View
231
Download
0
Tags:
Embed Size (px)
Citation preview
Snow HydrologySnow Hydrology
modified frommodified from
Don ClineDon Cline
for COMETfor COMET
modified frommodified from
Don ClineDon Cline
for COMETfor COMET
Why is Snow Important?
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
Snow Hydrology
• Understanding and predicting the physical processes of:
• Snow Accumulation
• Ablation
• Melt Water Runoff
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
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
Snowfall Formation
Snowfall FormationWater Vapor + Nucleus + T<0oC + Saturation
Nucleation
Ice Crystal
Snow Crystal
RimingSublimation Aggregation
Continued Growth
Sublimation Growth
Snow Crystal Formation
SectorPlates
Very Thick Plates
Hollow
SolidPrisms
SolidPrisms
Cups
-10 -20 -30 -40TEMPERATURE (oC)
0
Prisms
0
10
20
30
40
50
60
Dendrite
Sectored Plate
DendriticSectored Plate
Prism(Column)
Needle
A-AxisGrowth
C-AxisGrowth
Snow Cover Distribution
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
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 year-to-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
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
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
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
20-45%Greater SnowAccumulation
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.
Snow Cover Distribution• Open Environments
– Relative accumulation on various landscapes in an open grassland environment
• Normalized to snow accumulation on level plains under fallow
Landscape RelativeAccumulation
Level Plains Fallow 1.00 Stubble 1.15 Pasture (grazed) 0.60Gradual Hill and Valley Slopes Fallow 1.0 – 1.10 Stubble, hayland 1.0 – 1.10 Pasture (ungrazed) 1.25Steep Hill and Valley Slopes Pasture (ungrazed) 2.85 Brush 4.20Ridge and Hilltops Fallow, ungrazed pasture 0.40 – 0.50 Stubble 0.75Small Shallow Drainageways Fallow, stubble, pasture (ungrazed) 2.0 – 2.15Wide Valley Bottoms Pasture (grazed) 1.30Farm Yards Mixed Trees 2.40
Blowing Snow
Blowing Snow
• Two major hydrological influences of wind transport of snow:
Redistribution of Snow Water Equivalent
Loss of Water by Sublimation
Blowing Snow
• Four Factors
1. Shear Velocity
2. Threshold Wind Speed
3. Types of Transport
4. Transport Rates
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, , is equal to pau*2, where pa is the air density and u* is the friction (shear) velocity.
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):
u* =u10 1.18/41.7
u* =u10/26.5
u* =u10 1.30/44.2
Antarctic Ice Sheet
Snow-covered Lake
Snow-coveredFallow Field
u10 = 5 m/s
u* = 0.19
u* = 0.16
u* = 0.18
Blowing Snow
• Threshold Shear Velocity - Snow– u*t is the friction velocity at which snow transport begins
• depends on snow characteristics
0
0.2
0.4
0.6
0.8
1
1.2
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
u*
Antarctic Lake Field
Older, wind-hardened,dense or wet-snow:u*t = 0.25 - 1.0 m/s
Fresh, loose, dry snow,and during snowfall:u*t = 0.07 - 0.25 m/s
Blowing Snow
• Three Types of Transport
Creep
TYPE MOTION HEIGHT WINDSPEED
Saltation
TurbulentDiffusion
Roll
Bounce
Suspended
< 1 cm
1 cm - 10 cm
1 m - 100 m
<< 5 m/s
5 - 10 m/s
> 10 m/s
Blowing Snow
• Transport Rates– Approximately proportional to u10
3
• 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.
