Vegetation intercepts snow as a function of its winter leaf and stem area and the size of the snowfall event. Because sublimation and melt remove snow from canopies, interception (the snowfall trapped in the canopy, normally event-based) is distinguished from snow load (the snow held in the canopy at a particular time). Hedstrom and Pomeroy (1998) developed and field tested a model of snow interception and snow unloading of the following form (see Fig. 3.3):

ATMOSPHERE

Precipitation rate

CANOPY

* (q)/ Sublimation and evaporation rate

ATMOSPHERE

Precipitation rate

CANOPY

* (q)/ Sublimation and evaporation rate

SNOWPACK

Snow mass per unit area

SNOWPACK

Snow mass per unit area

Figure 3.3. Interception and unloading by vegetation.

where I is snow interception after unloading has occurred (kg m-2), I1 is interception at the start of unloading, UI is a snow unloading coefficient, t is time (s), LL is the maximum canopy snow load, LL,o is the initial snow load, P is the accumulated snowfall (kg m-2) and ccan is the canopy closure.

In a southern boreal forest, Hedstrom and Pomeroy (1998) found that e~Ult = 0.678 using empirical data with a weekly time resolution. LL can be determined using an empirical relationship developed by Schmidt and Gluns (1991), where:

and LLb is the maximum snow load per unit area of branch (kg m-2), ps is the density of fresh snow (kg m-3) and LAI is winter leaf and stem area index (m2 m-2). LLb ranges from 5.9 kg m-2 for spruce to 6.6 kg m-2 for pine. Equation (3.28) is an extension of the expression proposed by Calder (1990), which is based on formulations used for the interception of rainfall. Important differences from earlier formulations are the consideration of canopy snow load in reducing interception efficiency and the substantially high maximum canopy snow loads that are found, compared to maximum rainfall interception. Interception efficiency (I/P) varies from small values up to 0.6 for dense conifer canopies and declines with increasing storm snowfall amounts and initial canopy snow loads.

Snow in the canopy represents a "snow surface" with very different characteristics from snow on the ground (Harding and Pomeroy, 1996). For instance, it is relatively well-exposed to the atmosphere and subject to high net radiation because snow-filled canopies retain their low snow-free albedo (Pomeroy and Dion, 1996; Yamazaki et al., 1996), are aerodynamically rough (Lundberg et al., 1998), and are usually associated with slightly unstable surface boundary layers (Nakai et al., 1999). The varying snow load and degree of its exposure in the canopy means that snow-filled canopies do not behave as a continuous, saturated surface; this affects the free availability of moisture in the canopy. Nakai et al. (1999) accounted for this effect by varying the ratio of bulk transfer coefficients Cq / CH (see Equation 3.12) from 0.1 for the low moisture case (low snow load) to 1.0 for the high moisture case (high snow load). Lundberg et al. (1998) used a Penman-Monteith combination model with a resistance parameterization to estimate sublimation from snow-covered canopies, and found that the resistance of a snow-covered canopy had to be set at 10 times that of a rain-covered canopy to provide results that matched measurements. Parviainen and Pomeroy (2000) use a coupled calculation scheme where turbulent transfer from snow clumps is calculated (presuming no evaporation from the snow-free canopy) to provide a within-canopy humidity and temperature field that is then matched with the bulk transfer formulation for the whole canopy as shown in Equation (3.11). Both sensible and latent heat transfer from snow clumps are calculated from variants of Equation (3.11) where the terms CH and Cq are replaced with Chq and u (z) is replaced with the Sherwood and Nusselt numbers for turbulent transfer of heat and water vapor from particles respectively and solved with the assumption that sublimating snow clumps are in thermodynamic equilibrium at the ice-bulb surface temperature (Schmidt, 1991). Chq is then found as:

where Ll is snow load (equal to initial interception, I1, less any sublimation and unloading), kci is a dimensionless snow clump shape coefficient, rpt,n is a nominal snow particle radius (0.0005 m), p i is the density of ice (kg m-3), and x I is 1.0 less the fractal dimension of intercepted snow (Pomeroy and Schmidt, 1993). Pomeroy et al. (1998a) found x I is normally 0.4 for mature evergreens and Parviainen and Pomeroy (2000) report empirically derived values of kcl (given the nominal rpt,n) of 0.0114 for a mature conifer forest and 0.0105 for a young conifer plantation in western Canada.

ATMOSPHERE

* Precipitation rate

Sublimation rate of blowing snow

ATMOSPHERE

* Precipitation rate

Sublimation rate of blowing snow

Blowing snow transport

SNOWPACK

Snow mass per unit area

Blowing snow transport

SNOWPACK

Snow mass per unit area

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