Dry land before a flash drought is linked to stronger heat feedback during it

Why it matters

Forecast models for 2 weeks to 2 months ahead often miss flash droughts. The results suggest that getting the land’s prior moisture state right could improve them. Vegetation water content from satellites may be a convenient early signal in some regions.

What they did

The authors found flash droughts in 2000–2020 using daily satellite surface soil moisture at 0.25° resolution. They combined these events with satellite and reanalysis data on radiation, evaporation, land surface temperature, vegetation and air temperature. They used the gap between land surface and air temperature as a proxy for sensible heat flux, and split events into quartiles by its peak. They then tested three semi-arid African regions with a bootstrapped Mann-Whitney U test.

Key findings

• Net radiation rises as drought develops. After onset, sensible heat flux keeps rising while net radiation falls and latent heat flux declines, which indicates water limitation.
• Events with stronger evaporative stress show shifted surface energy budgets for 4 months both before and after onset, and also larger losses in vegetation water content and fluorescence.
• Vegetation and air temperature respond most to soil drying in semi-arid regions.
• In West Africa in summer, 9 % of events in the lowest heat-flux quartile exceed a 1.5σ air temperature anomaly, against 27 % in the highest quartile.
• In West Africa, 12 % of events with high precursor vegetation optical depth reach a peak anomaly above 1.5σ, against 27 % with low precursor values. East Africa shows the same direction, but southern Africa shows no robust link.

Limitations

• Events in densely forested areas are missing because satellites cannot observe surface soil moisture there.
• Some inputs are not purely observed: air temperature comes from reanalysis and latent heat flux comes from a model driven by satellite data. Land surface temperature is also missing under clouds.
• The precursor signal is not robust in southern Africa, and land information alone cannot predict onset, since rainfall deficits must also be forecast.

Glossary

• Flash drought: A drought that develops rapidly, over about 2 weeks to 2 months.
• Vegetation Optical Depth (VOD): A microwave-based measure of the water content of vegetation.
• Sensible heat flux: Heat passed from the land surface to the air, as opposed to heat used to evaporate water.
• Standardised anomaly: How far a value is from normal, in units of its usual variability (σ).