Comparative assessment of radio occultation-based refractivity measurements from the COSMIC mission and in-situ atmospheric measurements in equatorial Africa

The growing technological needs for multi-instrument datasets require proper understanding of the behaviour of the datasets relative to each other. This paper presents the first results of analysis on the relationship between in-situ ground refractivity measurements and Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) refractivity measurements in the African equatorial region. In-situ measurements of surface refractivity obtained from four atmospheric ground stations in the region are compared with COSMIC-1 refractivity measurements at 1 km altitude. The in-situ datasets cover the periods from years 2007 to 2014, and corresponding COSMIC-1 datasets over the same period was used. Datasets from the recently launched COSMIC-2 mission (October 2019–September 2020) were utilized to show that the typical differences between refractivity values measured at 0 and 1 km altitudes are about 48 N-units. Interestingly, time-coincident measurements from COSMIC-1 (at 1 km altitude) and from ground in-situ measurements indicate that there is a similar typical difference (about 52 N-units) between refractivity values at the two altitudes. The reason for using COSMIC-2 measurements is that the altitudes covered by COSMIC-1 measurements start from a minimum of 0.1 km, and even at this altitude, the COSMIC-1 measurements are very scanty that there are no coincident observations with the in-situ ground stations. This is why it became imperative to first use COSMIC-2 measurements which cover altitudes from as low as 0 km. The reason is to validate that the difference between COSMIC measurements at 0 and 1 km altitudes are equivalent/comparable to difference between in-situ ground measurements and COSMIC measurements at 1 km. These results indicate that the COSMIC measurements at 0 km are comparable to the in-situ ground measurements.