Effect of the Vertical Diffusion of Moisture in the Planetary Boundary Layer on an Idealized Tropical Cyclone

Previous numerical studies have focused on the combined effect of momentum and scalar eddy diffusivity on the intensity and structure of tropical cyclones.The separate impact of eddy diffusivity estimated by planetary boundary layer(PBL)parameterization on the tropical cyclones has not yet been systematically examined.We have examined the impacts of eddy diffusion of moisture on idealized tropical cyclones using the Advanced Research Weather Research and Forecasting model with the Yonsei University PBL scheme.Our results show nonlinear effects of moisture eddy diffusivity on the simulation of idealized tropical cyclones.Increasing the eddy diffusion of moisture increases the moisture content of the PBL,with three different effects on tropical cyclones:(1)an decrease in the depth of the PBL;(2)an increase in convection in the inner rain band and eyewall;and(3)drying of the lowest region of the PBL and then increasing the surface latent heat flux.These three processes have different effects on the intensity and structure of the tropical cyclone through various physical mechanisms.The increased surface latent heat flux is mainly responsible for the decrease in pressure.Results show that moisture eddy diffusivity has clear effects on the pressure in tropical cyclones,but contributes little to the intensity of wind.This largely influences the wind-pressure relationship,which is crucial in tropical cyclones simulation.These results improve our understanding of moisture eddy diffusivity in the PBL and its influence on tropical cyclones,and provides guidance for interpreting the variation of moisture in the PBL for tropical cyclone simulations.

Re-examination of Trends Related to Tropical Cyclone Activity over the Western North Pacific Basin

In order to re-examine some trends related to tropical cyclones（TCs） over the western North Pacific since 1949,the unreliable maximum sustained wind(V;) recorded in the 1949-1978 TC best-track data from the Shanghai Typhoon Institute was modified based on the wind-pressure relationships（WPRs） in this study. Compared to the WPR scheme based on the cyclostrophic balance,the WPR scheme based on the gradient balance could give a better fit to TCs under higher wind speeds and could introduce smaller estimated errors for TCs locating at higher latitudes as well as TCs landing on the continent.After the V;modification based on minimum sea-level pressure and TC center latitude,the revised annual number of category 4-5 typhoons shows no long-term trend,while the potential destructiveness measured by power-dissipation index decreases slightly,and this trend is not significant in the period 1949-2008.