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
Blowing Snow
• Sublimation Losses
30
25
252216
225020
Mean Annual Blowing Snow Sublimation
CANADA, 1970-1976Loss in mm SWE over 1 km
Blowing Snow
• Effect on Snow Characteristics– Mechanical fragmentation and sublimation
losses result in small, rounded particles– Windblown snow deposits are inherently more
dense
Snow crystalcollected during
snowfall undercalm winds
Windblown snowparticle collected during transport
2 mm
Blowing Snow
Snow Pack Characteristics
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
Snow Pack Characteristics
• Primary physical characteristics of deposited snow
DensityGrain Size
Grain Shape
Liquid Water Content
Impurities
StrengthHardness
Depth
Water Equivalent
Albedo
Temperature
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)
Density of Snow Cover
Snow Type Density (kg/m3)
Wild Snow
Ordinary new snow immediatelyafter falling in still air
Settling Snow
Average wind-toughened snow
Hard wind slab
New firn snow
Advanced firn snow
Thawing firn snow
10 to 30
50 to 65
70 to 90
280
350
400 to 550
550 to 650
600 to 700
Snow Depth for One Inch Water
98” to 33”
20” to 15”
14” to 11”
3.5”
2.8”
2.5” to 1.8”
1.8” to 1.5”
1.6” to 1.4”
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
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: • bonded, unbonded, bond size, clustered, number
of bonds per grain, oriented texture, arranged in columns
Snow Grain Shapes
Rime on Plate Crystal Early Rounding Faceted Growth Early Sintering (Bonding)
Wind-Blown Grains Melt-Freeze withNo Liquid Water
Melt-Freeze withLiquid Water
Faceted Layer GrowthHollow, Faceted Grain(Depth Hoar)
Electron Microscopyof Snow Crystals
Snow Pack Characteristics
• Grain Size– The average size of the characteristic
grains within a mass of snow• its greatest extension in mm
Term Size (mm)
FineMediumCoarseVery CoarseExtreme
Very Fine < 0.20.2 - 0.50.5 - 1.01.0 - 2.02.0 - 5.0
> 5.0
Snow Pack Characteristics
• Liquid Water Content– Wetness, Percentage by volume
Term Remarks
Moist
Wet
Very Wet
Slush
Dry
Approximate RangeUsually 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.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).
T = 0oC. The water can be pressed out by squeezing snow, but there is an appreciable amount of air (funicular regime).T = 0oC. The snow is flooded with water and contains a relatively small amount of air.
<3%
3-8%
8-15%
>15%
0%
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
Snow Characteristics
• Diurnal Temperature Gradients
0 -5 -10
0
20
40
60
80
100
120
140
Temperature (oC)
EveningDay
TemperatureProfile
Snow Surface
Snow Pack
Ground Surface
Snow Metamorphism
Why snow grains change...
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)
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
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)
Snow Metamorphism• Dry Metamorphism:
– Driven by water vapor movement in pores
– Vapor movement is driven by vapor pressure gradient, controlled by:
• 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
Snow Metamorphism
• Two Types of Dry Metamorphism:– Equitemperature (ET)
• Destructive - destroys crystal structure
– Temperature Gradient (TG)• Constructive - builds grains
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
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
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
Snow Energy Exchanges
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
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
– convection
Snow Energy Exchanges• (K-K) + (L - L) + Qe + Qh + Qg + Qp = Q
Albedo
Humidity
ENERGY
MASS
MELTING
REFREEZING
Snow
Rain
Vapor
Solar
ReflectedSolar
Incident/Emitted
Longwave
Wind
ConductionMelt Flow
CanopyShortwaveReduction
CanopyLongwaveEmissions
CanopyWind
Reduction
Thermally Active Soil Layer
Snow
TurbulentExchange
Solar
Temperature
Atmosphere
K
K
L
Qe Qh
Qg
Qp
L
Q
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
273 K (32oF)Total Energy Emitted: 315 Wm-2
Peak Wavelength: 10.5 m
Snow Energy Exchanges
• Radiation Energy Transfer– Equations and Terms
• Stefan-Boltzmann Law– Total Energy Emitted = T4
» where = emissivity,
» if = 1, referred to as a blackbody
» where = Stefan-Boltzmann constant, and
» where T = Temperature (Kelvin)
• Absorption = Emissivity• Reflectance = 1 -
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
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
Snow Energy Exchanges
• Reflective Properties of Snow
0.0 0.5 1.0 1.5 2.0 2.5 3.0
WAVELENGTH (microns)
0.0
0.2
0.4
0.6
0.8
1.0
r = 0.05 mmr = 0.2 mmr = 0.5 mmr = 1.0 mm
Snow Grain Radius (r)
Snow Energy Exchanges
• Shortwave Radiation Properties of Snow
100
80
60
40
0 5 10 15 20
AccumulationSeason
MeltSeason
Time since last snow fall (days)
100
80
60
40
0 5 10 15 20Summation Tmax since last snow fall (days)
Why does snow albedo decrease over time?
