Kinematic processes contributing to the intensification of anomalously strong North Atlantic jets

Abstract

Anomalously strong North Atlantic jets, defined in this study as jets with wind speeds exceeding 100 m·s−1, are notable due to their potential to induce high‐impact weather. This study examines the kinematic processes that contribute to the intensification of anomalously strong North Atlantic jets, as well as the variability in those processes across a large number of events. Anomalously strong jets are objectively identified during September–May 1979–2018 within the Climate Forecast System Reanalysis and composited to reveal the synoptic‐scale flow evolution associated with jet intensification. The analysis demonstrates that anomalously strong North Atlantic jets are most frequent during the winter compared with the fall and spring, and that their development is preceded by low‐level warm‐air advection, poleward moisture advection, and moist ascent within the warm conveyor belt of a surface cyclone beneath the equatorward jet‐entrance region. A diagnosis of the irrotational and nondivergent components of the ageostrophic wind within the near‐jet environment reveals that both wind components facilitate jet intensification via their nonnegligible contributions to negative potential vorticity (PV) advection and PV frontogenesis in the vicinity of the dynamic tropopause. Weather Research and Forecasting (WRF) model simulations of a jet event from December 2013 with and without latent heating further suggest that the ageostrophic wind field within the near‐jet environment is substantially modulated by latent heating. The foregoing results indicate that a diagnosis of jet intensification during anomalously strong jet events is dependent on an accurate representation of the cumulative effects of latent heating within the near‐jet environment.

The Remote Role of North‐American Mesoscale Convective Systems on the Forecast of a Rossby Wave Packet: A Multi‐Model Ensemble Case‐Study

“…Previous case‐studies have also identified that negative PV experiences an anti‐cyclonic advection path when in close proximity to the jet stream (Blanchard et al., 2021; Oertel et al., 2020). Winters (2021) proposed that different components of the wind field, such as the ageostrophic non‐divergent component of the wind, can play a more dominant role over the irrotational wind field in determining the advection of low values of PV toward the jet stream. Our case‐study illustrates that negative PV arises from regions of strong irrotational wind‐fields; however, it would be interesting to expand upon this research further by identifying whether different components of the wind field become more significant as the negative PV is advected further away from the center of the irrotational wind field.…”

“…The metric used in Equation is phase independent and tracks packets of error that propagate in a wave‐like motion (i.e., RWPs) and specifically pertains to rotational error due to the use of the streamfunction term. This makes the metric useful for identifying the initial development of forecast error onto the jet stream, which is dominated by non‐divergent wind flow (Keyser & Johnson, 1984; Winters, 2021). $$$WA{E}_{\psi }=\frac{1}{2}\left[{\left(\frac{\partial {\psi }_{e}}{\partial x}\right)}^{2}+{\left(\frac{\partial {\psi }_{e}}{\partial y}\right)}^{2}-{\psi }_{e}\left(\frac{{\partial }^{2}{\psi }_{e}}{\partial {x}^{2}}+\frac{{\partial }^{2}{\psi }_{e}}{\partial {y}^{2}}\right)\right]$$$ …”

“…The metric used in Equation 2 is phase independent and tracks packets of error that propagate in a wave-like motion (i.e., RWPs) and specifically pertains to rotational error due to the use of the streamfunction term. This makes the metric useful for identifying the initial development of forecast error onto the jet stream, which is dominated by non-divergent wind flow (Keyser & Johnson, 1984;Winters, 2021).…”

Jet stream dynamics from a potential vorticity gradient perspective: The method and its application to a kilometre‐scale simulation

“…Progress in understanding cross‐isentropic vertical mass transport in extratropical cyclones indicates that diabatic processes play a significant role in shaping the upper level PV gradient more often than was assumed previously (Wernli and Davies, 1997; Davies and Didone, 2013; Methven, 2015; Pfahl et al ., 2015; Saffin et al ., 2021). Recently, Winters (2021) adopted an Eulerian approach by using PV frontogenesis to study the evolution of extreme jets and identified diabatic processes as important contributors to jet streak formation. These findings are a prelude to a Lagrangian study of PV frontogenesis, because the integrated effects of adiabatic and diabatic tendencies along the flow are crucial for understanding the state of the instantaneous PV gradient field along the jet.…”