Assessment of the planetary boundary layer over the Northesatern Pacific Ocean: Impact of ducting and horizontal inhomogeneity on GNSS radio occultation measurements

In the northeastern Pacific Ocean, strong free tropospheric subsidence and cooler sea surface temperatures due to upwelling result in a distinctive planetary boundary layer (PBL), marked by a sharp temperature inversion and moisture gradient. This distinct subtropical eastern ocean region showcases a unique transition from a shallow stratocumulus-topped PBL near the southern California coast to a deeper trade cumulus PBL regime closer to Hawaii. The shallow PBL coupled with frequent cloudiness poses significant challenges for conventional space-based observations and simulations in weather and climate models. The Global Navigation Satellite System (GNSS) radio occultation (RO) technique excels in sensing the PBL due to its superior vertical resolution, global coverage, and all-weather observation capability. This dissertation is comprised of three major tasks aimed at assessing the potential and limitation of GNSS RO for PBL sensing over the northeastern Pacific Ocean. First, the RO refractivity data from the first Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC-I) for the years 2007 to 2012 were used to derive the PBL height (PBLH) climatology over the Northeastern Pacific Ocean. The PBL in this region is characterized by pronounced temperature inversions and moisture gradients across the PBLH, leading to dominant ducting conditions that introduce significant negative biases in RO refractivity retrievals. Consequently, the second task examines the characteristics of the elevated ducting layer along the transect between Los Angeles, California and Honolulu, Hawaii with high-resolution radiosondes from the MAGIC field campaign and ERA5 global reanalysis data. A systematic negative refractivity bias (N-bias) below the ducting layer is observed throughout the transect, peaking approximately 70 meters below the PBL height (−5.42%), and gradually decreasing towards the surface (−0.5%). Third, the noticeable horizontal inhomogeneity, especially near the PBLH along the transect, may introduce additional RO retrieval errors, warranting further investigation. Using MAGIC radiosonde observations, a 2-dimensional (2D) model of atmospheric refractivity is created which integrates key PBL parameters. An asymmetry index is introduced to measure the extent of horizontal inhomogeneity. Then multiple phase screen (MPS) simulations were carried out to assess the impact of ducting and horizontal inhomogeneity on GNSS RO soundings. Preliminary findings highlight ducting as the primary cause of negative N-bias in RO retrieval, while horizontal inhomogeneity within the PBL contributes an additional −1% near the PBL top. This research enhances understanding of RO data quality within the PBL, paving the way for improved RO data assimilation and advancing weather and climate prediction capabilities.