Parameterization of Turbulent Orographic Form Drag and Implementation in GRAPES

Due to the limited model resolution, scales of topographies below the model resolution are called sub-grid scale orography. Sub-grid scale orography plays important roles on the atmosphere from the thermal and dynamic aspects, and these effects are fed back to the model atmosphere through parameterization.Generally, the dynamic effects of sub-grid scale orography includes: Sub-grid orographic turbulent form drag caused by turbulent flow over hills; sub grid-scale form drag of orographic flow blocking, separating and lee vortex; buoyancy waves drag caused by breakdown of gravity-waves that excited by flow over mountains and transmitting upward in the upper atmosphere. With resolution raised, turbulent form drag becomes more important which is associated with small-scale orography. Given these facts, the characteristics and effects of the sub-grid orographic turbulent form drag and its parameterization are studied.Usually, effects of the turbulent form drag due to sub-grid scale orography in numerical models are simply considered as enhancing surface roughness, known as the effective roughness method. In recent years, according to asymptotic theory and numerical simulations, a new scheme of sub-grid orographic turbulent form drag is developed. As an independent physical process, the new scheme is of three-dimension features in the model, while the effective roughness method is only considered being on the surface level.It is significant to consider turbulent form drag in NWP model for completing physical processes of model and improving the model prediction of the surface layer. A single-column model is used to simulate the characteristics of turbulent form drag, comparing the advantages and disadvantages between the effective roughness method and direct parameterizing method. Two direct parameterization schemes are also compared, one is developed and applied in NWP model recently, and the other is based on ideal terrain. Finally, the form drag scheme that developed and applied in NWP model is implemented into the GRAPES regional model. Case studies are conducted to investigate the possible influences of form drag on the forecast of near surface wind fields and other variables.