17:15 〜 19:15
[PEM12-P05] Research on polar mesospheric cloud response to the water vapor injection by the 2022 Tonga eruption based on Himawari-8/9 observations
キーワード：極中間圏雲、フンガ・トンガ＝フンガ・ハアパイ火山の噴火、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 Earth's atmosphere. These clouds consist of water ice particles, which can be formed mainly around the high-latitude mesopause region during summer. The formation and loss of such water ice particles can be sensitive to variability in the background atmospheric conditions, such as temperature and water vapor. Thus, PMCs are considered to be an important indicator to understand the variability in the mesopause region.
The discoveries of PMCs were documented in late 1880s, during which industrial human activity led to a dramatic increase in greenhouse gas emissions, such as CO2 and CH4. Then, such greenhouse gas emissions might lead to the cooling and water vapor increase in the mesosphere. Thus, the possibility that the human activity-related effects might promote PMC activity has been long discussed. On the other hand, a large eruption of Krakatoa occurred during the same period, and the impact of eruptions on PMC activity has also long been discussed. In the proposed scenario, the eruption might promote PMC activity by cooling the mesosphere and injecting water vapor. However, large eruptions such as Krakatoa are rare, and the eruption-induced effects on PMC activity have never been well evaluated based on observational data.
On 15 January 2022, the large eruption of the submarine volcano Hunga Tonga-Hunga Ha'apai (HTHH) in the South Pacific injected an unprecedented amount of water vapor into the middle atmosphere, increasing its concentration by approximately 10% over HTHH, as observed by the Microwave Limb Sounder (MLS) onboard the Aura satellite. Then, it has been reported that the injected water vapor was spreading to higher latitudes and higher altitudes after the injection. If the injected water vapor can reach high-latitude mesopause regions, eruption-induced effects on PMC activity could be expected.
To examine this issue, in the present study, we have investigated eruption-induced effects on PMC activity based on satellite observations for the first time. In our data analysis, we have used PMC data from 2015 to 2024, obtained by the Japanese geostationary satellites, Himawari-8 and Himawari-9. In addition, temperature and water vapor volume mixing ratio (VMR) data in the mesospheric heights from Aura/MLS have been analyzed. As a result, the observed water vapor VMR showed a significant increase in January 2024 at the southern high latitudes (65-81ºS) and a significant increase in July 2024 at the northern high latitudes (65-81ºN). These results would indicate that the eruption-injected water vapor was spreading for two years after the eruption in January 2022, and it reached around the high-latitude mesopause regions in both hemispheres in 2024. On the other hand, the observed PMC occurrence rates (ORs) were strongly dependent on the observed temperature, and the relationships between the PMC ORs and the 2024 water vapor increases were unclear. Here, to focus on the water vapor injection effects, we have removed the temperature-dependent PMC OR variations with the analysis based on the linear regressions. After removing those, we found clear PMC OR enhancements due to the injected water vapor effects in 2024 in both hemispheres. In the latitudes of 65-81ºS in January 2024, the observed increase in the PMC OR was +15.5%, and the increase of +15.7% was observed in the water vapor VMR. Thus, these results would demonstrate the first observational evidence in the polar mesospheric cloud response to the eruption-induced water vapor injection.
The discoveries of PMCs were documented in late 1880s, during which industrial human activity led to a dramatic increase in greenhouse gas emissions, such as CO2 and CH4. Then, such greenhouse gas emissions might lead to the cooling and water vapor increase in the mesosphere. Thus, the possibility that the human activity-related effects might promote PMC activity has been long discussed. On the other hand, a large eruption of Krakatoa occurred during the same period, and the impact of eruptions on PMC activity has also long been discussed. In the proposed scenario, the eruption might promote PMC activity by cooling the mesosphere and injecting water vapor. However, large eruptions such as Krakatoa are rare, and the eruption-induced effects on PMC activity have never been well evaluated based on observational data.
On 15 January 2022, the large eruption of the submarine volcano Hunga Tonga-Hunga Ha'apai (HTHH) in the South Pacific injected an unprecedented amount of water vapor into the middle atmosphere, increasing its concentration by approximately 10% over HTHH, as observed by the Microwave Limb Sounder (MLS) onboard the Aura satellite. Then, it has been reported that the injected water vapor was spreading to higher latitudes and higher altitudes after the injection. If the injected water vapor can reach high-latitude mesopause regions, eruption-induced effects on PMC activity could be expected.
To examine this issue, in the present study, we have investigated eruption-induced effects on PMC activity based on satellite observations for the first time. In our data analysis, we have used PMC data from 2015 to 2024, obtained by the Japanese geostationary satellites, Himawari-8 and Himawari-9. In addition, temperature and water vapor volume mixing ratio (VMR) data in the mesospheric heights from Aura/MLS have been analyzed. As a result, the observed water vapor VMR showed a significant increase in January 2024 at the southern high latitudes (65-81ºS) and a significant increase in July 2024 at the northern high latitudes (65-81ºN). These results would indicate that the eruption-injected water vapor was spreading for two years after the eruption in January 2022, and it reached around the high-latitude mesopause regions in both hemispheres in 2024. On the other hand, the observed PMC occurrence rates (ORs) were strongly dependent on the observed temperature, and the relationships between the PMC ORs and the 2024 water vapor increases were unclear. Here, to focus on the water vapor injection effects, we have removed the temperature-dependent PMC OR variations with the analysis based on the linear regressions. After removing those, we found clear PMC OR enhancements due to the injected water vapor effects in 2024 in both hemispheres. In the latitudes of 65-81ºS in January 2024, the observed increase in the PMC OR was +15.5%, and the increase of +15.7% was observed in the water vapor VMR. Thus, these results would demonstrate the first observational evidence in the polar mesospheric cloud response to the eruption-induced water vapor injection.