Interaction of Low- and High-Frequency Transients in a Forecast Experiment with a General Circulation Model

Abstract

Two sets of 15-day numerical forecasts are performed with a general circulation model to examine aspects of the mutual interaction between high-frequency, baroclinic-wave variability and the low-frequency components of the atmospheric flow. A control run based on an initial field, arbitrarily chosen from the history tapes of a previous model integration and a forecast based on a time-filtered version of the same initial state are compared. The results indicate that the high-frequency variability of the flow in the latter forecast returns to normal amplitudes about one week after the initialization time, at which state it is only weakly correlated in space with the high-frequency component of the flow in the control run. The low-frequency components of the flow seems to behave differently depending on their zonal scale: Ultralong waves (wavenumber 1?3) are only weakly affected by the removal of the baroclinic activity from the initial conditions, while long waves (wavenumber 4?6) react to the removal of the baroclinic waves by drifting eastward faster than their counterparts in the control run.