Understanding and Quantifying Uncertainties Related to Counteracting Influence of Projected Reduction in Temperature Gradient and Increase in Atmospheric Moisture on Winter Storms and Cool Season Flooding

During the cool season, winter storms and inland flooding brought by extratropical
cyclones lead to substantial losses in life and property each year, accounting for many of the
billion-dollar weather and climate disasters that have plagued the U.S. How these extremes may
change under global warming is still uncertain, due to the counteracting influence of increase in
atmospheric moisture due to warming, and the reduction in pole-to-equator temperature gradient
due to enhanced high latitude warming. In this project, an integrated assessment on the multiple
physical processes that can impact these extremes will be conducted to understand and quantify
the uncertainties of these projections. The proposed studies will make use of the high resolution
global multi-model ensemble projections, a novel dataset that will become available from the
Coupled Model Intercomparison Project Phase 6 (CMIP6), as well as the innovative diagnostic
tools and metrics relating cyclone activity to weather extremes recently developed by the PI’s
research group.

As the planet warms under increased CO2 radiative forcing, Northern Hemisphere Polar
regions are warming faster than the global average, thus reducing the meridional temperature
gradient in the Northern Hemisphere, leading to weaker dynamical forcing for the extratropical
cyclones that cause weather extremes such as winter storms and cool season precipitation
extremes. However, increased moisture under warming can lead to increased latent heat release
within these storms, potentially enhancing their intensity. Recent studies have suggested that
previous generations of Global Climate Models (GCMs) may not have sufficient resolution to
correctly simulate the interactions between diabatic heating and storm dynamics, potentially
under-estimating the intensity of these storms in future projections. CMIP6 will provide, for the
first time, multi-model ensemble projections at a resolution high enough (about 25 km grid
spacing) to resolve these interactions. Over the past few years, the PI’s research group has
developed innovative process oriented diagnostics and metrics that quantitatively relate cyclone
variability and change to those of the weather extremes that these cyclones can cause, and can
thus make full use of these novel high resolution CMIP6 projections to conduct a process
oriented, integrated assessment of the uncertainties in the projected changes of these extremes
associated with the uncertainties in the underlying thermodynamical and dynamical processes.

This project is highly relevant to NOAA’s objective of providing the essential and
highest quality environmental information vital to our Nation’s safety, prosperity and resilience.
This proposal also directly responds to the Climate Program Office MAPP Program’s call for
building on CMIP6 results for improved depictions of 21st Century climate over the United
States, by developing integrated projections on how winter storms and cool season precipitation
extremes that adversely impact multiple regions of the U.S., as well as developing integrated
process-level understanding of these projected changes for the purpose of characterizing
associated uncertainties by relating these weather and climate extremes to the physical processes
that generate them.