own -- not used elsewhere)

     dflux_q = derivative of the flux of specific humidity 
               at the top of the lowest atmospheric layer with respect to 
               the specific humidity of that layer  (--/(m2 K))
               (defined in gcm_vert_diff_down -- not used elsewhere)

     delta_t = the increment in temperature in the lowest atmospheric
               layer (((i+1)-(i-1) if atmos model is leapfrog) (K)
               (defined in  gcm_vert_diff_down as the increment computed up
  