program
– Snow Water Equivalent (SWE) estimated
from attenuation of naturally occurring
terrestrial gamma radiation.
• Typical flight line is 16 km long, measuring a ground
swath 3000 m wide.
– Measures average SWE over area of ~5 km2
• 1800 flight lines throughout coterminous U.S.
• Two twin-engine aircraft fly ~900 lines/year.
NOHRSC
COMET
Snow Measurement
• Airborne Snow Survey Program
Atmosphere
Cosmic Rays
Radon Daughters
in Atmosphere
Gamma Radiation
reaches
Detector in Aircraft
Uncollided
Scattering
Gamma Radiation
Absorbed by Water
in the Snow Pack
Natural Gamma Sources
238
U Series,
NOHRSC
232
Th Series, 40K Series
Snow
Soil
COMET
Snow Measurement
• Airborne Snow Survey Program
NOHRSC
COMET
Snow Measurement
• Airborne SWE Measurement Theory
– Airborne SWE measurements are made
using the following relationship:
 100  111
1  C0
. M 
  g cm 2
SWE   ln
 ln 
A C
. M0  
 100  111
Where:
C and C0 = Uncollided terrestrial gamma count rates over snow
and dry, snow-free soil,
M and M0 = Percent soil moisture over snow and dry, snow-free soil,
A
NOHRSC
= Radiation attenuation coefficient in water, (cm2/g)
COMET
Snow Measurement
• Airborne SWE: Accuracy and Bias
Airborne measurements include ice
and standing water that ground
measurements generally miss.
RMS Agricultural Areas: 0.81 cm
RMS Forested Areas: 2.31 cm
NOHRSC
COMET
Airborne Snow Survey Products
NOHRSC
COMET
Airborne Snow Survey Products
.B GAMMA 990120 /SAIRF/SWIRF
:TO ------ Service Hydrologist (Please give HARDCOPY to SH)
:FROM ---- Tom Carroll, (612) 361-6610 ext 225, Minneapolis, Minnesota
:Visit our web page at www.nohrsc.nws.gov
:SUBJECT - AIRBORNE SNOW WATER EQUIVALENT DATA
990120222453
:----------------------------------------------------------------------: Total No. of flight lines sent = 10
:----------------------------------------------------------------------:Line
Survey
%SC
SWE
SWE %SM Est Fall %SM Pilot
:No.
Date
(in) (35%) (M) Typ Date (F) Remarks
:=======================================================================
MI113 DY990120 / 100 / 1.8 : 1.2, 25 SE
0 , 25 OLD CRUSTY SNOW
MI114 DY990120 / 100 / 2.3 : 1.7, 25 SE
0 , 25
MI115 DY990120 / 100 / 0.8 : 0.3, 25 SE
0 , 25 TOWN LINE RVR FRZ
MI116 DY990120 / 100 / 0.7 : 0.2, 25 SE
0 , 25 HOUGHTON LAKE FROZEN
MI117 DY990120 / 100 / 1.8 : 1.3, 25 SE
0 , 25
MI118 DY990120 / 100 / 1.6 : 1.0, 25 SE
0 , 25
MI121 DY990120 / 100 / 1.6 : 1.0, 25 SE
0 , 25 MUSKEGON RVR OPEN 90
MI123 DY990120 / 100 / 1.8 : 1.3, 25 SE
0 , 25
MI124 DY990120 / 100 / 1.9 : 1.4, 25 SE
0 , 25 TWIN RVR PRTLY OPN
MI138 DY990120 / 100 / 3.1 : 2.6, 25 SE
0 , 25
.END
Conditions on the ground observed over the survey area were of complete
snow cover with frozen lakes and many frozen rivers. Partially open
rivers are noted in the survey line comments.
