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Kai-Chih Tseng, Chung-Hsiung Sui, and Tim Li

wave dynamics with a wave–conditional instability of the second kind (wave-CISK)-type parameterization of convective heating (e.g., Lau and Peng 1987 ). The most unstable wave in such a simplified system is normally at a small wavelength, which is different from the observed planetary-scale circulation associated with MJOs. To remedy the scale selection problem, Wang (1988) and Wang and Li (1994) added friction-induced boundary layer convergence in the wave-CISK framework. In the wave

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James H. Ruppert Jr. and Fuqing Zhang

distinction in phase speed remains unclear; however, density current theory may at least shed light on the propagation rate of individual rainfall systems. This is explored next. Cross sections depicting the low-level atmospheric structure, both in the zonal and in the diagonal (cross-Sumatra) contexts, are provided in Figs. 11 and 12 . In the boundary layer over and west of Borneo, low-level isentropes (of constant θ υ ) reveal a localized surface-based cold anomaly extending from the landmass across

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Tim Li, Chongbo Zhao, Pang-chi Hsu, and Tomoe Nasuno

related to a preceding MJO event that circumnavigates around the global tropics (e.g., Lau and Peng 1987 ; Wang and Li 1994 ; Li and Wang 1994 ; Matthews 2000 , 2008 ) and processes due to local changes of atmospheric planetary boundary layer (PBL) moisture, circulation and sea surface temperature ( Kemball-Cook and Weare 2001 ; Jiang and Li 2005 ; Li et al. 2008 ; Ling et al. 2013 ; Sobel et al. 2014 ; Wang et al. 2015 ), or downward influence of midtropospheric potential vorticity ( Seo

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Elizabeth J. Thompson, Steven A. Rutledge, Brenda Dolan, and Merhala Thurai

across the warm pool ( Johnson et al. 1999 ; Rauber et al. 2007 ; Jakob and Schumacher 2008 ; Barnes and Houze 2013 ), where the atmosphere is conditionally unstable below the equivalent potential temperature θ e minimum ( Lilly 1960 ). However, this relatively shallow and weak oceanic convection is not dominant in coastal or continental boundary layers, which likely explains its underrepresentation in BR03 , BR09 , and TH10 , which consist of data mostly from midlatitude and subtropical land

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Rachel C. Zelinsky, Chidong Zhang, and Chuntao Liu

-interaction theory emphasizes the role of feedbacks among boundary layer frictional convergence, moisture, and wave dynamics ( Wang et al. 2016 ). These and other theories (e.g., Majda and Stechmann 2009 ; Yang and Ingersoll 2013 ; Fuchs and Raymond 2017 ) explain the propagation and scale selection of the MJO. No theory currently explains why it initiates in the first place or why it initiates over the Indian Ocean most of the time ( Zhang and Ling 2017 ). There are hypotheses on convective initiation of the

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Zhe Feng, Sally A. McFarlane, Courtney Schumacher, Scott Ellis, Jennifer Comstock, and Nitin Bharadwaj

obtain the best-estimate time–height fields of the three radar moments (i.e., reflectivity, Doppler velocity, and spectral width). The KAZR-ARSCL product has vertical and temporal resolution of 30 m and 4 s, respectively. Although KAZR-ARSCL provides cloud boundaries that are derived from a combination of KAZR measurements and observations from the micropulse lidar and ceilometer, they are not used for comparison purpose in this study because reflectivity measurements from S-Pol and SMART-R are

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Jun-Ichi Yano and Joseph J. Tribbia

section. Thus, where we have further added constant spatially homogeneous potential-vorticity anomalies and to the interior and the exterior, respectively, to the potential vorticity q . For further simplification, they divide the inner and the outer regions by a circular boundary with the angular radius centered at longitude and latitude . Keep in mind that this “radius” does not measure the actual modon vortex size. The latter further extends exponentially beyond the transitional angular

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Weixin Xu, Steven A. Rutledge, Courtney Schumacher, and Masaki Katsumata

significant temporal and spatial variations of convection during DYNAMO. However, the DYNAMO radar studies already reviewed were based on a single site and are not capable of addressing spatial variability. The current study is therefore intended to compare the properties of convection as simultaneously observed by the three DYNAMO C-band Doppler radars, situated at Gan, Revelle , and on board the R/V Mirai ( Mirai ). In this way, we can begin to investigate properties and spatial variations of the MJO

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Weixin Xu and Steven A. Rutledge

tropospheric humidity rapidly increase over approximately 3–7 days prior to the MJO onset. The current study aims to use ship-based measurements (~700 km from Gan Island) during DYNAMO to examine the evolution of convective population and the environment across the MJO life cycle and quantify their convective properties. Though ZH13 examined the population of different convective systems over Gan Island during DYNAMO, they mainly focused on 2–4-day rainfall episodes. We aim to complement their results by

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Tomoe Nasuno, Tim Li, and Kazuyoshi Kikuchi

presented in Fig. 4 . Subsidence and negative moisture anomalies at the beginning of the preconditioning periods ( Figs. 4a,b ) transitioned into upward motion with positive moisture anomalies 2–3 days before the date of convective initiation. In the boundary layer (950–1000 hPa), upward motion and warm anomalies started during the early preconditioning stage ( Figs. 4a,c ). These precursor signals were consistent with those in the climatological MJO ( Zhao et al. 2013 ), but with a slower evolution of

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