A study of intraseasonal variability in a one-dimensional version of the Goddard Atmospheric Global Climate Model.

摘要

A series of numerical experiments using the Goddard Laboratory for Atmospheres Global Climate Model (GLA-AGCM) are designed to study intraseasonal variabilities of the Madden-Julian (1972) type for an idealized tropical oceanic-atmospheric environment. A commentary on the GLA-AGCM parameterizations is presented as a means to justify the adaptation of the full three dimensional AGCM to a simpler model of a dynamically isolated vertical column. The remaining portions of the model include physical parameterizations for cumulus convection (Arakawa and Schubert, 1974), turbulent fluxes at the planetary boundary layer (Helfand and Labraga, 1988), and short and longwave radiative interactions (Harshvardhan et al., 1987). The set of model experiments use prescribed values for the sea surface temperature and the upper air wind profile, over an assumed warm and uncoupled tropical ocean.;The predominant time scales and the structure of the most noticeable fluctuations are dependent on the vertically integrated water vapor mass. Under low surface evaporation, a convective fast time scale of about 42 hours dominates the lower half of the troposphere and the transport of moisture to the upper layers. This convective regime tends to be warmer and moist all throughout, with a longer residence time. As the upper layers moisten, the absorption of shortwave radiation by water vapor aloft, and a reduced cooling due to longwave radiation emission, induce an overturning of the layers at the top of the troposphere, and a sudden transition to a colder and drier climate regime. The entire column is convectively active for a sustained period of time.;For larger amounts of precipitable water the system fluctuates non-periodically between two similarly defined regimes, but in time scales of about 20 to 60 days. In this case the drier regime has a longer residence time. A comparison of experiments with and without diurnal and seasonal solar cycles reveals that the predominant fluctuations and transitions occur even in the absence of the cyclic solar forcing. As opposed to the case of the real tropical atmosphere where moisture is provided mainly by evaporation and horizontal advection, these experiments suggest that the necessary conditions for the local enhancement of the intraseasonal variabilities are the availability of surface moisture and the residual accumulation of it in the upper troposphere.