March 23, 2022 - 3:00 pm
March 23, 2022 - 4:00 pm
CategoriesWeather and Climate Systems
Weather and Climate Systems
Sensitivity of the February 2021 U.S. Cold-Air Outbreak to Tropopause Polar Vortex Intensity
Wednesday, March 23
Tropopause Polar Vortices (TPVs) are coherent vortices along the tropopause, with cyclonic TPVs identified by a local minimum in temperature and local maximum in potential vorticity along the dynamic tropopause. TPVs are most often found in high latitudes poleward of 60ÂºN, but can be transported into the midlatitudes where they can impact a wide variety of synoptic-scale phenomena. One common TPV pathway out of the high latitudes is via Canada, where TPVs and their associated cold pools can then contribute to major midlatitude Cold Air Outbreaks (CAOs).
February 2021 featured a historic CAO across the United States, both in terms of magnitude and duration. This CAO and associated winter storms had substantial societal impacts, with extensive power and water outages across Texas and parts of Oklahoma, historic cold and multiple snow and ice events, and over 100 fatalities and an estimated $195 billion in damages. This CAO was associated with multiple TPVs transported southward into southern Canada, with a downstream upper-tropospheric block resulting in the TPVs lingering over the same region for multiple days, building a reservoir of cold air continually advected southward into the South Plains leeward of the Rockies.
A set of multiple Model for Prediction Across Scales (MPAS) simulations was used to assess the sensitivity of the CAO to the antecedent TPVs. In addition to a control simulation, three simulations weakened the TPV by imposing a heating rate near the tropopause of 5, 10 and 15 K per day, and three simulations strengthened the TPV by applying the reverse process with a cooling rate near the tropopause. Results show the importance of the interaction and merger of two TPVs over southern Canada to the longitude and southward extent of the CAO, suggesting a possible balance between how quickly the TPVs merge and the longitude at which they merge to maximize the impacts of the CAO in the South Plains.