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Abstract This study uses shipborne [R/V Roger Revelle and R/V Mirai ] radar, upper-air, ocean, and surface meteorology datasets from the DYNAMO field campaign to investigate the diurnal cycle (DC) of precipitation over the central Indian Ocean related to two distinct Madden-Julian oscillations (MJOs) observed. This study extends earlier studies on the MJO DC by examining the relationship between the DC of convective organization and the local environment and comparing these results on- and off-equator. During the suppressed phase on-equator, the DC of rain rates exhibited two weak maxima at 15 LT and 01 LT, which was largely controlled by the presence of sub-MCS nonlinear precipitation features (PFs). During the active phase on-equator, MCS nonlinear features dominated the rain volume, and the greatest increase in rain rates occurred between 21-01 LT. This maximum coincided with the maxima in convective available potential energy (CAPE) and sensible heat flux, and the column moistened significantly over night. Off-equator, the environment was much drier and there was little large-scale upward motion as a result of limited deep convection. The DC of rain rates during the active phase off-equator was most similar to the DC observed during the suppressed phase on-equator, while rainfall off-equator during the suppressed phase did not vary much throughout the day. The DC of MCS nonlinear PFs closely resembled the DC of rainfall during both phases off-equator, and the DC of environmental parameters, including sea surface temperature, CAPE, and latent heat flux, was typically much weaker off-equator compared to on-equator. 

Abstract The Propagation of Intraseasonal Tropical Oscillations (PISTON) experiment conducted a field campaign inAugust-October 2018. The R/V Thomas G. Thompson made two cruises in thewestern North Pacific region north of Palau and east of the Philippines. Using select field observations and global observational and reanalysis data sets, this study describes the large-scale state and evolution of the atmosphere and ocean during these cruises. Intraseasonal variability was weak during the field program, except for a period of suppressed convection in October. Tropical cyclone activity, on the other hand, was strong. Variability at the ship location was characterized by periods of low-level easterly atmospheric flow with embedded westward propagating synoptic-scale atmospheric disturbances, punctuated by periods of strong low-level westerly winds that were both connected to the Asian monsoon westerlies and associated with tropical cyclones. In the most dramatic case, westerlies persisted for days during and after tropical cyclone Jebi had passed to the north of the ship. In these periods, the sea surface temperature was reduced by a couple of degrees by both wind mixing and net surface heat fluxes that were strongly (~200 Wm −2 ) out of the ocean, due to both large latent heat flux and cloud shading associated with widespread deep convection. Underway conductivity-temperature transects showed dramatic cooling and deepening of the ocean mixed layer and erosion of the barrier layer after the passage of Typhoon Mangkhut due to entrainment of cooler water from below. Strong zonal currents observed over at least the upper 400 meters were likely related to the generation and propagation of near-inertial currents. 

Cloud‐to‐ground strokes, narrow bipolar events, and energetic in‐cloud pulses are known classes of high peak‐current lightning processes that occur in thunderstorms. Here, we report one more distinct class of high peak‐current events observed exclusively over mountainous terrain, usually above 2,000 m altitude, in the continental Unites States. These events, which we call mountain‐top energetic pulses (MEPs), are bipolar pulses with negative radiated field polarities. MEPs are generated between the high mountain tops and compact overhead thunderclouds. Evidence supports the hypothesis that MEPs are produced by terrain‐initiated upward positive leaders propagating in high electric fields due to the proximity of the low negative charge regions of the thunderstorms. This scenario further suggests the possibility that MEPs are associated with downward terrestrial gamma‐ray flashes, and their high peak currents imply that they may produce elves.