- Generated on Mon Feb 3 2020 19:58:04 for CLASSIC by
1.8.13
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CLASSIC
Canadian Land Surface Scheme including Biogeochemical Cycles
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Diagnoses snowpack visible and near-IR albedos given the all-wave albedo at the current time step. Calculates snowpack transmissivity for shortwave radiation. More...
Functions/Subroutines | |
| subroutine | snowalbedotransmiss (ALVSSN, ALIRSN, ALVSSC, ALIRSC, ALBSNO, TRSNOWC, ALSNO, TRSNOWG, FSDB, FSFB, RHOSNO, REFSN, BCSN, SNO, CSZ, ZSNOW, FSNOW, ASVDAT, ASIDAT, ALVSG, ALIRG, ILG, IG, IL1, IL2, JL, IALS, NBS, ISNOALB) |
Diagnoses snowpack visible and near-IR albedos given the all-wave albedo at the current time step. Calculates snowpack transmissivity for shortwave radiation.
In subroutine snowAging, called at the end of waterBudgetDriver, the change of total snow albedo over the current time step is calculated using an empirical exponential decay function, which has different coefficients depending on whether the snow is dry or melting. In this subroutine, if the ISNOALB switch is set to 0, the visible and near-IR components of the snow albedo are diagnosed from the total albedo. According to the literature (Aguado, 1985 [2] ; Robinson and Kukla, 1984; Dirmhirn and Eaton, 1975 [33]), the following represent typical snow albedos for fresh snow, old dry snow and melting snow:
\[ \begin{array} { | l | c | c | c | } \hline & \text{Total albedo} & \text{Visible albedo} & \text{Near-IR albedo} \\ \hline \text{Fresh snow} & 0.84 & 0.95 & 0.73 \\ \hline \text{Old dry snow} & 0.70 & 0.84 & 0.56 \\ \hline \text{Melting snow} & 0.50 & 0.62 & 0.38 \\ \hline \end{array} \]
The same decay function is assumed to apply to all three albedo ranges, so the relative location of the visible and near-IR albedos, \(\alpha_{s, VIS}\) and \(\alpha_{s, NIR}\), on the decay curve will be analogous to that of the total albedo, \(\alpha_{s, T}\). Thus, for dry snow:
\([\alpha_{s, VIS} - 0.84]/[0.95-0.84] = [\alpha_{s, T} - 0.70]/[0.84-0.70]\) \([\alpha_{s, NIR} - 0.56]/[0.73-0.56] = [\alpha_{s, T} - 0.70]/[0.84-0.70]\)
or, simplifying:
\(\alpha_{s, VIS} = 0.7857 \alpha_{s, T} + 0.2900\) \(\alpha_{s, NIR} = 1.2142 \alpha_{s, T} - 0.2900\)
For melting snow:
[ \(\alpha_{s, VIS} - 0.62]/[0.95-0.62] = [\alpha_{s, T} - 0.50]/[0.84-0.50]\) [ \(\alpha_{s, NIR} - 0.38]/[0.73-0.38] = [\alpha_{s, T} - 0.50]/[0.84-0.50]\)
or, simplifying:
\(\alpha_{s, VIS} = 0.9706 \alpha_{s, T} + 0.1347\) \(\alpha_{s, NIR} = 1.0294 \alpha_{s, T} - 0.1347\)
The above calculations are performed if the flag IALS is set to zero. If IALS is set to one, indicating that assigned snow albedos are to be used instead of calculated values, \(\alpha_{s, VIS}\) and \(\alpha_{s, NIR}\) are set to the assigned values ASVDAT and ASIDAT respectively. The sub-canopy values of visible and near-IR albedo are currently set equal to the open snowpack values (this is expected to change if a canopy litterfall parametrization is developed).
The transmissivity of snow under vegetation \(\tau_{s, c}\) is then calculated from the snow depth ZSNOW using Beer’s law, with an empirical extinction coefficient of \(25.0 m^{-1}\) derived from the literature (Grenfell and Maykut, 1977 [42] ; Thomas, 1963):
\(\tau_{s, c} = exp[-25.0 z_s]\)
If the ISNOALB switch is set to zero, the value of ALSNO in the first wavelength band is set to the previously calculated value of \(\alpha_{s, VIS}\) and the values for the remaining bands are set to the previously calculated value of \(\alpha_{s, NIR}\); the value of \(\tau_{s, g}\) is set to \(\tau_{s, c}\). If the ISNOALB switch is set to 1, a new parameterization for the snow albedo and transmissivity in four shortwave radiation bands (one visible and three near-IR) is used, according to Cole et al. (2017). This parameterization incorporates the effects of snow grain size and black carbon content, and makes use of lookup tables contained in the CCCma subroutines SNOW_ALBVAL and SNOW_TRANVAL.
