Interactions of Synoptic and Planetary Waves: Scale-Dependent Forcing of a GCM

Abstract

In order to better understand planetary wave?synoptic wave interactions in the atmosphere, and to develop a tool for further studies, the authors have applied a wavenumber-dependent external forcing to a general circulation model (GCM). The forcing constrains various length scales of the GCM to be close to those in the evolving analyses of the European Centre for Medium-Range Weather Forecasts and the National Centers for Environmental Prediction. The forcing acts either on the planetary waves (PW, defined as zonal wavenumbers 0?5), the synoptic waves (SW, defined as zonal wavenumbers 6?20), the synoptic waves plus the zonal mean (SW0), or the synoptic waves plus the zonal mean and wavenumber 1 (SW01). The form of the forcing is a linear relaxation to the (evolving) analyses with a time constant of 8 h. This forcing is applied only to the temperature and vorticity equations of the GCM, which has a spectral truncation of T42. Control integrations of length 30 days have been run starting on 15 December, 1 January, and 15 January for each of the 12 winters in the period 1982/83?1993/94. This set of 36 integrations was repeated for PW forcing, SW forcing, SW0 forcing, and SW01 forcing. The effectiveness of the SW forcing is measured by the mean zonal error variance of each wavenumber, normalized by the zonal variance in the analyses. This ratio is generally less than 0.2 when the analysis variance is large. The systematic error of the pentad-mean 500-hPa height is very small in the PW-forced experiments compared to the control. The error reduction is very modest in the SW-forced experiments, and the zonal mean bias is increased compared to the control. Implications regarding errors in the GCM formulation of the planetary wave system are discussed. Dramatic reduction in the systematic error occurs only for the SW01 experiment, indicating the importance of wavenumber 1 errors in the GCM. The very modest reduction of the random pentad mean height error in the SW forced experiments compared to the control reflects the instrinsically chaotic nature of the PWs. The 5-day mean streamfunction tendency due to bandpass transient SW?SW interactions in the control experiment tends to extend the Pacific jet too far east, and the Atlantic jet too far equatorward. The SW forcing reduces the systematic error in this transient?mean flow interaction, but systematic errors remain in the Atlantic, where the mean flow is in error. The PW-forced experiments show very low systematic error in this interaction, indicating 1) the strong steering effect of the PWs on the SWs, and 2) the ability of the GCM to simulate SWs realistically. The random error of the SW?SW transient?mean flow interaction emphasizes the intrinsic lack of predictability of the SWs. The 5-day mean streamfunction tendency due to bandpass transient PW?SW interactions in the control experiment tends to support excessive meridional flow in the eastern Pacific, and to force the Atlantic jet too far equatorward. The reduction of the systematic error of this interaction from its value in the control experiment is marginal in the SW, SW0, and SW01 experiments, but is much greater for the PW-forced experiment, emphasizing the steering of the SWs by the mean PWs. The reduction in the random error of the PW?SW tendency from its control value is also significant for the PW-forced experiments at high latitudes.