European high-impact weather caused by the downstream response to the extratropical transition of North Atlantic Hurricane Katia (2011)

Abstract

Tropical cyclones undergoing extratropical transition (ET) are thought to cause high‐impact weather (HIW) close to the transitioning tropical cyclone and in remote regions. However, no study so far clearly attributed European HIW to the downstream impact of North Atlantic ET. When Hurricane Katia underwent ET in September 2011, severe thunderstorms occurred downstream in Central Europe. We quantify the role of Katia in the European HIW, using numerical sensitivity experiments. Results show that Katia was crucial for the evolution of a narrow downstream trough. Large‐scale forcing for ascent ahead of this trough triggered deep convection. In the absence of ET, no trough was present over Europe and no HIW occurred. This study is the first unambiguous documentation that European HIW is caused by the downstream impact of North Atlantic ET and would not occur otherwise. It likewise corroborates the crucial role of ET in altering the large‐scale midlatitude flow in downstream regions.

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

“…However, major forecast uncertainty and error in the midlatitudes in current NWP models have recently been shown to be linked to a misrepresentation of the Rossby wave pattern and various authors pointed to the potential role of moist processes in large-scale flow modification (e.g., Grams et al 2011;Davies and Didone 2013;Rodwell et al 2013;Gray et al 2014;Teubler and Riemer 2016). Enhanced forecast uncertainty for the large-scale flow evolution has also been documented downstream of extratropical transition (ET; Jones et al 2003;Anwender et al 2008;Aiyyer 2015;Grams et al Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/ MWR-D-15-0419.s1. a Current affiliation: NOAA/Climate Program Office, Silver Spring, Maryland. 2015; Quinting and Jones 2016).…”

“…The divergent outflow and initial ridge building results from the diabatically driven strong ascent and associated latent heat release in the TC inner core and later at the midlatitude baroclinic zone yielding a net transport of lower-tropospheric air with low values of potential vorticity (PV) to the tropopause (e.g., DiMego and Bosart 1982;Bosart and Lackmann 1995;Torn 2010;Grams et al 2013b). Also, other weather systems such as predecessor rain events (PREs; Galarneau et al 2010;Moore et al 2013) and warm conveyor belts (WCBs; e.g., Carlson 1980;Madonna et al 2014b) occur surrounding an ET event, exhibit strong diabatic outflow, and may modify the upper-level Rossby wave pattern in a similar manner as the actual ET (Ahmadi-Givi et al 2004;Grams et al 2011;Bosart et al 2012;Lang and Martin 2013;Moore et al 2013;Madonna et al 2014a;Galarneau 2015;Teubler and Riemer 2016). The diabatic nature of this interaction alters the behavior of Rossby waves expected from a purely dry dynamical perspective.…”

“…Research during the last few decades has related the modulation of the midlatitude Rossby wave pattern during ET to the strong latent heat release by the transitioning TC. In essence, the upper-level divergent outflow of the transitioning TC impinging on the midlatitude waveguide enhances ridge building (e.g., Bosart and Lackmann 1995;Riemer et al 2008;Archambault et al 2013), in addition to the balanced flow, and triggers downstream dispersion of Rossby waves (e.g., Riemer and Jones 2010; Grams et al 2013a;Archambault et al 2015;Torn and Hakim 2015;Quinting and Jones 2016). The divergent outflow and initial ridge building results from the diabatically driven strong ascent and associated latent heat release in the TC inner core and later at the midlatitude baroclinic zone yielding a net transport of lower-tropospheric air with low values of potential vorticity (PV) to the tropopause (e.g., DiMego and Bosart 1982;Bosart and Lackmann 1995;Torn 2010;Grams et al 2013b).…”

“…In essence, the upper-level divergent outflow of the transitioning TC impinging on the midlatitude waveguide enhances ridge building (e.g., Bosart and Lackmann 1995;Riemer et al 2008;Archambault et al 2013), in addition to the balanced flow, and triggers downstream dispersion of Rossby waves (e.g., Riemer and Jones 2010; Grams et al 2013a;Archambault et al 2015;Torn and Hakim 2015;Quinting and Jones 2016). The divergent outflow and initial ridge building results from the diabatically driven strong ascent and associated latent heat release in the TC inner core and later at the midlatitude baroclinic zone yielding a net transport of lower-tropospheric air with low values of potential vorticity (PV) to the tropopause (e.g., DiMego and Bosart 1982;Bosart and Lackmann 1995;Torn 2010;Grams et al 2013b). Also, other weather systems such as predecessor rain events (PREs; Galarneau et al 2010;Moore et al 2013) and warm conveyor belts (WCBs; e.g., Carlson 1980;Madonna et al 2014b) occur surrounding an ET event, exhibit strong diabatic outflow, and may modify the upper-level Rossby wave pattern in a similar manner as the actual ET (Ahmadi-Givi et al 2004;Grams et al 2011;Bosart et al 2012;Lang and Martin 2013;Moore et al 2013;Madonna et al 2014a;Galarneau 2015;Teubler and Riemer 2016).…”

“…In contrast to previously studied aspects of ET, our focus expands from the transitioning TC to include a discussion of the role and character of diabatic outflow by other weather systems. A multiscale case study based on a combination of the observational composite approach of Archambault et al (2015) with the modeling case study approach of Grams et al (2013b) serves as a representative scenario for strong midlatitude flow amplification surrounding ET. This approach allows for a comprehensive, holistic view of the physical and dynamical processes governing midlatitude flow modification during and after ET.…”