Moisture retrievals from simulated zenith delay "observations" and their impact on short-range precipitation forecasts

The feasibility of assimilating the GPS total zenith delay into atmospheric models is investigated within the framework of the "Observing System Simul ation Experiment." The total zenith delay is made up of two terms: one is p roportional to the pressure at the site of the GPS groundbased receiver and the other to the overlying amount of water vapor. Using the MM5 mesoscale model and its adjoint, a set of 4-dimensional variational (4DVAR) experimen ts is performed. Results from the assimilation of simulated precipitable wa ter observations are used as the benchmark. The model domain covers Souther n California. The observations are simulated with a 10 km horizontal resolu tion model that includes full physics, while a 20-km resolution and a less comprehensive physics package are used in the 4DVAR experiments. Both, the IO-km and 20-km models employ the same set of 15 vertical levels. Moisture fields retrieved from the total zenith delay are found to compare very well with those retrieved from the precipitable water. Verified against the obs ervations, the vertically integrated moisture is found to be very accurate. An overall improvement is also achieved in the vertical profiles of the mo isture fields. The use of the so-called background term and model initializ ation are shown to greatly reduce the negative impact that the sole assimil ation of the total zenith delay can have on the pressure field and integrat ed water vapor. The adverse effect stems from the poor resolution of the to pography needed to evaluate the model pressure at the GPS sites. The analys is increments of all model fields are found to be similar to the counterpar ts obtained from the assimilation of the precipitable water. The same is tr ue for the short-range precipitation forecasts initiated from the 4DVAR-opt imal initial conditions.