17:15 〜 19:15
[PEM12-P03] Investigation on relationships among polar mesospheric cloud, temperature, and water vapor based on simultaneous observations by Himawari-8/AHI and Aura/MLS
キーワード：極中間圏雲、温度、水蒸気、ひまわり8号/AHI、オーラ/MLS
Polar mesospheric clouds (PMCs) are water ice clouds that can form in the mesopause region at altitudes of 80 to 85 km during summer, mainly in the polar region. Hence, the condition of supersaturation in water vapor would be vitally important for PMC formation, and it is supposed to be determined mainly by temperature and water vapor in the background atmosphere. Thus, it would be important to understand relationships among PMC, temperature, and water vapor. A previous study based on the latitudinal averaged analysis reported that the ice water content (IWC) in PMCs was generally controlled by temperature. In addition, they suggested that the relationship between the IWC and water vapor volume mixing ratio (VMR) changed seasonally from the cooling phase to the warming phase, implying more complexity in water vapor effects.
In this study, to advance our understanding of temperature and water vapor in PMC formation, we have investigated simultaneous observation events of the temperature, water vapor VMR, and PMC. The PMC data were obtained from limb-viewing PMC observations by Advanced Himawari Imager (AHI) onboard the geostationary satellite, Himawari-8. The field of view (FOV) of Himawari-8/AHI is fixed on the ground because of its geostationary orbit. The temperature and water vapor VMR data were obtained from the Microwave Limb Sounder (MLS) onboard the sun-synchronized polar orbit satellite, Aura. Thus, when the moving Aura/MLS FOV overlaps with the fixed Himawari-8/AHI FOV, we are able to obtain simultaneous observation events. In data analysis, we focused on the northern summer at higher latitudes. The analyzed data covered May to August in 2016-2022, and its latitudes were 80-81ºN. As for the Aura/MLS data, we selected data at 81.5 km.
As a result of the analysis, we obtained 12,953 simultaneous observation events, including 5,809 PMC detection events and 7,144 non-PMC events between 2016 and 2022. The obtained events were classified by grids with intervals of 2.5 K in temperature and 0.5 ppmv in water vapor VMR. Then, we calculated PMC occurrence rates for each grid. The results showed that the PMC occurrence rate was generally increasing with decreasing temperature. In cases of lower temperature (<152.5 K), the relationship between PMC occurrence rate and water vapor VMR was unclear. Then, in cases of higher temperature (>152.5 K), increases in the water vapor mixing ratio from 4.5 to 7.0 ppmv led to 17.9-44.2% increases in PMC occurrence rate. These results suggest that the supersaturation condition for the PMC formation can be satisfied in lower temperatures even with a relatively small amount of water vapor. On the other hand, in higher temperatures, a greater amount of water vapor would be needed to satisfy the supersaturation condition. Thus, our investigation implies an importance in the water vapor for PMC formation in such higher temperature conditions.
In this study, to advance our understanding of temperature and water vapor in PMC formation, we have investigated simultaneous observation events of the temperature, water vapor VMR, and PMC. The PMC data were obtained from limb-viewing PMC observations by Advanced Himawari Imager (AHI) onboard the geostationary satellite, Himawari-8. The field of view (FOV) of Himawari-8/AHI is fixed on the ground because of its geostationary orbit. The temperature and water vapor VMR data were obtained from the Microwave Limb Sounder (MLS) onboard the sun-synchronized polar orbit satellite, Aura. Thus, when the moving Aura/MLS FOV overlaps with the fixed Himawari-8/AHI FOV, we are able to obtain simultaneous observation events. In data analysis, we focused on the northern summer at higher latitudes. The analyzed data covered May to August in 2016-2022, and its latitudes were 80-81ºN. As for the Aura/MLS data, we selected data at 81.5 km.
As a result of the analysis, we obtained 12,953 simultaneous observation events, including 5,809 PMC detection events and 7,144 non-PMC events between 2016 and 2022. The obtained events were classified by grids with intervals of 2.5 K in temperature and 0.5 ppmv in water vapor VMR. Then, we calculated PMC occurrence rates for each grid. The results showed that the PMC occurrence rate was generally increasing with decreasing temperature. In cases of lower temperature (<152.5 K), the relationship between PMC occurrence rate and water vapor VMR was unclear. Then, in cases of higher temperature (>152.5 K), increases in the water vapor mixing ratio from 4.5 to 7.0 ppmv led to 17.9-44.2% increases in PMC occurrence rate. These results suggest that the supersaturation condition for the PMC formation can be satisfied in lower temperatures even with a relatively small amount of water vapor. On the other hand, in higher temperatures, a greater amount of water vapor would be needed to satisfy the supersaturation condition. Thus, our investigation implies an importance in the water vapor for PMC formation in such higher temperature conditions.