subroutine execute_model( model,
     &        y0, z0, v0, b0, R0, gamma_oo, ac_speed,
     &        runway_elev, surf_rho, i_hw, hw_avg,
     &        ntemp, tempz, temp, theta, rho, bvfsq,
     &        ncw, cwz, cw, cw_spln, nhw, hwz, hw, nq, qz, q,
     &        t1, t2, n_steps,
     &        nt_model, time_model,
     &        y_model_p, z_model_p, cir_model_p,
     &        y_model_s, z_model_s, cir_model_s,
     &        tld, tssd )

      implicit none

      include 'vpr20.h'

      real y0, z0, v0, b0, R0, gamma_oo, ac_speed,
     &     runway_elev, surf_rho, hw_avg,
     &     t1, t2, tld, tssd,
     &     tempz(ntemp), temp(ntemp), theta(ntemp), rho(ntemp),
     &     bvfsq(ntemp), cwz(ncw), cw(ncw), cw_spln(ncw),
     &     hwz(nhw), hw(nhw), qz(nq), q(nq),
     &     y_model_p(*), z_model_p(*), cir_model_p(*),
     &     y_model_s(*), z_model_s(*), cir_model_s(*), time_model(*)

      integer i_hw, ntemp, ncw, nhw, nq, n_steps, nt_model

      character(12) model

      real rnt, ntog, flag

      select case( model )

      case('viper30')
         call viper30(
     &     y0, z0, b0, R0, gamma_oo, ac_speed, i_hw,
     &     ntemp, tempz, theta, rho, bvfsq,
     &     ncw, cwz, cw, cw_spln, nhw, hwz, hw, nq, qz, q,
     &     t1, t2, n_steps,
     &     nt_model, time_model,
     &     y_model_p, z_model_p, cir_model_p,
     &     y_model_s, z_model_s, cir_model_s )
         tld = 0.0;  tssd = 0.0

      case('viper20' )
         rnt = float(ntemp)
         ntog = -float(n_steps+1)
         call viper20(
     &     y0,z0,v0,b0,zmf,zgf,gmf,grf,gng,
     &     rnt,tempz,temp,rho,bvfsq,
     &     ncw,cwz,cw,nhw,hwz,hw,nq,qz,q,
     &     tlag_fac,tige1_fac,
     &     t1,t2,ntog,
     &     runway_elev,surf_rho,ac_speed,i_hw,
     &     rte0, rte0_edr, rte0_n2,
     &     rte1, rte1_edr, rte1_n2,
     &     rte2, rte2_edr, rte2_n2,
     &     a_grav,
     &     vcdotsc,
     &     vbdotsb,
     &     a_ci,
     &     aeff_b,
     &     aeff_s,
     &     alink,
     &     a_ige_nge,
     &     a_db,
     &     a_particle,
     &     nt_model,time_model,
     &     y_model_p,z_model_p,cir_model_p,
     &     y_model_s,z_model_s,cir_model_s, flag,
     &     linear,hw_avg,tld,tssd)

         case default
            print *, 'ERROR: model ',trim(model),' is not valid'
            stop

      end select

      return
      end