A wave number dependent external forcing is applied to a general circulation model (GCM) in order to force certain components of the simulated circulation to be completely realistic. The forcing acts either on the planetary waves (PW, defined as zonal wave numbers 0-5) or the synoptic waves (SW, defined as zonal wave numbers 6-20), and takes the form of a linear relaxation to the (evolving) analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF) with a time constant of eight hours. The forcing is added to the temperature and vorticity equations of the GCM, which has a spectral truncation of T42.
Seasonal (control) simulations have been carried out starting on 15, 16 and 17 December for the winters 1985/86, 1988/89 and 1993/94. For each control simulation, corresponding PW forced and SW forced simulations have been carried out. In the 1985/86, and 1993/94 winter cases, the days 1-15 mean PWs in the PW-forced runs have a very small error with respect to the ECMWF analyses, but the days 16-30 mean PWs have much larger errors. In the 1988/89 case, the PWs in the PW forced runs are realistic for 30 days. In all cases, the days 1-15 mean PW errir in the SW-forced integrations is greater that in the PW forced runs, but is less than the control error.
The SW root-mean-square (RMS) errors in the SW-forced runs (PW-forced runs), averaged over days 1-15, are about one quarter (two thirds) that of the control error. The corresponding SW correlations with the analyses for days 1-15 are greater than 0.8 (0.6) almost everywhere in the Northern Hemisphere in the SW-forced (PW-forced) intergration.
It is shown that the linear wave forcing can lead to a spurious damping of the amplitude of the wave if the phase of the wave is incorrect. A direct amplitude/phase forcing is formulated, but it is highly nonlinear and can become singular in the wave components.
Complete copies of this report are available from:
Center for Ocean-Land-Atmosphere Studies
last update: 1 July 1996
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