# COSMIC-2 RO Profile Ending at PBL Top with Strong Vertical Gradient of Refractivity
## 1 Introduction
This is most likely to happen in the atmospheric ducts which usually appear near the PBL and are accompanied by large negative gradients of atmospheric refractivity. [PERSON] et al. [27] utilized the RO observations from one of the COSMIC receivers which was configured to track down to the height of straight line (HSL) of \(-\)350 km in the tropics (30\({}^{\circ}\)S-30\({}^{\circ}\)N) on 5-6 October 2010 to investigate the RO signals very deep in the earth's shadow, where HSL is defined as the height of a straight line that connects GPS and occulted LEO from the earth's surface. Through the wave optics modeling of these RO signals, it was found that the atmospheric ducts confined the RO signals in the ducting areas, especially for the ducts of wide horizontal expansion. Since the refractivity gradients in the tropical lower troposphere are strong due to the impact of water vapor and its complicated small-scale structures, the fluctuation of the RO signals is quite strong, and tracking RO signals becomes more difficult. [PERSON] et al. [28] emphasized the importance of understanding how deep the GPS RO signals can be observed. When processing COSMIC-2 RO data, there are some COSMIC-2 RO profiles which cannot penetrate below 1 to 2 km above the surface. Since the planetary boundary layer (PBL) of the atmosphere is about 1 to 2 km and the PBL top is often characterized by a strong temperature inversion and a sharp moisture gradient, a strong vertical gradient of atmospheric refractivity can appear at the PBL top. A relationship of high lowest altitudes of RO profiles to a strong vertical gradient of atmospheric refractivity is thus sought in this study. Two oceanic areas, one over the Southeast Pacific region (SEP) and the other over the South-Central Pacific (SCP), are selected. It is well known that the SEP area is characterized by a persistent boundary layer cloud deck [29] and has a frequent occurrence of ducting [30; 31] due to large negative refractivity gradient (\(dN/dr\)\(\leq-\)157 N-unit km\({}^{-1}\)) [32], where \(r\) is the distance from the perigee point of the RO ray path to the earth's center. The persistent cloud deck over SEP is formed by both the large-scale subsidence in subtropical oceans and coastal upwelling-induced cold sea surface temperature [33; 34]. Near the cloud deck, often accompanied by a sudden change in moisture and temperature, atmospheric ducts can easily occur. Some studies have used years of RO observations to investigate the atmospheric ducting events and found that RO refractivity over the SEP has a large bias and the ducts occur most frequently over the SEP [31; 35]. These situations that are prone to occur in the SEP area are proven to easily affect the downward penetration of RO rays. Based on the above considerations, the SEP is selected as an area suitable for finding a statistical relationship between the lowest RO observing heights and the vertical levels of strong gradients of refractivity. The SCP area is a mid-ocean area located to the west of the SEP and will be used as a comparison to the SEP and to see if similar conclusions are valid in an area other than the SEP. This paper is organized as follows. Sections 2 and 3 describe the data and the methodology, respectively. Results of the relationships of RO lowest heights to the vertical and horizontal gradients of refractivity and clouds are given in Section 4. Section 5 presents the discussion. The conclusion and future work are provided in Section 6.