Numerical Simulation And Influence Mechanism On The Track Of Typhoon Megi(2010)

In this study,synoptic analysis,numerical simulation and diagnostic analysis using model output data are conducted,in order to investigate the mechanism of track deflection of Typhoon Megi(2010)before and after it lands on the Luzon Island.Based on the analysis on the major systems affecting Megi and numerical simulation results of typhoon track,spectral nudging technique is adopted to reduce the error of simulated large-scale environmental field and improve the simulation results of Megi's north deflection period.Sensitivity tests are conducted by changing the nudging variables,height range and horizontal scale of wind field,so as to obtain the optimal parameter configuration for typhoon simulation and to analyze the main environmental factor as well as different height and scale of this main factor affecting the typhoon track especially in the deflection period.Based on the simulation results of spectral nudging tests,terrain sensitivity tests are conducted by varying the terrain height of Luzon Island.Temporal evolution of circulation structure and variable field is diagnosed to investigate the impact mechanism of high terrain on the typhoon track deflection.And the southward movement when the typhoon approaches and lands on the island is displayed.Synoptic analysis on the Megi's landing period as well as deflection period using NCEP 1°×1° reanalysis is conducted.It shows that the temporal evolution of the west Pacific subtropical high(SH),the continental high in the southeast of China(CH)and the meso-scale anticyclone southeast of the typhoon(SA)play an important role in the Megi's north deflection.Meanwhile,the cross-equatorial flow around Kalimantan Island continues enhancing and northing and joins in the cross-equatorial flow from the Indian Ocean,thus enhancing the south flow east of the typhoon and leading the typhoon to the north.Using WRF model,numerical simulation on Megi's southward landing process as well as north deflection process is conducted.The simulation result of control experiment(CTRL)precisely reflects the southward landing process,the deflection moment as well as the speed and direction of typhoon movement before the deflection.However,due to the simulated error of large-scale environmental field,the simulated error of Megi's north deflection process is significant.Spectral nudging technique is applied to the numerical simulation of Typhoon Megi to reduce the error of simulated large-scale environmental field.By choosing different physical variables in sensitivity tests,the key environmental factor to the track simulation is analyzed.It indicates that the outputs with SN can effectively reduce the track error of Megi,especially in its deflection period.The environmental wind field is the key factor influencing the typhoon track and its change.Although nudging the wind component of lower troposphere increases the track error during the landing period,it plays an important role in improving the track simulation during the deflection period.Besides,after adding the meso-scale information of environmental wind field,it sharply reduces the track error in the deflection process and relieves the error increasement during the landing period.By analyzing the steering flow and inner structure of typhoon itself using model output data to investigate the mechanism of terrain impact on the typhoon track,it shows that nudging environmental wind field can improve the simulation to mid-and low-latitude systems especially SA to acquire more accurate steering flow.Then the test with SN also influences the inner structure of typhoon itself.Nudging environmental wind field modifies the environmental vertical wind shear to change the location and symmetry of the severe inner convection,thus affecting the deflection of typhoon track.Based on the simulation results of spectral nudging tests,terrain sensitivity tests are conducted by varying the terrain height of Luzon Island.When the terrain height of Luzon Island is reduced to zero,the simulated typhoon track is in correlation with the steering flow.Impact of high terrain leads the typhoon to turn to south(north)when the typhoon approaches(leaves)the high terrain and the deflection degree increases with the terrain height increasing.Results of terrain sensitivity tests are diagnosed and backward trajectory analysis of the affected areas is conducted,in order to investigate the temporal evolution of circulation structure and variable field when the typhoon approaches and crosses the island.The result shows that convergence center occurs both in the windward and leeward slope,contributing to gale center in these areas.Positive vorticity area occurs in the leeward slope,indicating that secondary depression is induced and appeals to the typhoon.Meanwhile,the outer and inner circulation of typhoon are affected by the high terrain.Channeling effect contributes to gale centers and confluent trajectories between the high terrain and typhoon center.North(south)low-level jet in the windward(leeward)slope induced by the channeling effect leads the typhoon to the south(north).