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- Rename `g_tt` to `g_ttm`
- Fix use of `tt` (`ttm`) for describing snow melt in shared_processes.qmd.
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vers-w committed Dec 3, 2024
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2 changes: 1 addition & 1 deletion docs/model_docs/parameters_vertical.qmd
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Expand Up @@ -40,7 +40,7 @@ the `layered` profile, input parameter `kv` is used, and for the `layered_expone
| **`whc`** | water holding capacity as fraction of current snow pack | - | 0.1 |
| **`w_soil`** | soil temperature smooth factor | - | 0.1125 |
| **`cf_soil`** | controls soil infiltration reduction factor when soil is frozen | - | 0.038 |
| **`g_tt`** | threshold temperature for glacier melt | ᵒC | 0.0 |
| **`g_ttm`** | threshold temperature for glacier melt | ᵒC | 0.0 |
| **`g_cfmax`** | Degree-day factor for glacier | mm ᵒC$^{-1}$ Δt$^{-1}$| 3.0 mm ᵒC$^{-1}$ day$^{-1}$ |
| **`g_sifrac`** | fraction of the snowpack on top of the glacier converted into ice | Δt$^{-1}$ | 0.001 day$^{-1}$ |
| **`glacierfrac`** | fraction covered by a glacier | - | 0.0 |
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40 changes: 20 additions & 20 deletions docs/model_docs/vertical/shared_processes.qmd
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Expand Up @@ -17,18 +17,18 @@ parameter `tti` defines how precipitation can occur partly as rain or snowfall.
# https://gist.github.com/JoostBuitink/21dd32e71fd1360117fcd1c532c4fd9d#file-snowfall_fig-py # hide
-->

If precipitation occurs as snowfall, it is added to the dry snow component
within the snow pack. Otherwise it ends up in the free water reservoir, which represents the
liquid water content of the snow pack. Between the two components of the snow pack,
interactions take place, either through snow melt (if temperatures are above a threshold `tt`)
or through snow refreezing (if temperatures are below threshold `tt`).
If precipitation occurs as snowfall, it is added to the dry snow component within the snow
pack. Otherwise it ends up in the free water reservoir, which represents the liquid water
content of the snow pack. Between the two components of the snow pack, interactions take
place, either through snow melt (if temperatures are above a threshold `ttm`) or through
snow refreezing (if temperatures are below threshold `ttm`).

The respective rates of snow melt and refreezing are:

$$
\begin{align*}
Q_m &=& \subtext{\mathrm{cf}}{max}(T_a−\mathrm{tt})\, &&T_a > \mathrm{tt} \\
Q_r &=& \subtext{\mathrm{cf}}{max} \, \mathrm{cf}_r(\mathrm{tt}−T_a) &&T_a < \mathrm{tt}
Q_m &=& \subtext{\mathrm{cf}}{max}(T_a−\mathrm{ttm})\, &&T_a > \mathrm{ttm} \\
Q_r &=& \subtext{\mathrm{cf}}{max} \, \mathrm{cf}_r(\mathrm{ttm}−T_a) &&T_a < \mathrm{ttm}
\end{align*}
$$

Expand Down Expand Up @@ -61,34 +61,34 @@ into firn/ice (using the HBV-light model) and glacier melt (using a temperature
model).

The definition of glacier boundaries and initial volume is defined by two parameters. The
parameter `glacierfrac` gives the fraction of each grid cell covered by a glacier as a number
between zero and one. The state parameter `glacierstore` gives the amount of water (in mm w.e.)
within the glaciers at each grid cell. Because the glacier store (`glacierstore`) cannot be
initialized by running thFe model for a couple of years, a default initial state should be
supplied by adding this parameter to the input static file. The required glacier data can be
prepared from available glacier datasets.
parameter `glacierfrac` gives the fraction of each grid cell covered by a glacier as a
number between zero and one. The state parameter `glacierstore` gives the amount of water
(in mm w.e.) within the glaciers at each grid cell. Because the glacier store
(`glacierstore`) cannot be initialized by running the model for a couple of years, a default
initial state should be supplied by adding this parameter to the input static file. The
required glacier data can be prepared from available glacier datasets.

First, a fixed fraction of the snowpack on top of the glacier is converted into ice for each
timestep and added to the `glacierstore` using the HBV-light model (Seibert et al., 2018). This
fraction `g_sifrac` typically ranges from $0.001$ to $0.006$.

Then, when the snowpack on top of the glacier is almost all melted (snow cover $<
\SI{10}{mm}$), glacier melt is enabled and estimated with a degree-day model. If the air
temperature, $T_a$, is below a certain threshold `g_tt` ($\SIb{}{\degree C}$) precipitation
occurs as snowfall, whereas it occurs as rainfall if $T_a ≥$ `g_tt`.
temperature, $T_a$, is above a certain threshold `g_ttm` ($\SIb{}{\degree C}$) glacier melt
occurs.

With this the rate of glacier melt in mm is estimated as:

$$
Q_m = \subtext{g}{cfmax}(T_a − \subtext{g}{tt})\, ; \, T_a > \subtext{g}{tt}
Q_m = \subtext{g}{cfmax}(T_a − \subtext{g}{ttm})\, ; \, T_a > \subtext{g}{ttm}
$$

where $Q_m$ is the rate of glacier melt and $\SIb{\subtext{g}{cfmax}}{mm (\degree
C)^{-1}day^{-1}}$ is the melting factor. Parameter `g_tt` can be taken as equal to the snow
`tt` parameter. Values of the melting factor `g_cfmax` normally varies from one glacier to
C)^{-1}day^{-1}}$ is the melting factor. Parameter `g_ttm` can be taken as equal to the snow
`ttm` parameter. Values of the melting factor `g_cfmax` normally varies from one glacier to
another and some values are reported in the literature. `g_cfmax` can also be estimated by
multiplying snow `cfmax` by a factor between 1 and 2, to take into account the higher albedo of
ice compared to snow.
multiplying snow `cfmax` by a factor between 1 and 2, to take into account the higher albedo
of ice compared to snow.

## Rainfall interception
Both the Gash and Rutter models are available to estimate rainfall interception by the
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2 changes: 1 addition & 1 deletion docs/user_guide/model_config.qmd
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Expand Up @@ -18,7 +18,7 @@ water_holding_capacity = "WHC"
glacierstore = "wflow_glacierstore"
glacierfrac = "wflow_glacierfrac"
g_cfmax = "G_Cfmax"
g_tt = "G_TT"
g_ttm = "G_TT"
g_sifrac = "G_SIfrac"

[state.vertical]
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