The constrained heat engine of a tropical cyclone in vertical wind shear: Lagrangian analysis

Vertical shear of the environmental wind is a major factor for intensity and structural changes of tropical cyclones. The processes that govern these changes are still poorly understood and predicting intensity change of tropical cyclones remains a challenge for operational forecasters. Based on idealised numerical experiments, this work aims to advance our fundamental understanding of tropical cyclone – vertical shear interaction.

Recent research has strengthened the long-standing hypothesis that the frustration of the storm's thermodynamic cycle by intrusion of low-entropy, environmental air is the major constraint on intensity for storms in vertical wind shear. The different pathways through which environmental air enters the tropical cyclone circulation, however, have not been examined in detail yet. In particular, it is an open question to what extent different pathways contribute to the frustration of the thermodynamic cycle.

Here, an extensive trajectory analysis using model data with high temporal resolution is presented. Thermodynamic properties are evaluated along the individual trajectories that constitute the storms' secondary circulation. From the thermodynamic evolution along these trajectories, we construct thermodynamic diagrams representing the ‘heat engine' for tropical cyclones in vertically-sheared and quiescent environments. The diagrams provide a novel diagnostic to study the thermodynamic impact of vertical shear.

Our analysis shows a distinct weakening of the heat engine in the experiment with vertical shear. Besides the weakening, it is found also that vertical shear leads to structural changes, most prominently of the entropy gain in the frictional inflow layer. A ‘ventilation' of the heat engine at mid-levels is not diagnosed in this experiment.