Abstract
Abstract. Global Navigation Satellite System (GNSS) radio occultation (RO) measurements are promising in sensing the vertical structure of the Earth's planetary boundary layer (PBL). However, large refractivity changes near the top of PBL can cause ducting and lead to a negative bias in the retrieved refractivity within the PBL (below ∼ 2 km). To remove the bias, a reconstruction method with assumption of linear structure inside the ducting layer models has been proposed by Xie et al. (2006). While the negative bias can be reduced drastically as demonstrated in the simulation, the lack of high-quality surface refractivity constraint makes its application to real RO data difficult. In this paper, we use the widely available precipitable water (PW) satellite observation as the external constraint for the bias correction. A new framework is proposed to incorporate optimization into the RO reconstruction retrievals in the presence of ducting conditions. The new method uses optimal estimation to select the best refractivity solution whose PW and PBL height best match the externally retrieved PW and the known a priori states, respectively. The near-coincident PW retrievals from AMSR-E microwave radiometer instruments are used as an external observational constraint. This new reconstruction method is tested on both the simulated GNSS-RO profiles and the actual GNSS-RO data. Our results show that the proposed method can greatly reduce the negative refractivity bias when compared to the traditional Abel inversion.
Highlights
The planetary boundary layer (PBL) is the lowest layer of the atmosphere (∼ 2 km) and couples the surface to the free troposphere
The precipitable water (PW) values acquired from the three external sources (RAOB, European Centre for Medium-Range Weather Forecasts (ECMWF), AMSR-E) in all six cases are greater than the ones calculated from the negatively biased Abel-inverted profiles, which suggests dry biases in the Abel retrievals inside the boundary layer when ducting occurs
The slight negative bias caused by lower PW values (∼ 1 mm) could reduce its reliability, these results suggest that the ECMWF analysis can still be used to improve the retrieval under the trapping layer
Summary
The planetary boundary layer (PBL) is the lowest layer of the atmosphere (∼ 2 km) and couples the surface to the free troposphere. GNSS RO is a limb-sounding technique that precisely measures the GNSS signal phase delay received by low-Earth-orbiting satellites, through which the bending angle and accurate atmospheric refractivity profiles can be retrieved (Kursinski et al, 1997). This phenomenon, called ducting, occurs when the refractivity gradient dN/dr −157 (N -units km−1) and can be frequently observed in the subtropics below 2 km. The standard Abel inversion of the bending angle profile will always lead to a profile with no ducts and can cause a negative N-bias as large as 15 % below the ducting layer (Xie et al, 2010).
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