5:15 PM - 7:15 PM
[AAS02-P01] Effect of northward ageostrophic wind on PRE enhancement: Secondary circulation
★Invited Papers
Keywords:Typhoon PrePredecessor Rain Event, ageostrophic wind, Secondary Circulation
When a typhoon is located over the southern seas of Japan, heavy rainfall can occur on the mainland. This phenomenon is often described as a "typhoon stimulating the front." Similar cases of typhoon-induced remote precipitation enhancement have been observed overseas and are referred to as Predecessor Rain Events (PRE).
During the approach of Typhoon No. 18 in 2009, a significant northward ageostrophic wind crossing the contour lines was observed in western Japan. Saito (2019) [1] conducted numerical experiments using the JMA-NHM model with a horizontal grid spacing of 10 km and demonstrated that this northward ageostrophic wind could be reproduced in the numerical model (Figure 1, right). The study also showed that the phenomenon could be explained dynamically using the horizontal wind acceleration vector.
Saito and Matsunobu (2020) [2] focused on the water vapor flux associated with the northward wind from the typhoon. They found that while the ageostrophic wind was more pronounced in the upper levels, numerical experiments indicated that when the water vapor amount corresponding to the contribution of the ageostrophic wind was removed, precipitation in western Japan decreased significantly.
Saito et al. (2022) [3] conducted a follow-up study using a cloud-resolving model with a horizontal grid spacing of 2 km. They demonstrated that the presence or absence of upper-level moisture associated with the ageostrophic wind affects the amount of condensation, the thickness of the moist absolutely unstable layer (MAUL), and the strength of convection in Japan, ultimately influencing precipitation levels.
The aforementioned studies published in SOLA provided important insights into how northward ageostrophic winds associated with typhoons contribute to increased rainfall in Japan. However, the numerical experiments only examined the impact of the ageostrophic wind in terms of upper-level moisture increase due to water vapor transport.
It is known that when northward winds caused by ageostrophic flow occur at the entrance of a jet stream, large-scale ascending motion due to secondary circulation can develop, potentially enhancing precipitation. To investigate the extent to which ascending motion associated with secondary circulation of ageostrophic winds affects precipitation near Japan, an experiment is currently being conducted. A 5 km grid model is nested within a 10 km model, and only the ascending motion from a dry model is subtracted as a boundary condition before running the 5 km model to analyze the resulting changes in precipitation. The results of this experiment will be presented at the conference.
[1] Saito, K., 2019: On the northward ageostrophic winds associated with a tropical cyclone. SOLA, 15, 222-227.
[2] Saito, K. and T. Matsunobu, 2020: Northward ageostrophic winds associated with a tropical cyclone. Part 2: Moisture transport and its impact on PRE. SOLA, 16, 198-205.
[3] Saito, K., T. Matsunobu, and T. Oizumi, 2022: Effect of upper-air moistening by northward ageostrophic winds associated with a tropical cyclone on the PRE enhancement. SOLA, 18, 81-87.
During the approach of Typhoon No. 18 in 2009, a significant northward ageostrophic wind crossing the contour lines was observed in western Japan. Saito (2019) [1] conducted numerical experiments using the JMA-NHM model with a horizontal grid spacing of 10 km and demonstrated that this northward ageostrophic wind could be reproduced in the numerical model (Figure 1, right). The study also showed that the phenomenon could be explained dynamically using the horizontal wind acceleration vector.
Saito and Matsunobu (2020) [2] focused on the water vapor flux associated with the northward wind from the typhoon. They found that while the ageostrophic wind was more pronounced in the upper levels, numerical experiments indicated that when the water vapor amount corresponding to the contribution of the ageostrophic wind was removed, precipitation in western Japan decreased significantly.
Saito et al. (2022) [3] conducted a follow-up study using a cloud-resolving model with a horizontal grid spacing of 2 km. They demonstrated that the presence or absence of upper-level moisture associated with the ageostrophic wind affects the amount of condensation, the thickness of the moist absolutely unstable layer (MAUL), and the strength of convection in Japan, ultimately influencing precipitation levels.
The aforementioned studies published in SOLA provided important insights into how northward ageostrophic winds associated with typhoons contribute to increased rainfall in Japan. However, the numerical experiments only examined the impact of the ageostrophic wind in terms of upper-level moisture increase due to water vapor transport.
It is known that when northward winds caused by ageostrophic flow occur at the entrance of a jet stream, large-scale ascending motion due to secondary circulation can develop, potentially enhancing precipitation. To investigate the extent to which ascending motion associated with secondary circulation of ageostrophic winds affects precipitation near Japan, an experiment is currently being conducted. A 5 km grid model is nested within a 10 km model, and only the ascending motion from a dry model is subtracted as a boundary condition before running the 5 km model to analyze the resulting changes in precipitation. The results of this experiment will be presented at the conference.
[1] Saito, K., 2019: On the northward ageostrophic winds associated with a tropical cyclone. SOLA, 15, 222-227.
[2] Saito, K. and T. Matsunobu, 2020: Northward ageostrophic winds associated with a tropical cyclone. Part 2: Moisture transport and its impact on PRE. SOLA, 16, 198-205.
[3] Saito, K., T. Matsunobu, and T. Oizumi, 2022: Effect of upper-air moistening by northward ageostrophic winds associated with a tropical cyclone on the PRE enhancement. SOLA, 18, 81-87.