real
           time step , seconds
           if using leapfrog step, set equal to 2*dt = time(i+1) - time(i-1)

         u -- real, dimension(:,:,:)   zonal wind (m/sec)
         v -- real, dimension(:,:,:)   meridional wind (m/sec)
         T -- real, dimension(:,:,:)   temperature  (K)
         q -- real, dimension(:,:,:)   specific humidity 
                                         (non-dimensional -- Kg water/Kg air)
         tr -- real, dimension(:,:,:,:) tracers
                                     