Estimating planetary boundary layer height over the Tibetan Plateau using COSMIC radio occultation data

Many previous studies on the planetary boundary layer (PBL) over the Tibetan Plateau (TP) were based on sparse conventional observations, which led to limitations for overall understanding of the PBL over the TP. Global Positioning System Radio Occultation (GPS RO) measurements contain useful information about the PBL due to its high accuracy and high vertical resolution. COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) RO measurements from 2007 to 2013 were used to derive the PBL height over the TP. The top of the PBL is defined as the height at which the vertical gradient of the refractivity is the minimum. The PBL height derived from COSMIC RO is highly correlated with collocated radiosonde data with a correlation coefficient of 0.786, a mean PBL height difference of approximately 0.049 km and a root-mean-square difference near 0.363 km. The PBL height over the TP decreases from west to east with the amplitude of annual mean PBL height ranging with in 1.8 to 2.3 km over the central-western TP and 1.4 to 1.8 km over the eastern TP. The maximum PBL height is found over southern TP. In addition, the PBL height over the TP has a distinct seasonal variation. The PBL height is more than 2.0 km during winter over most TP regions. Following the approach of spring, the PBL height starts to shrink over most TP regions except southern TP, where the PBL height increases to 3.0 km due to the influence of the Indian monsoon. The PBL over the TP strengthens during summer with the PBL height more than 1.8 km. The PBL height reduces again during the autumn. There are two maxima over Taklimakan Desert and the monsoon region, which is located at the north and south of the TP, respectively. The maxima of PBL height occur at summer over the desert and April before the monsoon onset over the monsoon region. The convergence of horizontal winds and extensive ascending motions over the central and western TP provide the impetus for the development of the PBL, and the large-scale descending motions restrain the development of PBL over the eastern TP. The seasonal distributions of PBL height are consistent with that of sensible heat flux over the TP. The PBL height derived from COSMI RO have similar spatial and temporal patterns with that from ERA-Interim (European Centre for Medium-Range Weather Forecasts Reanalysis Interim), but the PBL height derived from ERA-Interim is underestimated almost everywhere over the TP. The GPS RO can capture the structure of PBL over TP very well. However, the presence of strong inversion layer, or cloud with large liquid or ice water content, may cause uncertainties in the derived PBL height.