Performance evaluation of planetary boundary layer structure from Weather Research and Forecasting model over Seoul Metropolitan area for a dry event in summer

Accurate prediction of planetary boundary layer height (PBLH) is important in weather forecasting as well as air quality prediction. We evaluated Weather Research and Forecasting model performance to simulate planetary boundary structure against observation from ceilometer, wind lidar, radiometer and flux tower at an urban site located in Seoul Metropolitan area for the period of 11 to 13 August 2016. Study case is characterized by clear sky and weak synoptic wind, which is favorable condition for developing of sea breeze. PBLH was estimated from two different observations: from ceilometer and radiometer observation. Simulated maximum PBLH was compared with ceilometer derived PBLH and diurnal pattern of PBLH was compared with that of radiometer derived PBLH. The maximum PBLH is well captured for first two days but is underestimated on the last day. It is also noted that PBLH in late afternoon and early evening is underestimated on the first two days while it is well simulated on the last day. We compared surface sensible heat flux between model and observation. In late afternoon and early evening, surface sensible heat flux is well simulated in the model. This indicates that surface sensible heat flux is not a factor for the underestimated PBLH. To investigate the cause for the different performance of PBLH depending on day, we compared wind structure between model and observation. General features such as timing and vertical extent of sea breeze are well captured by model. However, intensity of simulated sea breeze is much stronger in the model than observation. Stronger sea breeze results in stronger cold advection and hence cooling. Particularly, on the first day, vertical extent of sea breeze is confined in the lower part of PBL. Rapid decay of PBL in the model is due to enhanced cooling in the lower boundary layer in the late afternoon. On the other hand, on the last day when westerly wind exists throughout the boundary layer, PBL does not show rapid decay in late afternoon. This study suggests the importance of good simulation of sea breeze on accurate PBLH prediction.