MY SIMULATOR

Forest to Desert

Rain shadow, lapse rate and moving biome boundaries.

INTERACTIVE 3D

Pin -Elev 0 mT 0 CP 0 mmPER 0FPS 0

TEMPERATURE0 C

RAINFALL0 mm

ARIDITY (PET/P)0

WARMING☀+0.0 K

Forest-to-Desert Ecotone Lab: Rain Shadow, Lapse Rate & Biome Boundaries

A model valley: snowy mountains and conifer forest in the west, a river with wetlands and grassland in the middle, desert in the east. Change the warming and the moisture of the wind and watch the rain, the aridity and the biome boundaries move. Click the land to read the climate of any point.

Simple climate model, illustrative values. Distances, heights and numbers are model values for a stylised valley, not a real place or a forecast. Animals, trees and relief are drawn much bigger than real.

Scenario

Climate settings

Pinned point

Valley report (along the river terrace)

Elevation along the valley

Rain and potential evaporation

Where species fit

Missions

Independent numerical checks

Also run outside the browser: node src/check.js (lapse rate, Holdridge ratio, moisture conservation over a synthetic ridge and over every row of the real terrain grid, ecotone width on an exponential gradient, boundary shift under warming).

Model values and simplifications: one-dimensional moisture flux blowing west to east over the height grid, rain from upslope lift P_oro = P_u · (Q_i − Q_i+1)/Δx with Q_i+1 = Q_i e^{−k_s s Δx} (s = upslope gradient, capped), a continental rain gradient that fades with distance and is reduced in the lee by the moisture already used, and a river subsidy for the wet strip along the river. Temperatures fall with height at a fixed lapse rate and rise slightly to the east [assumption]. All constants are illustrative [check against a source]. The vertical relief is exaggerated about three times.

Controls

Drag / one finger: orbit; wheel / pinch: zoom; Hero / Valley / Map / Desert buttons (keys 1-4) move the camera; Space pauses the wind.
Click or tap the land to pin a point and read its height, temperature, rain, aridity ratio and biome.
The Warming slider adds extra degrees to the whole valley; the settings below change the moisture of the wind, the valley temperature and the lapse rate. Scenario buttons load ready-made cases.

The science

Lapse rate. Air cools as it rises, about 6.5 K per kilometre of height in the standard atmosphere, so mountains are cold: T = T₀ − Γ Δh. The snow line is simply where the annual mean falls near 0 °C.

Rain shadow. Moist air pushed up a slope cools, drops its water and arrives on the far side dry. In the model, the moisture flux Q loses a fraction on every uphill step; the same amount appears as rain, so water is conserved (checked on every row of the terrain). The lee side then gets less rain than the windward side.

Aridity ratio. In the Holdridge life-zone scheme the potential evapotranspiration is PET = 58.93 · T_bio mm per year (T_bio = annual mean temperature with values below 0 °C and above 30 °C set to the limits) and the ratio PER = PET/P separates forest (below about 1), grassland and steppe (1–2), desert scrub (2–4) and desert (above 4) [check thresholds against a source].

Ecotone. The transition zone between two biomes is not a line. Here its width is the distance over which the aridity ratio rises from 0.5 to 3. If the ratio grows exponentially with distance, w = L ln 6 where L is the e-folding length; the check scans exactly that.

Moving boundaries. A boundary sits where PER crosses a fixed value. If the temperature along the valley changes by ΔT and the rainfall is unchanged, the crossing moves by Δx = −ΔT/(dT/dx): warming pushes the forest line toward the wetter, colder mountains, and the desert edge toward the river.

Species. Each animal and plant has a Gaussian “niche” in temperature and ln(PER); the maps show where its suitability is high. These are illustrative values, not field data.

Not modelled: seasons, soils, fire, grazing, rivers feeding back on rainfall, wind change with height, real topography and climate data.