Snow model¶
SnowMapper uses the Factorial Snow Model, FSM1 (Essery, 2015), in an in-house Rust implementation (fsm-rs) that matches the reference Fortran code. FSM is an energy-balance model: it computes every heat and water flux in and out of the snowpack, hour by hour.
What the model computes¶
flowchart LR
SW["Sunshine × (1 − albedo)"] --> E((Energy<br/>balance))
LW[Longwave in − out] --> E
H["Sensible + latent heat<br/>from air and wind"] --> E
G[Ground heat] --> E
E -->|surplus| ML[Melt]
E -->|deficit| C[Cooling / refreeze]
SF[Snowfall] --> PK[(Snowpack<br/>up to 3 layers)]
R[Rain] --> PK
ML --> PK
PK -->|meltwater + rain drain out| RO[Runoff]Each hour, for each terrain unit:
- Mass: snowfall adds snow. Rain and meltwater percolate, can refreeze in a cold pack, and drain out as runoff once the pack's water-holding capacity is exceeded. Sublimation removes mass.
- Energy: absorbed sunshine (depends on albedo), longwave exchange, turbulent sensible and latent heat (depends on wind and on the temperature and humidity difference), and heat from the ground. A surplus at 0 °C melts snow.
- Albedo: high for fresh snow. It decays as the snow ages and faster when it is melting. Fresh snowfall resets it. This feedback is a big part of spring melt.
- Density: fresh snow settles and compacts over time, which sets depth for a given SWE.
- Soil: several layers beneath the snow conduct heat and freeze or thaw.
Model outputs are averaged to daily values for the maps.
Variables¶
| Output | Meaning |
|---|---|
| SWE | water equivalent of the snowpack, mm |
| Snow depth | m (cm on maps) |
| Runoff | water leaving the snowpack or bare ground: melt + rain |
| Snowmelt / rain runoff | the two parts of runoff, where available |
| Albedo | surface reflectivity |
| Surface / soil temperature | °C |
Glaciers¶
FSM1 has no glacier ice. Above the equilibrium line, snow that survives summer carries over and the pack keeps growing year on year. Such units are flagged and excluded from climatologies and anomalies.
Some regions use GFSM, our glacier extension of FSM. It adds an ice layer under glacier units, so that ice melt and runoff from glaciers are represented. See Known limitations for what remains.
Fractional snow cover¶
To compare with satellites, the model's snow depth is converted to a fraction of snow-covered area within a unit. A thin pack in rough terrain only covers part of the ground. This is the Model fSCA layer.
Snowpack detail at selected sites¶
Where the board offers SNOWPACK sites, the Swiss SNOWPACK model (Lehning et al.) runs at single points on the same forcing. It resolves many layers, grain types and stability indices. It complements FSM; it does not drive the maps.