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[ACG32-P03] Zonal snow clouds over the Sea of Japan enhanced by zonal high sea surface temperatures
Keywords:Hokkaido, cirrocumulus clouds, Domain Meteorological Models, Pressure adjustment mechanism, Vertical mixing mechanism
1. Introduction
In Iwamizawa, Sorachi District, Hokkaido, westerly winds in winter tend to bring in snow clouds from Ishikari Bay located to the east. Previous studies have shown that zonal snow clouds extending in a north-south direction from the Mamiya Strait to the west coast of Hokkaido and snow clouds extending in a northwest-southeast direction from downwind of the coastal mountains to the west coast of Hokkaido are likely to appear in winter. These bands are called the Okabayashi Band and the Muramatsu Band, respectively. The snow clouds that develop along the west coast of Hokkaido often bring heavy snowfall to the Sea of Japan side of Hokkaido, with annual snowfall in Iwamizawa reaching 624 cm on average.
Iwamizawa, one of the heaviest snowfall areas in Hokkaido, experienced the largest snowfall in its recorded history from December 21 to 23, 2023. At that time, banded snow clouds associated with the Okabayashi band mainly in the first half of the period and the Muramatsu band mainly in the second half of the period were observed. In addition, a band of warmer-than-normal sea water temperatures was distributed over the Sea of Japan along the western coast of Hokkaido, corresponding to the banded snow clouds. The purpose of this study is to clarify the influence of the zonal high sea surface temperature region on the zonal snow clouds in this case.
2. data and analysis method
Numerical experiments were conducted using the domain meteorological model WRF ver. 4.5 to verify the effect of sea surface temperature on snow clouds and snow cover. ERA5 was used for initial and boundary values, and OISST was used for lower boundary values. The computational domain is 600 km in both the east-west and north-south directions centered at 44°N and 140°E. The horizontal resolution is 1 km, the number of vertical layers is 33, and the computational period is from midnight on the 20th, the day before the heavy snowfall, to 18:00 on the 23rd. The results are compared with the results obtained by integrating the actual SSTs (Ex. ctl) and the results obtained by modifying only the SSTs to the climatological values from 1991 to 2020 (Ex. clim).
3. Results and Discussion
The cumulative snowfall at Iwamizawa from 0:00 on the 21 to 18:00 on the 23 was 65 cm in the Ex. ctl and 50 cm in the Ex. clim, and the difference in cumulative snowfall between the ctl and Ex. clim was positive in most areas, especially from the Ishikari Bay coast to the Sorachi region, reaching over 40 cm in some areas.
Next, we examined the lower-level horizontal convergence at 12 hours, when the Okabayashi and Muramatsu bands were distinct. The results showed that the lower-level horizontal convergence associated with the banded snow clouds was enhanced in the Ex. ctl. Detailed hourly analysis revealed that the Okabayashi band lasted longer along the west coast of Hokkaido in the Ex. ctl and took longer to move eastward and become obscured after landing in Hokkaido. The Muramatsu band was found to have enhanced hourly convergence.
The average wind speed during the 12-hour period when the Okabayashi band was well-defined showed that the wind speed in Ex. ctl decreased zonally near the Okabayashi band, indicating that westerly winds were blowing in this region in Ex. clim and that easterly wind anomalies were created by the high sea surface temperature region in the band. This may be caused by a pressure adjustment mechanism due to the strengthening of the east-west sea-land heat contrast between the low temperature onshore Hokkaido and the high temperature west coast of Hokkaido caused by rising sea water temperatures. On the other hand, the average wind speed during the 12-hour period when the Muramatsu band was clear was enhanced by the wind speed of Ex. ctl in the band around the Muramatsu band. This result suggests that the vertical mixing mechanism, caused by the increase in the vertical temperature difference between the cold air from the continent and the warmer sea surface temperature, may have been responsible for the enhanced wind speeds and the enhanced convergence.
4. Conclusion
In this study, numerical experiments were conducted to clarify the effect of zonal high sea surface temperatures on zonal snow clouds in the case of the largest snowfall in Iwamizawa. The results show that the zonal high SSTs increased snowfall and precipitation by 20 to 30%, contributing to the largest snowfall in recorded history. Furthermore, the results suggest that the zonal high sea surface temperatures increased the duration of the Okabayashi band by a pressure adjustment mechanism and enhanced the Muramatsu band by a vertical mixing mechanism.
