rather than in the broader sense used by meteorologists, which additionally denotes sedimentation of the condensates (hereafter we use the term in this broader sense). Although there is no mention of sedimentation by Lewis, the treatment does imply certain assumptions. Starting below the cloud base and working upwards, at each computational level the Lewis model assumes that all the condensate remains at the level where it appears. Considered in the framework of a parcel in an updraft, the Lewis model assumes that all condensate rains out with a fallspeed matching the updraft velocity. Were sedimentation slower, condensate would be transported upward (as discussed by Weidenschilling & Lewis 1973); were sedimentation faster, condensate would be transported downward. Hence, the Lewis (1969) assumption regarding sedimentation is an unstated compromise between those two possibilities.

We implement the Lewis model by starting below the cloud base (where $q_{\rm c} = 0$ and $q_{\rm v} = q_{\rm below}$) and condensing all vapor in excess of saturation at each successive level upward: $$\begin{aligned}
%%
\label{lewis_qc}
q_{\rm c}(z) & = & \max( 0, q_{\rm v}(z-\Delta z) - q_{\rm s}(z) ) \\
%%
\label{lewis_qv}
q_{\rm v}(z) & = & \min( q_{\rm v}(z-\Delta z), q_{\rm s}(z) ) 
%%\end{aligned}$$ where 