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  1. Abstract

    Observed Madden–Julian oscillation (MJO) events are examined with the aid of regional model simulations to understand the role of cloud radiative effects in the MJO development. The importance of this role is demonstrated by the absence of the MJO in the model simulations that contain no cloud radiative effects. Comparisons of model simulations with and without the cloud radiative effects and observation help identify the major processes arising from those effects. Those processes develop essentially from heating in the upper troposphere due to shortwave absorption within anvil clouds in the upper troposphere and the convergence of longwave radiation in the middle to upper troposphere, with a peak at 300 hPa, during deep convection. First, that heating adds extra buoyancy and accelerates the rising motion in the upper troposphere in deep convection. The vertical acceleration in the upper troposphere creates a vacuum effect and demands for more deep convection to develop. Second, in response to that demand and required by mass balance arises the large-scale horizontal and vertical mass, moisture, and energy convergence. It strengthens deep convection and, with the feedback from continuing cloud radiative effect, creates conditions that can perpetuate deep convection and MJO development. That perpetuation does not occur however because those processes arising from the cloud radiative heating in the upper troposphere stabilize the troposphere until it supports no further deep convection. Weakening deep convection reduces cloud radiative effects. The subsequent reduction of the vacuum effect in the upper troposphere diminishes deep convection completing an MJO cycle. These results advance our understanding of the development of the MJO in the radiative–convective system over warm waters in the tropics. They show that while the embryo of intraseasonal oscillation may exist in the system its growth/development is largely dependent on cloud radiative effects and feedbacks.

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  2. Abstract

    A new mechanism is proposed as a potential cause for the one‐third of warm season severe nocturnal convection in the US Great Plains that develops in environments without the presence of air‐mass boundaries of fronts or mesoscale systems. This mechanism is tested in two‐ and three‐dimensional models. Results show strong ascent (∼1.0 m·s−1), sufficient for nocturnal convection initiation, arising from interactions of mean westerly zonal wind with the vertical shear of a northern vortex and also perturbation westerly winds that are created by the Coriolis torque on the Great Plains southerly low‐level jet. The interaction involving the northern vortex results in organized strong ascent on the east side of the vortex from the near‐surface level to the top of the model atmosphere, and also a weak upward acceleration near the centre of the vortex. In simulations with westerly wind perturbations, strong and organized ascent occurs above and on the east side of the westerly perturbation winds. The upward motion in these simulations relies on both mechanical forcing from non‐hydrostatic pressure perturbations and buoyant acceleration caused by interactions of the westerly zonal wind and the vertical shear in the vortex or the perturbation westerly wind. Statistical tests confirm that these interactions, not the northern vortex or westerly perturbation itself and related shear, are essential for the simulated vertical motion. Additional sensitivity analysis indicates robust ascent across a wide range of westerly perturbation or northern vortex strengths. The vertical motion profile is not sensitive to the horizontal grid spacing of the model, at least at or below 4 km, but to the morphology of westerly wind perturbations. The latter suggests where improvement could be made to increase the accuracy of model prediction of nocturnal convective storms in the US Great Plains.

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  3. Abstract

    Fungal polyketides display remarkable structural diversity and bioactivity, and therefore the biosynthesis and engineering of this large class of molecules is therapeutically significant. Here, we successfully recode, construct and characterize the biosynthetic pathway of bikaverin, a tetracyclic polyketide with antibiotic, antifungal and anticancer properties, inS. cerevisiae. We use a green fluorescent protein (GFP) mapping strategy to identify the low expression of Bik1 (polyketide synthase) as a major bottleneck step in the pathway, and a promoter exchange strategy is used to increase expression of Bik1 and bikaverin titer. Then, we use an enzyme-fusion strategy to directly couple the monooxygenase (Bik2) and methyltransferase (Bik3) to efficiently channel intermediates between modifying enzymes, leading to an improved titer of bikaverin at 202.75 mg/L with flask fermentation (273-fold higher than the initial titer). This study demonstrates that the biosynthesis of complex fungal polyketides can be established and efficiently engineered inS. cerevisiae, highlighting the potential for natural product synthesis and large-scale fermentation in yeast.

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