A Dynamical Initialization Scheme For Binary Tropical Cyclones And Simulations

Binary tropical cyclones(TCs)prediction is more difficult because binary TCs are more complicated than one TC.The interaction of vortices is as important as the vortex structure and large-scale environmental field for forecasting binary TCs.The Fujiwhara effect of binary TCs can change the tracks,evolutions of intensities and structures.The advection of vortices and interaction between binary TCs and large-scale environmental field can change the result of binary TCs predictions.Therefore,considering the interaction of binary TCs is an important key to improve the performance of forecasting binary TCs.To improve numerical binary TCs forecasts,one way is to obtain accurate initial conditions.However,the initial conditions of binary TCs are interpolated from global analyses or the forecasting data from meteorological centers.These data are usually to weak and to large,sometimes having the wrong location compared to real vortices,which may increase errors of predictions.To improve the initial conditions of binary TCs,this paper proposes a dynamical initialization scheme for binary vortices(BVDI)used in the simulation of binary tropical cyclones by the Weather Research and Forecasting(WRF)model.In this scheme,one TC is regarded as the environmental vortex during the initializing of the other TC using a dynamical initialization scheme for single vortex(HTDI),which is a modification of the dynamical initialization scheme for TCs developed by Cha and Wang(CW13).The initial vortex size and water vapor adjustments are introduced in the HTDI scheme to ensure deepening the vortex and making the vortex size accord with the observed data.Both TCs are initialized using SVDI for double times to ensure the adjustment of each one to the other in the large-scale environment field based on Fujiwhara effect.To test the performance of BVDI,3 clusters of forecast simulations were conducted for 11 pairs of binary TCs over the northwest Pacific during 2002 and 2012 with different initial conditions including:(?)the FNL data(CTRL),(?)the CW13 scheme,and(?)the BVDI scheme.There is limitation that the WRF model does not allow two moving nests and we need to get rid of this bug by modifying the model to achieve the two domains moving with the binary TCs.The results showed that the mean absolute position and intensity errors of BVDI were 90.2 km and 6.2 m s-1,which reduced tracks and intensities errors by 35.2%and 56.6%compared with CTRL,and also performed better than the direct extension of CW13 or HTDI scheme to binary TCs.A case study,super Typhoon Saomai(2006)and Tropical Storm Bopha(2006),demonstrated that the simulated tracks and intensities of this pair of binary TCs were accord with the best-track data,whose mean absolute position and intensity errors were 53.1 km and 5.4 m s-1,respectively.The BVDI scheme could capture the features of trajectories of this pair of binary TCs and the evolution of intensities and structures(e.g.,rapid intensification and concentric eyewall)were also similar to the best-track data and radar observations.In addition,other simulation with different initial time also showed the better performance of BVDI.These results indicate that it is necessary to improve the representation of binary TCs in the initialization phase,include their interactions in future forecasts.Using the piecewise potential vorticity(PV)inversion,we found that Saomai and Bopha moved under the control of subtropical anticyclone and monsoon trough.The interaction of Saomai and Bopha was indirect.The circulation of Saomai made Bopha move southward and the circulation of Bohpa made Saomai move northward.The sensitivity tests showed that without Saomai in the initial condition,Bopha moved northward.And without Bopha in the initial condition,Saomai moved southward.The lagre-scale flow was different from that with Saomai and Bopha in the initial condition.Therefore,the exsitance of Saomai and Bopha changed the large-scale flow,and then changed the tracks themselves.