Sensitivity of a global climate model to the critical Richardson number in the boundary layer parameterization

The critical bulk Richardson number (Ri(cr)) is an important parameter in planetary boundary layer (PBL) parameterization schemes used in many climate models. This paper examines the sensitivity of a global climate model, the Beijing Climate Center atmospheric general circulation model, to Ri(cr). The results show that the simulated global average of PBL height increases nearly linearly with Ri(cr), with a change of about 114m for a change of 0.5 in Ri(cr). The surface sensible (latent) heat flux decreases (increases) as Ri(cr) increases. The influence of Ri(cr) on surface air temperature and specific humidity is not significant. The increasing Ri(cr) may affect the location of the Westerly Belt in the Southern Hemisphere. Further diagnosis reveals that changes in Ri(cr) affect stratiform and convective precipitations differently. Increasing Ri(cr) leads to an increase in the stratiform precipitation but a decrease in the convective precipitation. Significant changes of convective precipitation occur over the Intertropical Convergence Zone, while changes of stratiform precipitation mostly appear over arid land such as North Africa and Middle East.