Simulation of orographic effects with a Q uasi‐3‐ D Multiscale Modeling Framework: Basic algorithm and preliminary results

The global atmospheric models based on the Multi‐scale Modeling Framework (MMF) are able to explicitly resolve subgrid‐scale processes by using embedded 2‐D Cloud‐Resolving Models (CRMs). Up to now, however, those models do not include the orographic effects on the CRM grid scale. This study shows that the effects of CRM grid‐scale orography can be simulated reasonably well by the Quasi‐3‐D MMF (Q3D MMF), which has been developed as a second‐generation MMF. In the Q3D framework, the surface topography can be included in the CRM component by using a block representation of the mountains, so that no smoothing of the topographic height is necessary. To demonstrate the performance of such a model, the orographic effects over a steep mountain are simulated in an idealized experimental setup with each of the Q3D MMF and the full 3‐D CRM. The latter is used as a benchmark. Comparison of the results shows that the Q3D MMF is able to reproduce the horizontal distribution of orographic precipitation and the flow changes around mountains as simulated by the 3‐D CRM, even though the embedded CRMs of the Q3D MMF recognize only some aspects of the complex 3‐D topography. It is also shown that the use of 3‐D CRMs in the Q3D framework, rather than 2‐D CRMs, has positive impacts on the simulation of wind fields but does not substantially change the simulated precipitation.