Eyewall Tilt Viewed from a Moist Adiabatic Ascending Perspective

According to the gas state equation, the increase in volume of a moist adiabatically ascending air parcel is caused by latent heating and ambient pressure decrease. The results show that the contribution of pressure decrease to the change of parcel volume is more than 80% at lower levels in a subtropical environment and it further increases with height because of the dilution of latent heating. The increase in volume of a rising air parcel is redistributed between vertical thickness and horizontal area through the variation of vertical velocity. This implies that when a specific environmental thermal profile is given, the change of the horizontal area of an ascending parcel is related to the vertical velocity uniquely. Basing on this analysis, a simple model with respect to eyewall tilt can be set up. Given steady sinking motion in the eye and the ascending motion in the eyewall maximized in the mid-troposphere, the profile of equivalent potential temperature expands (shrinks) at high (low) level, which is related with deceleration (acceleration) of vertical velocity, qualitatively representing the eyewall tilt. It is confirmed that there is no relationship between the eyewall tilt and tropical cyclone intensity, and the eyewall tilt is associated with TC size. This eyewall tilt model is built without any balance constraint and can be used in nonbalance situation, complementing the understanding of eyewall tilt dynamics from an alternative view.