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Publisher: Weinheim [u.a.] : Wiley × WGL Institute: PIK ×

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    On the genesis and dynamics of Madden–Julian oscillation‐like structure formed by equatorial adjustment of localized heating
    (Weinheim [u.a.] : Wiley, 2022) Rostami, Masoud; Zhao, Bowen; Petri, Stefan
    By means of a new multilayer pseudo‐spectral moist‐convective thermal rotating shallow‐water (mcTRSW) model in a full sphere, we present a possible equatorial adjustment beyond Gill's mechanism for the genesis and dynamics of the Madden–Julian oscillation (MJO). According to this theory, an eastward‐propagating MJO‐like structure can be generated in a self‐sustained and self‐propelled manner due to nonlinear relaxation (adjustment) of a large‐scale positive buoyancy anomaly, depressed anomaly, or a combination of these, as soon as this anomaly reaches a critical threshold in the presence of moist convection at the Equator. This MJO‐like episode possesses a convectively coupled “hybrid structure” that consists of a “quasi‐equatorial modon” with an enhanced vortex pair and a convectively coupled baroclinic Kelvin wave (BKW), with greater phase speed than that of dipolar structure on an intraseasonal time‐scale. Interaction of the BKW, after circumnavigating the entire Equator, with a new large‐scale buoyancy anomaly may contribute to excitation of a recurrent generation of the next cycle of MJO‐like structure. Overall, the generated “hybrid structure” captures a few of the crudest features of the MJO, including its quadrupolar structure, convective activity, condensation patterns, vorticity field, phase speed, and westerly and easterly inflows in the lower and upper troposphere. Although moisture‐fed convection is a necessary condition for the “hybrid structure” to be excited and maintained in the proposed theory in this study, it is fundamentally different from moisture‐mode theories, because the barotropic equatorial modon and BKW also exist in “dry” environments, while there are no similar “dry” dynamical basic structures in moisture‐mode theories. The proposed theory can therefore be a possible mechanism to explain the genesis and backbone structure of the MJO and to converge some theories that previously seemed divergent. By means of a new multilayer pseudo‐spectral moist‐convective thermal rotating shallow‐water model in a full sphere, we present a mechanism in which geostrophic adjustment of large‐scale localized heating in the lower troposphere over the equatorial zone can lead to generation of a structure similar to that of the Madden–Julian oscillation.
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    The influence of aggregation and statistical post‐processing on the subseasonal predictability of European temperatures
    (Weinheim [u.a.] : Wiley, 2020) Straaten, Chiem; Whan, Kirien; Coumou, Dim; Hurk, Bart; Schmeits, Maurice
    The succession of European surface weather patterns has limited predictability because disturbances quickly transfer to the large-scale flow. Some aggregated statistics, however, such as the average temperature exceeding a threshold, can have extended predictability when adequate spatial scales, temporal scales and thresholds are chosen. This study benchmarks how the forecast skill horizon of probabilistic 2-m temperature forecasts from the subseasonal forecast system of the European Centre for Medium-Range Weather Forecasts (ECMWF) evolves with varying scales and thresholds. We apply temporal aggregation by rolling-window averaging and spatial aggregation by hierarchical clustering. We verify 20 years of re-forecasts against the E-OBS dataset and find that European predictability extends at maximum into the fourth week. Simple aggregation and standard statistical post-processing extend the forecast skill horizon with two and three skilful days on average, respectively. The intuitive notion that higher levels of aggregation capture large-scale and low-frequency variability and can therefore tap into extended predictability holds in many cases. However, we show that the effect can be saturated and that there exist regional optimums beyond which extra aggregation reduces the forecast skill horizon. We expect such windows of predictability to result from specific physical mechanisms that only modulate and extend predictability locally. To optimize subseasonal forecasts for Europe, aggregation should thus be limited in certain cases.