Inner-and Outer-Rainband Structures Of Tropical Cyclones, And Outer Rainband Formation And Quasi-periodic Behavior

The structures of inner and outer spiral rainbands, the formation mechanism andquasi-periodic nature of the outer rainbands simulated with TCM4in a quiescent environmentare investigated in the thesis, and the impact of quasi-periodic behavior of the simulated outerrainbands on tropical cyclone (TC) structure and intensity is examined as well.The simulated inner and outer spiral rainbands in the control simulation (CTL) are firstcompared. The inner rainbands are generally active immediately outside the eyewall in therapid filamentation zone, while the outer rainbands are active in regions outside about3timesthe radius of maximum wind. The inner rainbands are characterized by the convectivelycoupled vortex Rossby waves (VRWs). The movement of the outer rainbands follows thelow-level vector winds associated with the azimuthally averaged low-level flow and theradially outward cross-band flow caused by the downdraft-induced cold pool in the boundarylayer. Convective cells in outer rainbands are typical of convective systems and movecyclonically and radially outward (inward) at large (small) radii. Net upward vertical masstransports (VMTs) appear throughout the depth of the troposphere in the whole inner-rainbandregion, while net downward VMTs are found below4km height in the outer-rainband region.In the whole inner-rainband region, only a very shallow layer with net horizontal convergenceappears below2-km height, while a deep layer with net convergence is found below7.5-kmheight with net divergence aloft in the outer-rainband region. The inner rainband shows twotangential wind maxima, respectively, located near the top of the inflow boundary layer and immediately below the upper tropospheric outflow layer. A secondary horizontal windmaximum occurs at about4-km height on the inner edge of the outer rainband. Distinctfeatures of the upwind, middle, and downwind sectors of the outer rainband are alsodiscussed.In CTL, outer spiral rainbands are preferably initiated near the60-km radius, or roughlyabout three times the radius of maximum wind (RMW). After initiation, they generallypropagate radially outward with a mean speed of~5m s-1. They are reinitiatedquasi-periodically with a period between22h and26h in the simulation. While the innerspiral rainbands, which form within a radius of about three times the RMW, are characterizedby the convectively coupled VRWs, the formation of outer spiral rainbands, namely,rainbands formed outside a radius of about three times the RMW, is much more complicated.It is shown that outer spiral rainbands are triggered by the inner-rainband remnantsimmediately outside the rapid filamentation zone and inertial instability in the uppertroposphere. The preferred radial location of initiation of outer spiral rainbands is understoodas a balance between the suppression of deep convection by rapid filamentation and thefavorable dynamical and thermodynamic conditions for initiation of deep convection.The quasi-periodic occurrence of the outer spiral rainbands simulated in CTL is found tobe associated with the boundary layer recovery from the effect of convective downdrafts andthe consumption of convective available potential energy (CAPE) by convection in theprevious outer spiral rainbands. Specifically, once convection is initiated and organized in theform of outer spiral rainbands, it will produce strong downdrafts and consume CAPE. Theseeffects weaken convection near its initiation location. As the rainband propagates outwardfurther, the boundary layer air near the original location of convection initiation takes about10h to recover by extracting energy from the underlying ocean. Convection and thus newouter spiral rainbands will be initiated near a radius of about three times the RMW. This willbe followed by a similar outward propagation and the subsequent boundary layer recovery,leading to a quasi-periodic occurrence of outer spiral rainbands. In response to thequasi-periodic appearance of outer spiral rainbands, the storm intensity experiences a similarquasi-periodic oscillation with its intensity or intensification rate starting to decrease after about4h of the initiation of an outer spiral rainband. The results provide an alternativeexplanation or one of the mechanisms that are responsible for the quasi-periodic(quasi-diurnal) variation in the intensity and in the area of outflow-layer cloud canopy ofobserved TCs.The influence of outer-core surface entropy fluxes (SEFs) on outer rainband activity isinvestigated through conducting sensitivity numerical experiments. The sensitivityexperiments with the outer-core SEF artificially increased and decreased by20%, respectively,also simulate quasi-periodic outer rainband behavior. The larger negative horizontal advectiondue to the greater radial wind and the positive contribution by asymmetric eddies lead to alonger period of outer rainband activity in the SEF-enhanced experiment. The activity ofhealthy outer rianbands in the SEF-reduced simulation significantly limits the TC intensity,whereas such an intensity suppression influence is not pronounced in the SEF-enhancedexperiment. As diabatic heating in outer rainbands enables to strengthen the outer-coretangential wind, the quasi-periodic activity of outer rainbands contributes to thequasi-periodic variations of the inner-core size of the TCs.