In this paper we present results of several preliminary numerical experiments to study the mechanism of the observed variabilities in wind and convection associated with super cloud clusters(SCC), westerly wind bursts and 30-60 day oscillations in the western Pacific region. Using the Lau and Peng(1987) model, we show that the generation of eastward propagating precipitation pattern associated with the 30-60 day osillaition can be identified as a SCC. During its development stage, the SCC is accompanied by convective clusters moving in opposite direction(westward) to the SCC. Our results suggest that these westward propagating cloud clusters are associated with Rossby waves produced at the development stage of the 30-60 day disturbance due to mutual adjustment of the large scale flow and heating. If the boundary forcing is zonally symmetric and in the absence of other external forcings, the SCC eventually settles down to an organized eastward propagating precipitation pattern accompanied by a planetary east-west circulation cell in the equatorial zonal plane. It is found that intraseasonal oscillaition in the tropical atmosphere is a multi-scale process. Three basic spatial scales are identified, i.e., synoptic scale motions associated with a cloud cluster(1000-2000km), size of the SCC complex(-2000-4000km) and planetary circulation scale(wavenumber 1 and 2). The first is associated with westerly wind burst, double cyclone formation and high-frequency fluctuations of order of several days. The second and third are associated with slow eastward propagation associated with the 30-60 day oscillaition.
The relationship between SCC and westerly wind burst is studied by examining the detailed spatial and temporal variation of the model SCCs as they propagate over tonally varying sea surface temperature. It is found that wave-CISK heating in the lower troposphere as well as surface heating are crucial in leading to the strongly asymmetric low level westerly wind bursts similar to those observed in the real atmosphere. The implication of the present results in terms of the development of a unified theory of low frequency oscillation is also discussed.
Meteorological Society of Japan