A Diagnostic Study on Interactions Between Atmospheric Thermodynamic Structure and Cumulus Convection over the Tropical Western Pacific Ocean and over the Indochina Peninsula

Abstract

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 trimodal 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 uppertroposphere (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 Corresponding author: Dr. Yukari N. Takayabu, Center for Climate Systems Research, the University of Tokyo, 5-1-5 Kashiwano-ha, Kashiwa, Chiba, 277-8568, Japan. E-mail: yukari@ccsr.u-tokyo.ac.jp ( 2006, Meteorological Society of Japan 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.

TRMM-Observed Shallow versus Deep Convection in the Eastern Pacific Related to Large-Scale Circulations in Reanalysis Datasets

“…Takayabu et al (2006) showed the existence of a significant meridional divergence around 500-600 hPa, which almost coincides with the diagnosed level of detrainment, in addition to the deep meridional cell of the local Hadley circulation over the western Pacific. In addition, Wu (2003) showed that shallow heating profiles effectively drive shallow circulations, which range from the surface to ;500 hPa.…”

“…Nolan et al (2007) show that shallow return flows have importance in terms of the moisture transport in and out of the ITCZ, so it is meaningful to understand the behavior of the largescale circulation over the eastern Pacific in detail. Note that the SMCs appear not only over the eastern Pacific but also over the central and western Pacific, Atlantic, Africa, and the Indian subcontinent (e.g., Takayabu et al 2006;Zhang et al 2006Zhang et al , 2008. In addition, SMCs are observed not only over ITCZ regions, which we will examine in this study, but also in monsoon heat low regions such as the West Africa monsoon region .…”

Statistical Study on Cloud Top Heights of Cumulonimbi Thermodynamically Estimated from Objective Analysis Data during the Baiu Season

“…Therefore downdrafts from deep convection have to cause a large decrease of the y à e , (>20 K) at the middle level to generate the cumulus congestus. However since it is unrealistic, the evaporation effect of hydrometeors below the middle level is emphasized (e.g., Takayabu et al 2006). On the other hand, the small G below the middle level around the Baiu frontal zone is brought through latent heat release associated with convective activities, described above.…”

Statistical Analysis of Oceanic Rainfall Characteristics in the Baiu Season Utilizing TRMM PR Data

“…We then obtain a mean value of CAPE6 for all Baiu seasons from 1998 to 2006 (CAPE6-M), which is 350.3 J kg -1 . The thermodynamic conditions for an individual day are determined from the sign of the anomaly of CAPE6 from CAPE6-M. To calculate CAPE, the lowest level is selected for the starting level of the parcel since previous studies indicated that the diagnosed cloud base is not too sensitive to the selection of the air at 900 to 1000 hPa levels (Takayabu et al 2006), so that mostly 1000 hPa is selected. When the level of free convection (LFC) is not diagnosed, CAPE is set to zero.…”