Observation system simulation experiments (OSSEs) for assimilation of the planetary boundary‐layer height (PBLH) using the EnSRF technique

Abstract

AbstractThe planetary boundary‐layer height (PBLH) is a key parameter that is very important in numerical weather and air quality predictions. The current LiDAR networks make it possible to provide potential PBLH observations, and assimilating the parameter will be helpful to improve the forecast of variables within the planetary boundary layer (PBL). This study first carried out idealized experiments on PBLH assimilation through observation system simulation experiments (OSSEs). The ensemble square root filter (EnSRF) is applied to assimilate the simulated PBLH based on the Weather Research and Forecast (WRF) model. This study mainly focused on two issues: which variables can be effectively improved by assimilating the PBLH, and whether there are differences in the assimilation effects above and within the PBL in the vertical direction. The results show that during the daytime within the PBL, PBLH assimilation could effectively improve the simulation and forecasting of model variables, including perturbation potential temperature (pt), water vapour mixing ratio (qv) and perturbation geopotential (ph); meanwhile, PBLH assimilation could strengthen turbulence mixing and result in a warmer and drier PBL. While the vertical and horizontal wind speeds (w, u and v) cannot be effectively updated by assimilating the PBLH, they could be indirectly improved by model dynamical adjustment when pt, qv and ph are improved. Above the PBL top, most of the above six variables cannot be effectively improved by assimilating the daytime PBLH. In contrast, at night, only u and v within the PBL could be directly updated by the low nocturnal PBLH. In this study, the simplest and most idealized schemes are selected based on idealized experiments; nevertheless, the results lay a foundation for later vertical and horizontal localization research and for the study of LiDAR‐retrieved PBLH assimilation in the future, which is the ultimate goal of this series of works.