A vertical velocity () damping term is used to prevent reflection of acoustic waves off the upper model boundary and is defined as a Raleigh damping term
where is a characteristic timescale and only varies in the vertical and is zero except in the uppermost layers where it increases approaching a maximum value at the upper boundary. This is akin to a Raleigh damping to the basic state value () to avoid reflections off the model top (downward). This term is included in the term in Eq. 32.