Planetary boundary layer climatological features over North China Plain: Derived from intensive experiment data

A field campaign was carried out in the North China Plain (NCP) spanning 4 years and formed a dataset of more than 2,700 planetary boundary layer (PBL) sounding samples (872 in summer and 1,841 in winter). Based on these data, fundamental aspects of the PBL climatology over this region were investigated. First, the ensemble mean features of the PBL were revealed. The maximum PBL height in the daytime reaches 1,079 versus 786 m (summer vs. winter, hereafter the same), while about 200 m at nighttime. The bulk mean temperature and specific humidity across the PBL in summer are significantly higher than those in winter (24.0 vs. -0.5 degrees C; 8.9 vs. 1.7 g center dot kg(-1), respectively). Mean wind speed within the PBL keeps almost the same in winter and summer (4.5 m.s(-1)) but with a seasonal difference in wind direction (southerly vs. northwesterly). Second, significant diurnal variations within/above the PBL were quantified. Day-night temperature contrast is largest near the ground (7.9 vs. 5.1 degrees C), and it decreases with height to almost a constant above the PBL. Diurnal variation amplitude of specific humidity is about 1.0 versus 0.3 g center dot kg(-1). Diurnal variation of winds also supports the extended influence of the earth surface across the PBL. Third, the local influence on the PBL over the NCP is also presented. PBL height is higher in the plain centre; temperature is lower in coast than inland; winds in the PBL are strongly modulated by local or mesoscale circulations including the plain wind and sea breeze; low-level jets are common at summer night over the central and eastern parts of the NCP. Analysis reveals possible energy and water vapour exchange across the PBL, and additional heating mechanism above the PBL (e.g., solar radiation absorption).