Behavior of Stable Surface Layer in the NCEP Global Forecast System

In the GFS model, the parameterization of turbulent fluxes for momentum, heat and moisture within the stable surface layer was developed by Long (1986), based on the surface layer scheme in the Geophysical Fluid Dynamics Laboratory's ‘E2' physics package. This study examined the GFS's surface layer parameterization scheme, especially for a strong stable condition on which turbulence would be weak or even disappear because of high near surface atmospheric stability. Cases for snow-free or deep snowpack were investigated. The result shows that the decoupling and excessive cooling might appear in late afternoon and nighttime, which resulted in a large cold bias of the surface temperatures and the cold bias could be sustained for several hours. At the same time, due to little heat downward transport from the atmosphere to the land, it produced a warm bias of the first model layer, even in layers of the lower troposphere. The modifications were proposed to include updated roughness length and preventing the coupling system not to be decoupled. Sensitivity tests demonstrate substantial reduction of errors in near-surface 2-m air temperature forecasts using the proposed modifications.