A Mechanistic Study Of Tropical Cyclone Scale Differences In The Northwest Pacific And North Atlantic

Tropical cyclone(TC)is a kind of natural disaster that can cause heavy damage to human life and property.The size of a TC is an important parameter to estimate how large the area its impact will be.Due to the lack of historical observational data of the TC size,the understanding on the TC size has been lagging behind that of the track and intensity.Based on the summer mean background state and synoptic scale perturbation derived from reanalysis data,using the Weather Research and Forecasting Model(WRF),this study systematically analyzes the reasons for the TC size difference between the western North Pacific(WNP)and North Atlantic(NA),and the mechanism is revealed.The WRF model is used to examine the destructive potential and size of TC Shanshan(2006)at various horizontal resolutions with different cumulus parameterization(CP)schemes.The main conclutions are summarized as follows:(1)Observational studies have shown that TC size in the WNP is statistically larger than that in the NA.In this study we conduct idealized simulations for TC developments using highresolution WRF model to understand the reason behind the size difference and relative contributions from among temperature,moisture and wind fields to modulate the TC size.Simulation results shows that the WNP environment is favor to evolve a larger TC size than the NA environment.The temperature profile is the dominant factor in controlling the TC size with its influence about twice larger than from the specific humidity or the wind fields.The background climatological state in the WNP has a higher SST and a lower tropopause temperature than in the NA.This condition is favorable for more intense TCs.This more unstable atmospheric condition is the reason behind the larger size observed in the WNP.(2)Different synoptic scale perturbation types in the two basins can affect the TC size as well.Numerical model experiments show that the synoptic wave train(SWT)environment is favor to have a larger TC at an equilibrium state.Compared with the easterly wave(EW),the surface wind speed of the SWT is larger.Therefore,the SWT generates more moisture in the outer region than EW through surface evaporation process.This favors the development of stronger convection in the outer region.The enhanced convection leads to greater diabatic heating,which lower the local sea level pressure(SLP),which changes surface radial pressure gradients in the inner-core and outer-core of the vortex.On the one hand,the falling of the SLP decreases the surface radial pressure gradient in the inner-core.As a result,the radial wind is weakened and then the radius of maximum wind(RMW)extends outwards.On the other hand,the lower SLP increases the radial pressure gradient in the outer region,strengthening the radial wind outside.The wind convergence favors stronger development of convection in the outer region.Through this positive feedback,inflow in the outer region is further strengthened.This accelerates local tangential wind and thus enlarges the TC size.(3)The WRF is used to examine the destructive potential of TC at various horizontal resolutions(7.5 km – 1 km)with different CP schemes.It is found,the calculated Power Dissipation Index(PDI)increases while the size-dependent destructive potential(PDS)decreases as the grid spacing decreases for all CP-scheme simulations,which indicates a weak model convergence in both PDI and PDS calculations.Moreover,it is change of the storm intensity and inner-core size that lead to the non-convergence of PDI and PDS respectively.The mechanism that influence the model convergence is revealed.It is found that in the NOCP,KFEX,and BMJ experiments,when the resolution gradually increases,more convection is resolved,and the simulated maximum diabatic heating near the TC eyewall becomes larger,leading to rapid pressure decrease and pressure gradient increase in the corresponding area.On the one hand,the above processes keep increasing the simulated TC intensity,and result in stronger non-convergence of the PDI simulation.On the other hand,larger pressure gradient force would reduce the RMW according to the radial wind balance equation.The area of diabatic heating becomes closer to the TC center due to the decrease in the RMW,leading to larger pressure decrease in the TC center.Such a positive feedback loop keeps increasing the pressure gradient and decreasing the RMW until a new stable equilibrium of radial force in the eyewall is reached.As a result,the model simulation of the TC size becomes non-convergent,which eventually leads to non-convergent PDS simulation.The Grell-Freitas scheme exhibits a stronger convergence in the simulations of TC intensity and size,although the convergence in PDS is relatively weak,but is closer to the truth.