A data analysis study on the atmospheric disturbances associated with shallow convection over the eastern tropical Pacific

In this study, differences in characteristics of rain systems between the eastern Pacific (EP) ITCZ and the western Pacific (WP) warm pool are quantitatively analyzed, and examined in relation to the large-scale environment and synoptic-scale disturbances. At first, characteristics of Tropical Rainfall Measuring Mission (TRMM) precipitation feature data are analyzed over the tropical Pacific in the boreal autumn from 1998 to 2007. Precipitation features are classified into four types according to their areas and maximum echo top heights. Deep rains from unorganized tall systems and very tall organized systems are dominant over the WP warm pool, where a relatively deep lower tropospheric convergence field (1000-400 hPa) exists. In contrast, deep rain from moderately tall organized systems and shallow rain from congestus, which are highly correlated with the shallow convergence field (1000-925 hPa), are dominant over the EP ITCZ.

Secondly, characteristics of synoptic-scale disturbances over the EP are examined, and the results are compared with those over the WP. Spectral peaks of total precipitable water (TPW) and vertical velocity at 850 hPa are found at the periods of 3-7 days over the EP. The spectral peak of OLR is also found at the same periodicity. In contrast, over the WP, the spectral peak of OLR is pronounced, but peaks of TPW and vertical velocity at 850 hPa are not found. Over the EP, synoptic-scale disturbances have a coupled structure of a vortex with its center near ITCZ and a Mixed Rossby-Gravity- (MRG-) wave-type disturbance with a composite analysis. Interestingly, the disturbance is associated with both deep convection and congestus, which generate kinetic energy of the disturbance in the upper and in the lower troposphere, respectively. Examining diabatic heating in relation to the coupled disturbance over the EP, deep heating with its peak at ~7.5 km is large in the northeastern part of the vortex, into which region relatively deep cross-equator southerly winds associated with the MRG-wave-type disturbance effectively flow. These results suggest that deep rain from organized system is maintained with the existence of a deep convergence associated with the coupled disturbance in the shallow convergence field over the EP.