NNNN
NOHRSC
COMET
Snow Measurement
• Satellite Hydrology Program
AVHRR
0.0
1.0
2.0
3.0
WAVELENGTH (microns)
4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0
0.0
1.0
2.0
3.0
4.0 5.0
GOES
NOHRSC
6.0
7.0
8.0
9.0 10.0 11.0 12.0 13.0
WAVELENGTH (microns)
AVHRR and GOES Imaging Channels
COMET
Snow Measurement
• Remote Sensing of Snow Cover
AVHRR Ch. 1
AVHRR Ch. 2
1.6 micron
(NOAA 16)
1.0
Snow Grain Radius (r)
r = 0.05 mm
r = 0.2 mm
r = 0.5 mm
r = 1.0 mm
0.8
0.6
Optically
Thick
Clouds
0.4
0.2
0.0
0.0
GOES
Ch. 1
0.5
1.0
1.5
2.0
2.5
3.0
WAVELENGTH (microns)
NOHRSC
COMET
Snow Measurement
• NOAA-15 1.6 Micron Channel
NOHRSC
COMET
Snow Measurement
• NOAA-16 1.6 Micron Channel
Snake River Valley, Idaho
SNOW
Visible Channel
NOHRSC
1.6 micron Channel
COMET
Satellite Hydrology Products
• Satellite Areal Extent of Snow Cover
NOHRSC
COMET
Satellite Hydrology Products
• Snow Cover by Elevation
NOHRSC
COMET
Satellite Hydrology Products
• Snow Cover by Basin
.BR MSP 990121 DM012018 DC01212234 /SAIPZ
:---------------------------------------------------------------------:National Weather Service - Office of Hydrology
:National Operational Hydrologic Remote Sensing Center
:Chanhassen, Minnesota
(612) 361-6610
:---------------------------------------------------------------------:Satellite Areal Extent of Snow Cover (percent), Elevation Zones (1000ft)
:Composite Analysis 9901181615 - 9901211830
:
:BASIN
SA
Name
:
ezone1
ezone2
ezone3
ezone4
ezone5
AFRA3L
0.0 : AGUA FRIA - ROCK SPRINGS
: 2.0- 5.0 5.0- 7.0
:
0.0
0.0
ALMA3L
0.0 : ALAMO RESERVIOR
: 1.2- 4.0 4.0- 6.6
:
0.0
0.0
LKPA3L
0.0 : AGUA FRIA - LAKE PLEASANT
: 1.6- 4.0 4.0- 7.0
:
0.0
0.0
NOHRSC
COMET
Satellite Hydrology Products
• Snow Water Equivalent (SWE) Analysis
NOHRSC
COMET
Satellite Hydrology Products
• SWE Analysis by Basin
.BR MSP 990122 DM012018 DC01220349 /SWIPZ
:---------------------------------------------------------------------:National Weather Service - Office of Hydrology
:National Operational Hydrologic Remote Sensing Center
:Chanhassen, Minnesota
(612) 361-6610
:---------------------------------------------------------------------:Estimated Snow Water Equivalent (inches), Elevation Zones (1000ft)
:Composite Analysis 9901190000 - 9901212400
:
:BASIN
SW
Name
:
ezone1
ezone2
ezone3
ezone4
ezone5
AFMA3
0.0 : AGUA FRIA NR MAYER
: 3.6- 5.5 5.5- 7.6
:
0.0
0.0
AFPU1
7.8 : AMERICAN FORK NR AMERICAN FORK
AFRA3L
0.0 : AGUA FRIA - ROCK SPRINGS
: 2.0- 5.0 5.0- 7.0
:
0.0
0.0
ALEC2
7.1 : EAST R - ALMONT
: 8.0- 9.0 9.0-10.0 10.0-13.1
:
3.8
5.6
8.8
NOHRSC
COMET
Snow Modeling
• Point Models
– Degree Day Methods
– Semi-Physical Methods (e.g. SNOW-17)
• Distributed Models
– Physically Based
– Gridded or Polygon Discretization
– Assimilation Systems (e.g. SNODAS)
NOHRSC
COMET
Snow Hydrology
E-mail
[email protected]
NOHRSC
WWW
http://www.nohrsc.nws.gov
COMET