Dissipative Heating In Tropical Cyclone Observations And Its Impact On Secondary Eyewall Formation

Dynamic and numerical studies of tropical cyclones have shown that dissipative heating has a significant impact on the structure and intensity changes of tropical cyclones.This study estimates the characteristics of dissipative heating in the tropical cyclone boundary layer with an internal-boundary-layer structure using tower observations.The influence of dissipative heating in different regions of the vortex circulation on secondary eyewall formation is also investigated with high-resolution numerical experiments.The main conclusions are as follows.Observational results show that there is an internal boundary layer in the typhoon boundary layer when the cyclones made landfall near Zhangpu,Fujian Province.The dissipation rate within the internal boundary layer is much larger than that above the layer,so the vertical integrated dissipative heating containing the internal boundary layer cannot be estimated based on the turbulent spectra method.In addition,the dissipative heating estimated based on the balance assumption above the internal boundary layer is smaller than that estimated by the turbulent spectra method.The dissipative heating integrated between different heights increases with increasing wind speed.Near-surface wind speed in different regions of the tropical cyclone circulation can produce varying dissipative heating.Therefore,in this study we further explore the influence of dissipative heating in different regions of the tropical cyclone circulation on secondary eyewall formation using high-resolution numerical modeling.The secondary eyewall formation strikingly differs in the numerical experiments.In the experiment where dissipative heating in the eyewall region is removed,the potential vorticity ring of inner rainbands can shrink radially inward due to larger potential vorticity flux convergence.Eventually,this potential vorticity ring merges with the potential vorticity ring related to the primary eyewall.The convection in the primary eyewall is,thus,enhanced,leading to an increase in the diabatic heating vertical gradient.As a result,the primary eyewall tilts more radially outward.Such a significantly tilting eyewall produces much evaporation cooling in the lower troposphere and suppresses inner rainband activity.Finally,axisymmetrization of the outer rainband convection predominantly contributes to the secondary eyewall formation in this experiment.Comparatively,the process of secondary eyewall formation in the experiment where dissipative heating is removed over the whole region is similar to the results of the above experiment.In the experiment with dissipative heating in the rapid filamentation zone removed,the potential vorticity ring of inner rainbands contracts radially inward slowly due to relatively small potential vorticity flux divergence.Later on,this ring does not merge with the potential vorticity ring associated with the primary eyewall before the formation of the secondary eyewall.Therefore,weaker convection is observed in the eyewall.The small vertical gradient of diabatic heating leads to a less outward tilting of the eyewall causes.Evaporative cooling due to the eyewall precipitation is less harmful to the inner rainband activity.Resultantly,the inner rainbands propagate radially outward and becomes axisymmetric,resulting in secondary eyewall formation.In contrast,in the experiment where dissipative heating in the outer core is removed,both inner and outer rainbands are active,and the axisymmetrization of convection in inner and outer rainbands plays an important role in the formation of the secondary eyewall.The finding of this study indicate that dissipation can only lead to a small increase in nearsurface temperatures(～0.2 K),comparable to the conventional temperature observation error.However,it can still greatly affect secondary eyewall formation by adjusting spiral rainband activities.Therefore,small thermodynamical perturbations in the boundary layer can significantly influence the inner-core structure of numerically simulated tropical cyclones,especially the eyewall structure change.