Snow Energy Exchanges
• Atmospheric (Longwave) Radiation
CLOUD, T = 0oC
SNOW, T = 0oC
CLEAR DRY AIR, T = 0oC
Total Energy Emitted = T4
EmissivityAir 0.60 - 0.70Water, Ice, Snow 0.92 - 0.97
Net Energy LossFrom Snow Pack No Net Energy Loss
From Snow Pack
Snow Energy Exchanges
• Atmospheric (Longwave) Radiation
500
400
300
200
0 5 10 15 20
Surface Air Temperature (oC)
25 30-5-10
Relative Humidityat Surface
OvercastSky Radiation
Clear Sky Radiation
Snow Radiation
030100
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
Snow Energy Exchanges
• Turbulent Energy Exchange– Sensible and Latent Heat Fluxes– Boundary layer– Function of wind, temperature, humidity
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
• In both cases, there must be mechanical turbulence to maintain the vapor pressure gradient.
Snow Energy Exchanges• Latent Heat (condensation or sublimation)
– Vapor Pressure Gradients over Snow
0 10 20-10-200
5
10
15
20
Temperature (oC)
Melting
Snow Surface
Saturation vapor pressure of a meltingsnow cover at 0oC is about 6 mb.
x
yy
A
Most of the time the atmosphere is not saturated, and air samples would plotto the right side of the curve (e.g. “A”).
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).
Vapor Pressure at the snow surface is generally at or very near the saturation level.
Snow Energy Exchanges• Latent Heat (condensation or sublimation)
– Are water losses due to sublimation important to snow hydrology?
x
yy
A
Any time the vapor pressure of the air fallswithin the dark blue area, a vapor pressuredeficit exists and sublimation is possible.
0 10 20-10-200
5
10
15
20
Temperature (oC)
Melting
Snow Surface
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)
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.
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 Wm-2
– Very small compared to 800 Wm-2 Incident Solar Radiation!
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:– 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.
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.
0 -5 -10
0
20
40
60
80
100
120
140
Temperature (oC)
EveningDay
TemperatureProfile
Snow Surface
Snow Pack
Ground Surface
If the snow temperature below the surface is less than 0oC, refreezing will occur.
When the snow pack becomes isothermal at 0oC (“ripens”), snowmelt can occur as long as energy is supplied and the snow pack does not cool.
Nightime refreezing of melt water is common due to cooling of the snow pack - results in complex changes to internal energy of snow pack.
IsothermalSnow Pack
Energy Flux Partitioning
Energy Flux Partitioning
Water Flow Through Snow
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
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
• Very efficient mechanism for draining the snow pack
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
Water Flow Through Snow• Flow through Heterogeneous
Snow– Preferential Flow Paths
• Dye studies reveal vertical channels or macropores in most natural snowpacks
– Ice Layers• Develop from surface melt or refreezing• Relatively impermeable• Forces ponding of water and lateral
flow
Ice Lens
Water Flow
Ice Lenswith Ponding
Preferential Flow Paths
Water Flow Through Snow• Liquid Water Transmission
Melt and rain water arelagged and attenuated as they move through the snow cover.
Function of depth, density, ice layers, grain size, and refreezing.
122 123 124 125 126 127 128 129 1300
2
4
6
130 131 132 133 134 135 136 137 1380
2
4
6
138 139 140 141 142 143 144 145 1460
2
4
6
146 147 148 149 150 151 152 153 1540
2
4
6
Snow Melt at SurfaceOutflow from Base
Niwot Ridge, ColoradoMay 2-30, 1995
Day of Year
Rain
Fate of Snowmelt
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.
Snowmelt in this case behaves much like rainfall would.
Surface Melt
Thawed Soil
Fate of Snowmelt
• Depends on slope, snow, and soil conditions
Snowmelt encountering frozensoil at the base of the snow pack, or other impediments to infiltration, may pond at the snow/soil interface.
Surface Melt
Frozen Soil
Ponding
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.
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.
Surface Melt
Thickening of Basal Flow Layer
Snow Measurement
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.
Snow Measurement
Grain Size
Hardness
Density
Depth
Temperature
Chemistry
Stratigraphy
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.
