Beta-effect on the evolution of tropical cyclone

The influence of the variable planetary vorticity on the evolution of tropical cyclone has been discussed a lot in the literatures. It is well known that, the tropical cyclone in the northern hemisphere moves northwestward under the impact of β-effect. However, as the role of β-effect played in the intensification of tropical cyclone is concerned, it seems that no consensus has been achieved. In this work, the β-effect on the development of tropical cyclone is re-investigated with the cloud-resolving simulation with the Weather Research and Forecast (WRF-ARW) model.

It is found that the cyclone developed in the environment with variable planetary vorticity is weaker while wider than its counterpart developed in the f-plane. Such differences are mainly induced by the meridional gradient of planetary vorticity across the cyclone circulation (βy) instead of the latitudinal variation of the cyclone center (f_c ) in the expression of Coriolis paramenter in β-plane: f=f_c+βy. Due to the meridional variation of planetary vorticity across the cyclone, β-shear develops gradually along with the intensification of the cyclone. As a result, the convection in the downshear left part of the cyclone (east-northeast quadrant of cyclone) is more active and wider than that in other quadrants of cyclone, which causes considerable low-level diabatic cooling in this area. Along with the intensification of the cyclone, the vertical wind shear is greatly weakened after it reaches the peak value and shifts anticyclonically due to the significant changes occurred in the upper-level circulation. Simultaneously, the cyclone is axi-symmetrized and the cold air appeared in the east-northeast quadrant of cyclone expands azimuthally and finally circles the primary eyewall and forms an annular with high relative humidity and low CIN. Subsequently, convection burst in this region. It is the huge latent heat released outside of the primary eyewall that is responsible for the weak intensity and large size of tropical cyclone in the environment with variable planetary vorticity.

Further sensitive experiments show that the intensification ratio of the cyclone in the environment with variable planetary vorticity depends on the structure of the initial vortex. In the case that the initial vortex is shallow, the β-effect tends to slow down the development of the cyclone remarkably while the β-effect seems to play a trivial role in the spin-up of cyclone when the initial vortex is very deep. Despite of the different developing speeds, both cyclones evolved from shallow and deep vortices have wider circulation while weaker maximum intensity in the environment with variable planetary vorticity as compared with that developed in the f-plane.