Sensitivity of North American Numerical Weather Prediction to Initial State Uncertainty in Selected Upstream Subdomains

Abstract

In this study the impact of initial uncertainty in localized regions on midrange forecast sensitivity over North America is studied. The local regions considered are the North American domain and two areas upstream, one covering the northeast Pacific and another extending farther west to include most of the North Pacific. The University of Utah Global Model and an estimate of initial uncertainty given by the differences between ECMWF and NCEP reanalyses are used. Control runs are performed with NCEP initial data globally. The effect of initial uncertainty on the control forecasts is simulated by a change of initial data from NCEP to ECMWF reanalysis first globally, and then only inside or only outside the selected domains. The impact of local initial uncertainty is quantified in comparison to the impact of initial uncertainty over the whole globe. Results from 17 cases show that regional state differences are less important than global state differences, unless the considered region covers most of the North Pacific.