A self‐weakening mechanism for tropical cyclones. (26th September 2017)

• Record Type: Journal Article
• Authors: Corsaro, Carlo M.; Toumi, Ralf
• Is Part Of: Quarterly journal of the Royal Meteorological Society. Volume 143:Number 707(2017)
• Page Start: 2585
• Page End: 2599
• Publication Date: 2017-09-26
• Subjects: tropical cyclone -- beta effect -- self‐weakening -- dry intrusion -- recirculation -- back‐trajectories; Meteorology -- Periodicals; 551.5
• DOI: 10.1002/qj.3109
• Languages: English
• ISSNs: 0035-9009

Abstract

Abstract : A mechanism leading to the self‐weakening of tropical cyclones is proposed using the Weather and Research and Forecasting model. A comparison between an experiment with variable Coriolis parameter f and one on an f ‐plane shows that, after the initial intensification, the former is characterized by a smaller intensity. As opposed to the tropical cyclone on the f ‐plane, the one with variable f weakens significantly after reaching maturity. Analyses of the 3D circulation show that the main reason for the weakening is dry intrusion in the mid‐upper troposphere from the west. Once the dry intrusion reaches the inner vortex, strong downdraughts reduce the high equivalent potential temperature in the boundary‐layer inflow. The subsequent updraughts in the eyewall, characterized by lower equivalent potential temperature, are considerably reduced and, consequently, the secondary circulation weakens. Back‐trajectories are used to determine the origin of the dry intrusion. It is found that the air parcels expelled from the storm deep convection into the outflow layer recirculate anticyclonically back into the vortex, causing a self‐weakening of the tropical cyclone. A time span for the recirculation of at least 48 h allows the air parcels to sink substantially before reaching the vortex circulation. Some implications of the intrinsic nature of this process are briefly discussed. Abstract : Air parcels expelled from the eyewall of a tropical cyclone may recirculate back into the tropical cyclone circulation, as shown in the image, because of the meridional gradient of the Earth vorticity and the related β ‐effect. The dry nature of this recirculation causes downdraughts that reduce the equivalent potential temperature in the boundary‐layer inflow. The subsequent updraughts in the eyewall are considerably reduced and thus the tropical cyclone weakens.