9:15 AM - 9:30 AM
[HTT16-02] Daily variation in winter precipitation isotopes over Hirosaki and associate atmospheric circulation - Case study of a winter season 2019/2020 -
Keywords:snowfall, isotope, water vapor
The precipitation stable isotope ratio is an index that allows inverse estimation of the phase change and circulation process that the water has undergone. Therefore, understanding the variability of precipitation stable isotope ratios, especially snowfall stable isotope ratios, will improve the accuracy of paleoclimate reconstructions and lead to a better understanding of present-day hydrological processes, but it is also necessary to analyze them meteorologically because they are affected by atmospheric circulation. A regional isotope circulation model Iso-RSM (Yoshimura et al., 2010), which incorporates known isotope fractionation processes into an atmospheric general circulation model and a regional meteorological model, can be able to reproduce the isotope ratio variations. In this study, we used modeled isotope ratios of precipitation and water vapor to improve our understanding of the observed isotope variations, and for the processes that did not reproduce the isotope ratio variations, the observed isotope ratio results can be used to improve the model.
In this paper, we report the results of the observed variations of winter precipitation in Hirosaki during the last winter (2019/2020). There were few days of snowfall due to a warm winter and less snowfall, but precipitation from the previous day was collected from December 24th to March 17th, including rainfall days. The oxygen and hydrogen isotope ratios of 40 samples were measured by Picarro spectroscopy at Research Institute for Humanity and Nature. δ18O varied between -1.39‰ and -18.87‰, and δD varied between -9.9‰ and -142.2‰. The trend of isotopic variation of precipitation in Hirosaki by Iso-RSM was qualitatively consistent from mid-January to early March, while that of d-excess was not consistent with observation except for a period in February.
Based on the observed precipitation stable isotope ratios and the water vapor transport field by ERA5 reanalysis, we found the following.
(1) when the isotope ratio is low and the d-excess is high, precipitation is brought to Hirosaki by the rapid evaporation of cold, dry air from Eurasia over the Japan Sea (no moist air from the south);
(2) when the isotope ratio is high and the d-excess is low, precipitation is brought by warm, moist air masses from the southwest;
(3) when the isotope ratio is high and the d-excess is high, the northwest monsoon passes near the Korean Peninsula and the rapid merging of evaporated water vapor in the Sea of Japan and moist air from the southwest is observed.
During the period of the study, we identified three periods that show the precipitation isotope ratio gradually increased over several days. These trends are reproduced in the Iso-RSM. Period-1: on January 10th to 14th, water vapor from (3) mixed with water vapor from (1) resulting in little change in d-excess; Period-2: on February 9th to 13th, water vapor from (2) mixed with water vapor from (1), resulting in an increase in isotope ratio but a decrease in d-excess; Period-3: on February 17th to 21st, air masses from (1) merged with air masses from (3), resulting in a decrease in d-excess; and on February 17th to February 21st, air masses from (1) merged with water vapor from (1), resulting in a decrease in d-excess. During the period-3, a low-pressure system moved northward (17th -19th ) and a local circulation effect (20th -21st) which was strongly affected by Shirakami mountains seems to affect the precipitation isotope characteristics. In order to clarify the cause of the decrease in d-excess during the period, vertical precipitation isotope ratio in the clouds and cloud development process according to the topography should be considered.
In the winter of 2020/2021, we measured not only daily snowfall but also water vapor isotopes once or twice a day. The comparison between the two years and the snowfall process that can be understood from the water vapor isotope variation will also be reported at the meeting.
In this paper, we report the results of the observed variations of winter precipitation in Hirosaki during the last winter (2019/2020). There were few days of snowfall due to a warm winter and less snowfall, but precipitation from the previous day was collected from December 24th to March 17th, including rainfall days. The oxygen and hydrogen isotope ratios of 40 samples were measured by Picarro spectroscopy at Research Institute for Humanity and Nature. δ18O varied between -1.39‰ and -18.87‰, and δD varied between -9.9‰ and -142.2‰. The trend of isotopic variation of precipitation in Hirosaki by Iso-RSM was qualitatively consistent from mid-January to early March, while that of d-excess was not consistent with observation except for a period in February.
Based on the observed precipitation stable isotope ratios and the water vapor transport field by ERA5 reanalysis, we found the following.
(1) when the isotope ratio is low and the d-excess is high, precipitation is brought to Hirosaki by the rapid evaporation of cold, dry air from Eurasia over the Japan Sea (no moist air from the south);
(2) when the isotope ratio is high and the d-excess is low, precipitation is brought by warm, moist air masses from the southwest;
(3) when the isotope ratio is high and the d-excess is high, the northwest monsoon passes near the Korean Peninsula and the rapid merging of evaporated water vapor in the Sea of Japan and moist air from the southwest is observed.
During the period of the study, we identified three periods that show the precipitation isotope ratio gradually increased over several days. These trends are reproduced in the Iso-RSM. Period-1: on January 10th to 14th, water vapor from (3) mixed with water vapor from (1) resulting in little change in d-excess; Period-2: on February 9th to 13th, water vapor from (2) mixed with water vapor from (1), resulting in an increase in isotope ratio but a decrease in d-excess; Period-3: on February 17th to 21st, air masses from (1) merged with air masses from (3), resulting in a decrease in d-excess; and on February 17th to February 21st, air masses from (1) merged with water vapor from (1), resulting in a decrease in d-excess. During the period-3, a low-pressure system moved northward (17th -19th ) and a local circulation effect (20th -21st) which was strongly affected by Shirakami mountains seems to affect the precipitation isotope characteristics. In order to clarify the cause of the decrease in d-excess during the period, vertical precipitation isotope ratio in the clouds and cloud development process according to the topography should be considered.
In the winter of 2020/2021, we measured not only daily snowfall but also water vapor isotopes once or twice a day. The comparison between the two years and the snowfall process that can be understood from the water vapor isotope variation will also be reported at the meeting.