Impacts of using thermal stratification dependent critical bulk Richardson number in a PBL scheme of a climate model

The planetary boundary layer height (PBLH) is a crucial component in almost all planetary boundary layer (PBL) parameterizations, and the PBLH is calculated using a method based on the bulk Richardson number (RiB), which utilizes a threshold value of RiB called the critical bulk Richardson number (Ricr). In most of the PBL schemes, Ricr prescribed as one single value, and its dependency on thermal stratification has been ignored. In the present study, an effort has been made to incorporate Ricr based on different stratification conditions following a study by Zhang et al. (2014) in the PBL parameterization proposed by Holtslag and Boville (1993) of the National Centre for Atmospheric Research Community Atmosphere Mode version 5 (NCAR-CAM5). The modified scheme is evaluated over Indian land and associated different climatic zones in simulating PBLH, surface turbulent fluxes, near-surface atmospheric variables, and precipitation during the winter (DJF), pre-monsoon (MAM), monsoon (JJA), and post-monsoon (SON) seasons. The simulations with the default and modified schemes have been carried out at a spatial resolution of ~1o for a period of six years, discarding the first year as spin-up time and considering the last five-year simulation for the analysis. The study reveals that the modified scheme is able to produce more accurate estimates for PBLH than the default scheme compared to the ERA5 reanalysis dataset over Indian land, which further enhances the accuracy of turbulent transport of heat, moisture, and momentum inside the PBL under various atmospheric stability regimes. The modified scheme noticeably improved the simulation of surface sensible and latent heat fluxes, surface air temperature, and precipitation compared to the default scheme over Indian land during all four seasons.