Medium range forecasts using cut-cells: a sensitivity study

Abstract

In this study, 10-day forecasts of Atlantic lows are investigated by comparing the forecasts of the standard Climate version of the Lokal Modell using terrain-following coordinates to that of the cut-cell Lokal Modell with z coordinates (LMZ). As indicated by idealized tests of the atmosphere at rest and mountain wave tests, LMZ exhibits a much better approximation of the lower boundary condition due to the use of the cut-cell approximation. An important difference to the terrain-following coordinate is that with cut-cells, a single steep mountain as defined different from 0 at only one point, has a reasonable up–downwind scheme and does not become unstable. We are interested in the large-scale structure of Atlantic lows, which are under the influence of the high orography of Greenland. The steep orography of Greenland has a considerable impact on these lows, even though they are positioned to a large part over water. It is found that at Day 6, both models yield similar forecasts, which is consistent with the fact that Day 6 is in the useful forecast range of both models. At Day 10, the forecasts are quite different, indicating a sensitivity of the 10-day forecast to the discretization in question.