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When the Rain Stops: Drought on Subseasonal and Longer Timescales

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Session Description: 

Drought presents daunting scientific challenges, not only for observing and understanding the processes and mechanisms that produce it, but also for prediction, adaptation, and informing policy decisions. Droughts can have numerous severe impacts on a variety of timescales related to water and food security. Unlike other weather and climate extremes, drought has many definitions that often are framed by impacts that unfold on different timescales. Drought has mostly been studied on interannual to decadal timescales, however, and so-called “flash droughts” on the subseasonal to seasonal timescale (roughly 30 to 90 days) have been largely overlooked, even though they can produce acute impacts that can quickly overwhelm emergency resources with dire consequences.

These events can amplify negative impacts occurring during longer-timescale (interannual or decadal) dry periods, or longer term drying associated with climate change. Longer timescale drought events represent a cumulative risk, which is intensified at the onset of a flash drought when longer term dry conditions have already exhausted relief supplies, and funding resources have already been stretched to the limit. The onset of a flash drought under these conditions can ramp up extreme impacts in a short period of time, with profound implications for adaptive management.

Onset of a subseasonal to seasonal drought may be associated with synoptic-scale weather phenomena such as blocking. Termination is an open initial value problem, and it is unclear what kinds of precipitation events can “break” a subseasonal to seasonal drought. Duration of a flash drought also raises questions: can such a drought persist beyond a season or year, and how long can such a drought last? Flash droughts expose issues involved with water and evaporative demand, as well as emergency response and long term vulnerability for food security.

Understanding the processes involved with flash droughts requires enhanced observations. Prediction of flash droughts is a challenge, but efforts at subseasonal to seasonal initialized prediction can be leveraged. Such predictions are in the early stages and the degree of skill for this kind of drought phenomena is unclear.

This session will address four topics related to subseasonal to seasonal drought: science (understanding, modeling and prediction), observations, impacts and response. Experts from fields of physical science, impacts and policy will be brought together to address questions related to these four topics.