Simulated 1D-variational assimilation of ground based GPS measurements of total zenith delay
Higgins, M.
GPS measurements of position are affected by atmospheric refractivity and so can be used to estimate atmospheric total zenith delay. Such measurements are becoming increasingly common. The most natural quantity to assimilate is total zenith delay. It is possible to assimilate integrated water vapour but there are more assumptions made in the processing sequence and the estimates of total zenith delay have smaller errors. The forward operator for the assimilation is simply expressed in terms of temperature and relative humidity on discrete model levels. This operator is weakly non-linear. The Jacobian is also easily computed. Model estimates of total zenith delay have errors of about 10mm. Any observation with no greater error can be assimilated and will add information to NWP models. Tests using simulated observations with a standard deviation of 4mm show that almost all of the information in the measurement is used to constrain the model water vapour fields. This is determined by the sensitivity of the measurement to changes in water vapour and the structure of the model error covariance.
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