In Iwamizawa, Sorachi District, Hokkaido, westerly winds in winter tend to bring in snow clouds from Ishikari Bay located to the east. Previous studies have shown that zonal snow clouds extending in a north-south direction from the Mamiya Strait to the west coast of Hokkaido and snow clouds extending in a northwest-southeast direction from downwind of the coastal mountains to the west coast of Hokkaido are likely to appear in winter. These bands are called the Okabayashi Band and the Muramatsu Band, respectively. The snow clouds that develop along the west coast of Hokkaido often bring heavy snowfall to the Sea of Japan side of Hokkaido, with annual snowfall in Iwamizawa reaching 624 cm on average.
Iwamizawa, one of the heaviest snowfall areas in Hokkaido, experienced the largest snowfall in its recorded history from December 21 to 23, 2023. At that time, banded snow clouds associated with the Okabayashi band mainly in the first half of the period and the Muramatsu band mainly in the second half of the period were observed. In addition, a band of warmer-than-normal sea water temperatures was distributed over the Sea of Japan along the western coast of Hokkaido, corresponding to the banded snow clouds. The purpose of this study is to clarify the influence of the zonal high sea surface temperature region on the zonal snow clouds in this case.
2. data and analysis method
Numerical experiments were conducted using the domain meteorological model WRF ver. 4.5 to verify the effect of sea surface temperature on snow clouds and snow cover. ERA5 was used for initial and boundary values, and OISST was used for lower boundary values. The computational domain is 600 km in both the east-west and north-south directions centered at 44°N and 140°E. The horizontal resolution is 1 km, the number of vertical layers is 33, and the computational period is from midnight on the 20th, the day before the heavy snowfall, to 18:00 on the 23rd. The results are compared with the results obtained by integrating the actual SSTs (Ex. ctl) and the results obtained by modifying only the SSTs to the climatological values from 1991 to 2020 (Ex. clim).
3. Results and Discussion
The cumulative snowfall at Iwamizawa from 0:00 on the 21 to 18:00 on the 23 was 65 cm in the Ex. ctl and 50 cm in the Ex. clim, and the difference in cumulative snowfall between the ctl and Ex. clim was positive in most areas, especially from the Ishikari Bay coast to the Sorachi region, reaching over 40 cm in some areas.
Next, we examined the lower-level horizontal convergence at 12 hours, when the Okabayashi and Muramatsu bands were distinct. The results showed that the lower-level horizontal convergence associated with the banded snow clouds was enhanced in the Ex. ctl. Detailed hourly analysis revealed that the Okabayashi band lasted longer along the west coast of Hokkaido in the Ex. ctl and took longer to move eastward and become obscured after landing in Hokkaido. The Muramatsu band was found to have enhanced hourly convergence.
The average wind speed during the 12-hour period when the Okabayashi band was well-defined showed that the wind speed in Ex. ctl decreased zonally near the Okabayashi band, indicating that westerly winds were blowing in this region in Ex. clim and that easterly wind anomalies were created by the high sea surface temperature region in the band. This may be caused by a pressure adjustment mechanism due to the strengthening of the east-west sea-land heat contrast between the low temperature onshore Hokkaido and the high temperature west coast of Hokkaido caused by rising sea water temperatures. On the other hand, the average wind speed during the 12-hour period when the Muramatsu band was clear was enhanced by the wind speed of Ex. ctl in the band around the Muramatsu band. This result suggests that the vertical mixing mechanism, caused by the increase in the vertical temperature difference between the cold air from the continent and the warmer sea surface temperature, may have been responsible for the enhanced wind speeds and the enhanced convergence.
4. Conclusion
In this study, numerical experiments were conducted to clarify the effect of zonal high sea surface temperatures on zonal snow clouds in the case of the largest snowfall in Iwamizawa. The results show that the zonal high SSTs increased snowfall and precipitation by 20 to 30%, contributing to the largest snowfall in recorded history. Furthermore, the results suggest that the zonal high sea surface temperatures increased the duration of the Okabayashi band by a pressure adjustment mechanism and enhanced the Muramatsu band by a vertical mixing mechanism.