Abstract
<title>Abstract</title> <p>Planetary boundary layer (PBL) parameterisations are major sources of uncertainty in climate modelling because they regulate land–atmosphere exchanges of heat, moisture, and momentum. This study evaluates the influence of two PBL schemes on the seasonal climatology of southeastern Australia using ERA5-forced Weather Research and Forecasting regional climate models (RCM) at 4-km resolution: the local Mellor–Yamada–Nakanishi–Niino (MYNN2) scheme and hybrid local–nonlocal Asymmetric Convective Model version 2 (ACM2). Although both RCMs exhibit biases in seasonal-mean daily maximum and minimum temperatures, daily maximum temperature is more sensitive to PBL parameterisation. Compared with MYNN2, ACM2 reduces the cold bias in daily maximum temperature while producing a generally warmer climate. Additionally, MYNN2 overestimates seasonal precipitation during most seasons, whereas ACM2 shows substantially smaller biases except during summer. We show that a positive land–atmosphere feedback can explain the warmer and drier climate simulated by the ACM2 RCM. Stronger nonlocal turbulent mixing in the ACM2 scheme deepens the PBL, enhancing the entrainment of dry free-tropospheric air. The resulting reduction in boundary-layer humidity suppresses cloud liquid water, allowing more shortwave radiation to reach the surface. Increased surface heating raises near-surface temperature and promotes further PBL growth, which in turn strengthens entrainment and reinforces the drying and cloud reduction. Reduced precipitation further decreases soil moisture, sustaining the warm–dry feedback. These findings highlight the strong sensitivity of RCM simulations to PBL parameterisation, thus providing process-based guidance for future high-resolution climate modelling.</p>