The Key Role of Diabatic Outflow in Amplifying the Midlatitude Flow: A Representative Case Study of Weather Systems Surrounding Western North Pacific Extratropical Transition

Abstract

Recurving tropical cyclones (TCs) undergoing extratropical transition (ET) may substantially modify the large-scale midlatitude flow pattern. This study highlights the role of diabatic outflow in midlatitude flow amplification within the context of a review of the physical and dynamical processes involved in ET. Composite fields of 12 western North Pacific ET cases are used as initial and boundary conditions for high-resolution numerical simulations of the North Pacific-North American sector with and without the TC present. It is demonstrated that a three-stage sequence of diabatic outflow associated with different weather systems is involved in triggering a highly amplified midlatitude flow pattern: 1) preconditioning by a predecessor rain event (PRE), 2) TC-extratropical flow interaction, and 3) downstream flow amplification by a downstream warm conveyor belt (WCB). An ensemble of perturbed simulations demonstrates the robustness of these stages. Beyond earlier studies investigating PREs, recurving TCs, and WCBs individually, here the fact that each impacts the midlatitude flow through a similar sequence of processes surrounding ET is highlighted. Latent heat release in rapidly ascending air leads to a net transport of low-PV air into the upper troposphere. Negative PV advection by the diabatically driven outflow initiates ridge building, accelerates and anchors a midlatitude jet streak, and overall amplifies the upper-level Rossby wave pattern. However, the three weather systems markedly differ in terms of the character of diabatic heating and associated outflow height, with the TC outflow reaching highest and the downstream WCB outflow producing the strongest negative PV anomaly.

The North Atlantic Waveguide and Downstream Impact Experiment

“…Figure 1 portrays an idealized North Atlantic flow situation that could result in HIW in the form of high winds and heavy precipitation over northern Europe. In addition to Rossby waves amplifying through baroclinic instability, diabatic processes are able to modify upper-tropospheric PV at the level of the midlatitude jet stream, which impacts the wavelength and amplitude of the downstream Rossby wave development (e.g., Massacand et al 2001;Knippertz and Martin 2005;Grams et al 2011;Teubler and Riemer 2016).…”

“…1) tend to amplify the upperlevel downstream ridge and to intensify the jet stream by strengthening the PV gradient (Archambault et al 2013). If the outflow layer is higher, the negative PV anomaly is stronger and more of the air mass enters the anticyclonic branch of the WCB flowing into the downstream ridge (Grams and Archambault 2016). In addition, a sharp peak in longwave radiative cooling near the tropopause, associated with a step change in water vapor, creates a reinforcement of the positive PV anomaly in upper-level troughs (Chagnon et al 2013) and plays a key role in maintaining and strengthening TPVs (Cavallo and Hakim 2012).…”

“…They form frequently over the eastern North Atlantic and Europe (e.g., Wernli and Sprenger 2007), and several studies have reported their relevance for triggering HIW, in particular heavy precipitation (e.g., Martius et al 2006;Chaboureau and Claud 2006;Grams and Blumer 2015). Synoptic wave breaking events are also important for the large-scale flow itself as they reinforce weather regimes such as blocking ridges (Michel and Rivière 2011;Spensberger and Spengler 2014).…”

Enhanced Tropospheric Wave Forcing of Two Anticyclones in the Prephase of the January 2009 Major Stratospheric Sudden Warming Event

“…From a more synoptic perspective, extratropical trough-ridge patterns are often associated with strong diabatic processes that influence the patterns themselves and the evolution of the downstream flow (e.g., Davis et al 1993;Grams et al 2011). The most intense latent…”

“…Ascent within these vigorous cross-isentropic airstreams leads to a net transport of lower-tropospheric air with low Ertel potential vorticity (PV;Ertel 1942) into the upper troposphere. Recent research highlights that the resulting upper-level outflow of such airstreams is important for the amplification of Rossby waves in the upper troposphere and for the onset of blocking anticyclones (Pfahl et al 2015;Grams and Archambault 2016;Teubler and Riemer 2016).…”