Impacts Of Air-sea Coupled Processes On Tropical Cyclones Simulation And The Relative Mechanisms

In the past four decades or so,evident progress has been made in the track forecast of tropical cyclones(TCs)relying on the increasing observations.However,the intensity forecast,especially in the future 24 hours,still lags obviously.This is mainly because TCs'intensity change is determined by complicated internal processes and external forcing,including the air-sea interaction.As one of the Chinese offshore basins which is famous for TCs activities every year,the South China Sea(SCS)impacts TCs significantly with its complicated ocean conditions and thus challenges the forecasts of rapid intensification(RI)TCs.Therefore,improving the skill of forecasting the RI TC passing SCS is both scientifically important and practically urgent.In this paper,a series of coupled model simulations were conducted to understand the interactions between the RI TC and both open sea and coastal basins of SCS,revealing the important role of the enthalpy flux in the development of TCs.In addition,numerical simulations combined with the in-situ observations were used to further check the effects of surface exchange coefficient(Cd)on surface heat flux and intensification of TC.The impacts of different local sea surface temperature(SST)distributions on the RI typhoon Hato(2017)were studied using both oceanic coupled and uncoupled high-resolution cloud-resolving atmospheric models.As a category 3 landfalling typhoon that moved west-northwestward across the northern SCS,Hato rapidly intensified prior to its landfall and induced significant impacts on the SCS.The fully coupled WRF_ROMS model reproduced both TC features and ocean responses reasonably well.Results from the azimuthal mean latent heat flux and tangential wind tendency analysis indicated that,when cold core SST existed with large radial SST gradient,intense surface latent heat flux would thus arise at the outer edge of the eyewall.Strong evaporation and updrafts were facilitated in the outer rainbands rather than the core region,resulting in inhibited intensification and lagged onset of RI.However,when the core SST was uniformly hot,interactions between the primary circulation and secondary circulation became the ma-jor mechanism for the inner-core evolution.Evident LHF generating at almost all radii with active evaporation near the inner edge of the eyewall.Following the intensified eye-wall updrafts,boundary layer inflows penetrated inward outside the RMW,transporting massive moisture,evident radial eddy momentum and the diabatic PV to the core region,and,finally triggered the onset of RI hours earlier.Therefore,the high sensitivity of the typhoon inner-core evolution to the radial SST profiles associated with the enthalpy flux was demonstrated.The coastal ocean response and feedback to Typhoon Hato were studied based on high-resolution numerical simulations using both the coupled and uncoupled cloud-resolving model.Hato mpacted the coastal water column greatly and caused warm and cold patches on SST over the continental shelf to the right of the track.This feature was well captured in an air-sea coupled model experiment.The coastal SST warming was found to be re-lated to a two-layer oceanic circulation across the continental shelf forced by the onshore surface wind stress to the right of the storm track.The associated onshore surface currents imposed a warm temperature advection and downwelling,leading to the SST warming in the inner sea shelf,as diagnosed from an ocean temperature budget analysis.A sensitivity experiment,in which the typhoon vortex was removed from the initial conditions,further confirmed that it was the strong onshore wind stress to the right of the storm track that forced the onshore surface currents and the SST warming in the inner sea shelf.Results from an atmosphere-only model experiment with the typhoon-forced coastal warm SST anomalies removed demonstrate that the typhoon-induced coastal warm SST anomalies contributed partly to the rapid intensification of Typhoon Hato prior to its landfall over South China and also slowed down the weakening of Hato at and shortly after its landfall.The study above indicated that surface enthalpy flux could significantly impact the evolution of TC intensity and structure.Though the surface exchange coefficient(Cd)plays the key role in calculating the enthalpy flux,the uncertainties of Cd under high wind scenarios still challenge the accurate estimation of surface heat flux in the numerical models due to inadequate in-situ observations.Therefore,a series of coupled simulations combined with the in-situ observations were conducted to assess the effect of Cd and neg-ative ocean feedback(SSC)on surface heat flux and intensification of typhoon Kalmaegi(2014).Compare with the buoy observations,the Charnock scheme was the best parame-terization used in reproducing TC intensity,the difference between 2 m temperature and SST and heat flux changes in the coupled model while other parameterizations got much stronger TCs.Distinct sensitivity of heat-flux distribution and variations to SSC was found through sensitive experiments,while the direct link to the change of Cd was not obvious.Since Cd impact low-level wind greatly without changing the SSC and heat flux directly,the enthalpy flux change was mainly caused by the wind-induced surface heat exchange.In addition,the Cd could also impact the thermo structures of the ocean mixed layer(OML)by inserting momentum flux through the surface.Larger Cd was found to contribute to more transfer of momentum flux to the deeper layers of the ocean,leading stronger upwellings of deep cold seawater even after typhoon passage and thus continuous cooling of the OML(slow change process).The local heat flux was therefore lessened.This process is more intense to the right of the typhoon,which is consistent with the right-hand-side stronger wind of the typhoon.In summary,the development of TCs is highly sensitive to the air-sea coupled pro-gresses.To understand the mechanisms behind both intensity and structure changes of the TCs,it is of necessity to consider the characteristics of interactions between different ocean basins and TCs,associating with the process of energy exchange between the air-sea interface.The development of a high-resolution coupled air-sea model is an urgent requirement in forecasting the RI TCs.At the same time,an accurate,Cd parameteriza-tion scheme is highly needed in the numerical model to accurately quantify the heat flux estimation during the forcing period.