Assessing the influence of lower stratospheric dynamics in North American winter extreme events

During the last few winters, the media portrayed the polar vortex as the source of extreme

cold air and winter weather across the U.S. These polar vortex events were responsible for

thousands of flight cancellation, ground transportation disruptions, record breaking spot natural

gas prices that impacted consumer prices, as well as dozens of fatalities. With millions of dollars

in economic losses, understanding and assessing the factors that contribute to these extreme events

is critical to assessing their likelihood of occurrence in a future climate. In examining these events,

we need to consider that the polar vortex consists of a tropospheric component and a stratospheric

component, these components communicate intermittently with dramatic consequences via

troposphere-stratosphere dynamic wave coupling.

Broader impact and relevance: The proposed research is addresses the call for the

NOAA/CPO FY20 Competition 7: Multi-program-Explaining Climate Extreme Events:

Developing a Rapid Assessment Capability and Understanding the Causes and Mechanisms of

Extreme Events. The research outcome will directly address the attribution and assessment of

extreme events in response to Competition 7, type 1 proposal. This type 1 research focuses on

increasing our process-based understanding of the climate mechanisms that influence the extreme

event in the cool-season by using machine learning to identify features and processes occurring in

either, or both, the troposphere and the stratosphere. This research focuses on explaining the

occurrence of extreme cool-season events that are known to contribute to the growing list of U.S.

Billion Dollar Disasters. The results of the proposed work have the potential to inform the

resources needed to obtain environmental information (e.g., observational requirements or

computational resources requirements) vital to our Nation's safety, prosperity and resilience

The proposed research is motivated by literature from the weather and climate forecasting

communities, where it is largely accepted that including a well-resolved stratosphere results in

improved model forecast skill, though the mechanisms responsible for this increased skill is a

current active research area. The uncertainty in the mechanisms is due to the fact that during

troposphere-stratosphere wave coupling, the dispersive nature of waves means they can be

transmitted, absorbed, reflected or refracted (or some combination of the former) in three

dimensions contingent their wavenumber. The proposed work addresses the problem of our limited

knowledge of the three-dimensional Rossby wave dynamics in the tropopause and stratosphere by

evaluating these dynamics during periods when they are known to be important, during periods of

extreme cool-season weather. The research will identify historical cases for examination from

existing NOAA databases and will examine these cases from climatological analysis as well as by

applying machine learning tools. The analysis will accomplish four key research objectives: (a)

Conduct a climatological assessment of the three-dimensional troposphere-stratosphere wave

coupling contributions to cool-season extreme events; (b) Identify distinct categories of three-

dimensional, troposphere-stratosphere wave coupling that lead to these extreme events; (c) Assess

the impact of low-frequency variability on the frequency of occurrence of the categories identified

in (b); and (d) Evaluate the contributions and sensitivities to important features identified in the

three-dimensional, troposphere-stratosphere wave coupling contributions extreme events.