Initiation of shallow convection in the Black Forest mountains

The temporal evolution and spatial distribution of shallow convection over complex terrain are examined by observational data and compared with model calculations using the Local Model of the Deutscher Wetterdienst with 2.8 km (LM2.8) and 7 km horizontal resolution (LM7). For this, the 01 June 2002 was chosen from the VERTIKATOR (Vertical Exchange and Orography) measurement campaign performed in the northern Black Forest mountains. On this day shallow convection was restricted to the mountainous regions. This can be explained by the fact that the conditions of initiation of convection, i.e. that the near-surface temperature reaches the convective temperature, are only met at upper sites of the mountains. Additionally, the initiation of convection over the mountain ridges is enhanced by up-slope and up-valley winds, generating convergence zones over the ridges. The resulting vertical lifting supports convective processes. The areas of convergence primarily comply with the areas of convection, as confirmed by satellite images from NOAA. 2.8 and LM7 simulations do not show significant differences concerning the large-scale meteorological conditions. However, LM2.8 better reproduces the near-surface temperature and humidity at most sites leading to a more realistic representation of convective clouds. Another important difference concerns the near-surface winds, that is only LM2.8 reproduces the main characteristics of the thermally induced wind systems which prove to support convection. Convergence zones and upward lifting is simulated by LM2.8 above the mountain crests resulting in the accompanied formation of clouds. The onset and position of simulated convective clouds generally fit with the observations. In contrast to LM2.8, LM7 simulates clouds above the transition zone between the northern Black Forest and the Rhine valley, where the large-scale easterly flow converges with the channelled northerly wind over the Rhine valley, that is, the position does not agree with the observations.