The Study Of Sub-grid Orographies Effects Parameterization In Numerical Model

Due to the limited model resolution, scales of topographies below the model resolution are called sub-grid scale orographies. Sub-grid scale orographies have important roles that can not be ignored on the atmosphere in the thermal and dynamic aspects. Its effects may only be fed back to the model atmosphere through parameterization.This study focuses on parameterization of dynamic effects of sub-grid scale orographies. In general, the dynamic effects of sub-grid scale orographies include: (1) sub-grid orographic turbulent form drag caused by turbulent flow over hills; (2) sub grid-scale form drag of orographic flow blocking, separating and lee vortex; (3) buoyancy waves drag caused by breakdown of gravity-waves that excited by flow over mountains and transmitting upward in the upper atmosphere. This paper focuses on the characteristics and effects of the sub-grid orographic turbulent form drag and its parameterization.Usually, the effects of the turbulent form drag due to sub-grid scale orographies in numerical models are simply considered as the enhanced 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 has been developed. As an independent physical process, the new scheme is of three dimensional features in the model, while the effective roughness method is only considered being on the surface level.For purpose of understanding the advantages and disadvantages between the'effective roughness method'and'direct parameterization method', comparison studies have been conducted at first in this paper. Furtheremore, we compared two direct parameterization schemes by a single-column model. The sensitivity of the two schemes to horizontal scales of orographies and model resolutions by setting the ideal terrain in the single-column model and GRAPES regional model has been discussed. Finally, the two orographic form drag schemes are implemented into the GRAPES regional model. Case studies have been conducted to investigate the possible influences of form drag on the forecast of near surface wind fields and other variables.