Toward Improving the Simulation and Prediction of Extratropical Northern Hemisphere Blocking Patterns and Extreme Weather

Steven M. Cavallo, University of Oklahoma (Co-Investigator)

James H. Ruppert, University of Oklahoma (Co-Investigator)

James J. Benedict, Los Alamos National Laboratory (Co-Investigator)

Extreme weather events (e.g., heatwaves, heavy precipitation, drought) have significant socioeconomic costs by, for example, threateningpublic safety, destabilizing supply chains, and damaging infrastructure. Skillful predictions of such events at the subseasonal-to-seasonal (S2S) timescale (i.e., forecasts with a lead of 2 to 8 weeks) present enormous value for preparation and loss mitigation. One factor affecting skillful S2S forecasts of extreme weather is accurate representation of mid-latitude atmospheric blocking patterns – i.e., stagnant, anomalous circulation patterns (typically high pressure systems) that persist for days to weeks and enhance probabilities of abnormal and extreme weather over large areas of the Northern Hemisphere.Several processes are important for blocking initiation and maintenance, including large-scale wave breaks in the upper troposphere (i.e., similar to ocean waves breaking on the shore)and remote effects from tropical and high-latitude climate variability. Unfortunately, S2S forecasting models and climate models poorly capture such processes important for predicting these blocking patterns, leaving considerable uncertainty in their predictions.

In this project, we will focus on (a) the dynamical and thermodynamical processes that lead to the development and maintenance of blocking patterns, (b) how the ability of a model to simulate those processes affects that model's forecast skill of extreme weather events on the S2S timescale, and (c) quantifying the relative contributions of different tropical and high-latitude atmospheric processes to simulating atmospheric blocking patterns. The work will use multiple sources of data and techniques, including analysis with atmospheric reanalysis data, operational S2S forecast models of past weather (called reforecasts), and performing targeted experiments with the latest version of the Energy Exascale Earth System Model (E3SM). By the end of the project, we will provide a comprehensive assessment of mechanisms that are most important for forming and maintaining Northern Hemisphere blocking episodes, with the aim of making more skillful forecasts and better future climate projections of extreme weather. The proposed research will consist of three (3) research activities.

• ResearchActivity#1willuse the ERA5 dataset (an atmospheric reanalysis product that reproduces global atmospheric conditions from 1950 to near present) to study the observed evolution of blocking patterns and relate them to modes of climate variability, tropical convection patterns, and upper-level vortices in the polar atmosphere (called tropopause polar vortices, which have important implications for extreme weather).We will identifyblockingepisodesacrosstheNorthernHemisphereusingestablishedalgorithmsandperform statistical analyses with several variables (e.g., geopotential heights, jet-stream winds, mid-troposphericheatingrates). We will characterize the frequency,intensity,andduration of these blocking episodes in the 20th and early 21st centuries.WewillrepeattheseanalyseswithexistingrunsoftheE3SM over a similar time period (i.e., the historical experiment) to benchmarkitsperformanceinsimulatingblockingpatternsandassociatedprocesses.
• ResearchActivity#2 will focus on examining the S2S operational model reforecasts for these blocking episodes and how the models represent them. We will performa sensitivityanalysis