Impact of boundary layer processes on systematic winter temperature bias in a regional climate model

This study investigated the source of systematic errors in air temperature at 2 m (SAT) in Far East Asia during years with strong East Asian winter monsoon (EAWM) using the Weather Research and Forecasting (WRF) regional climate model. Three planetary boundary layer (PBL) parameterizations were employed — Mellor-Yamada-Janjić (MYJ), Yonsei University (YSU), and Asymmetric Convective Model version 2 (ACM2). The results showed notable nighttime SAT biases over Manchuria (MC) for the strong East Asian winter monsoon years, with the ACM2 scheme producing smaller cold biases than the MYJ and YSU schemes. The analysis of cold biases in lower-level air temperature indicated that the PBL schemes influenced the vertical temperature distribution along with nocturnal radiative cooling, indirectly leading to SAT errors. Mechanically-driven PBL turbulent mixing, influenced by wind shear and overestimated synoptic variables, was identified as a significant factor influencing lower-layer air temperature and SAT errors, along with land surface processes. In essence, weaker PBL mixing reduced the nighttime SAT (~21%) and skin temperature (~19%) errors over MC, particularly the nighttime SAT (~26%) errors on cold wave days, even if it still has the systematic winter temperature bias.