Lidar observations of pyrocumulonimbus smoke plumes in the UTLS over Tomsk (Western Siberia, Russia) from 2000 to 2017

摘要

Large volcanic eruptions with the volcanic explosivityindex (VEI) ≥ 3 are widely known to be the strongest source oflong-lived aerosol in the upper troposphere and lower stratosphere (UTLS).However, the latest studies have revealed that massive forest (bush) firesrepresent another strong source of short-term (but intense) aerosolperturbations in the UTLS if combustion products from the fires reach thesealtitudes via convective ascent within pyrocumulonimbus clouds (pyroCbs).PyroCbs, generated by boreal wildfires in North America and northeastern Asiaand injecting smoke plumes into the UTLS, have been intensively studiedusing both ground- and space-based instruments since the beginning of the21st century. In this paper, we focus on aerosol layers observed in the UTLS overTomsk (56.48∘ N, 85.05∘ E, Western Siberia, Russia) thatcould be smoke plumes from such pyroCb events occurring in the2000–2017 period. Using the HYSPLIT trajectory analysis, we have reliably assignednine aerosol layers to 8 out of more than 100 documented pyroCb events,the aftereffects of which could potentially be detected in the UTLS overTomsk. All the eight pyroCb events occurred in the USA and Canada: one eventper year occurred in 2000, 2002, 2003, 2013, 2015, and 2016, whereas two eventsoccurred in 2017. No plumes from pyroCbs originating in the boreal zone ofSiberia and the Far East (to the east of Tomsk) were observed in the UTLSover Tomsk between 2000 and 2017. We conclude that the time durations forpyroCb plumes to be detected in the UTLS using ground-based lidars are lessthan about a month, i.e., plumes from pyroCbs generated by wildfires to theeast of Tomsk can significantly diffuse before reaching the Tomsk lidarstation by the westerly zonal transport of air masses. A comparativeanalysis of the contributions from pyroCb events and volcanic eruptions withVEI ≥ 3 to aerosol loading of the UTLS over Tomsk showed the following.Plumes from two or more pyroCbs that have occurred in North America in asingle year are able to markedly increase the aerosol loading compared tothe previous year. The annual average value of the integrated aerosolbackscatter coefficient Bπ,532a increased by 14.8 %in 2017 compared to that in 2016 due to multiple pyroCbs occurring in BritishColumbia (Canada) in August 2017. The aftereffects of pyroCb events arecomparable to those of volcanic eruptions with VEI ≤ 3, but evenmultiple pyroCbs can hardly compete with volcanic eruptions with VEI = 4.