The Total Water Uniform PDF in Ice Nucleation

One requirement for ice nucleation in Thompson MP is a constant supersaturation. When grid size is small the constant supersaturation works fine. However, in the large size grids, significant subgrid variability of total water exists and needs to be accounted for. A total water uniform PDF is used to take subgrid variability into consideration, which allows the ice nucleation to occur even when the grid averaged supersaturation is below the constant supersaturation. The PDF method can also partially be used to address the scale awareness of the scheme. Experiments will be conducted to show the impacts of the PDF method.