2006 Volume 84A Pages 151-169
Interactions between the convective activity and the atmospheric thermodynamic structures are analyzed utilizing upper-air rawinsonde observations obtained by R/V Mirai, R/V Kaiyo, R/V Natsushima, of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) over the western tropical Pacific Ocean, and those over three GEWEX Asian Monsoon Experiment (GAME) stations: Chiang Mai, Non Khai, and Ubon Ratchathani. Special emphases are placed on understanding the correlation between convection and the atmospheric thermodynamic structures in relation to the recent findings of tri-modal cloud levels over the warm ocean (e.g., Johnson et al. 1999) and to the cloud diagnostics proposed by Raymond and Blyth (1992).
We first examine the relationships between a convection index and thermodynamic structure indices. A large correlation is found between the convective activity and lower-tropospheric (600-800 hPa) humidity, while there is no significant correlation between the convective activity and Convective Available Potential Energy (CAPE) or Convective Inhibition (CIN).
Next, we apply a cloud diagnostic model introduced by Raymond and Blyth (1992) (referred to as RB92) to the observed profiles. As a result, it is shown that there are fundamentally 3 peaks of detrainment levels, which are lower-troposphere (near 900 hPa), mid-troposphere (near 450 hPa), and upper-troposphere (near 150 hPa), over ocean as well as over land. In the soundings over ocean, when the lower-troposphere (600-800 hPa) is dry, there is a tendency for simultaneous existence of stable layers both in the lower-troposphere and in the mid-troposphere. Such atmospheric thermodynamic structure is diagnosed as favorable for strengthened detrainments in the low- and mid-troposphere and weakened in the upper-troposphere.
Finally, meridional winds are composited to the north and to the south of the maximum convective activity in the Inter Tropical Convergence Zone (ITCZ) region, respectively, over the tropical western Pacific Ocean. It is confirmed with upper-air soundings that there is a significant meridional divergence near the melting layer level in the mid-troposhere around 500-600 hPa and a significant meridional convergece near 350-400 hPa, in addition to the lower-tropospheric convergence and the upper-tropospheric divergence of the local Hadley Circulation. These additional circulations in the mid-troposphere are consistent with detrainment profiles diagnosed for observed atmospheric profiles utilizing RB92 cloud model. After all, it is strongly suggested that the cloud microphysics, such as melting and freezing, play significant roles in determining the large-scale circulation.
