Sensitivity to PBL parameterizations on the marine layer cloud simulations in the southern Indian Ocean

In this study, a suite of ten Weather Research and Forecasting (WRF) model simulations is conducted using various choices of local and non-local representations of vertical mixing adopted in the planetary boundary layer (PBL) parameterizations to assess the boreal summertime marine layer cloud environment in the southern Indian Ocean (SIO). The local, non-local, and hybrid type PBL schemes used in the simulations are generally able to produce the pertinent features of the marine layer embedded with stratocumulus clouds in the SIO region, viz., nocturnal coverage of low clouds with warm cloud tops, well-mixed marine layer, inversion capping, strong temperature and moisture jumps across the inversion layer, cloud water generation and its coupling with cloud-top radiative cooling, as conceptually envisaged in earlier investigations. However, there is a large spatial variability noticed in the simulated low clouds over the SIO region. While the satellite diagnosed cloud liquid water paths (LWPs) are noted as high as 0.4 mm in the region 60° E–90° E, 15° S–25° S, the simulated cloud LWPs show a large mesoscale variability exhibiting a range of values between 0.04 and 2.8 mm, corresponding to cloud depths as small [high] as 200 [1700] m. It is further noted that LWPs reproduced from hybrid PBL schemes employed with eddy diffusivity mass flux (EDMF) formulation are closer to the observed estimates in the SIO region. While the non-local PBL mixing schemes tend to produce more organized sheeted low-cloud layers, the local closure schemes tend to produce more discreteness in the low-cloud decks and also exhibit substantial cloud depth variability. In addition, the coupling of in-cloud turbulence in association with cloud-top radiative cooling and sub-cloud layer turbulence are not uniquely reproduced by these schemes. The inferences from this study suggest that non-local/hybrid type of PBL parameterizations accounting for stratocumulus cloud-top driven mixing processes shows greater promise in the reproduction of low-clouds in the SIO region.