Effects Of Vertical Wind Shear On Inner-Core Thermodynamics Of The Tropical Cyclone

Over the past thirty years,the intensity forecast of tropical cyclones(TCs)has made a little progress.TC-environment interaction is one of the most important processes which result in the uncertainty of intensity forecast.Of all the environmental factors,the vertical wind shear(VWS)has been widely recognized as important for the TC intensity change.The VWS can induce strong asymmetries,both in the dynamical and thermodynamic structure in the TC's inner-core region,leading to the intensity change of TC.It's found by the previous studies that the TC intensity change is closely related to the thermodynamic evolution.Therefore,complete understanding of the effect of VWS on the inner-core thermodynamic evolution of the tropical cyclone is important for the improvement of TC intensity forecast.The tropical cyclone extracts energy from the ocean and converted it into the kinetic energy through the "heat engine",in which the moist entropy is an important thermodynamic state variable.In the real atmosphere,during the development of TCs,the microphysics includes sophisticated ice processes.Therefore,to provide a comprehensive method to explore the characteristics of thermodynamic evolution of the TCs,it's necessary to derive a formula of moist entropy which includes the ice processes under the conditions of non-equilibrium phase changes.Starting from the basic thermodynamic laws,this study derives three formulas that include the ice processes,through the detailed analysis of moist entropy change during the phase change.We found that the moist entropy is related with the entropy of dry air and the total water substance,the heat absorption and release and the change of chemical potentials during the non-equilibrium phase changes.Three governing equations of moist entropy including the ice processes have been derived and proved to be equivalent.The equivalent potential temperature has been defined and its relationship with moist entropy is derived.The formulations of moist entropy have been simplified through reasonable assumptions and the governing equations of moist entropy have also been derived,providing a convenient method to diagnostic inner-core thermodynamic evolution of the TCs.In order to study the characteristics of thermodynamic evolution of the tropical cyclone in the VWS and investigate its relationship with the TC intensity,a 3D full physic mesoscale numerical model is used to perform idealized simulations,using an idealized vortex and balanced baroclinic field as the initial condition.We found that the TC intensity is highly correlated with the radial gradient of moist entropy across the eyewall from the axisymmetric point of view.There is an increase of moist entropy within the low-to-mid level eyewall while little change of entropy is found in the boundary layer(BL)and outside the eyewall,when the VWS is absent or weak.Three primary characteristics of inner-core moist entropy evolution are found in the presence of strong VWS:entropy reduction within the mid-level eyewall and the BL,and entropy increase outside the eyewall above the BL.The three features work cooperatively to reduce the radial gradient of moist entropy across the eyewall,and thus the TC intensity.Detailed budget analysis of moist entropy suggests that it's the shear-induced asymmetries that determine the inner-core thermodynamic evolution.Radial eddy flux has been considerably enhanced and the vertical eddy flux has a second maximum in the mid-level eyewall which does not appear in the absence of VWS.The asymmetric convection is organized in such a way that the lower-entropy air is located in the region where the radial inflow and relative downward motion dominate while the higher-entropy air coincides with the outflow and updrafts.Therefore,not only does the radial eddy flux intrude the lower-entropy air into the eyewall,but the vertical eddy flux can also frustrate the moist entropy in the mid-level eyewall.This suggests that a complex 3D numerical model will be required to evaluate the ventilation hypotheses.In the BL,strong convection is concentrated in the downshear-left quadrant with persistent precipitation,flushing the mid-level low entropy air into the BL.The low entropy air is then swept into the eyewall through asymmetric BL inflows.The low entropy air enhances the thermodynamic disequilibrium between the ocean and atmosphere,thus more energy is extracted.The restoring mechanism of the ocean cannot recover the low entropy downflux when the VWS is strong,thus resulting in the entropy reduction in the BL.In addition to the above two pathways for the mid-level ventilation,an increase of moist entropy is found outside the eyewall above the BL after the imposition of VWS,which is mostly contributed by the convection outside the eyewall initiated by the VWS.Vertical eddy advection associated with these convective activities transports higher-entropy air(mostly contributed by moisture)upwards into the region above the BL,and then further decreases the radial gradient of moist entropy across the eyewall.This feature is believed to be partly responsible for the intensity decrease,and to be related to a potential new pathway that restricts TC intensity in the vertically sheared environmental flow.Considering about the complexity of the environmental VWS in the real atmosphere,this study continues to investigate the inner-core thermodynamic evolution in different shear flows.It's found that the dynamical and thermodynamic structure of vortex scale is largely determined by the orientation of VWS.The entropy in the mid-level eyewall and BL is decreased while the entropy outside the eyewall and above the BL is increased by the VWS.In addition to the low entropy downflux,the radial eddy advection of low entropy air could also contribute to the BL entropy reduction.The entropy increase outside the eyewall from shear-right to downshear side is largely contributed by the wavenumber-1 structure,suggesting the important role of shear-induced rainbands outside the eyewall.Different characteristics are also found in the different shear flows:the entropy reduction in the BL is not obvious when the mean flow at low levels is counter-aligned with the VWS.Further investigation suggests that this difference is largely determined by the degree of inner-core asymmetries in the different shear flows,which could be reasonably interpreted by decomposing the shear flow into a uniform flow and a basic shear flow.