Tracer Simulation Using a Global General Circulation Model: Results from a Midlatitude Instantaneous Source Experiment

Abstract

An 11-level general circulation model with seasonal variation is used to perform an experiment on the dispersion of passive tracers. Specially constructed time-dependent winds from this model are used as input to a separate tracer model. The methodologies employed to construct the tracer model are described. The experiment presented is the evolution of a hypothetical instantaneous source of tracer on 1 January with maximum initial concentration at 65 mb, 36°N, 180°E. The tracer is assumed to have no sources or sinks in the stratosphere, but is subject to removal processes in the lower troposphere. The experimental results reveal a number of similarities to observed tracer behavior, including the average poleward-downward slope of mixing ratio isopleths, strong tracer gradients across the tropopause, intrusion of tracer into the Southern Hemisphere lower stratosphere, and the long-term interhemispheric exchange rate. The model residence times show behavior intermediate to those exhibited for particulate radioactive debris and gaseous C14O2. This suggests that caution should be employed when either radioactive debris or C14O2 data are used to develop empirical models for prediction of gaseous tracers 'Which are efficiently removed in the troposphere. In this experiment, the tracer mixing ratio and potential vorticity evolve to very high correlations. Mechanisms for this correlation are discussed. The zonal mean tracer balances exhibit complex behavior among the various transport terms. At early stages, the tracer evolution is dominated by eddy effects. Later, a very large degree of self-cancellation between mean cell and eddy effects is observed. During seasonal transitions, however, this self-cancellation diminishes markedly, leading to significant changes in the zonal mean tracer distribution. A possible theoretical explanation is presented. For this tracer dispersion problem, probably the most significant model shortcoming is the inability of the general circulation model to produce the midwinter stratospheric sudden warming phenomenon.