A comparison of momentum mixing models for the planetary boundary layer

In a convective planetary boundary layer (PBL), nonlocal closure PBL models represent the turbulent mixing as nonlocal mixing induced by large‐scale eddies in addition to local mixing between adjacent grid levels. Recently, the nonlocal mixing approach formulated with a counter‐gradient term has been developed for the momentum transport in many numerical models, such as the Weather Research and Forecasting (WRF) model and the UK Met Office Unified Model. This study reviews a few momentum mixing models, provides alternative mixing formulations to address the potential problem in momentum transport, and analyzes the wind and momentum flux profiles predicted by these momentum mixing models. The predictions for momentum mixing from two nonlocal closure PBL models, i.e., the Yonsei University (YSU) PBL model and the modified Frech and Mahrt (FMX) PBL model, and from one local closure PBL model (YSU‐local) are compared, using the WRF model. The YSU model, in which the nonlocal momentum flux is aligned with the surface wind shear, is found to often predict larger vertical wind shear than the local momentum mixing model YSU‐local in the interior of the convective PBL. In contrast, the FMX model produces more well‐mixed wind profiles that agree better with observations and large eddy simulations results than the YSU and YSU‐local models, especially when the sign of the bulk wind shear over the convective PBL is opposite to that of the surface wind shear.