# COSMIC-2 RO Profile Ending at PBL Top with Strong Vertical Gradient of Refractivity
## 4 Results
### Impact of Strong Horizontal Gradient of Refractivity  
The horizontal gradient of refractivity is in general about two to three orders of magnitude smaller than the vertical gradient of refractivity. However, the relatively strong horizontal gradient along the tangent direction could also affect the propagation of the RO rays. In this section, we only use the RO data with weak vertical gradients of \(N-\overline{N}^{\mathrm{obs}}(r)\) to explore the impact of strong horizontal gradients so that the influence of strong vertical gradients is excluded. Two COSMIC-2 RO profiles will be examined first. The RO3 profile is a typical RO profile located in an area with a large horizontal gradient of refractivity at a certain height, which may result in multipath propagation in the impact parameter domain and make the retrieval of refractivity impossible below this height [41; 42; 43]. The RO4 profile is located near the RO3 in both time and space but reached the surface and is chosen for comparison. Figure 9 shows the vertical profiles of the bending angle, refractivity, temperature, and specific humidity for the two profiles from the COSMIC-2 RO observations and the ERA5 reanalysis. The ERA5 profiles are smoother than the observed RO profiles for all four variables. The bending angle calculated by the Abel-transform from the ERA5 reanalysis for the RO4 profile (black dashed curve in Figure 9a) compares well with the observed  
Figure 8: Spatial distribution of COSMIC-2 ROs in August 2020 within the SEP (left panels) and SCP (right panels) areas with the vertical gradient of \(N-N(r)\) less than \(-65\) N-unit km\({}^{-1}\), in which (**a**,**b**) the lowest height and (**c**,**d**) PBL height are indicated, respectively. bending angle (black solid curve in Figure 9a), but the ERA5 bending angle profile for the RO3 (red dashed curve) differs from the RO3 observation (red solid curve). Moreover, the refractivity and temperature profiles from the COSMIC-2 observations and the ERA5 reanalysis (Figure 9b,c) are very close, while the specific humidity profiles (Figure 9d) differ significantly, especially for the RO3 (red curves). To understand the reasons for the above differences, the atmospheric conditions near the two profiles are analyzed below. Figure 10a shows the horizontal distributions of the geopotential height and refractivity at 775 hPa, which are the closest pressure levels to the RO3's lowest altitude. It can be seen that the RO3 is located at a place where the refractivity changes dramatically along its tangent direction near the perigee (black arrow). In the vertical cross-section along the tangent direction of the RO3 perigee (Figure 10b), an abnormal distribution of upward protrusion of refractivity can be seen at the lowest altitude of the RO3. This distribution results in a strong horizontal gradient of refractivity at the lowest height of the RO3 profile, which is shown in Figure 11. The RO3 ray path passed a streak of strong horizontal gradient of refractivity. In the vertical cross-section, there is a zone of strong horizontal gradients of refractivity located at and below the lowest RO3 altitude. Therefore, the strong horizontal  
Figure 9: Vertical profiles of **(a)** bending angle, **(b)** refractivity, **(c)** temperature and **(d)** specific humidity from COSMIC-2 RO observation (solid) and ERA5 reanalysis (dashed) for RO3 and RO4, which are located at (37.11°S, 96.27°W) and (31.56°S, 84.29°W) in the SEP area and observed at 1433 and 1526 UTC 30 August 2020, respectively. gradient of refractivity is considered as a possible reason for the RO3 not to go further down to the earth's surface. Figure 11: _Cont._  
Figure 10: (**a**) Horizontal distribution of refractivity (shaded) and geopotential height (contour, unit: gpm, contours) at 775 hPa, and (**b**) vertical cross-section of total refractivity (shaded), wet refractivity (solid curves) and dry refractivity (dashed curves) along the tangent direction (black arrow) at the perigee point (black open circle) of RO3.