have a question about da_interpolation function da_to_zk_new

[ivaNov-0042]

In line 30 and 34 of file var/da/da_interpolation/da_to_zk_new.inc, why the first indices of mdl_v are all kts.

...
   if (v_interp_optn == v_interp_p) then
      if (anal_type_verify) then
         do n=1,num
            do k=1,nlevels
               if (obs_v(k) > mdl_v(kts,n)) then 
                  ! below the lowest level:
                  zk(k,n) = r_ktsplus - &
                     (obs_v(k)-mdl_v(kts+1,n))/(mdl_v(kts,n)-mdl_v(kts+1,n))
               else if (obs_v(k) < mdl_v(kts,n)) then ! why this is kts instead of kte
                  ! above the highest level:
                  zk(k,n) = r_kteminus + &
                     (obs_v(k)-mdl_v(kte-1,n))/(mdl_v(kte,n)-mdl_v(kte-1,n))
               else
                  do kk = kts,kte-1
                     if (obs_v(k) <= mdl_v(kk,n) .and. obs_v(k) >= mdl_v(kk+1,n)) then
                        zk(k,n) = real(kk) + (mdl_v(kk,n) - obs_v(k))/(mdl_v(kk,n) - mdl_v(kk+1,n))
                        exit
                     end if
                  end do
               end if
            end do
         end do
      else
...

[weiwangncar]

@liujake Please see if you have a quick answer for this question.