ree-time level (leapfrog) schemes
               units up to user
           
         t -- real, dimension(:,:,:)
            temperature ( degrees K)
              used only for computation of density 
              shape must conform to that of xi

         q -- real, dimension(:,:,:)
            specific humidity (dimensionless)
              used only for computation of density 
              shape must conform to that of xi
              (set = 0 to ignore effects of vapor on density)

         diff -