One of the most challenging problems in predicting the Madden–Julian oscillation (MJO) is the initiation of large-scale convective activity associated with the MJO over the tropical Indian Ocean. The lack of observations is a major obstacle. The Dynamics of the MJO (DYNAMO) field campaign collected unprecedented observations from air-, land-, and ship-based platforms from October 2011 to February 2012. Here we provide an overview of the aircraft observations in DYNAMO, which captured an MJO initiation event from November to December 2011. The National Oceanic and Atmospheric Administration (NOAA) WP-3D aircraft was stationed at Diego Garcia and the French Falcon 20 aircraft on Gan Island in the Maldives. Observations from the two aircraft provide a unique dataset of three-dimensional structure of convective cloud systems and their environment from the flight level, airborne Doppler radar, microphysics probes, ocean surface imaging, global positioning system (GPS) dropsonde, and airborne expendable bathythermograph (AXBT) data. The aircraft observations revealed interactions among dry air, the intertropical convergence zone (ITCZ), convective cloud systems, and air–sea interaction induced by convective cold pools, which may play important roles in the multiscale processes of MJO initiation. This overview focuses on some key aspects of the aircraft observations that contribute directly to better understanding of the interactions among convective cloud systems, environmental moisture, and the upper ocean during the MJO initiation over the tropical Indian Ocean. Special emphasis is on the distinct characteristics of convective cloud systems, environmental moisture and winds, air–sea fluxes, and convective cold pools during the convectively suppressed, transition/onset, and active phases of the MJO.

The Madden-Julian Oscillation (MJO) is an atmosphere-ocean coupled phenomenon characterized by an alternating pattern of enhanced and suppressed precipitation initiating in the Indian Ocean (IO) and propagating eastward across the Maritime Continent (MC) and western Pacific (Madden & Julian, 1971, 1972 Zhang, 2005). The precipitation pattern is accompanied by changes in surface winds and upper ocean temperature. Although it is the leading source of tropical intraseasonal variability, its prediction remains a major challenge in numerical weather prediction (

The Madden-Julian Oscillation (MJO) is the leading source of tropical intraseasonal variability with a wide range of impacts on global weather and climate (Zhang, 2013). The MJO is characterized by an alternating large-scale pattern of active and suppressed precipitation with corresponding enhanced westerly and easterly zonal winds (Madden & Julian, 1971, 1972. Active convection associated with the MJO typically initiates over the Indian Ocean (IO), propagates eastward over the Maritime Continent (MC) and into the western Pacific before decaying. Within the large-scale MJO convective envelope, deep convection is predominately organized in mesoscale convective systems or cloud clusters (Chen et al., 1996;Takayabu, 1994;Zhang, 2005). The mesoscale convective systems can enhance surface westerly winds through the downward transport of momentum into the atmospheric boundary layer

Large-scale convection associated with the Madden-Julian Oscillation (MJO) initiates over the Indian Ocean and propagates eastward across the Maritime Continent (MC). Over the MC, MJO events are generally weakened due to complex interactions between the large-scale MJO and the MC landmass. The MC barrier effect is responsible for the dissipation of 40-50\% of observed MJO events and is often exaggerated in weather and climate models. We examine how MJO propagation over the MC is affected by two aspects of the MC -its land-sea contrast and its terrain. To isolate the effects of mountains and landsea contrast on MJO propagation, we conduct three high-resolution coupled atmosphere-ocean model experiments: 1) control simulation (CTRL) of the 2011 November-December MJO event, 2) flattened terrain without MC mountains (FLAT), and 3) no-land simulation (WATER) in which the MC islands are replaced with 50 m deep ocean. CTRL captures the general properties of the diurnal cycle of precipitation and MJO propagation across the MC. The WATER simulation produces a more intense and smoother-propagating MJO compared with that of CTRL. In contrast, the FLAT simulation produces much more convection and precipitation over land (without mountains) than CTRL, which results in a stronger barrier effect on MJO propagation. The land-sea contrast induced land-locked convection weakens the MJO's convective organization. The land-locked convective systems over land in FLAT are more intense, grow larger, and last longer, which is more detrimental to MJO propagation over the MC, than the mountains that are present in CTRL.

The Maritime Continent (MC) is a unique region of thousands of islands in the tropical Pacific warm pool with a very dynamic distribution of topography and terrain, and one of the main drivers of the global general circulation (Ramage, 1968). It lies at the intersection of many scales of atmospheric and oceanic variability, from decadal (El Niño-Southern Oscillation), to seasonal (monsoons), intraseasonal (the Madden-Julian Oscillation (MJO), Madden & Julian, 1971, 1972, and some of the strongest diurnal cycles in the world (Moron et al., 2015). Kikuchi and Wang (2008) classify the diurnal cycle (DC) over the MC into the coastal regime under which systems of land-and sea-breezes drive precipitation location and intensity, modified by the background circulation, orography, and coastline orientation (Abbs & Physick, 1992). Differential solar heating during the day induces a sea breeze circulation around islands and precipitation begins to form on the coast, then propagate inland from noon to evening; precipitation over the neighboring oceans is suppressed (Miller et al., 2003