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Snow HydrologySnow Hydrology

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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


• 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


• Primary physical characteristics of deposited snow

DensityGrain Size

Grain Shape

Liquid Water Content

Impurities

StrengthHardness

Depth

Water Equivalent

Albedo

Temperature


• 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”


• 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


• 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


• 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


• 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


• 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


• 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


• 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 -


• 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


• Radiation Energy Transfer– Longwave Radiation

• Radiation from the earth and atmosphere• Wavelength 4 - 100 m• Peak Intensity (300 K) ~ 10 - 12 m


• 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)


• 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?


• 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


• 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


• 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


• 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)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


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


OpticallyThick

Clouds

1.6 micron

0.0 0.5 1.0 1.5 2.0 2.5 3.0


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


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


.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


• Snow Water Equivalent (SWE) Analysis


.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)

Snow Hydrology modified from Don Cline for COMET modified from Don Cline for COMET.

Documents

snow hydrology

magnitude of snow

blowing snow slide

wild snow ordinary new

blowing snow effect

snow surface conditions

availability of erodible

dense snow crystal