The Forcing and Balance of Zonally Symmetric Modes in a Global Model

Abstract

The temporal behavior of the zonal wavenumber 0 Hadley modes in the U.S. Navy's operational numerical weather prediction model is investigated. Time series of individual gravitational normal modes are examined in forecasts using different initialization strategies to determine the existence and nature of any nonlinear balance and the impact of initial conditions. A form of diabatic initialization based on a least-squares fit to the modes' trajectories in the forecast model is developed. A straightforward interpretation of the results, particularly how they relate to the principles of nonlinear normal-mode initialization, is afforded via a simple prognostic equation for a single gravitational mode. A limited number of behavior types is observed, which varies depending on the temporal and meridional scales of the modes. Only the largest-scale zonally symmetric modes show any evidence of diabatic balance and may thus be suitable candidates for diabatic initialization. Convective heating is the primary stationary forcing for these modes. Most medium-scale modes, whose natural periods may be close to the diurnal period, behave in a forced, wavelike manner due, apparently, to near-resonant diurnal forcing. Those modes with the smallest temporal and meridional scales exhibit both balanced and forced behavior. The dominant forcing for these modes appears to be adiabatic.