Abstract
In a series of collaborative Russian–American works (Levina and Montgomery, 2009–2015), we applied the fundamental ideas of self-organization in turbulence with broken mirror symmetry, the so-called “helical” turbulence. In this context, tropical cyclogenesis is considered as a threshold extreme event in the three-dimensional helical moist convective atmospheric turbulence of a vorticity-rich environment of a pre-depression zone. This allowed us to discover a large-scale vortex instability and answer the question “When will cyclogenesis commence given a favorable tropical environment?”. The new instability emerges against the background of seemingly disorganized convection, without a well-defined center of near-surface circulation and noticeably precedes the formation of a tropical depression. This can give the fundamental ground and quantitative substantiation for the term “Potential Tropical Cyclone” as a beginning of TC genesis. In the present work, we explore in detail the crucial role of special convective coherent structures of cloud scales—vortical hot towers (VHTs)—in the formation and maintenance of the secondary circulation and, therefore, of the whole mesoscale vortex system. On this basis, we propose how the onset of large-scale instability, i.e., the beginning of TC genesis, can be diagnosed exactly and distantly with VHTs patterns in the field of temperature (satellite data) and vertical helicity (cloud-resolving numerical analysis). The present research is intended to contribute to a recently initiated development of operational diagnosis of the beginning of TC genesis based on GOES Imagery and supported by cloud-resolving numerical modeling.
Highlights
The lack of a universally accepted definition of tropical cyclogenesis and the related problem of accurate diagnosis of the birth of hurricane vortices remain among the major challenges of modern tropical meteorology
The approach was applied for post-processing of data from idealized near-cloud- resolving numerical simulations of tropical cyclogenesis [13], where the authors proposed a new scenario of tropical cyclone formation based on the upscale organization of convective processes—“a vortical hot tower (VHT) route to tropical cyclogenesis”
Our interpretation of tropical cyclogenesis as the pre-depression large-scale helicalvortex instability allows for providing the exact time when cyclogenesis commences and quantifying the chaotic influence resulting from moist convection
Summary
The lack of a universally accepted definition of tropical cyclogenesis and the related problem of accurate diagnosis of the birth of hurricane vortices remain among the major challenges of modern tropical meteorology. The most accurate diagnosis of the formation of TC is currently obtained by direct measurements from aircraft in the area of an incipient vortex To our knowledge, such approach has Environ. The genesis event can be accurately diagnosed only based on an atmospheric process, or a series of such processes, that can provide a quantitative threshold criterion. To this purpose, in our collaborative Russian–American works [2,3,4], we applied a theoretical hypothesis on the turbulent vortex dynamo [5,6]. Similar to two other well-known large-scale instabilities in helical turbulence, the alpha-effect in magnetohydrodynamics [7] and the anisotropic kinetic alpha (AKA)-effect in non-conducting fluid [8], the vortex dynamo theory gives an excitation threshold of the instability [5,6,9] and quantitative estimations for the corresponding atmospheric parameters [6]
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