1. In both ENS and HRES small amounts of snow on the ground tend to take too long to melt, even if the temperature of the overlying air is well above zero. This is because, for melting purposes, the snow that there is is assumed to be piled up high in one segment of a gridbox. For smaller nominal depths, the pile becomes higher, though at the same time covers a much smaller fraction of the box. The reason this is used is to improve the handling of screen temperature; by confining the snow to gridbox segments the impact on the temperature of that snow is reduced, and on average we find smaller errors and biases in 2m temperature as a result. The main downside is that snow cover pictures can look misleading, particularly at longer leads (when they can not of course be rectified by observational data). The cut-off above which snow is assumed to cover the full grid box is a 10cm depth - this is why a green hue used on standard snow depth charts on the web, which suggests to the eye the presence of some vegetation, disappears at 10cm.
2. In marginal snow situations, when precipitation at the surface comprises both rain and snow, the snow component accumulates as lying snow. In the vast majority of cases this is wrong - it should melt instantaneously. This behaviour occurs because small snow depths within the model are assumed to be piled up into a small segment of a gridbox, and as such it is very difficult for them to melt quickly (as in item 1 above).
Deze problemen zijn in de aankomende situatie(s) ook mogelijk een factor.
2. In marginal snow situations, when precipitation at the surface comprises both rain and snow, the snow component accumulates as lying snow. In the vast majority of cases this is wrong - it should melt instantaneously. This behaviour occurs because small snow depths within the model are assumed to be piled up into a small segment of a gridbox, and as such it is very difficult for them to melt quickly (as in item 1 above).
Deze problemen zijn in de aankomende situatie(s) ook mogelijk een factor.
