Planetary boundary layer profiling with spaceborne GNSS radio occultation and advances in radio occultation from aerial platforms

The Global Navigation Satellite System (GNSS) radio occultation (RO) is an effective tool for Earth atmosphere measurements that provide spatially quasi-random, temporally constant sampling of atmospheric thermodynamic profiles. The RO technique precisely measures GNSS signal bending and refraction when passing through stratified layers of the atmosphere, which can be related to vertical profiles of atmospheric pressure, temperature, and moisture content. The thermodynamic vertical structure of the atmosphere can easily be observed using GNSS RO from stratosphere down to the planetary boundary layer (PBL). The most widely used GNSS RO datasets are the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC-1) and COSMIC-2. The PBL height (PBLH), a key parameter in weather and climate system, is affected by numerous physical processes within the boundary layer. Specifically, the PBLH over land exhibits large spatial and temporal variation across different geographical regions. However, regular observations of the PBL and PBLH are typically limited to sparse radiosonde profiles and low-resolution infrared/microwave sounders. The Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) research site provides a unique terrestrial location with 6-hourly radiosondes to examine the terrestrial PBL. COSMIC-1 RO can observe diurnal and seasonal variations in the terrestrial PBLH over the SGP region. Annual mean diurnal amplitude of approximately 250 m in the terrestrial PBLH was observed by COMSIC-1, with maxima occurring at around 15:00 local solar time, which is consistent with the colocated radiosonde profiles. Seasonal changes in the PBLH diurnal cycles ranging from approximately 100 m to 400 m were also observed. While the investigation of the terrestrial PBL and the PBLH using GNSS RO is difficult because of topography, GNSS RO observations in tropical cyclones (TCs) are less understood. GNSS RO profiles have not been adequately compared against in-situ observations (dropsondes), and therefore, the quality of RO profiles from both GNSS RO is not well documented, despite studies using GNSS RO for analyses in the TC environment. Clear vertical gradients in dropsonde water vapor pressure, absolute temperature, and refractivity are present in the lowest regions of theTC troposphere that indicate existence of thermodynamic boundary layers in TCs. Overall median refractivity difference between GNSS RO (COSMIC-1 and COSMIC-2) and colocated dropsondes is less than 0.1% with median absolute deviation (MAD) of 0.2%. Near-surface (2 km and below) overall negative refractivity biases of -1.60% (MAD: 0.44%) for COSMIC-1 and -1.24% (MAD: 0.27%) for COSMIC-2 are detected, which are mostly unaffected by colocation criteria. GNSS RO fractional refractivity difference also shows negative correlation with moisture content below 1.5 km and implies that water vapor pressure impacts the magnitude of N-bias in the lowest 1.5 km of TCs. Tropical cyclone boundary layer heights (TCBLHs) are also derived from GNSS RO, dropsonde, and model reanalysis profiles with the simple gradient method. TCBLH derived from refractivity matches the TCBLH derived from water vapor pressure in dropsondes, indicating consistency between the variables used to retrieve the TCBLH. However, the gradient method for determining TCBLH works best in the presence of strong vertical gradients in water vapor pressure but requires additional refinement for complicated multi-gradient-layer environments near TCs. Traditional spaceborne RO receiver satellites are custom-built and are expensive to launch but can provide high-vertical resolution global sampling. Airborne RO (ARO) platforms, on the other hand, can provide high-frequency local sampling of ROs around weather events with comparable data quality. Balloon-borne RO (BRO) has not been thoroughly explored but has the potential to minimize costs by using commercially available off-the-shelf (COTS) components and by removing fuel costs and the need for custom components. Additionally, a controlled balloon platform can potentially remain aloft much longer (e.g., weeks), resulting in thousands of ROs. Atmospheric refractivity retrievals from BRO payloads developed by Night Crew Labs, LLC (NCL) obtained during two flight campaigns (World View and ZPM-1) are presented. Modifications to traditional spaceborne retrieval methods for in-atmosphere receivers are also discussed, including the derivation of the partial bending angle designed to emulate retrieved bending angle from spaceborne platforms. COTS payloads show promise for high-quality refractivity profiles in the troposphere from both geometric optics (GO) and full spectrum inversion (FSI) retrieval methods. World View results show near-zero overall median refractivity retrieval difference compared to colocated ERA5 reanalysis refractivity profiles (MAD: 2.28%). ZPM-1 median refractivity difference is positively biased (~2.5%, MAD: 2.61%) due to multiple reasons, such as decreased platform control resulting from power failures during flight. We conclude that COTS RO payloads on balloon platforms are worth further investigation for use to obtain large quantities of RO profiles due to their comparatively low-cost and the extended flight times. Additionally, balloon RO platforms with advanced RO receivers have the potential to observe the lower troposphere including the planetary boundary layer (PBL) phenomena with high-density observations.