Transition of Near-Ground Vorticity Dynamics During Tornadogenesis

Although much is known about the environmental conditions necessary for tornadogenesis, the near-ground vorticity dynamics during the tornadogenesis process itself are still somewhat poorly understood. For instance, seemingly contradicting mechanisms responsible for large near-ground vertical vorticity can be found in the literature. Broadly, these mechanisms can be sorted into two classes, one being based on upward tilting of mainly baroclinically produced horizontal vorticity in descending air (here called downdraft mechanism), while in the other the horizontal vorticity vector is abruptly tilted upward practically at the surface by a strong updraft gradient (referred to as in-and-up mechanism). In this study, full-physics supercell simulations and highly idealized simulations show that both mechanisms play important roles during tornadogenesis. Pretornadic vertical vorticity maxima are generated via the downdraft mechanism, while the dynamics of a fully developed vortex are dominated by the in-and-up mechanism. Consequently, a transition between the two mechanisms occurs during tornadogenesis. This transition is a result of axisymmetrization of the pretornadic vortex patch and intensification via vertical stretching. These processes facilitate the development of the corner flow, which enables production of vertical vorticity by upward tilting of horizontal vorticity practically at the surface, i.e. the in-and-up mechanism. The transition of mechanisms found here suggests that early stages of tornado formation rely on the downdraft mechanism, which is often limited to a small vertical component of baroclinically generated vorticity. Subsequently, a larger supply of horizontal vorticity (produced baroclinically or via surface drag, or even imported from the environment) may be utilized, which marks a considerable change in the vortex dynamics.

A Comparison of the Vorticity Dynamics Governing the Oceanic Bomb Cyclone of 4–5 January 1989 and the Super Derecho of 8 May 2009

Convection-allowing simulations of two warm seclusion cyclones are used to elucidate the vorticity dynamics that contribute to intensification of these systems. The rapidly intensifying oceanic “bomb” cyclone on 4–5 January 1989 and the super derecho on 8 May 2009 are the subject of this study. While these systems occupy different spatial scales, they both acquire characteristics of a warm seclusion cyclone. The aim of this study is to compare the basic structure and determine the dynamics driving increases in system-scale vertical vorticity during the intensification of these systems. Results from a vorticity budget show that system-scale stretching and the lateral transport of vertical vorticity to the cyclone center contribute to increases of system-scale low-level vertical vorticity during the intensification of the oceanic cyclone. The intercomparison of the oceanic cyclone and the super derecho shows that the relative contributions to increases in system-scale vertical vorticity by stretching and tilting as a function of height differ among the two cases. However, the lateral transport of vertical vorticity to the cyclone center is a key contributor to increases in low-level system-scale vertical vorticity for both cases. We hypothesize that this process may be common among a wide array of intense cyclonic systems across scales ranging from warm seclusion extratropical cyclones to some mesoscale convective systems.

Whither the Chukchi Slope Current?

Recent measurements and modeling indicate that roughly half of the Pacific-origin water exiting the Chukchi Sea shelf through Barrow Canyon forms a westward-flowing current known as the Chukchi Slope Current (CSC), yet the trajectory and fate of this current is presently unknown. In this study, through the combined use of shipboard velocity data and information from five profiling floats deployed as quasi-Lagrangian particles, we delve further into the trajectory and the fate of the CSC. During the period of observation, from early September to early October 2018, the CSC progressed far to the north into the Chukchi Borderland. The northward excursion is believed to result from the current negotiating Hanna Canyon on the Chukchi slope, consistent with potential vorticity dynamics. The volume transport of the CSC, calculated using a set of shipboard transects, decreased from approximately 2 Sv (1 Sv ≡ 106 m3 s−1) to near zero over a period of 4 days. This variation can be explained by a concomitant change in the wind stress curl over the Chukchi shelf from positive to negative. After turning northward, the CSC was disrupted and four of the five floats veered offshore, with one of the floats permanently leaving the current. It is hypothesized that the observed disruption was due to an anticyclonic eddy interacting with the CSC, which has been observed previously. These results demonstrate that, at times, the CSC can get entrained into the Beaufort Gyre.