Snow and Vegetation
J h P
John
Pomeroy
Canada Research Chair in Water Resources and Climate Change
Centre for Hydrology
y
gy
University of Saskatchewan,
Saskatoon, Canada
Snow Dynamics
z
z
z
Snowfall
Accumulation and
Redistribution by
Wind and
Vegetation
Snow Ablation by
melt & sublimation
Forest and shrub
effects on
accumulation and
melt
Snow Regimes
F
Forest
t Snow
S
–
O
Open
Snow
S
Blowing Snow Transport Over Complex
T
Terrain
i
QSUBLIM
WINDWARD HILL,
BARE GROUND,
GROUND
GRASS
QTRANSP
LEEWARD HILL,
FOREST
SHRUB,
DEPRESSION
BLOWING SNOW
IF CAPACITY/THRESHOLD
IS EXCEEDED
WINDWARD
HILL
LEEWARD
HILL
QSUBLIM
QSUBLIM
QTRANSP
Research Basins:
• Wolf Creek
• Marmot Creek
TOPOGRAPHIC
DEPRESSION
BARE
GROUND
QTRANSP
SHRUB
GRASS
FOREST
Linear simulation of westerly flow over Wolf Creek, Yukon
Windspeed
Direction
3000
2500
2000
1500
1000
500
0
0
3 km
Essery and Pomeroy, in preparation
500
1000
1500
2000
2500
3000
Simulation of Hillslope Snowdrift
3 km
Distributed Blowing Snow Model
Snowdrift Simulation
200
SWE (m
mm)
Observed
100
50
0
1420
Eleva
ation (m)
Simulated
150
0
100
200
300
400
300
400
H i
Horizontal
t l di
distance
t
((m))
1400
1380
1360
1340
0
100
200
Horizontal distance (m)
Snowmelt Rate
z
z
z
At a p
point controlled by
y
energy inputs, snow
internal energy and
available snow mass
Over some area controlled
by the spatial distribution
of snow mass (including
snow covered area) and
energy inputs
Strong influence of
topography and vegetation
Coupled Mass and Energy
Equations
E
ti
for
f Snowmelt
S
lt
z
MELT of SWE
SWE, S = QM / λf
z
Melt Energy
gy QM =
Q* + QH – QE + QP – QG – dU/dt
z
z
z
z
z
z
z
Q* Net radiation (+ to snow surface) (=K↓
(=K↓-K↑+L↓K↑+L↓-L↑)
QH Sensible energy (+ towards snow surface)
QE Latent energy
gy (+
( awayy from snow surface))
QP Advected energy from precipitation (+ to snow)
QG Ground heat flux (+ downward from snow)
dU/dt Internal energy change (+ loss from melt)
λf latent heat of fusion
Real World Snowpacks
Snow Exchange Realistic View
z
z
Longwave exchange
only at surface
Penetration of
shortwave
K↓
K↑
L↓
P i it ti
Precipitation
L↑
QH
QA
z
Air flow through
porous structure
z
z
z
Convection
Wind pumping
Advection of energy
from bare ground,
plant stalks
Ti
QE
Ts
SNOWPACK
QG
dU/d
dU/dt
Melt
Some Problems with Snowmelt
Calculations
z
z
z
z
z
z
Heterogeneity of atmospheric and radiative exchange
Longwave exchange occurs at snow surface, shortwave
exchange occurs in upper snow layers.
Where are atmospheric exchanges occurring? Not only
the snow surface
Coupling between snow surface and internal energetics
poorly understood.
Mixture of “wet”
wet snow preferential flow zones and impeding
layers (~0 oC) and “cold” snow zones (<0 oC)
Difficulty in coupling snow and frozen ground energetics,
phase change and mass transfer
f
Turbulence generation
mechanisms in mountains
upper level winds
strong shear zone
transported turbulence
valley winds
tributary valley
winds
surface winds (internal B-L)
flux tower in clearing
QH = -ρ cp (Ts-Ta) f(u)
QE = -ρ λs (qa-q*(Ts)) f(u), where surface saturated is at Ts
Slope Forest Transmissivity
τ a function of
LAI,
Foliage inclination
C
Crown
coverage
Slope,
Aspect,
Solar azimuth,
Solar elevation
South Face Forest
50
100
150
200
250
azimuth angle
300
350
25
40
55
70
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
10
Model of solar
radiation
transmission
through
continuous
evergreen
canopy on
slopes
elevation anglle
70
25
40
55
x
01
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
10
elevation anggle
85
85
North Face Forest
50
100
150
200
250
azimuth angle
300
350
Incoming Longwave in Mountains
30
Percent increase in
longwave irradiance due
to terrain emission due to
sky view factor (Vf) and
surface temperature (Ts).
25
Ts (°C)
20
Air temperature is 0°C
and the clear sky
emissivity is 0.65
15
10
5
0
0.5
0.6
0.7
0.8
0.9
1
Sk View
Sky
Vi
F
Factor
t Vf
Sicart et al. 2006 Hydrological Processes
Thermal IR Image
Wolf Creek shrrubs
Marmot Creek leve
el forest
Solar radiation to snow beneath shrubs and trees
1000
400
SW (W/m )
300
2
2
SW (W//m )
800
600
400
200
0
123
200
100
0
124
125
74
Day (2003)
Pomeroy et al., J Hydrometeorol, 2008
75
Day (2005)
76
Modelling Sub-alpine Shortwave Radiation
Lidar and canopy delineation
Shadow simulation
Essery et al. (2007). Journal of Hydrometeorology.
Upscaling Sub-alpine Shortwave Radiation
Simulated skyview
Simulated skyview
Clear
600
Overcast
Measured
2
SW (W/m )
500
Modelled
400
300
200
100
0
84
85
Day
y (2003)
(
)
Essery et al. (2007). Journal of Hydrometeorology.
86
Hot Trees
Longwave Exitance Pine Stand
350
330
Exiitance W/m²²
310
290
270
250
Needles
230
Trunk
210
190
Sky
170
Air
150
74
74.5
75
75.5
76
Julian Day
76.5
77
77.5
78
LiDAR
Marmot Creek “honeycomb” clearings and spruce forest
Valley
400
0.8
300
0.6
200
100
0.4
0
0.2
-100
-200
105
110
115
120
Day
125
130
135105
110
115
120
Day
125
130
0
135
Snow depth
h (m)
2
Sensible Heatt (W/m )
S
Plateau
How Well Do Models Simulate Snow?
Snowmip2 http://xweb.geos.ed.ac.uk/~ressery/SnowMIP2/results.html