Balanced And Unbalanced Dynamics Of Tropical Cyclone Intensification

The balanced dynamics is largely applied to explain the mechanism of tropical cyclone(TC)intensification.However,some studies showed that TC intensification in the boundary layer could not be explained by the balanced dynamics,but rather the result of surface friction and its induced unbalanced dynamics.To investigate the balanced and unbalanced contributions to TC intensification and improve our understanding on TC intensification,the individual effects of diabatic heating in the eyewall and surface friction in the bounary layer to the spinup of the simulated storm are analyzed using the Tropical Cyclone Model(TCM,Version 4).The results confirm that the balanced response to eyewall heating is the major mechanism for TC intensification and the radial distribustions of diabatic heating and inertial stability in the balanced dynamics can largely explain TC intensification,even within the boundary layer where the balanced assumption is invalid due to the presence of surface friction.The effect of surface friction and its induced unbalanced processes to TC intensification are examined.The results show that surface friction largely enhanced the boundary layer inflow and the contractionof the radius of maximum wind(RMW),the positive tangential wind tendency resulting from the frictionally induced inward absolute angular momentum(AAM)transport in the bounary layer is not large enough to offset the negative tendency due to the direct fricitonal loss of AAM to the surface.As a result,the tendency near the RMW is negative,that means,the net effect of surface friction in the boundary layer to TC intensification is negative.Results from the Sawyer-Eliassen equation suggest that the balanced response to diabatic heating in the eyewall is the major mechanism to TC intensification and the unbalanced dynamics due to the presence of surface friction seems to spin up the tangential wind in the surface layer near the RMW where the flow is strongly subgradient while spin down the tangential wind immediately above where the flow is strongly supergradient.Although surface friction shows an overall net negative effect on TC intensification,it plays a critical role in producing the structure of the supregradient wind in the boundary layer.The importance of the radial location of eyewall heating can be further evidenced by results from an analysis on the contribution of surface entropy fluxes in the eye region to TC intensification rate(IR).The results show that surface entropy fluxes in the eye region contributes about 42%to the IR of the simulated storm.The number of Convective bursts(CBs)and the rainfall rate averaged in the inner-core region did not display significant differences with the change of surface entropy fluxes in the eye region.This suggests that the effect of the near-surface high energy air in the eye region on the simulated TC IR was not through the modifications to the overall strength of eyewall convection as previously hypothesized rather than resulting in a quicker eyewall contraction and the larger RMW.The surface entrapy fluxes can initiate convection near the inner edge of the eyewall and facilitates eyewall contraction,leading to higher inner-core inertial stability and thus higher dynamical efficiency of eyewall heating in spinning up the tangential winds near the RMW and larger IR of the simulated TC.Finally,based on the balanced dynamics,the role of the inner and outer wind profiles of an initial TC vortex is exmamined.The results show that the wind inside the RMW determines the timing of TC spinup.Stronger winds inside the RMW favors earlier spin up of the tangential wind when compared with the weaker wind inside the RMW.It is because that the stronger inner wind in the eye accelerates the air evaporation and thus increases the entropy of the air.When the air entrains into the eye wall,it will effectively promote the deep convection,resuting more diabatic heat release in the eyewall region.Simultaneously,there is a sharp contraction of the RMW which increases the inner-core inertial stability,rasing the efficiency of heating rate in the eyewall and thus more TC intensification.The wind outside the RMW plays an important role in the intensity and size of a TC both.The stronger outer wind in a TC leads to larger surface entropy fluxes therein,which is favorable to the formation of outer spiral rainbands.The diabatic heat released from the outer rainbands can increase the radial inflow and tangential wind in the outer region,which contributues negatively to the intensification of the TC while positively to its size increase.