Day 1Hour 0ITCZ 0 degRain now 0 mm/hCloud top 0 kmFPS 0

Drag to orbit, wheel or pinch to zoom. The thunderhead is drawn from the parcel model: base = lifting condensation level, top = where the rising parcel stops being buoyant. Forest clearings grow with the deforestation slider.

Equatorial Climate: ITCZ Shift, Afternoon Convection and Rainforest Water Balance

Why does the equator get two rainy seasons, why do storms build at 3 pm, and what happens to the river when the forest goes?

Site and climate

Forest

Time

Live readouts

ITCZ latitude and the site

Daily rain and evapotranspiration through the year

Parcel ascent at the afternoon temperature maximum

Soil water store through the year

Experiment: latitude vs rain seasons

Experiment: clear the forest

Independent numerical checks

Controls

Drag or one finger: orbit; wheel or pinch: zoom.
Time scale is simulated hours per real second: 1 h/s watches a storm form, 240 h/s runs ten days per second.
Latitude moves the site relative to the ITCZ; moisture recycling controls how much forest loss dries the boundary layer.

The science

Sun and ITCZ. Declination delta = 23.44 deg x sin(2 pi (d - 80)/365). The equator is overhead twice a year (equinoxes), so daily insolation has two maxima. The convergence zone follows the sun with a lag: phi_ITCZ = A sin(2 pi (d - 80 - lag)/365) with A = 12 deg and lag = 30 days over land (illustrative). Rain concentrates where the site is near the ITCZ: weight exp(-((phi - phi_ITCZ)/8 deg)^2).

Daily insolation. Q = S0/pi (d0/d)^2 (h0 sin phi sin delta + cos phi cos delta sin h0), S0 = 1361 W/m2; the page integrates the hourly flux numerically to compare.

Parcel model. Saturation vapour pressure (Bolton) e_s = 6.112 exp(17.67 T/(T + 243.5)) hPa; dew point from relative humidity; lifting condensation level from Bolton's formula (Espy rule 125 m per kelvin is the cross-check). Above it the parcel cools at the moist-adiabatic rate Gamma_m = g (1 + L r_s/(R T)) / (c_p + L^2 r_s eps/(R T^2)). CAPE = integral of g (T_p - T_e)/T_e dz up to the equilibrium level; environment lapse rate 6.2 K/km up to 13 km. Rain per day is 17 mm x min(1.6, CAPE/1500) x ITCZ weight, plus 2 mm/day x weight of stratiform rain, averaged over a five-member +-1.2 K temperature ensemble. These rain constants are tuned to give a rainforest-like total (about 1600 mm/yr) and are illustrative, not fitted to a station.

Water balance. A soil bucket of 400 mm (illustrative): dW/dt = P - ET - Q. Evapotranspiration is Priestley-Taylor, ET = alpha Delta/(Delta + gamma) Rn/lambda, alpha = 1.26 for forest and 0.8 for pasture, throttled when the bucket is below 55 %. Runoff is overflow plus a drainage term that grows when forest is removed. Forest loss also lowers boundary-layer humidity by the moisture-recycling slider (0.05 per unit of forest lost, illustrative), which lifts the condensation level and weakens CAPE.

Not modelled: ocean-atmosphere coupling, Walker circulation, MJO, orography, aerosols, real rainfall observations. Cloud and forest are stylised. This is a teaching model; for real data use station or satellite records. Models built with Blender (trees, basin, river, cloud puffs) and rendered with Three.js.