Experiments with large number of GNSS-RO observations through the ROMEX collaboration in the Met Office NWP system

Abstract. Over recent years there has been an increase in the number of GNSS-RO observations available for use in numerical weather prediction (NWP). The Radio Occultation Modelling Experiment (ROMEX) was set up to assess the impact of increasing numbers of GNSS-RO observations and to provide evidence for a further increase in the number. Unlike previous studies, ROMEX gathered a large set of real observations to test the impact, rather than use simulated observations. Tests assimilating the ROMEX observations into the Met Office's NWP system showed negative impacts on forecast quality. This was largely due to a bias in the forecasts of geopotential height in the troposphere. This bias is shown to be due to the impact of GNSS-RO observations in the stratosphere. The forward operator for GNSS-RO has a long ``tail'' meaning that the NWP system is able to adjust the height of observations in the stratosphere by changing the tropospheric state (i.e. raising or lowering the height of the observation). Thus, the NWP system can adjust to a systematic difference between the model and observations in the stratosphere by creating a bias in the tropospheric state. Therefore, it was necessary to adjust the refractivity operator used in the forward model to reduce this bias in the forecast state. After much experimentation it was decided to alter the refractivity coefficients in the operator by 0.1 % and 3.5 % for k1 and k2, respectively. When using this adjusted operator it was possible to demonstrate the beneficial effect of assimilating the ROMEX observations. Additional modifications were also applied to the processing of GNSS-RO observations, including vertical smoothing of the observed profiles, and a bias correction at high altitudes to correct for errors within the NWP model. With this modified operator various experiments were conducted to assess impact of increases to the total number of observations. It was shown that the addition of the extra ROMEX observations provided a substantial improvement in the forecast quality. This is particularly true in the southern-hemisphere extra-tropics where the largest benefits were seen for the additional data. The benefit seen for a given number of additional observations varied substantially with the region being considered and the data against which the verification was being performed. Overall the largest forecast improvements were seen when assimilating 20,000 occultations per day, although this may be connected to the quality of the observations being excluded when reducing the ROMEX dataset to this number.