Reevaluation of long‐term Umkehr data and ozone profiles at Japanese stations

Abstract

Umkehr observations have been routinely conducted at Japanese stations, Sapporo, Tsukuba, Kagoshima, and Naha, and the Antarctic station, Syowa, for more than 50 years. Umkehr data are a valuable source of information on long‐term changes in the ozone vertical profile; however, the Umkehr record at Japanese stations has evident discontinuities. The majority of the discontinuities are related to the exchange of instruments for calibration (for total ozone measurements) and the replacement of instruments. These discontinuities may be related to the difference in instrument characteristics. In this article, reevaluation of the long‐term Umkehr data in Japanese network is done by assessment of instrument‐related changes in compared N values that exhibit solar zenith angle and total ozone dependence in addition to the step changes. The systematic errors are evaluated by simultaneous intercomparisons of each instrument with the reference instrument. Through this reevaluation, most discontinuities in a station's Umkehr time series are successfully corrected, and new sets of ozone vertical profiles are derived. The ozone profiles retrieved by two available Umkehr retrieval algorithms are compared with ozonesonde observations at every station and ozone lidar observations at Tsukuba. The results show that the revised Umkehr ozone profiles show improved consistency with both types of auxiliary ozone observations as compared to the old data sets, especially with regard to ozonesonde observations (difference of less than 5%). Trend analyses of the revised Umkehr ozone profile time series show a significant decrease in stratospheric ozone over Japan during the 1980s. It also varies between stations, with Naha showing the least significant trend among Japanese stations and Sapporo exhibiting as much as 6% of ozone decline per decade. In addition, a positive and statistically significant trend is detected in tropospheric ozone column at Naha (∼5.5% per decade) and Tsukuba (∼3.5% per decade) stations over the last 20 years, but no significant trend is observed over Sapporo.