vic model status blowing snow and lake algorithms princeton meeting december 4, 2006
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Predictive model of the sublimation from blowing snow
SWE > 0?Snow mass
and energy balance
Snowaccumulation
YesBlowingsnow
sublimation
No
ee QMP
dt
dW
vQp
eve QQpMP
dt
dW
• Derived from existing small-scale blowing snow models (Pomeroy et al. 1993 and Liston and Sturm 1998).
• Mass concentration of suspended transport based on power law relationship (Kind 1992).
•Particle sublimation rate proportional to the undersaturation of water vapor.
= VIC snow model
• Soil node temperatures solved via heat diffusion equation (Cherkauer and Lettenmaier 1999)
• Constant flux or constant temperature options
• Imposed temperature distribution at each node allows spatial variation of infiltration capacity and active layer depth across the grid cell (Cherkauer et al. 2001)
On-going work at UW
• Confirmed functionality of constant flux solution
• Revise distribution of soil thermal nodes to improve stability
• Introduce ground ice parameterization
Predicting the effects of lakes and wetlands
• Lake energy balance based on:– Hostetler and Bartlein
(1990)
– Hostetler (1991)
• Lake ice cover (Patterson and Hamblein)
• Assumptions:– One “effective” lake for
each grid cell;
– Laterally-averaged temperatures.
Lake surface energy balance
Mean daily values, June-August 2000
Mean diurnal values, June-August 2000‘Lake 1’, Arctic
Coastal Plain, Alaska
Wetland Algorithm
soilsaturated
land surface runoff &
baseflow enters lake
evaporation depletes soil
moisture
lake recharges
soil moisture
History
• Original model - documented (briefly) in Cherkauer et al. (2003)
• Subsequent revisions (incorporated into VIC 4.1.0 r3 and documented in Bowling et al. manuscript):– Lakes can disappear/reappear– Lake profile description and thermal
solution nodes separated– Lake runoff rate more physically described
Water Table
Previously VIC did not calculate the water table depthAverage depth to water table calculated for each vegetation type Summation of depth of saturated layers and depth of excess soil moisture for unsaturated layer
VIC Simulations
VIC top layer moistureVIC 2nd layer moistureVIC water table
Observations
• Observations show rain pulse penetrating to water table quickly
• Issue of moisture transfer to depth?
or• Lateral inflow
from flooded ditch?
Lateral Exchange
Previously the lake could not recharge the local groundwaterEquilibrium soil moisture is calculated to determine flow directionBaseflow can go either into or out of the lake in a given time step
Baseflow out of lake is at maximum rate
Lake Extent
Previously, maximum water extent fixed by inputs elevation curve supplied for this
wetland fraction only emerging land had static
characteristics never worked with snow bands
Wetland now considered a subset of each vegetation type Same elevation curve applies to all
vegetation classes? Lake area can be calculated
separately for each veg class, or collapsed back to one effective lake
Could be a nightmare to calibrate
Lake extent scenarios
• Three scenarios defined:1) Variable extent/defined
maximum, e.g. as defined by Bowling et al. (2002)
2) Constant extent, as used by Su et al. (2005)
3) Variable extent/unlimited growth
• Maximum depth adjusted such that scenarios 1 and 2 have equal volume
Grid CellFractional Area
Fra
ctio
nal D
epth
Sub-Lake Energy Exchange
Previously, heat fluxes in the soil below the lake were not resolved Normal VIC implementation for
exposed wetland soil (these are values output)
Appropriate soil heat flux algorithm called for sub-lake soil Assumes that soil layers are
preserved under the lake When lake reaches a threshold depth,
energy balance must be solved for combined water/soil layer for stability