Towards a climatology of orographic induced wave drag in the stable boundary layer over real terrain

The stable boundary layer (SBL) is of particular interest for numerous environmental issues as air quality, aviation, fog forecasting, wind energy engineering, and climate modelling. Unfortunately the current understanding of the SBL is still rather poor, and progress is slow. The relatively poor understanding of the SBL is a direct consequence of the multiplicity of small-scale processes which may occur at the same time in the SBL. One of such a processes is generation of orographic induced gravity waves. In night-time stable conditions undulating orography in the landscape may trigger gravity wave propagation. Until now the quantitative role of orographic induced gravity wave drag to the total momentum budget of the SBL is rather limited, and solely originates from high resolution numerical studies over idealized terrain and for idealized forcing. At the same time it is realized that large-scale weather forecast models encounter problems with forecasting winds and temperatures in the stable boundary layer. Therefore it is tempting to further investigate the role of gravity wave drag on the SBL in a climatological sense, and as such this study extends earlier results from Steeneveld et al (2009) for a broader range of weather conditions and time frame. In order to do so, we run the high resolution WRF single column model in for the Great Plains (USA) area, which is characterized by small scale orography (amplitude ~10 m, wave length <5 km). The forecasted wind and temperature fields are forwarded to a linear wave model which estimates near surface wave drag. In this module, the contributions of the individual Fourier modes of the terrain to the wave drag are considered for each wind sector. Also it allows for analyzing critical levels, and wave stress divergence, which might trigger flow acceleration, and turbulence. It appears that the estimated surface wave drag is substantial and of comparable magnitude as the turbulent drag, in particular for relatively calm nights. The simulations suggest that wave drag can be important and need to be parameterized in large-scale models.