A Framework for Flash Drought Progression

Not all droughts are the same.  In some cases, drought rapidly intensifies at subseasonal to seasonal scales with significant impacts to agriculture and water resources along with the increased propensity for heatwaves and wildfires.  Soil moisture is a critical drought variable, and the desiccation of the terrestrial surface is an effective resource for monitoring drought development and associated impacts.  During flash drought, soil moisture can play two critical roles: (1) drought enhancement via dry soils, enhanced sensible heat flux, reduced evaporation, and enhanced vapor pressure deficit and, (2) drought resistance via moist soils that cool the surface via evaporation and decreased vapor pressure deficit.  Thus, a fundamental question is at what point during flash drought development does the environment transition from drought resistance (a negative feedback) to drought enhancement (a positive feedback) and vice versa? Further, how do surface-layer processes impact the development of the planetary boundary layer (PBL) during this transition?  Finally, what is the overall relationship between atmospheric demand, evaporative stress, terrestrial desiccation, and precipitation in the progression of flash drought.  This study provides a conceptual framework that captures the critical processes that drive flash drought progression (and moderation) through a relative parameter space approach. Using this framework, the onset, development, intensification, moderation, and termination of flash drought can be diagnosed by the complex interactions between terrestrial and atmospheric variables.