Comparison of Daily Ozonesonde Measurements and Chemical Reanalyses Over South Korea Based on 2021 Pre‐ACCLIP Data: An Ozone Intrusion Case

Abstract This study investigates an ozone intrusion event observed during the Pre‐Asian Summer Monsoon Chemical and Climate Impact Project in August 2021, using 26 consecutive daily ozonesonde measurements over South Korea. A pronounced enhancement in total column ozone was observed between 17 and 19 August, which can be largely attributed to an ozone intrusion in the upper troposphere–lower stratosphere (UTLS), accounting for approximately 60% of the increase. The upper tropospheric circulation patterns demonstrate a clear signature of anticyclonic Rossby wave breaking (AWB) on the northeastern edge of the Asian summer monsoon anticyclone, aligned with the summertime jet stream. This AWB, accompanied by a cut‐off low and tropopause folding, facilitated downward transport of stratospheric ozone into the upper troposphere. In addition, the ozone variability is investigated in two chemical reanalysis data sets: Modern‐Era Retrospective Analysis for Research and Applications, Version 2 (MERRA‐2) and European Centre for Medium‐Range Weather Forecasts (ECMWF) Atmospheric Composition Reanalysis 4 (EAC4). MERRA‐2 and EAC4 capture the ozone intrusion event with relevant synoptic‐scale circulation patterns and ozone variability. However, discrepancies of ozone data in the chemical reanalyses were found in vertical ozone structures and persistence in the troposphere. MERRA‐2 better represented the secondary ozone peak in the UTLS but underestimated lower‐tropospheric ozone. In contrast, EAC4 showed a systematic positive bias particularly in the stratosphere and near the surface. Continued integration of temporally high‐resolution ozone measurements is beneficial for understanding synoptic‐scale ozone variability and evaluating emerging chemical reanalyses.