| Although tropical cyclone (TC) track forecasts have been improved substantially over the past couple decades, few improvements have been achieved for TC intensity forecasts (Rappaport et al.2009). Rapid intensification (RI) events are particularly challenging in that they are almost always missed by operational forecasts (Elsberry et al.2007).The lack of observation of the inner core in RI TCs limits the intensity forecast on large degree. Based on ground-based Doppler radar data with high-temporal resolution, our studies focus on two near-coast RI cases, i.e., Vicente(1208) and Meranti(1010). Using GBVTD (Ground-based Velocity Track Display Technique), ARPS (Advanced Regional Prediction System) and two-dimensional nondivergent barotropic model, we discuss the inner-core structure and evolution of those TCs during their RI period.The large-scale environment of Vicente and Meranti were favorable for their RI occurrence except for the vertical wind shear during the RI onset of Meranti. Time evolution of their inner-core characteristics show that Vicente experienced axisymmetric RI, while Meranti experienced two stages of RI:the axisymmetric and the asymmetric RI. Due to the limit of data, GBVTD results of Vicente are just able to show the structure during the last 6 hours in RI period, which have many similarities with the structure of Meranti in axisymmetric stage. The RI processes of them were accompanied with a contracting eyewall during the last few hours before RI ended. This phenomenon resulted in the strengthening of symmetric tangential wind according to the conservation of angular momentum. In addition, angular momentum fluxes from the inflow at low level (1-2km) were transported to the eyewall region during RI. However, the low-level inflow was truncated when RI ceased. Furthermore, both of the vertical vorticity profile of Meranti and Vicente shifted from an unstable ring to a monopole configuration. Meanwhile, there were 2 (and more) wavenumber structure in eyewall, and the eye was polygonal or elliptic. This confirm the theory that barotropic instability introduces PV mixing between eye and eyewall, which has subtle influence on TC intensity. RI ended after landfalling.The comparison between two RI stages shows the intensification rate during asymmetric RI is more efficient than that during axisymmetric stage. Meranti intensified from 22 m·s-1 to 28 m·s-1 in 9 hours. It was wavenumber-1 component that dominated the asymmetric structure. Furthermore, vertical wind shear (12.4 m·s=1 when Meranti’s RI beginning) determine the location and the intensity of wavenumber-1 asymmetry. Comparing the former studies with results based on high temporal and spatial resolution of this article, the author concluded the reasonable mechanism for asymmetric RI TC. In the moderate-to-strong shear environment, deep convection occurs mainly in downshear-left quadrant, generating substantial diabatic heating in the downshear region. Under suitable configuration of inner-core structure, the downshear vortices will be entrained cyclonically inward and result in RI. It’s the interaction betweem axisymmetric inner-core structure of TC and mesoscale vortex that control the intensification process. This infrequent case provided observation of RI during stronge shear period, it suggest that inner core structure is vital for intensity of TC. |
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