The Forcing of Gravitational Normal Modes by Condensational Heating

Abstract

Time series of normal mode coefficients were determined by projection of data produced at every time step for 64 days of a long climate simulation with the NCAR Community Climate Model. Harmonic dials and power spectra for selected gravitational modes were examined. One result is that the greatest power for gravitational modes is typically at the longest periods examined, but noteworthy relative maxima also occur near a mode's resonant period and corresponding time-computational period. For most naturally fast modes, the power at long periods tends to be many times greater than the power near the resonant period, implying that the behavior of these modes may be characterized as approximately balanced. For naturally slow modes, such as Kelvin modes, however, the portion of power near the mode's resonant period is often nonnegligible, implying that these modes are characterized by quasi-linear, wavelike propagation, rather than by either diabatic or adiabatic balance behavior. For the same simulated period, the forcing due to convective and stable-layer condensation was also projected onto the normal modes, and resulting harmonic dials and power spectra were examined. Typically, this power peaks at the longest periods and is approximately proportional to the period squared. A response of each mode to its forcing by condensational heating was determined by assuming that linear damping acted on each mode with an e-folding period of 5 days and that this forcing was periodic. Results indicate this forcing has sufficient power at all periods to explain the near-resonant peaks in the observed power spectra of most gravitational model. In other words, the departure of the behavior of most modes from that of slow, nearly balanced motion may be explained as a consequence of the spatial and temporal characteristics of condensational heating, and the model's diabatic forcing destroys rather than creates balance. An important implication is that diabatic nonlinear normal mode initialization is a basically incorrect procedure for strengthening a tropical circulation otherwise weakened by applying an adiabatic initialization scheme.