Snow Measurement
• Airborne Snow Survey Program
Natural Gamma Sources
238U Series, 232Th Series, 40K SeriesSoil
Snow
Atmosphere
Radon Daughtersin Atmosphere
Cosmic Rays
Uncollided
Gamma RadiationAbsorbed by Waterin the Snow Pack
Gamma Radiationreaches
Detector in Aircraft
Scattering
Natural Gamma Sources
238U Series, 232Th Series, 40K SeriesSoil
Snow
Atmosphere
Radon Daughtersin Atmosphere
Cosmic Rays
Uncollided
Gamma RadiationAbsorbed by Waterin the Snow Pack
Gamma Radiationreaches
Detector in Aircraft
Scattering
Snow Measurement• Airborne Snow Survey Program
Snow Measurement
• Airborne SWE Measurement Theory– Airborne SWE measurements are made using the following relationship:
SW EA
C
C
M
Mg cm
1 100 1 11
100 1 110
0
2ln ln.
.
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 = Radiation attenuation coefficient in water, (cm2/g)
Snow Measurement
• Airborne SWE: Accuracy and Bias
Airborne measurements include ice and standing water that ground measurements generally miss.
RMS Agricultural Areas: 0.81 cmRMS Forested Areas: 2.31 cm
Airborne Snow Survey Products
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 FROZENMI117 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 90MI123 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 .ENDConditions on the ground observed over the survey area were of completesnow cover with frozen lakes and many frozen rivers. Partially openrivers are noted in the survey line comments.NNNN
Snow Measurement• Satellite Hydrology Program
WAVELENGTH (microns)
WAVELENGTH (microns)AVHRR
GOES
0.0 1.0 4.02.0 3.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0
0.0 1.0 4.02.0 3.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0
AVHRR and GOES Imaging Channels
Snow Measurement• Remote Sensing of Snow Cover
0.0 0.5 1.0 1.5 2.0 2.5 3.0
WAVELENGTH (microns)
0.0
0.2
0.4
0.6
0.8
1.0
AVHRR Ch. 2AVHRR Ch. 1
GOESCh. 1
r = 0.05 mmr = 0.2 mmr = 0.5 mmr = 1.0 mm
Snow Grain Radius (r)
OpticallyThick
Clouds
1.6 micron
0.0 0.5 1.0 1.5 2.0 2.5 3.0
WAVELENGTH (microns)
0.0
0.2
0.4
0.6
0.8
1.0
AVHRR Ch. 2AVHRR Ch. 1
GOESCh. 1
r = 0.05 mmr = 0.2 mmr = 0.5 mmr = 1.0 mm
Snow Grain Radius (r)
OpticallyThick
Clouds
1.6 micron(NOAA 16)
Snow Measurement• NOAA-15 1.6 Micron Channel
Snow Measurement• NOAA-16 1.6 Micron Channel
Visible Channel 1.6 micron Channel
SNOW
Snake River Valley, Idaho
Satellite Hydrology Products• Satellite Areal Extent of Snow Cover
Satellite Hydrology Products
• Snow Cover by Elevation
Satellite Hydrology Products
.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 ezone5AFRA3L 0.0 : AGUA FRIA - ROCK SPRINGS : 2.0- 5.0 5.0- 7.0 : 0.0 0.0ALMA3L 0.0 : ALAMO RESERVIOR : 1.2- 4.0 4.0- 6.6 : 0.0 0.0LKPA3L 0.0 : AGUA FRIA - LAKE PLEASANT : 1.6- 4.0 4.0- 7.0 : 0.0 0.0
• Snow Cover by Basin
Satellite Hydrology Products
• Snow Water Equivalent (SWE) Analysis
Satellite Hydrology Products
.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 ezone5AFMA3 0.0 : AGUA FRIA NR MAYER : 3.6- 5.5 5.5- 7.6 : 0.0 0.0AFPU1 7.8 : AMERICAN FORK NR AMERICAN FORKAFRA3L 0.0 : AGUA FRIA - ROCK SPRINGS : 2.0- 5.0 5.0- 7.0 : 0.0 0.0ALEC2 7.1 : EAST R - ALMONT : 8.0- 9.0 9.0-10.0 10.0-13.1 : 3.8 5.6 8.8
• SWE Analysis by Basin
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)