# Atmospheric GNSS RO 1D-Var in Use at UCAR: Description and Validation
## 3 Results
### Comparison to High Vertical Resolution Radiosondes  
Despite being useful for evaluating the overall quality of 1D-Var retrievals, NWP data--especially the analyses--tend to be correlated with 1D-Var retrievals in their errors mainly because RO observations are used in common by data assimilation and 1D-Var. A solution to this problem is to compare 1D-Var retrievals with independent data, e.g., radiosonde observations. On that ground, we compared 1D-Var retrievals with high vertical resolution (1 s sampling rate; \(\sim\)5 m in depth) radiosonde profiles in close proximity (within 150 km and 90 min) of COSMIC profiles for the period from May 2006 to August 2015. These radiosonde profiles are from the stations operated with the Vaisala RS92 system and by the National Oceanic Atmospheric Administration (NOAA), available online: [[ftp://ftp.ncdc.noaa.gov/pub/data/urs-data](ftp://ftp.ncdc.noaa.gov/pub/data/urs-data)]([ftp://ftp.ncdc.noaa.gov/pub/data/urs-data](ftp://ftp.ncdc.noaa.gov/pub/data/urs-data)) (accessed on 31 October 2022). The radiosonde observation network spans multiple latitudes, including tropical areas and the Alaska region. This radiosonde data set offers an excellent representation of small-scale atmospheric structures, thanks to the balloon's precise position recorded at every moment of the flight [38]. Figure 14 shows mean differences (lower panels) and standard deviations (upper) from the radiosonde observations. As a benchmark for 1D-Var retrievals, we also compare the dry RO temperature and pressure (denoted DRY). At heights above 12 km where water vapor is scarce, DRY shows a remarkable agreement with the collocated RS92 profiles. The dry temperature is cooler than the radiosonde temperature, but by less than 0.6 degree, with a standard deviation ranging from 1.0 to 2.3 degrees. The dry pressure also exhibits a slight (\(<\)0.25%) negative bias and a small (0.3-1.1%) standard deviation. OLD follows DRY very closely in the temperature and pressure in this height range, showing almost identical mean differences and marginally smaller standard deviations. The compatibility with DRY in stratospheric heights is what we intended while designing OLD; the primary purpose of OLD was to extend DRY to lower, moisture-abundant heights. The agreement between DRY and radiosonde in the lower altitudes deteriorates rapidly with decreasing height. On the contrary, OLD maintains small mean differences and standard deviations from radiosonde temperature and pressure down to the surface. Furthermore, NEW shows significantly better agreements with radiosondes than OLD does, throughout the entire height range, in both measures of the mean difference and the standard deviation, and in all variables compared, confirming the improvement over OLD demonstrated earlier in comparison with ECMWF analysis. The pressure produced by NEW exhibits no apparent bias up to the height that balloon bursts start deteriorating radiosonde pressures (\(\sim\)27 km). NEW shows significantly smaller pressure standard deviations than OLD by a factor of 3 at the surface. The pressure field, which describes the atmospheric mass distribution, is larger in spatial scales and varies more slowly than the temperature and moisture fields. As a result, the representativeness error is less of a concern when comparing the pressures. In addition, the pressure error corresponds to the temperature and moisture errors accumulated in a relatively deep layer for which the real atmosphere remains close to a hydrostatic balance. Therefore, the pressure error is a good metric for evaluating the overall performance of RO retrieval. It is encouraging that NEW shows noticeably better agreements with radiosonde than OLD in pressure. Humidity comparison is challenging because atmospheric humidity varies enormously in space and time, e.g., radiosondes flown together often show relative humidity differences of a few tens of percent, e.g., [84]. Despite that, 1D-Var humidities demonstrate good agreements with radiosonde humidities.