Initial snow depth in SnowDegreeDay and SnowEnergyBalance

Problem: Show that setting initial snow depth h0_snow has no effect in SnowDegreeDay and SnowEnergyBalance.

Import the Babel-wrapped SnowEnergyBalance and SnowDegreeDay components and create instances:



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from cmt.components import SnowEnergyBalance, SnowDegreeDay
seb, sdd = SnowEnergyBalance(), SnowDegreeDay()

Initialize the components with cfg files that, for simplicity, use the same time step and run duration:



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seb.initialize('./input/snow_energy_balance-1.cfg')
sdd.initialize('./input/snow_degree_day-1.cfg')

Store initial values of model time and maximum snowpack depth from the two components:



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time = [sdd.get_current_time()]
sdd_snow_depth = [sdd.get_value('snowpack__depth').max()]
seb_snow_depth = [seb.get_value('snowpack__depth').max()]

print time, sdd_snow_depth, seb_snow_depth

Advance both models to the end, saving the model time and maximum snowpack depth values at each step:



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while sdd.get_current_time() < sdd.get_end_time():
    seb.update()
    sdd.update()
    time.append(sdd.get_current_time())
    seb_snow_depth.append(seb.get_value('snowpack__depth').max())
    sdd_snow_depth.append(sdd.get_value('snowpack__depth').max())

Check the values:



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print time



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print seb_snow_depth



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print sdd_snow_depth

Here's the key point: the snow depth in each model after the first update is set by the equation on line 506 of snow_base.py. After the first update, the snow depth evolves according to the physics of the component. See:



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rho_H2O = 1000.0

h_swe = sdd.get_value('snowpack__liquid-equivalent_depth').max()
rho_snow = sdd.get_value('snowpack__z_mean_of_mass-per-volume_density').max()
print h_swe * (rho_H2O / rho_snow)  # sdd

h_swe = seb.get_value('snowpack__liquid-equivalent_depth').max()
rho_snow = seb.get_value('snowpack__z_mean_of_mass-per-volume_density').max()
print h_swe * (rho_H2O / rho_snow)  # seb

Plot the snow depth time series output from each component:



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%matplotlib inline
import matplotlib.pyplot as plt



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fig = plt.figure()
ax = fig.add_subplot(1, 1, 1)
ax.plot(time, seb_snow_depth, 'r', time, sdd_snow_depth, 'b')

ax.set_xlabel('Time (s)')
ax.set_ylabel('Snowpack depth (m)')
ax.set_title('Snowpack depth vs. time')

ax.set_xlim((time[0], time[-1]))
ax.set_ylim((0.49, 0.51))

Finalize the components:



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seb.finalize(), sdd.finalize()

Result: Setting the initial snow depth h0_snow in the TopoFlow snow components has no effect. The value is overwritten in the first time step by a value calculated from SWE and the densities of snow and water.