# COSMIC-2 RO Profile Ending at PBL Top with Strong Vertical Gradient of Refractivity
## 4 Results
### Two Typical Cases  
The vertical variations of the bending angle, refractivity, temperature, and specific humidity at the locations of two chosen RO profiles are compared between the COSMIC-2 observations and the ERA5 reanalysis in Figure 3. The RO1 profile is a typical RO affected by a strong local vertical gradient of refractivity. Since the vertical resolution of the ERA5 reanalysis (shown by the ticks on the right y-axis) is much coarser than that of the COSMIC-2 RO observations, the ERA5 vertical profiles (dashed curves) seem smoother than the RO profiles (solid curve) for all four variables shown in the figure. The ERA5 reanalysis does not include the bending angle, so the dashed curves in Figure 3a are calculated by the Abel transformation based on the ERA5 refractivity profiles (see Section 3). The COSMIC-2 bending angle observations differ greatly from the ERA5 reanalysis at the heights where the observed bending angle changes greatly, such as at the impact height of 3 km and 5.5 km in Figure 3a. As seen in Figure 3b-d, the vertical variations of refractivity, temperature, and specific humidity between RO observations and the ERA5 reanalysis are much more consistent from 1 to 8 km than the lower troposphere (e.g., below 2 km). Furthermore, when a COSMIC-2 RO profile did not reach the surface, the ERA5 reanalysis illustrated the atmospheric conditions near and below the RO lowest height. The RO1 is located near a shortwave trough of the geopotential height at 900 hPa, which is the pressure level closest to the lowest height of RO1 (Figure 4a). The RO observing time is 0637 UTC 11 August 2020. The refractivity at 900 hPa along the tangent direction of the RO1 ray path near the perigee point changes little. However, the vertical gradients of refractivity near the RO1 perigee point are strong. Figure 4b shows the vertical cross-section along the ray tangent direction at RO1 perigee point for total refractivity (color shaded), wet refractivity (solid black curve), and dry refractivity (dashed black curve). The distribution of the dry refractivity is nearly uniformly distributed in the vertical, while the  
Figure 3: Vertical profiles of (**a**) bending angle, (**b**) refractivity, (**c**) temperature and (**d**) specific humidity from COSMIC-2 RO observations (solid) and the ERA5 reanalysis (dashed) for the RO1 and RO2 profiles, which are located at (6.47\({}^{\circ}\)S, 91.81\({}^{\circ}\)W) and (3.61\({}^{\circ}\)S, 98.99\({}^{\circ}\)W) in the SEP area and observed at 0637 UTC and 0719 UTC 11 August 2020, respectively. wet refractivity varies greatly, and the contours are denser near the lowest height of RO1 than elsewhere, representing a large vertical gradient. We may examine the horizontal and vertical distributions of the vertical gradient of the wet refractivity (Figure 5), which corresponds to Figure 4. It can be seen that there is a local area of strong vertical gradients of wet refractivity in the southwest part of a short-wave trough where the RO1 was located. If we examine the vertical variation of the vertical gradient of wet refractivity, a ducting band of strong negative vertical gradient of refractivity of more than \(-157\) N-unit km\({}^{-1}\) appeared right below the lowest observing height of the RO1, which could limit the RO ray perigees below these altitudes. Therefore, RO1 is a typical RO profile for which the GPS radio signals cannot be distinguished at the GPS receiver due to the presence of a strong vertical gradient of refractivity in the low troposphere. The second example, RO2, is a COSMIC profile which reached the surface. The RO2 was located to the west of the RO1 and observed at 0719 UTC 11 August 2020, which is only 40 min later than the occurrence of the RO1. The RO2 was located within a ridge area where the vertical gradients of refractivity are small (figure omitted). Thus, the RO2 profile can extend all the way from the top of the atmosphere to the earth's surface.