L. Gantner

Numerical errors above steep topography: A model intercomparison

This paper compares the behaviour of three mesoscale models (MMS, Penn State University/NCAR, LM, German Weather Service, and KAMM2, IMK Karlsruhe) in the vicinity of a steep isolated mountain under zero-wind conditions. The setup of the simulations is such that the model atmosphere would remain at rest for all times in the absence of numerical errors. Our results show that the errors occurring in the models under these idealized conditions differ strongly between the models. In the MM5 and the LM, the isentropes become heavily disturbed above the mountain after a few hours of integration, and peak vertical motions of several m s -1 are encountered. In KAMM2, however, isentropes remain virtually undisturbed and vertical motions do not exceed 5 cm s -1 except close to the upper boundary. Sensitivity tests indicate that the numerical errors in the MM5 disappear almost completely when the horizontal diffusion of temperature is computed truly horizontally rather than along the sloping coordinate surfaces. Experiments with moist physics show that the numerical errors over steep mountains can induce substantial amounts of spurious rain. The largest values are found for the LM, followed by the original MM5. The KAMM2 and the modified MM5 version produce only negligible amounts of rain.

The diurnal circulation of Zugspitzplatt: observations and modeling

Zugspitzplatt is a gently sloping plateau in the Wetterstein massif with a steep eastward oriented slope which leads down to the narrow and deep Reintal. Such combinations of plateaus and valleys are ubiquitous in the Alps. A e eld campaign has been conducted and a surface station has been installed to observe the diurnal circulation of this plateau. Nocturnal oute ow as well as an ine ow of a few hundred meters depth during the day are found at the eastern edge of the plateau on fair weather days. A rather extreme decrease of the dew point tends to occur during oute ow late in the night. This indicates that air from relatively large heights descends down to the plateau to e ow over the edge. Simulations with a mesoscale model have been performed for the days of the e eld campaign. The model is capable of simulating successfully the trajectories of the pilot balloons released near the edge. The model calculations suggest that the ine ow towards the plateau during the day ascends from Reintal and includes air from outside the Wetterstein massif. This ine ow normally reaches the western end of Zugspitzplatt. Zusammenfassung

Modelling Convection over West Africa

The dramatic change in the region of the West African Monsoon (WAM) from wet conditions in the 50s and 60s to much drier conditions from the 70s to the 90s represents one of the strongest inter–decadal signals on the planet in the 20th century. Marked inter-annual variations in the recent decades have resulted in extremely dry years with devastating environmental and socio-economic impacts. Vulnerability of West African societies to climate variability is likely to increase in the next decades as demands on resources increase due to the rapidly growing population. The situation may be exacerbated by the effects of climate change, land degradation, water pollution and biomass burning. Furthermore, the WAM has an impact on the downstream tropical Atlantic by providing the seedling disturbances for the majority of Atlantic tropical cyclones and on the global climate as one of the world’s largest source regions of mineral dust and of fire aerosol. Motivated by the need to develop strategies to reduce the socio–economic impacts of climate variability and change in the WAM, the integrated European project African Monsoon Multidisciplinary Analysis (AMMA) aims to improve our knowledge and understanding of the WAM on daily to interannual timescales and thus improve our ability to forecast the weather and climate in the West African region.