Aq R Le S G M

where, R = Radiation balance (shortwave and longwave)

LE = Latent heat

S = Sensible heat

G = Ground heat

Positive energy fluxes are defined as going into the snowpack.. When AQ is negative the snowpack is cooling; when AQ positive the snowpack is either warming (when the temperature is below 0°C) or melting (when snow temperature is 0°C). Under typical conditions and for this sensitivity analysis, the ground heat flux is negligible and, because there is no precipitation, the advected heat component is zero.

We perform a series of model runs with a range of albedo treatments, seasonal boundary conditions and atmospheric conditions. The energy fluxes and melt rates for the different conditions and model formulations are then compared to evaluate the impact of different albedo parameterizations.

As input data to drive the model, we use a 12-day hourly meteorological data set collected in New Hampshire during the winter of 1987 (Jordan, 1991). Boundary conditions include air temperature, incoming solar radiation, incoming longwave radiation, relative humidity and wind speed. The model computes the reflected solar radiation based on the albedo parameterization. To simulate spring conditions, we modify the winter values by increasing the incoming shortwave radiation by 15%, daylight duration by 2 hours, and air temperatures (in Kelvin) by 6%. Figure 2 shows the solar irradiance and temperature trends (presented as daily mean values). Days 1-3 in the input data are cloudy and relatively warm, followed by one day of clearing skies and cooler temperatures. Day 5 is cloudy and relatively cold, followed by several days of clear skies and the coldest temperatures.

Figure 2. Daily mean air temperature and solar irradiance used for the winter (solid) and spring (dashed) "mixed" cases of the SNTHERM model runs. These values were computed from the hourly input values that were used in the model run

Table 1 shows the different albedo treatments and seasonal and atmospheric conditions used in this sensitivity exercise. The fixed albedo values all represent broadband albedo. The lowest albedo value is commonly used for melting snow while 0.70, 0.75, and 0.80 are commonly used as values for dry snow. The Marshall (1989) parameterization contains visible (0.4-0.7 |iim) and near-infrared (0.7-2.5 jam) components and computes a as a function of surface grain size, solar zenith angle and diffuse/direct irradiance. Neither soot concentrations nor thin snow cases are considered. Positive feedbacks between snow albedo and air temperature are not included in these experiments. These feedbacks would tend to further enhance the flux effects so the differences described in Section 2.3 should be viewed as something of a lower bound. Initial conditions are identical for the winter and spring scenarios: snowpack temperature -2°C; snow bulk density 250 kg m"2; and snow grain radius 160 |im.

This sensitivity study includes six unique 12-day sets of meteorological conditions: winter/clear, winter/cloudy, winter/mixed, spring/clear, spring/ cloudy, and spring/mixed. To represent clear (cloudy) conditions, observations from Day 9 (Day 3) were replicated for 12 days. "Mixed" scenarios include the unaltered set of meteorological conditions. The cloudy case (Day 3) is representative of a 0.7 fractional mid-level altostratus cloud cover. For each 12-day run, the first two days are used for model spin up. Results shown in Section 2.3 are for the last 10-days of each run.

Table 1. SNTHERM model run treatments_

_Atmospheric conditions:_Clear, Cloudy, Mixed_

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