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Understanding Predictability of Flash Drought over the United States

This proposal aims to investigate causes and predictability of flash droughts over the US. Here flash droughts refer to short-term drought events that have a rapid onset and occur on a time scale of weeks to months, among which the 2012 Great Plains drought is an excellent example. Often occurring during spring and summer, the typical growing season for crops across much of the US, flash droughts can have substantial impacts on agriculture. Recent research by the PIs has shown that leading modes of subseasonal atmospheric circulation variability play a key role in atmospheric circulation and surface meteorological variability on subseasonal time scales and that they have been crucial in the development of recent short-term warm season droughts and heat waves over the US (e.g. the 1988 and 2012 summer droughts). A proper representation of these processes requires a GCM to have a correct simulation of warm season mean state, particularly the Northern Hemisphere (NH) jet streams. Given that many current GCMs, including the NASA GEOS 5 GCM, are deficient in this regard, the PIs have successfully removed much of the mean bias in the NASA GEOS-5 AGCM relative to MERRA-2 reanalysis by applying 6-hourly climatological corrections (relative to MERRA-2) to model basic state variables within the free-running AGCM.

Our proposed work builds on the above development, and has two major thrusts. First, we will investigate causes and physical mechanisms of past flash droughts over the US, and explore potential sources of predictability. This task includes (i) determining key regional processes that led to the development of past US flash droughts and diagnosing their physical origin, and (ii) investigating the separate effects of regional persistent precipitation deficits and heat waves on US flash drought development. Second, we will investigate the predictability of US flash droughts. This includes (i) assessing the prediction skill and predictability of past US flash droughts in the seasonal North American Multi-Model Ensemble (NMME) and subseasonal NMME forecasting systems, (ii) investigating how well the potential sources of predictability identified in the first task and their effects on US flash drought development are represented in the seasonal NMME and subseasonal NMME; and (iii) investigating the impact of model mean bias on forecast skill and predictability estimate of US flash drought, by contrasting the NASA GMAO operational subseasonal forecasts performed using the standard GEOS-5 coupled model (as part of the subseasonal NMME) and a parallel suite of subseasonal forecasts produced with a mean bias-corrected GEOS-5 coupled model.

The link between our work and the NOAA operational drought prediction will be established through our involvement in the NIDIS Drought Prediction and Forecasts Working Group (as a co-Chair and a member). We will communicate with NCEP CPC regularly to ensure that our research findings are applied to improve NOAA operational drought outlook as appropriate. The proposed work directly targets the focus area “Advancing drought understanding, monitoring and prediction” solicited by FY 2017 NOAA MAPP Program. The expected outcome of the proposed work is an improved understanding of the predictability of development of US droughts on subseasonal time scales. It will also contribute to NOAA’s long-term goal of climate adaptation and mitigation through “Improved scientific understanding of the changing climate system and its impacts”.

Climate Risk Area: Water Resources

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