Influence of Low-Level, High-Entropy Air in the Eye on Tropical Cyclone Intensity: A Trajectory Analysis

Abstract

As suggested by previous studies, the entrainment of the low-level high-entropy eye air can provide additional energy for tropical cyclone (TC) intensification, but the previous trajectory analysis only indicated that considerable air parcels below the eye inversion could be entrained into the eyewall. In the present study, the 1 min output data from a semi-idealized experiment are utilized to quantitatively evaluate the relative importance of the entrainment of the high-entropy eye air by enhancing the eyewall convection.

It is confirmed that a significant amount of high-entropy eye air below the eye inversion can be entrained into the eyewall. The entrainment occurs favorably on the quadrants of enhanced eyewall convection and is enhanced in the presence of small-scale disturbances in the inner edge of the eyewall. However, the eyewall air parcels below 3 km experience a fast cycling. There are 84.4 % and 7.7 % eyewall air from the low-level inflow and the middle-level dry environment, respectively. The low-level high-entropy eye air only accounts for 1.7 % of the eyewall air, whereas 6.2 % eyewall air remains in the eyewall below 3 km during the 90 min period. The eye air from the low-level high-entropy reservoir accounts for 5.8 % of the equivalent potential temperature change below 3 km and 4.5 % of the total mass transport at 3 km in the TC eyewall. The present study suggests that the low-level high-entropy air from the eye has little direct influence on TC intensity through enhancing the eyewall convection by providing relatively small mass and thermodynamic contributions.