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

Abstract

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 114 m 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.

Comparison of the Atmospheric and Oceanic Boundary Layers During Convection and their Latitudinal Dependence

“…Ri b is larger in OBL than ABL and larger at ϕ10 than ϕ40 in ABL. Note that Ri 0 b = 0.5 for the ABL and Ri b = 0.3 for the OBL have been suggested as the optimal values (Large et al, 1994;Troen, 1986), while values in the range 0.25-1.0 have been examined (e.g., van Roekel et al, 2018;Zhang et al, 2015). It is also worthwhile to mention that the present results do not show that Ri c decreases with f in the OML, contrary to the suggestion by McWilliams et al (2009).…”

Introduction to the Special Section on Fast Physics in Climate Models: Parameterization, Evaluation, and Observation

“…CAM5 also fails to produce the observed diurnal cycle in low clouds, not exclusively due to the model coarse grid spacing that does not resolve Manus Island. Potential influence of Planetary Boundary Layer (PBL) parameterization on precipitation and other meteorological variables are also reported in Zhang et al ().…”

“…Eight papers are devoted to the complex topics of PBL processes, turbulence structure, surface properties, and the multiscale couplings between land surface, atmosphere, clouds and precipitation. Zhang, N. et al () examine the sensitivity of the Beijing Climate Center GCM to the critical bulk Richardson number (Ricr) in the PBL parameterization schemes. The results show that the simulated PBL height increases nearly linearly with increasing Ricr.…”

Sensitivity of precipitation and atmospheric low-level circulation patterns to domain size and choice of parameterization schemes in RegCM4.4 over Central America

“…The boundary layer height (BLH) in the Holtslag scheme is diagnosed in terms of the critical Richardson number (Ric), which means that this parameter also affects the turbulent fluxes and eddy diffusivity (Holtslag et al 1990). A reduction of Ric decreases the BLH, as shown by Zhang et al (2015).…”

“…The physical parameterizations follow those in Sections 2.2.1 and 2.2.2. Two additional values of Ric are used over both land and ocean: 0.20 (which is similar to the lowest value in Zhang et al 2015) and 0.10. By default, Ric in RegCM4.4 is equal to 0.25.…”