Large-Scale Environmental Characteristics And Inner-core Dynamical Processes Associated With Rapid Intensification Of Tropical Cyclones In The South China Sea

During the past three decades or so?significant progress has been achieved in track forecasting of tropical cyclones(TCs)while the ability for intensity forecast is still limited,especially for the rapid intensification(RI).Note the RI is defined as the 24-h increase in maximum sustained surface wind speed by 15 m s-1 as in previous studies.This problem turns to be even worse when the RI takes place in the South China Sea(SCS),one of Chinese off-shore basins famous for its frequent TC activity,since landfalling may occur within two or three days after the formation and intensification of the TCs.Considering these facts,improving the forecast skill for the RI TCs in SCS is both scientifically important and practically imperative.In this study,the large-scale environment and the key inner-core dynamics associated with RI TCs in the SCS have been examined in a systematic way via statistical analysis,fine-resolution simulation as well as verification by Doppler radar observation.The synoptic flow patterns and the significant environmental factors for the rapidly intensifying TCs in SCS have been extracted to build up an empirical conceptual model for RI forecast in SCS.In addition,the key dynamical processes under the RI flow patterns have been studied in details.First,the typical synoptic flow patterns and environmental factors that favor the RI of TCs in the SCS have been identified and combined to build a statistically empirical conceptual forecast model.Eighteen RI cases have been selected based on all TCs formed in the SCS between 1981 and 2011.They are consisted of 4.9%(2.3%)of tropical depressions(tropical storms).Six low-level synoptic flow patterns favoring RI have been identified based on the 18 RI cases.In the monsoon season very few TCs experience RI due to large vertical wind shear(VWS).Most RI cases occurred in the post-monsoon season when the mid-latitude troughs often penetrated into the SCS whereas the southwesterly monsoon flow still prevailed in the southern SCS.Compared with those of non-RI cases,the mean initial conditions of RI cases include weak VWS and relatively strong upper-tropospheric forcing from mid-latitude troughs.Several criteria of significant environmental factors for RI are statistically identified based on all TC samples.An empirical conceptual model for RI forecast has been set up based on the low-and upper-level RI flow patterns and RI criteria.It is found none of the non-RI TCs could satisfy both the RI flow patterns and RI criteria,suggesting the empirical forecast model could be employed to predict RI in the SCS.To further understand the specific dynamical processes under different synoptic flow patterns,a typical example for the identified RI flow pattern in monsoon season in SCS,i.e.,Typhoon Vicente(2012),has been examined using cloud-resolving fine-resolution simulation and Doppler radar observation.It is found that the important features of Vicente,including the extreme RI process,the sudden deflection,TC inner-and outer-core structures and spiral rainband,have been successfully reproduced in the Advanced Weather Research and Forecasting Model(ARW-WRF)simulation.The evolution of axisymmetric inner-core radar reflectivity and primary circulation of the simulated Vicente before landfalling are quantitatively compared with radar observed reflectivity and retrieved circulation from MGBVTD method(Modified Ground Based Velocity Track Display).The results show that the simulated axisymmetric inner-core structures correlate well to their observed counterparts.Both observation and simulation results show that the RI of Vicente consisted of two Stages:(?)asymmetric Stage(i.e.RI onset),represented by relatively slow intensification associated with a distinct eyewall contraction;(?)axisymmetric Stage with slight eyewall contraction but efficient storm intensification.Results from a system-scale tangential momentum budget indicate that the primary spinup mechanism during Stage I is the radial eddy momentum transport,which is beneficial to accelerate primary circulation within radius of maximum wind(RMW)and eyewall contraction.In contrast,the principal spinup mechanism during Stage ? is mainly ascribed to the forced secondary circulation in the response of diabatic heating within eyewall and boundary layer friction,which efficiently advected the absolute angular momentum radially inward and vertically upward to increase the primary circulation around the eyewall region throughout the troposphere.Further budget diagnosis based on scale separation reveals that the interaction between large-scale environmental circulation and vortex-scale vorticity anomalies has dominated the role of primary spinup mechanism during Stage I.In the upper-troposphere,two types of favorable TC-trough interaction persisted during the RI onset.In addition,the superposition of trough-line of easterly trough played a driving role in reducing the large-scale vertical wind shear and setting up double outflow channels with large divergence just prior to RI occurrence,both of them are favorable for the spinup of primary circulation.Further analysis relates to the inner-core dynamical processes show that vertical alignment is the direct cause of RI occurrence.There are two stages of vertical alignment.In the first stage,successive moist convection within the downshear convergence-shearing line promoted the formation of several mesovortices.They were advected cyclonically under the torodial background flow and their spatial distribution within inner-core helped to shift the low-level vortex center towards downshear side since the strongest mesovortex resided in downshear region.In the second stage,the mid-level vortex was strengthened through the axisymmetrization of convectively generated vorticity anomalies while the dominant low-level mesovortex triggered the "downshear reformation" mechanism,under which the precession between low-and mid-level vortices was accelerated.Both the tilt and local vertical wind shear decreased dramatically when mid-level vortex entered into the upshear region.Note vertical alignment in the second stage is also responsible for the notable inner-core bottom-up process.In general,the RI occurrence of Vicente(2012)results from the cooperative work between large-scale environment and the inner-core dynamical processes.