Evolution of the water ice abundance relative to the total number of hydrogen nuclei at the midplane (blue) and at 5 scale heights above the CPD midplane (red), and in the case of the gas parcel which drifts from z = 5H downwards to the midplane over a period of 100 yr (grey). The period in which the altitude of the gas parcel was iteratively lowered towards the midplane is highlighted in light green.