Lee, M.-I., I.-S. Kang, and B.E. Mapes
JMSJ, 81, 963-992.
Using an aqua-planet version of an atmospheric general circulation model (AGCM), the dependence of the tropical intraseasonal oscillation (ISO) simulation on the cumulus parameterization was examined with three different cumulus schemes - Relaxed Arakawa-Schubert (RAS), Kuo, and moist convective adjustment (MCA). The simulated intensity and propagation characteristics of the ISO are significantly affected by the choice of cumulus scheme. The MCA scheme simulates a strong eastward propagating 30-day oscillation (wave speed ~15 m/s), composed of intense point-like convective storms. On the other extreme, the RAS scheme barely simulates any eastward propagating wave, and its precipitation occurs in a low-intensity drizzling pattern. The Kuo simulation lies in between. It is hypothesized that a more stringent convection criterion in the RAS scheme would produce stronger variability. Specifically, the influence of introducing a critical value of the cumulus entrainment rate was examined, following Tokioka et al. (1988). This entrainment constraint causes clear eastward-moving large-scale waves to emerge, with dominant zonal wavenumber 1 and a period of about 30 days. Boundary-layer frictional moisture convergence to the east and divergence to the west of the convective region suggests that a frictional Kelvin wave-CISK mechanism is important to these waves.
A PDF file of this article is available.