Study of the katabatic winds in middle latitudes through a high-resolution mesoscale model

The stable boundary layer over complex terrain is strongly influenced by thermally driven flows. Therefore, the understanding of valley and slope winds is needed in order to analyze its evolution at night. Such wind circulations can be studied numerically through the complete set of equations that describes the phenomena (i.e., a katabatic wind) or even analytically if several simplifications are applied in advance. Besides, the use of large eddy simulations (LES) is also useful in order to understand the internal mechanism of these currents, but all these techniques isolate the problem from external influences.

In the present study another approach is taken. The katabatic winds are studied using a high-resolution mesoscale simulation from two different complex areas in middle latitudes: the Duero basin in the northern Spanish plateau and the isle of Mallorca in the western Mediterranean Sea. These simulations have been performed with large detail within the lower 1000 meters above the ground. Thus, their characteristics can be analyzed within a more real context and under the influence of other usual phenomena such as the formation of cold pools or their interaction with large scale wind circulations.

Moreover, the hydraulic model approach is applied by reducing the simulated katabatic wind features to vertical bulk quantities using the framework set out in Mahrt (1982), Renfrew (2004) and Haiden and Whiteman (2005). This technique leads to (i) an objective comparison between the two different katabatic flows and (ii) an analysis of the goodness of this simplification in order to use it as a tool for the parametrization of the subgrid-scale transports in climate and NWP models.