Evaluation of a New Orographic Gravity Wave Drag Scheme for the Community Earth System Model (CESM)

The parameterization of orographic gravity wave drag (OGWD) in NCAR’s recently released Community Earth System Model version 2 (CESM2) has been substantially modified with respect to that in earlier versions of the model. The modifications include incorporation of topographic anisotropy (ridge orientation) and representations of near surface nonlinearities such as flow-splitting and enhanced drag from downslope windstorm regimes. This is the first substantial modification to the OGWD parameterization in the NCAR model since the first version of Community Climate Model was released in 1989.

In this presentation we describe a three-pronged evaluation of the new scheme. We examine standard metrics from free-running climate simulations. These simulations use the Whole Atmosphere Community Climate Model (WACCM) configuration of CESM2. Results from sensitivity experiments comparing old and new OGWD schemes will be shown. We will also describe results from simulations that use nudging to ERA-Interim reanalyses to constrain the large scale flow. Nudging experiments enable direct comparisons of parameterized gravity wave activity with both satellite and balloon observations of small-scale variability. In addition, the amplitude and pattern of nudging tendencies obtained during these runs can provide insight into parameterization deficiencies.