The Behavior of Gravitational Modes in Numerical Forecasts with the NCAR Community Climate Model

Abstract

Characteristics of gravitational-wave noise in noninitialized forecasts were investigated with the NCAR Community Climate Model. Forecasts were begun from FGGE analyses. The behavior of individual, gravitational normal modes was examined. In particular, time series of parameters which describe their behavior in terms of single transient plus quasi-stationary components were determined from the observed behavior of the parameters. Most external and first internal modes were well described by these components. However, shallower modes appeared to require several transient components for an adequate description. The observed frequencies of the external and first internal modes were approximately their natural frequencies (determined as eigenvalues of the model equations linearized about a resting state), after allowances were made for effects of advection and the numerical time-integration scheme. The behavior of shallower modes was complicated by the choice of a simple basic state to define the modes, which did not consider realistic advection. The model's shallowest modes had a strong response to surface drag, indicating a diabatic adjustment in the lowest model levels.