Mechanisms for an in-phase relationship between convection and low-level zonal wind and the slow propagation of the convectively coupled Kelvin wave (CCKW) are investigated by analyzing satellite-based brightness temperature and reanalysis data and by constructing a simple theoretical model. Observational data analysis reveals an eastward shift of the low-level convergence and moisture relative to the CCKW convective center. The composite vertical structures show that the low-level convergence lies in the planetary boundary layer (PBL) (below 800 hPa), and is induced by the pressure trough above the top of PBL through an Ekman-pumping process. A traditional view of a slower eastward propagation speed compared to the dry Kelvin waves is attributed to the reduction of atmospheric static stability in mid-troposphere due to the convective heating effect. The authors’ quantitative assessment of the heating effect shows that this effect alone cannot explain the observed CCKW phase speed. We hypothesize that additional slowing process arises from the effect of zonally asymmetric PBL moisture. A simple theoretical model is constructed to understand the relative role of the heating induced effective static stability effect and the PBL moisture effect. The result demonstrates the important role of the both effects. Thus, PBL-free atmosphere interaction is important in explaining the observed structure and propagation of CCKW.
Kelvin wave Indian Ocean Planetary boundary layer Ekman pumping Theoretical model
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The authors thank valuable discussions with Dr. Guosen Chen and Dr. Lei Zhang. This study is jointly supported by China National 973 project 2015CB453200, NSFC Grant 41475084, ONR Grant N00014-16-12260, Jiangsu NSF Key project (BK20150062), Jiangsu Shuang-Chuang Team (R2014SCT001), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the International Pacific Research Center sponsored partially by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). The present results were obtained using the CLAUS archive held at the British Atmospheric Data Centre, produced using ISCCP source data distributed by the NASA Langley Data Center. This is SOEST Contribution Number 9639, IPRC Contribution Number 1197 and ESMC Number 113.
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1.Key Laboratory of Meteorological Disaster, Ministry of Education (KLME)/Joint International Research Laboratory of Climate and Environmental Change (ILCEC)/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD)Nanjing University of Information Science and TechnologyNanjingChina
2.International Pacific Research Center, and School of Ocean and Earth Science and TechnologyUniversity of HawaiiHonoluluUSA