Why did you not anticipate the cloudburst in my district?

As we wrote earlier, the horizontal distance between adjacent nodes (grid step) of the UM model is 4 kilometres. The relationship between model spatial step and resolution is such that the smallest perturbation that can be described by the model has a length of two grid steps. Thus, for the UM model with a grid step of 4 km, all phenomena smaller than 8 km are sub-grid in nature, i.e. they cannot be resolved explicitly. The only way to determine their possible occurrence is through parameterisation, i.e. statistical linkage between large-scale variables and sub-grid processes. Hence the difficulty in accurately describing the occurrence of a local storm in the 4 km model.

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