| subroutine snowalbedotransmiss | ( | real, dimension(ilg), intent(inout) | ALVSSN, |
| real, dimension(ilg), intent(inout) | ALIRSN, | ||
| real, dimension(ilg), intent(out) | ALVSSC, | ||
| real, dimension(ilg), intent(out) | ALIRSC, | ||
| real, dimension(ilg), intent(inout) | ALBSNO, | ||
| real, dimension(ilg), intent(inout) | TRSNOWC, | ||
| real, dimension (ilg,nbs), intent(out) | ALSNO, | ||
| real, dimension(ilg,nbs), intent(out) | TRSNOWG, | ||
| real, dimension(ilg,nbs), intent(in) | FSDB, | ||
| real, dimension(ilg,nbs), intent(in) | FSFB, | ||
| real, dimension(ilg), intent(in) | RHOSNO, | ||
| real, dimension (ilg), intent(in) | REFSN, | ||
| real, dimension (ilg), intent(in) | BCSN, | ||
| real, dimension (ilg), intent(in) | SNO, | ||
| real, dimension (ilg), intent(in) | CSZ, | ||
| real, dimension (ilg), intent(in) | ZSNOW, | ||
| real, dimension (ilg), intent(in) | FSNOW, | ||
| real, dimension(ilg), intent(in) | ASVDAT, | ||
| real, dimension(ilg), intent(in) | ASIDAT, | ||
| real, dimension (ilg), intent(in) | ALVSG, | ||
| real, dimension (ilg), intent(in) | ALIRG, | ||
| integer, intent(in) | ILG, | ||
| integer, intent(in) | IG, | ||
| integer, intent(in) | IL1, | ||
| integer, intent(in) | IL2, | ||
| integer, intent(in) | JL, | ||
| integer, intent(in) | IALS, | ||
| integer, intent(in) | NBS, | ||
| integer, intent(in) | ISNOALB | ||
| ) |
| [in] | nbs | Number of modelled shortwave radiation wavelength bands |
| [in] | isnoalb | Switch to model snow albedo in two or more wavelength bands |
| [out] | alvssc | Visible albedo of snow on ground under vegetation canopy [ ] |
| [out] | alirsc | Near-IR albedo of snow on ground under vegetation canopy [ ] |
| [out] | alsno | Albedo of snow in each modelled wavelength band [ ] |
| [out] | trsnowg | Transmissivity of snow in bare areas to shortwave radiation \([ ] (\tau_{s,g})\) |
| [in,out] | trsnowc | Transmissivity of snow under vegetation to shortwave radiation \([ ] (\tau_{s, c})\) |
| [in,out] | alvssn | Visible albedo of snow pack on bare ground \([ ] (alpha_{s, VIS})\) |
| [in,out] | alirsn | Near-IR albedo of snow pack on bare ground \([ ] (alpha_{s, NIR})\) |
| [in] | alvsg | Near-IR albedo f bare ground [ ] |
| [in] | alirg | Visible albedo of bare ground [ ] |
| [in] | fsdb | Direct solar radiation in each modelled wavelength band \([W m^{-2}]\) |
| [in] | fsfb | Diffuse solar radiation in each modelled wavelength band \([W m^{-2}]\) |
| [in] | zsnow | Depth of snow \([m] (z_s)\) |
| [in] | fsnow | Fractional coverage of snow on grid cell [ ] |
| [in] | asvdat | Assigned value of visible albedo of snow pack – optional [ ] |
| [in] | asidat | Assigned value of near-IR albedo of snow pack – optional [ ] |
| [in] | refsn | Snow grain size [m] |
| [in] | bcsn | Black carbon mixing ratio \([kg m^{-3}]\) |
| [in] | csz | Cosine of solar zenith angle [ ] |
| [in] | sno | Mass of snow pack \([kg m^{-2}]\) |
| [in] | rhosno | Density of snow pack \([kg m^{-3}]\) |
| [in,out] | albsno | All-wave albedo of snow pack \([ ] (\alpha_{s, T})\) |
Convert the units of the snow grain size and BC mixing ratio Snow grain size from meters to microns and BC from \(kg BC/m^3\) to ng BC/kg SNOW
1.8.13