Multiscale Interactive Processes Underlying the Heavy Rainstorm Associated with a Landfalling Atmospheric River

The heavy precipitation in Northern California—brought about by a landfalling atmospheric river (AR) on 25–27 February 2019—is investigated for an understanding of the underlying dynamical processes. By the peaks in hourly accumulation, this rainstorm can be divided into two stages (Stage I and Stage II). Using a recently developed multiscale analysis methodology, i.e., multiscale window transform (MWT), and the MWT-based theory of canonical transfer, the original fields are reconstructed onto three scale windows, namely, the background flow, synoptic-scale and mesoscale windows, and the interactions among them are henceforth investigated. In both stages, the development of the precipitation is attributed to a vigorous buoyancy conversion and latent heating, and besides, the instability of the background flow. In Stage I, the instability is baroclinic, while in Stage II, it is barotropic. Interestingly, in Stage I, the mesoscale kinetic energy is transferred to the background flow where it is stored, and is released back in Stage II to the mesoscale window again, triggering intense precipitation.

The asymmetric eddy-background flow interaction in the North Pacific storm track

<p>Using a recently developed methodology, namely, the multiscale window transform (MWT), and the MWT-based theory of canonical transfer and localized multiscale energetics analysis, we investigate in an eddy-following way the nonlinear eddy-background flow interaction in the North Pacific storm track, based on the ERA40 reanalysis data from ECWMF. It is found that more than 50% of the storms occur on the northern flank of the jet stream, about 40% are around the jet center, and very few (less than 5%) happen on the southern flank. For storms near or to the north of the jet center, their interaction with the background flow is asymmetric in latitude. In higher latitudes, strong downscale canonical available potential energy transfer happens, especially in the middle troposphere, which reduces the background baroclinicity and decelerates the jet; in lower latitudes, upscale canonical kinetic energy transfer intensifies at the jet center, accelerating the jet and enhancing the middle-level baroclinicity. The resultant effect is that the jet strengthens but narrows, leading to an anomalous dipolar pattern in the fields of background wind and baroclinicity. For the storms on the southern side of the jet, the baroclinic canonical transfer is rather weak. On average, the local interaction begins from about 3 days before a storm arrives at the site of observation, achieves its maximum as the storm arrives, and then weakens.</p>

New Perspectives on the Generation and Maintenance of the Kuroshio Large Meander

The internal dynamical processes underlying the Kuroshio large meander are investigated using a recently developed analysis tool, multiscale window transform (MWT), and the MWT-based canonical transfer theory. Oceanic fields are reconstructed on a low-frequency mean flow window, a mesoscale eddy window, and a high-frequency synoptic window with reference to the three typical path states south of Japan, that is, the typical large meander (tLM), nearshore non-large meander (nNLM), and offshore non-large meander (oNLM) path states. The interactions between the scale windows are quantitatively evaluated in terms of canonical transfer, which bears a Lie bracket form and conserves energy in the space of scale. In general, baroclinic (barotropic) instability is strengthened (weakened) during the tLM state. For the first time we found a spatially coherent inverse cascade of kinetic energy (KE) from the synoptic eddies to the slowly varying mean flow; it occupies the whole large meander region but exists only in the tLM state. By the time-varying multiscale energetics, a typical large meander is preceded by a strong influx of mesoscale eddy energy from upstream with a cyclonic eddy, which subsequently triggers a strong inverse KE cascade from the mesoscale window to the mean flow window to build up the KE reservoir for the meander. Synoptic frontal eddies are episodically intensified due to the baroclinic instability of the meander, but they immediately feed back to the mean flow window through inverse KE cascade. These results highlight the important role played by inverse KE cascades in generating and maintaining the Kuroshio large meander.

