Severe weather in southern Europe and severe flooding in Pakistan in August 2022: Linked blocking-related extreme weather events
Lance
Bosart
University at Albany/SUNY
Talk
Widespread severe weather occurred across southern Europe on 18 August 2022 in conjunction with a rare serial derecho. This derecho left a trail of destruction along a 1500+ km pathway between the French Island of Corsica northeastward to northern Italy and from there to Austria, and Slovakia. The forward speed of the derecho exceeded 100 km/h. Reported measured wind gusts associated with this derecho included 225 km/h at Marignana, 206 km/h at L'Île-Rousse, and 197 km/h at Calvi. Several people were killed or injured. Widespread power outages and significant urban dislocation was reported along the derecho’s pathway. This derecho was associated with an unusually strong summer 500-hPa cutoff cyclone that deepened southeastward across France into the western Mediterranean Sea and northern Italy. This cutoff cyclone subsequently turned northeastward and weakened as it moved toward central Europe on the western side of a strong and persistent blocking anticyclone that was situated over higher-latitude eastern Europe and western Russia. The aforementioned weakening of this cutoff cyclone, unanticipated by the Hungarian National Meteorological Service, resulted in the firing of two top officials after forecasts of severe weather on Hungary’s most important holiday failed to materialize.
Meanwhile, on the eastern side of the aforementioned blocking anticyclone a parade of troughs that had crested this anticyclone dropped south-southeastward toward a persistent trough that was situated across eastern Iran, Afghanistan, Pakistan, and the Tibetan Plateau. A persistent quasi-stationary 700-hPa cutoff cyclone associated with this trough was located on the border of Pakistan and India. This cutoff cyclone facilitated a strong southwesterly that enabled the northeastward transport of very moist air (precipitable water values > 60 mm and integrated water vapor transport values > 1000 kg/m/s) across the Arabian Sea toward western India and from there northward toward Pakistan. The aforementioned moist southwesterly flow across the Arabian Sea was enhanced by a merger with a westward-directed equatorial flow of very moist air from the western tropical Pacific Ocean. Preliminary evidence suggests that extraordinary heavy rains across Pakistan were associated more with duration than with rainfall intensity, given the persistent large-scale flow blocking that existed from eastern Europe to western Asia during much of August 2022. Three key circulation features were associated with this extreme weather event: (1) an unusually persistent, high-amplitude, quasi-stationary ridge centered over eastern Europe and western Russia, (2) a transport of anomalously high precipitable water from the Arabian Sea toward Pakistan and northwestern India, and (3) an unusually intense upper-level westerly jet located over northern China with a well-defined equatorward jet-entrance region situated over northern Pakistan and northwestern India. It is likely that this equatorward jet-entrance region facilitated persistent deep ascent above anomalously high precipitable water values between 60–70 mm.
Meanwhile, on the eastern side of the aforementioned blocking anticyclone a parade of troughs that had crested this anticyclone dropped south-southeastward toward a persistent trough that was situated across eastern Iran, Afghanistan, Pakistan, and the Tibetan Plateau. A persistent quasi-stationary 700-hPa cutoff cyclone associated with this trough was located on the border of Pakistan and India. This cutoff cyclone facilitated a strong southwesterly that enabled the northeastward transport of very moist air (precipitable water values > 60 mm and integrated water vapor transport values > 1000 kg/m/s) across the Arabian Sea toward western India and from there northward toward Pakistan. The aforementioned moist southwesterly flow across the Arabian Sea was enhanced by a merger with a westward-directed equatorial flow of very moist air from the western tropical Pacific Ocean. Preliminary evidence suggests that extraordinary heavy rains across Pakistan were associated more with duration than with rainfall intensity, given the persistent large-scale flow blocking that existed from eastern Europe to western Asia during much of August 2022. Three key circulation features were associated with this extreme weather event: (1) an unusually persistent, high-amplitude, quasi-stationary ridge centered over eastern Europe and western Russia, (2) a transport of anomalously high precipitable water from the Arabian Sea toward Pakistan and northwestern India, and (3) an unusually intense upper-level westerly jet located over northern China with a well-defined equatorward jet-entrance region situated over northern Pakistan and northwestern India. It is likely that this equatorward jet-entrance region facilitated persistent deep ascent above anomalously high precipitable water values between 60–70 mm.
Presentation file
bosart-lance-blocking.pdf
(6.09 MB)
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