Simulations have been performed with a dry global primitive-equation model on a 144 x 72 horizontal grid and 21 vertical levels. The simulated observing system has only radiosonde, SATEM and aircraft reports. In an initial experiment, it is assumed that the forecast model has no error. It is found that the predicted error levels for the guess fields are much lower than what is typically observed in data-assimilation procedures at operational centers.
It is investigated to what extent the above results are a consequence of the modest resolution of the forecast model and of the use of a simple model forcing. It is found that these model-error components are indeed responsible for the low simulated error levels.
By accounting for model error, error levels that are similar to those typically observed can be obtained. However, to arrive at reliable quantitative statements about the observational network, it will be necessary to first perform experiments with real data in order to calibrate the model-error component.