An atmosphere-only model system for making seasonal prediction and projecting future intensities of landfalling tropical cyclones(TCs)along the South China coast is upgraded by including ocean and wave models.A total ...An atmosphere-only model system for making seasonal prediction and projecting future intensities of landfalling tropical cyclones(TCs)along the South China coast is upgraded by including ocean and wave models.A total of 642 TCs have been re-simulated using the new system to produce a climatology of TC intensity in the South China Sea.Detailed comparisons of the simulations from the atmosphere-only and the fully coupled systems reveal that the inclusion of the additional ocean and wave models enable differential sea surface temperature responses to various TC characteristics such as translational speed and size.In particular,interaction with the ocean does not necessarily imply a weakening of the TC,with the coastal bathymetry possibly playing a role in causing a near-shore intensification of the TC.These results suggest that to simulate the evolution of TC structure more accurately,it is essential to use an air-sea coupled model instead of an atmosphere-only model.展开更多
Observational evidence is presented that during Extratropical Transition(ET) of Tropical Cyclones(TCs),Downstream Development(DD) is frequently underway.We show that DD results in rapid changes to the environment.A cr...Observational evidence is presented that during Extratropical Transition(ET) of Tropical Cyclones(TCs),Downstream Development(DD) is frequently underway.We show that DD results in rapid changes to the environment.A critical flow change is the development of a low-level trough,sandwiched between two developing anticyclones.The trough appears to merge with the storm,seemingly holding it upright and allowing it to withstand the damaging effects of wind shear.In this way the storm can eventually reach the favourable equatorward entrance region of the upper jet.To evaluate the mechanism,two sets of simulations have been run:one using high-resolution,full physics integrations and another using coarse-resolution with dry physics and the TC removed from the initial condition.We show that the dry dynamics can establish the large scale environment to enable the transition to proceed.The process can produce:(a) the deep,vertically-aligned,low-level pressure trough that merges with the storm,and(b) a partial inhibition through subsidence to embedded convection,allowing the boundary layer to moisten via(i) sustained surface fluxes,and(ii) enhanced horizontal moisture flux convergence from the environmental flow changes.This produces potential for more intense convective activity and vortex resiliency even in relatively strong,deep vertical wind shear.展开更多
Increased understanding of the importance of TC structure in dynamical,climatological and prediction studies makes determination of TC size important. A new algorithm for the objective estimation of the radius of oute...Increased understanding of the importance of TC structure in dynamical,climatological and prediction studies makes determination of TC size important. A new algorithm for the objective estimation of the radius of outermost closed isobar(roci) has been developed. The new method uses storm position and global analyses of mean sea level pressure to compute a mean(axisymmetric) roci. This radius can be used,together with the central pressure,for the construction of a synthetic vortex that is initialized in a numerical prediction model. The method also has important applications in dynamical and climatological studies of TC intensity,size and structure. The algorithm is robust and capable of estimating roci,even in the case of a weak system that may not have a closed isobar in the global analysis. The values produced by the new method are shown to be more consistent than the corresponding operational estimates which are subjective and produced under strong time constraints. Statistical comparison between subjective and objective estimates gives a mean absolute difference of 110 km,which given the difficulty in making a subjective estimate,is satisfactory. In addition,even though limitations exist with the estimates of vortex parameters like the radius to gales(r34),comparison with estimates from an extended best track data set provides independent evaluation of the scheme. Mean absolute difference for r34 for around 3200 cases is near 80 km,even though the best track estimates are subjective and the objective r34 is estimated only from storm central pressure and the objective roci. This validation suggests that the algorithm can be used to obtain useful size estimates of TCs.展开更多
基金supported by Hong Kong Research Grants Council Grant CityU E-CityU101/16supported by the Natural Environment Research Council/UKRI(Grant No.NE/V017756/1).
文摘An atmosphere-only model system for making seasonal prediction and projecting future intensities of landfalling tropical cyclones(TCs)along the South China coast is upgraded by including ocean and wave models.A total of 642 TCs have been re-simulated using the new system to produce a climatology of TC intensity in the South China Sea.Detailed comparisons of the simulations from the atmosphere-only and the fully coupled systems reveal that the inclusion of the additional ocean and wave models enable differential sea surface temperature responses to various TC characteristics such as translational speed and size.In particular,interaction with the ocean does not necessarily imply a weakening of the TC,with the coastal bathymetry possibly playing a role in causing a near-shore intensification of the TC.These results suggest that to simulate the evolution of TC structure more accurately,it is essential to use an air-sea coupled model instead of an atmosphere-only model.
文摘Observational evidence is presented that during Extratropical Transition(ET) of Tropical Cyclones(TCs),Downstream Development(DD) is frequently underway.We show that DD results in rapid changes to the environment.A critical flow change is the development of a low-level trough,sandwiched between two developing anticyclones.The trough appears to merge with the storm,seemingly holding it upright and allowing it to withstand the damaging effects of wind shear.In this way the storm can eventually reach the favourable equatorward entrance region of the upper jet.To evaluate the mechanism,two sets of simulations have been run:one using high-resolution,full physics integrations and another using coarse-resolution with dry physics and the TC removed from the initial condition.We show that the dry dynamics can establish the large scale environment to enable the transition to proceed.The process can produce:(a) the deep,vertically-aligned,low-level pressure trough that merges with the storm,and(b) a partial inhibition through subsidence to embedded convection,allowing the boundary layer to moisten via(i) sustained surface fluxes,and(ii) enhanced horizontal moisture flux convergence from the environmental flow changes.This produces potential for more intense convective activity and vortex resiliency even in relatively strong,deep vertical wind shear.
基金supported by the National Oceanographic Partnership Program(NOPP)and the US Office of Naval Research(ONR)under Award No.:N000141010139.
文摘Increased understanding of the importance of TC structure in dynamical,climatological and prediction studies makes determination of TC size important. A new algorithm for the objective estimation of the radius of outermost closed isobar(roci) has been developed. The new method uses storm position and global analyses of mean sea level pressure to compute a mean(axisymmetric) roci. This radius can be used,together with the central pressure,for the construction of a synthetic vortex that is initialized in a numerical prediction model. The method also has important applications in dynamical and climatological studies of TC intensity,size and structure. The algorithm is robust and capable of estimating roci,even in the case of a weak system that may not have a closed isobar in the global analysis. The values produced by the new method are shown to be more consistent than the corresponding operational estimates which are subjective and produced under strong time constraints. Statistical comparison between subjective and objective estimates gives a mean absolute difference of 110 km,which given the difficulty in making a subjective estimate,is satisfactory. In addition,even though limitations exist with the estimates of vortex parameters like the radius to gales(r34),comparison with estimates from an extended best track data set provides independent evaluation of the scheme. Mean absolute difference for r34 for around 3200 cases is near 80 km,even though the best track estimates are subjective and the objective r34 is estimated only from storm central pressure and the objective roci. This validation suggests that the algorithm can be used to obtain useful size estimates of TCs.
