After sunset under clear skies, the land surface cools rapidly, and a stable boundary layer (SBL) develops. Generally, the dominant physical processes in the SBL are turbulent mixing, radiative cooling, gravity waves, katabatic flows, drainage flows, and effects of land surface heterogeneity. The large variety of SBL physical processes, their nonlinearity, and their interactions, hampers our present ability to understand and to model the SBL on larger spatial scales such as in operational weather forecast and climate (NWP) models.
This project aims to understand and to quantify the role of small-scale orographically induced gravity-wave drag on the development of the SBL over land. In addition, a wave drag parameterization for use in NWP models will be developed and tested. As such we will use a sophisticated atmospheric modeling system at high horizontal and vertical resolution. We aim to systematically simulate a range of SBL wind and cooling regimes for idealized and real world surface orography. As such, we obtain systematic knowledge of how the orographically induced gravity-wave drag affect the SBL development for a range of realistic conditions. Next we will verify whether these simulations fulfill the 3D linear gravity-wave theory. With the set of model simulations it is our intention to derive simplified expressions for the wave drag for different wind speed and landscape conditions, and a climatology of wave drag effects. The newly developed wave drag descriptions will be tested in coarse resolution NWP models. |
