講演情報
[PEM16-18]Research on polar mesospheric cloud variations associated with the 2022 HTHH eruption based on Himawari/AHI observation
*森山 陽介1、津田 卓雄1、安藤 芳晃1、鈴木 秀彦2、中川 広務3、西山 尚典4、田中 良昌4、村田 健史5,6、Yue Jia7 (1.電気通信大学、2.明治大学、3.東北大学、4.国立極地研究所、5.信州大学、6.総合地球環境学研究所、7.NASA/GSFC)
キーワード：
極中間圏雲、フンガ・トンガ＝フンガ・ハアパイ噴火、ひまわり8号/9号、Aura/MLS
Polar mesospheric clouds (PMCs), also known as noctilucent clouds (NLCs), are the highest clouds in the terrestrial atmosphere. These clouds consist of water ice particles forming at an altitude of 80–85 km during the polar summer. The formation and disappearance of PMC are considered to be sensitive to temperature and water vapor. For over a century, the potential relationship between large volcanic eruptions and PMC activity has been discussed. Especially, the injection of water vapor into the middle and upper atmosphere by massive volcanic eruptions, such as the 1883 Krakatoa, is considered to contribute to the active PMC formation. However, due to limited observations and the rarity of large eruptions, the link between volcanic eruptions and PMCs remains unclear. In January 2022, the Hunga Tonga-Hunga Ha'apai volcano erupted. It is reported that a massive amount of water vapor was injected into the stratosphere (altitude 55–58 km), observed by the Aura/Microwave Limb Sounder (MLS). The injected water vapor diffused and transported, reaching the mesopause region at high latitudes in the Southern Hemisphere (SH) two years later. Thus, the 2022 HTHH event would be a nice opportunity to examine the relationship between large volcanic eruptions and PMC activities.
To investigate this event, we analyzed the PMC occurrence rate (OR) data from the Himawari-8/Advanced Himawari Imager (AHI) and Himawari-9/AHI, as well as temperature and water vapor volume mixing ratio (H2O VMR) data from the Aura/MLS. For both hemispheres, we extracted these datasets within 65–81ºS/N and 77.8–87.9 km altitude and calculated monthly averages for January in the SH and July in the Northern Hemisphere (NH). To focus on the effects of eruption-injected water vapor on the year-to-year PMC OR variations, we then reproduced the temperature-dependent PMC OR variability components. Then, we removed the reproduced components and derived residuals as temperature-independent components in the PMC OR variability. As a result, we found a significant temperature-independent PMC OR increase (+14.0–15.5%) in January 2024, i.e., two years after the 2022 HTHH eruption, in the SH. The obtained temperature-independent PMC OR increase was associated with a significant increase in H2O VMR (+0.87±0.04 ppmv compared to 2016–2021 mean). These findings would represent the first observational evidence of a relationship between eruption-originated water vapor injection and PMC activity. However, this increase in H2O VMR accounted for only about 22–29% of the observed temperature-independent PMC OR increase, when estimated PMC OR from the relative humidity with respect to ice (RHI). For more quantitative understanding, it would be necessary to consider not only the temperature and H2O VMR, but also other factors such as mesospheric dust and atmospheric waves in future studies.
To investigate this event, we analyzed the PMC occurrence rate (OR) data from the Himawari-8/Advanced Himawari Imager (AHI) and Himawari-9/AHI, as well as temperature and water vapor volume mixing ratio (H2O VMR) data from the Aura/MLS. For both hemispheres, we extracted these datasets within 65–81ºS/N and 77.8–87.9 km altitude and calculated monthly averages for January in the SH and July in the Northern Hemisphere (NH). To focus on the effects of eruption-injected water vapor on the year-to-year PMC OR variations, we then reproduced the temperature-dependent PMC OR variability components. Then, we removed the reproduced components and derived residuals as temperature-independent components in the PMC OR variability. As a result, we found a significant temperature-independent PMC OR increase (+14.0–15.5%) in January 2024, i.e., two years after the 2022 HTHH eruption, in the SH. The obtained temperature-independent PMC OR increase was associated with a significant increase in H2O VMR (+0.87±0.04 ppmv compared to 2016–2021 mean). These findings would represent the first observational evidence of a relationship between eruption-originated water vapor injection and PMC activity. However, this increase in H2O VMR accounted for only about 22–29% of the observed temperature-independent PMC OR increase, when estimated PMC OR from the relative humidity with respect to ice (RHI). For more quantitative understanding, it would be necessary to consider not only the temperature and H2O VMR, but also other factors such as mesospheric dust and atmospheric waves in future studies.