Theory 30/Earth, Climate & Energy /Climate extremes
Heat Waves Need Dry Soil
Heat waves are forecast from the atmosphere. The land surface may decide how hot it gets.
Heat waves require a soil moisture deficit threshold before they can amplify, so the same atmospheric setup produces a heat wave over dry ground and a mild spell over wet ground.
01The evidence
Heat wave amplitude correlates with antecedent soil moisture, and the same synoptic pattern yields different maxima over wet and dry regions.
02The tension
Forecast models emphasise the atmospheric pattern, and soil moisture is initialised coarsely, so its amplifying effect is under-represented.
03The mechanism
Evaporation cools the surface. When soil moisture falls below the level at which evaporation can be sustained, the incoming radiation heats the ground instead, and the sensible heat flux amplifies the anomaly.
That makes the deficit the switch, and predicts a threshold below which the atmosphere alone sets the maximum.
04What this is not
Those forecasts use the pattern. This uses the land surface, which explains amplitude differences under the same pattern.
05Predictions
- P1Amplitude should depend on antecedent soil moisture.
- P2A threshold should separate amplified from unamplified events.
- P3Wet ground should suppress the same pattern.
06Refutation
Amplitude independent of soil moisture would restore the atmospheric model.
07What it changes
It would improve heat wave forecasting.