17:15 〜 19:15
[AAS02-P15] Movement of a two-dimensional barotropic vortex due to effective-β-gyre
キーワード：熱帯低気圧、進路、βジャイア、数値シミュレーション
1. Introduction
A tropical cyclone (TC) generally moves following the large-scale atmospheric flow in the environmental field, known as the steering flow. However, its movement is also affected by the flow due to the interaction between a storm and a large-scale vorticity. One such effect is the β-gyre effect, which moves a TC to the northwest direction according to the meridional gradient of planetary vorticity in the northern hemisphere.
If we recall that the conventional β-gyre effect stems from the conservation of the reference absolute vorticity, a horizontal gradient of large-scale relative vorticity can play a role on the TC movement as well as planetary vorticity. However, only a few studies have focused on such effects. In this study, we extended the concept of the β-gyre to the "effective-β-gyre", which considers the gradient of large-scale relative vorticity in addition to the planetary vorticity gradient. As a first step, we examined the movement of a 2D barotropic vortex under the influence of the effective-β-gyre, assuming a basic field with relative vorticity gradients on both the f-plane and β-plane.
2. Method
We used the SPMODEL provided by the GFD-DENNOU Club. The governing equation is a 2D non-divergent barotropic vorticity equation that considers the dissipation of vorticity due to viscosity. The numerical calculations were performed using spectral methods, and time integration was done using the fourth-order Runge-Kutta method. The domain is 9600 km by 9600 km, and a time step is 0.02 day.
For the specific setup, we assumed a basic field with north-south relative vorticity gradients on both the f-plane and β-plane. We varied the vortex radius (R = 100, 200, 300, 400, 500 km) and intensity (ζ’max = 2.9×10-4, 5.8×10-4, 11.6×10-4 s-1).
3. Results
In the basic field with a positive vorticity region to the north and a negative vorticity region to the south. On the f-plane, the barotropic vortex moved to the northeast direction. This drift showed a tendency to move more quickly to the northwest direction relative to the basic field as the vortex radius and intensity increased. On the β-plane, a similar trend was seen, but the barotropic vortex moved even more quickly to the northwest direction compared to the f-plane.
Thus, even on the f-plane, without the planetary vorticity gradient, the effective-β-gyre causes the barotropic vortex to drift, with the degree of drift depending on the vortex radius and intensity. On the β-plane, this drift was more pronounced compared to the f-plane. This result implies that in real atmospheric conditions, such as the southern edge of the Pacific high or the westerly jet, where a TC is embedded in the gradient of large-scale relative vorticity in the environmental field, the effective-β-gyre may influence in the movement of TC.
A tropical cyclone (TC) generally moves following the large-scale atmospheric flow in the environmental field, known as the steering flow. However, its movement is also affected by the flow due to the interaction between a storm and a large-scale vorticity. One such effect is the β-gyre effect, which moves a TC to the northwest direction according to the meridional gradient of planetary vorticity in the northern hemisphere.
If we recall that the conventional β-gyre effect stems from the conservation of the reference absolute vorticity, a horizontal gradient of large-scale relative vorticity can play a role on the TC movement as well as planetary vorticity. However, only a few studies have focused on such effects. In this study, we extended the concept of the β-gyre to the "effective-β-gyre", which considers the gradient of large-scale relative vorticity in addition to the planetary vorticity gradient. As a first step, we examined the movement of a 2D barotropic vortex under the influence of the effective-β-gyre, assuming a basic field with relative vorticity gradients on both the f-plane and β-plane.
2. Method
We used the SPMODEL provided by the GFD-DENNOU Club. The governing equation is a 2D non-divergent barotropic vorticity equation that considers the dissipation of vorticity due to viscosity. The numerical calculations were performed using spectral methods, and time integration was done using the fourth-order Runge-Kutta method. The domain is 9600 km by 9600 km, and a time step is 0.02 day.
For the specific setup, we assumed a basic field with north-south relative vorticity gradients on both the f-plane and β-plane. We varied the vortex radius (R = 100, 200, 300, 400, 500 km) and intensity (ζ’max = 2.9×10-4, 5.8×10-4, 11.6×10-4 s-1).
3. Results
In the basic field with a positive vorticity region to the north and a negative vorticity region to the south. On the f-plane, the barotropic vortex moved to the northeast direction. This drift showed a tendency to move more quickly to the northwest direction relative to the basic field as the vortex radius and intensity increased. On the β-plane, a similar trend was seen, but the barotropic vortex moved even more quickly to the northwest direction compared to the f-plane.
Thus, even on the f-plane, without the planetary vorticity gradient, the effective-β-gyre causes the barotropic vortex to drift, with the degree of drift depending on the vortex radius and intensity. On the β-plane, this drift was more pronounced compared to the f-plane. This result implies that in real atmospheric conditions, such as the southern edge of the Pacific high or the westerly jet, where a TC is embedded in the gradient of large-scale relative vorticity in the environmental field, the effective-β-gyre may influence in the movement of TC.