Sensitivity of the February 2021 U.S. Cold-Air Outbreak to Tropopause Polar Vortex Intensity

February 2021 featured a historic cold air outbreak (CAO) across the United States, noteworthy for its extreme magnitude and duration. In combination with two associated winter storms producing anomalous snow and ice, substantial societal impacts resulted, including extensive power and water outages across Texas and parts of Oklahoma. This CAO was associated with multiple tropopause polar vortices (TPVs), or sub-synoptic scale coherent vortices based along the tropopause and commonly found in the Arctic, which were transported out of the Arctic into southern Canada. Subsequently, two intense TPVs gradually merged over several days, building a reservoir of cold air that continually progressed southward into the southern Great Plains via cold air damming. Given the known association of TPVs with CAOs, the role of these TPVs and their intensities in the CAO magnitude and evolution is investigated in this study.

An analysis of backward air parcel trajectories from the southern Great Plains using the NOAA HYSPLIT model reveals that despite the merger of two TPVs, a disproportionate number of low-tropospheric parcels originated from only a single TPV. Next, a series of numerical modeling experiments using the Model for Prediction Across Scales (MPAS) is used to assess the sensitivity of the CAO evolution to the intensity of the TPVs. Experiments are designed to weaken and strengthen the TPV by imposing heating and cooling rates of 5, 10, and 15 K per day near the tropopause. Results show a non-linear relationship between TPV intensity and the CAO magnitude in the southern Great Plains. Simulations with stronger TPVs are associated with a stronger cold pool but merge earlier and track more poleward than simulations with weaker TPVs. Finally, comparing these simulations to observations shows a persistent 2-meter temperature warm bias during the initial phase of the CAO associated with underestimating the magnitude of the low-tropospheric inversion, consistent with a similar warm bias in operational models.