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H. Bellenger, K. Yoneyama, M. Katsumata, T. Nishizawa, K. Yasunaga, and R. Shirooka

of several projects, including CINDY2011, DYNAMO, the Atmospheric Radiation Measurement Program (ARM) MJO Investigation Experiment (AMIE), and the Littoral Air–Sea Process (LASP) experiment. The observed increase of moisture in the lower troposphere prior to the triggering of the convectively active phase of the MJO ( Johnson et al. 1999 ; Kemball-Cook and Weare 2001 ; Benedict and Randall 2007 ; Thayer-Calder and Randall 2009 ; Riley et al. 2011 ; Cai et al. 2013 ) is one of the fundamental

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Matthew A. Janiga and Chidong Zhang

it propagates while the associated latent heat release generates teleconnection patterns that affect global weather and climate (e.g., Zhang 2005 , 2013 ). The ability of global operational and climate models to capture moisture–convection interactions within this convective envelope is closely related to their being able to simulate its growth and propagation (e.g., Bechtold et al. 2008 ; Hirons et al. 2013b , a ; Kim et al. 2014 ; Klingaman et al. 2015 ). Observational studies have

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Tomoe Nasuno, Tim Li, and Kazuyoshi Kikuchi

, representation of the MJO in numerical models (even state-of-the-art operational models) is not satisfactory ( Hung et al. 2013 ; Zhang et al. 2013 ). In particular, the accurate simulation and forecast of convective initiation of the MJO is a difficult task ( Seo et al. 2009 ; Gottschalck et al. 2010 ). A number of studies have addressed the idea that moisture accumulation is the key to initiation and control of the MJO ( Bladé and Hartmann 1993 ; Kemball-Cook and Weare 2001 ; Maloney et al. 2010

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Simon P. de Szoeke, James B. Edson, June R. Marion, Christopher W. Fairall, and Ludovic Bariteau

homogeneous temperature structure observed in the tropical free troposphere is modeled by the weak temperature gradient (WTG) assumption ( Sobel and Bretherton 2000 ; Sobel et al. 2001 ). Even in the case of weak temperature gradients, moisture anomalies increase the moist static energy of the tropospheric column (e.g., Maloney 2009 ). “Moisture mode” theories for the MJO consider the feedback between moisture anomalies and convection ( Sugiyama 2009 ; Hannah and Maloney 2014 ; Benedict et al. 2014

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Sharon L. Sessions, Stipo Sentić, and David J. Raymond

, 2016 ). More importantly, the vertical gradient of vertical mass flux becomes larger at low levels, which by mass continuity, concentrates convergence near the lowest, moistest levels. This increases the column moisture and precipitation rate. Both of these mechanisms involve relatively strong vorticity and moisture anomalies that are not present during the initial stages of the convective events observed during DYNAMO. Nevertheless, several studies have acknowledged the role of large

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James H. Ruppert Jr. and Richard H. Johnson

population alone. For instance, while large-scale subsidence and horizontal moisture advection, exert control over column humidity, and therefore over moist convection, clouds can reduce column radiative cooling. This reduction can in turn reduce large-scale subsidence (e.g., Mapes 2001 ), assuming negligible temperature variations, thereby providing a link between clouds and the large-scale column moisture source ( Chikira 2014 ). Local processes that augment moist convection (e.g., mesoscale organized

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Michael S. Pritchard and Christopher S. Bretherton

1. Introduction The Madden–Julian oscillation (MJO) has been hypothesized to be a “moisture mode.” Moisture is especially fundamental to the dynamics of tropical convective disturbances because equatorial scalings of the primitive equations include latent heating active at leading order and weak temperature gradient balance ( Sobel et al. 2001 ). Energetic thermodynamic feedbacks can thus occur between moisture anomalies and their effect through deep convection on circulations, evaporative

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Paul E. Ciesielski and Richard H. Johnson

/V Mirai , reveals similar biases at low-levels (not shown). The model low-level cool and dry bias and the surface moist bias are present at all the island sounding sites (not shown). The near-surface model cool bias has been attributed to excessive longwave cooling associated with the water vapor continuum in the model ( Nagarajan and Aiyyer 2004 ). In the dropsondes Daley (2012) analyzed near DG on 4 December, a near-surface increase in moisture, referred to as a “moisture toe,” was found in about

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Simon P. de Szoeke and Eric D. Maloney

). Differences among the models are not easily attributed to individual processes. It is likely that the MJO owes its existence to a mixed summation of weakly unstable modes. In moisture mode theories, small positive feedbacks to the column moist static energy (MSE) budget induced by the responses of radiation ( Johnson et al. 2015 ; Del Genio and Chen 2015 ) and surface fluxes to convection may be all that is needed to overcome weak atmospheric gross moist stability ( Raymond and Fuchs 2009 ) and grow

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Walter M. Hannah, Brian E. Mapes, and Gregory S. Elsaesser

” for that reason ( Mapes 2000 ). Modulation of convection’s depth by midlevel moisture is also implicated in this instability of vertical wavenumber 1 ( Kuang 2008 ). The MJO is thought to have fundamentally different dynamics from such CCWs for several reasons. First, its propagation does not fall along linear dispersion curves characteristic of the dipole wave mode in the equatorial region, and its propagation speed is so slow that advection by observed winds, both mean and MJO-variable anomalies

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