Sensitivity of boundary layer parameterization to the critical bulk Richardson number in a climate model over India and its adjoining regions

The planetary boundary layer (PBL) scheme is required in almost all numerical weather prediction and general circulation models to parameterize the vertical diffusion of mass, moisture and momentum, in which the critical bulk Richardson number (Ri_cr) is a crucial component. This study investigates the sensitivity of the Holtslag and Boville (HB) PBL scheme to the Ri_cr in the National Centre for Atmospheric Research Community Atmosphere Model version 5 (NCAR-CAM5) over India and its adjoining regions. According to the HB PBL scheme, turbulent motion in the PBL is caused by surface heating because of incoming solar radiation. The simulations were conducted using various values of Ri_cr (0.19, 0.25, 0.35, and 0.5) as well as the default scheme (Ri_cr = 0.30) at a horizontal resolution of 1^o. A total of five sets of simulations have been conducted, each for six years; the first year was discarded as spin-up time, and the last five years of simulations are considered for the analysis. The study spans DJF (December–January), MAM (March–May), JJA (June–August), and SON (September–November) seasons over the study domain. The study shows that the HB PBL is highly sensitive to the value of Ri_cr employed in the simulation. The results demonstrate that over the whole study domain (20S-40N, 40E-140E), as well as just over land and ocean, the mean planetary boundary layer height (PBLH) increases nearly linearly with increasing Ri_cr during all four seasons. The surface sensible and latent heat fluxes exhibit opposite behaviour towards the increment in the values of Ri_cr. The 2-m temperature, wind, and specific humidity changes are marginal and insignificant during all seasons. Moreover, the total precipitation averaged over the whole domain also shows increasing behaviour as Ri_cr increases during all four seasons.

Keywords: PBL parameterization, Critical bulk Richardson number, Climate model, Surface turbulent fluxes, Precipitation