Analysis Of The Energy Evolution Characteristics And Transformation Mechanism Of Typhoon "Rainbow" (2015) Offshore And During Its Landfall

Typhoons and the accompanying gale,heavy rain,storm surge as well as other disastrous weather pose a great threat to the safety of people's lives and property and social stability in coastal areas.China is located on the west coast of the Northwest Pacific and is one of the countries most affected by typhoons.In recent decades,the prediction of typhoon tracks has improved significantly while the situation with intensity have been relatively lagging.This is mainly due to the numerous factors that affect intensity changes,and the physical mechanisms of local energy circulation and vortex intensity changes within typhoons are still unclear.Further explorations of the key processes in the inner core and related physical mechanisms associated the evolution of typhoon have important scientific research and application value.Typhoon Mujigae in 2015 is selected as a case,which is successfully reproduced with the mesoscale numerical model WRF(Weather Research and Forecast),including the generation,development,movement,and landfall.The basic physical quantities such as temperature,wind field,and air pressure are decomposed into symmetric component and asymmetric component along the azimuth.On this basis,the energy is broken down into three components that contain complete information about the energy.From the perspective of the wind speed(kinetic energy),different properties of wind field(vorticity,divergence and deformation),and the co-evolution of the kinetic and thermodynamic fields(potential vorticity),the relevant non-hydrostatic equilibrium budget equations in the cylindrical coordinate system are derived.On this basis,the development and evolution mechanism before and after the typhoon makes landfall are discussed,and the main conclusions obtained are as follows:(1)The non-hydrostatic equilibrium kinetic energy(KE)equation is deduced.It is used to study the evolution and transformation mechanism of local energy.The KE is decomposed into three parts: symmetric component,interaction component and asymmetric component,in which the interaction KE acts as an intermediate variable to connect the interaction between symmetric component and asymmetric component.Diagnostic results of the symmetric KE equation show that the contributions of the total symmetric terms and the asymmetric processes are comparable.In the enhancement phase,both promote the growth of the KE in eyewall,corresponding to the conversion from the interaction energy to the symmetric component,while in the attenuation phase,the negative contribution of the asymmetric processes gradually exceeds the positive contribution of the symmetric processes with the conversion from the symmetric KE to the interaction component in the eyewall,which promotes the weakening of the symmetric kinetic energy.Among the symmetric terms,the radial flow toward lower pressure in the inflow layer is the major generation,which is partly dissipated by the friction and partly transported to the eyewall by the secondary circulation.The KE generated by the symmetrical buoyancy work is mainly located in the middle and upper troposphere.Among the asymmetric processes,the processes that play a decisive role in this term are the radial asymmetric advection term and the vertical asymmetric transport term in which the vertical asymmetric transport as well as upscale energy cascade process is dominant in the strengthening stage and the radial asymmetric transport as well as downscale energy cascade process is dominant in the attenuation stage.(2)The non-hydrostatic equilibrium available potential energy(APE)equation is deduced.The diagnosis results show that the main factors affecting the local change of APE are the baroclinic transformation with KE and the source and sink related to diabatic heating.With Boussinesq approximation,the APE components and KE components are transformed into each other through the coupling effect of vertical velocity and disturbance potential temperature.The source and sink of APE mainly depend on the coupling effect of diabatic heating and disturbance potential temperature.For symmetric APE,the coupling of vertical ascending motion and diabatic heating in middle and high level of eyewall convections is the main production process while the ascending motion of warm zone in eyewall convection is an important energy source of symmetric KE consume symmetric APE;For asymmetric APE,in the enhancement stage,the coupling effect of the asymmetric vertical velocity and diabatic heating acts as source in eyewall and sink inside,which is corresponding to the transformation between the asymmetric APE and KE due to the asymmetric vertical velocity and disturbance potential temperature,but opposite phase.The situation is inverse inside.In the attenuation stage,the inner diabatic heating sink enhances.Correspondingly,the conversion of asymmetric KE to asymmetric APE is dominant.(3)The influence of divergence and deformation on the vortex is analyzed Using the vorticity pseudo-energy,divergence energy and deformation energy equations in cartesian coordinate system.The results of the regional average show that in the boundary layer,divergence energy is transformed into vorticity enstrophy due to the vorticity-divergence transition term,vorticity enstrophy is transformed into deformation energy due to the vorticity-deformation transition term,and deformation energy is transformed into divergence energy due to the divergence-deformation transition term.In the free atmosphere above,the direction of energy pathway is opposite,forming a closed loop from divergence energy to deformation energy,then to vorticity enstrophy.The vorticity-deformation identical term and the divergence-deformation identical term promote the coordinated development of pseudo energy.In terms of the composition of each item,the main influencing factors include divergence coupling process,advection coupling process and tiltingcoupling process.(4)The non-hydrostatic vorticity enstrophy equation is deduced.It is used to study the main processes and physical mechanisms that affect the local vortex intensity.The diagnosis result shows that the large value of the symmetric vorticity enstrophy is mainly concentrated on the inner side of the maximum wind speed radius at the lower levels.In the enhancement phase,the symmetric process mainly depends on the convergence near the low-level eyewall and the upward tilt of the horizontal vorticity inside the eyewall updraft.The contribution of the asymmetric processes mainly come from the asymmetric vorticity advection.The asymmetric radial advection leads to the conversion of the interaction component to the symmetrical component promoting the enhancement of local rotation;In the attenuation stage,the overall change of the symmetry process is not significant,and the asymmetric processes promote the inward contraction of the ring structure at the lower level and accelerates in the middle.The rotation of the system weakened,and the symmetric vorticity is transformed into the interaction component.(5)Finally,the non-hydrostatic potential vorticity(PV)equation is deduced.It is used to study the evolution characteristics and physical mechanism of local PV.The diagnosis results show that the symmetric processes that affect the local changes of symmetric PV include the generation below the symmetric diabatic heating center and the dissipation above,as well as the redistribution by the symmetric secondary circulation.The influence of the asymmetric processes mainly come from the interaction between the symmetric PV and the interaction PV,in which the vertical flux of radial asymmetric vorticity dominates in the enhancement stage,guiding the interaction PV to the symmetric PV,the radial flux of vertical asymmetric vorticity dominates in the decay phase,and the effect is opposite.Asymmetric diabatic heating directly influences the asymmetric PV,and then affects the symmetric PV and the evolution of the system through the interaction component.