On the Inverse Relationship between the Boreal Wintertime Pacific Jet Strength and Storm-Track Intensity

Previous studies show that in boreal winters when the Pacific jet is extremely strong, the Pacific storm track is, however, unexpectedly weak. Using a recently developed technique, namely, the multiscale window transform (MWT), and the MWT-based localized multiscale energetics analysis, we investigate in this study the underlying mechanism of this counterintuitive phenomenon, based on ERA-40 data. It is found that most of the synoptic storms are generated at latitudes far north of the jet core, which lowers the relevance of the jet strength to the storm-track intensity, and the inverse relationship between the Pacific jet strength and storm-track intensity is mainly attributed to the internal dynamics. In the strong jet state, on one hand, the jet is narrow, and thus the jet winds at high latitudes are weak, resulting in weak baroclinic instabilities and hence reduced eddy growth rate; on the other hand, although baroclinic instabilities are strong at the jet core, inverse kinetic energy (KE) cascades are even stronger (by 43%). The resultant effect is that more eddy energy is transferred back to the background flow, leaving an overall weak storm track in a strong Pacific jet. In addition, diabatic processes are found to account for the inverse relationship: it is greatly weakened (by 25%) in the strong-core jet state. Apart from these, we also find that the role that barotropic canonical transfer plays in the inverse relationship is opposite to that in the formation of the midwinter minimum (MWM), another counterintuitive phenomenon in the Pacific storm track.

Multiscale Dynamical Processes Underlying the Wintertime Atlantic Blockings

The wintertime atmospheric blocking over the Atlantic is investigated using a newly developed methodology—namely, localized multiscale energy and vorticity analysis (MS-EVA)—and the theory of canonical energy transfer. Through a multiscale window transform (MWT), the atmospheric fields from the ERA-40 data are reconstructed on three-scale ranges or scale windows: basic-flow window, blocking window, and synoptic window. The blocking event is obtained by compositing the wintertime blocking episodes, and a clear westward-retrograding signal is identified on the blocking window. Likewise, the local multiscale energetics following the signal are composited. It is found that a life cycle of the blocking-scale kinetic energy (KE) may be divided into three phases: onset phase, amplification phase, and decay phase. Different phases have different mechanisms in play. In general, pressure work and the canonical transfer from the synoptic eddies initiate the generation of the blocking, while the latter contributes to its amplification. The blocking decays as the system transports the KE away and as it converts the KE into available potential energy (APE) through buoyancy conversion. For the APE on the blocking window, its evolution experiences two maxima and, correspondingly, two phases can be distinguished. In the first maximum phase, the dominating mechanism is baroclinic instability; in the second, buoyancy conversion takes place. These are also the mechanisms that cause the warm core of the blocking in the troposphere.

Canonical Transfer and Multiscale Energetics for Primitive and Quasigeostrophic Atmospheres

The past years have seen the success of a novel and rigorous localized multiscale energetics formalism in a variety of ocean and engineering fluid applications. In a self-contained way, this study introduces it to the atmospheric dynamical diagnostics, with important theoretical updates and clarifications of some common misconceptions about multiscale energy. Multiscale equations are derived using a new analysis apparatus—namely, multiscale window transform—with respect to both the primitive equation and quasigeostrophic models. A reconstruction of the “atomic” energy fluxes on the multiple scale windows allows for a natural and unique separation of the in-scale transports and cross-scale transfers from the intertwined nonlinear processes. The resulting energy transfers bear a Lie bracket form, reminiscent of the Poisson bracket in Hamiltonian mechanics; hence, we would call them “canonical.” A canonical transfer process is a mere redistribution of energy among scale windows, without generating or destroying energy as a whole. By classification, a multiscale energetic cycle comprises available potential energy (APE) transport, kinetic energy (KE) transport, pressure work, buoyancy conversion, work done by external forcing and friction, and the cross-scale canonical transfers of APE and KE, which correspond respectively to the baroclinic and barotropic instabilities in geophysical fluid dynamics. A buoyancy conversion takes place in an individual window only, bridging the two types of energy, namely, KE and APE; it does not involve any processes among different scale windows and is hence basically not related to instabilities. This formalism is exemplified with a preliminary application to the study of the Madden–Julian oscillation.