engleski

Prognostic deep convection interaction with mountains in 2-8 km resolution

The schemes that parametrize the contribution of deep moist convection assume that the convective cells occupy a small portion of the grid cell as long as the model resolution is above the so called grey zone usually taken to be in the range from 3 to 8 km or similar. It is further assumed that models that run with grid meshes lower than 3 km fully resolve motions associated with deep convection and that the parametrization of deep convection is not needed in high resolution. However, convective motions associated to atmospheric fronts are large and organised so that they can be resolved with horizontal grids larger than 8 km. On the other hand, moist convective motions that result in precipitation can have horizontal extent of less than 3 km and are not resolved with models using that horizontal resolution. The extent of the grey zone is therefore dependent on the weather. A scheme that parametrizes deep convection in a way that the above assumption is not needed has been implemented in the ALADIN model. The scheme is modular multi-scale micro-physics and transport (3MT) scheme. It is used for operational forecast in the Croatian Meteorological and Hydrological Service. Both moisture convergence and CAPE closure are available as well as a combination of them, the first one performs better for most cases of severe convection, while using the other is better for cases with light sub-grid showers. The combined closure is the one used operationally since it allows optimal forecast in both weather patterns and tuning of the schemes performance. The operational turbulence scheme uses prognostic TKE and interacts with deep convection in multiple ways through closure and fluxes. The scheme also parametrizes contribution of shallow convection. The prognostic deep convection scheme is used for operational forecast in CMHS with horizontal resolution of 2, 4 and 8 km. Over flat terrain, precipitation amount moves from the convective to the resolved precipitation with increasing resolution. But this is not generally valid over mountains, especially in cases of severe torrential rain, when higher resolution model also resolves more details, mountain tops and ridges raise higher and this affects the triggering of precipitation and the total amount forecast by the model.

parametrization ; deep convection ; resolved precipitation

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