Abstract (eng)
During the extreme Pacific Northwest June 2021 heat wave previous all-time 2-m temperature records were broken by 5 K or more. The highest 2-m temperature of 49.6°C on June 30 was measured in Lytton, a village in the Canadian province British-Columbia (50.2°N). To address the question why this event was so anomalous, I applied the Lagrangian dispersion model FLEXPART to obtain analysis data of 15-day backward trajectories and to trace meteorological parameters and physical quantities along them. This master thesis includes a case study of the PNW June 2021 event and an analysis of the 32 most extreme PNW heat events between 1960 and 2021. First, large portions of air mass associated with the PNW June 2021 event originated from subtropical regions in the western North Pacific and thus unusually far southern and western compared to previous PNW heat events. Secondly, the moisture uptake and subsequent latent heat release due to condensation along the Meiyu-Baiu-Front and a warm conveyor belt contributed decisively to the intensity of the PNW June 2021 event. Consequently, the potential temperature increased to about 50 °C in Lytton on June 30, 2021, 7 – 11 K above those values during previous most extreme heat events’ peak over the Pacific Northwest. Moreover, the latent heat release, which occurred only before the PNW June 2021 event, contributed to the onset of an intense persistent atmospheric blocking over the Pacific Northwest. In addition, positive soil-moisture-temperature feedback was favored by low moisture conditions and led to the development of a higher boundary layer top on June 30, 2021. Therefore, adiabatic heating by descent and mixing occurred to a greater extent than during previous PNW heat events. Thus, to summarize, two superimposed main driving processes could be identified only during the June 2021 PNW event. This explains why the 2-m temperature maxima on June 30, 2021, over the Pacific Northwest deviated by 5 K or more from previous 2-m temperature records.