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Julia H. Keller
,
Christian M. Grams
,
Michael Riemer
,
Heather M. Archambault
,
Lance Bosart
,
James D. Doyle
,
Jenni L. Evans
,
Thomas J. Galarneau Jr.
,
Kyle Griffin
,
Patrick A. Harr
,
Naoko Kitabatake
,
Ron McTaggart-Cowan
,
Florian Pantillon
,
Julian F. Quinting
,
Carolyn A. Reynolds
,
Elizabeth A. Ritchie
,
Ryan D. Torn
, and
Fuqing Zhang

Abstract

The extratropical transition (ET) of tropical cyclones often has an important impact on the nature and predictability of the midlatitude flow. This review synthesizes the current understanding of the dynamical and physical processes that govern this impact and highlights the relationship of downstream development during ET to high-impact weather, with a focus on downstream regions. It updates a previous review from 2003 and identifies new and emerging challenges and future research needs. First, the mechanisms through which the transitioning cyclone impacts the midlatitude flow in its immediate vicinity are discussed. This “direct impact” manifests in the formation of a jet streak and the amplification of a ridge directly downstream of the cyclone. This initial flow modification triggers or amplifies a midlatitude Rossby wave packet, which disperses the impact of ET into downstream regions (downstream impact) and may contribute to the formation of high-impact weather. Details are provided concerning the impact of ET on forecast uncertainty in downstream regions and on the impact of observations on forecast skill. The sources and characteristics of the following key features and processes that may determine the manifestation of the impact of ET on the midlatitude flow are discussed: the upper-tropospheric divergent outflow, mainly associated with latent heat release in the troposphere below, and the phasing between the transitioning cyclone and the midlatitude wave pattern. Improving the representation of diabatic processes during ET in models and a climatological assessment of the ET’s impact on downstream high-impact weather are examples for future research directions.

Open access
Andreas Schäfler
,
George Craig
,
Heini Wernli
,
Philippe Arbogast
,
James D. Doyle
,
Ron McTaggart-Cowan
,
John Methven
,
Gwendal Rivière
,
Felix Ament
,
Maxi Boettcher
,
Martina Bramberger
,
Quitterie Cazenave
,
Richard Cotton
,
Susanne Crewell
,
Julien Delanoë
,
Andreas Dörnbrack
,
André Ehrlich
,
Florian Ewald
,
Andreas Fix
,
Christian M. Grams
,
Suzanne L. Gray
,
Hans Grob
,
Silke Groß
,
Martin Hagen
,
Ben Harvey
,
Lutz Hirsch
,
Marek Jacob
,
Tobias Kölling
,
Heike Konow
,
Christian Lemmerz
,
Oliver Lux
,
Linus Magnusson
,
Bernhard Mayer
,
Mario Mech
,
Richard Moore
,
Jacques Pelon
,
Julian Quinting
,
Stephan Rahm
,
Markus Rapp
,
Marc Rautenhaus
,
Oliver Reitebuch
,
Carolyn A. Reynolds
,
Harald Sodemann
,
Thomas Spengler
,
Geraint Vaughan
,
Manfred Wendisch
,
Martin Wirth
,
Benjamin Witschas
,
Kevin Wolf
, and
Tobias Zinner

Abstract

The North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) explored the impact of diabatic processes on disturbances of the jet stream and their influence on downstream high-impact weather through the deployment of four research aircraft, each with a sophisticated set of remote sensing and in situ instruments, and coordinated with a suite of ground-based measurements. A total of 49 research flights were performed, including, for the first time, coordinated flights of the four aircraft: the German High Altitude and Long Range Research Aircraft (HALO), the Deutsches Zentrum für Luft- und Raumfahrt (DLR) Dassault Falcon 20, the French Service des Avions Français Instrumentés pour la Recherche en Environnement (SAFIRE) Falcon 20, and the British Facility for Airborne Atmospheric Measurements (FAAM) BAe 146. The observation period from 17 September to 22 October 2016 with frequently occurring extratropical and tropical cyclones was ideal for investigating midlatitude weather over the North Atlantic. NAWDEX featured three sequences of upstream triggers of waveguide disturbances, as well as their dynamic interaction with the jet stream, subsequent development, and eventual downstream weather impact on Europe. Examples are presented to highlight the wealth of phenomena that were sampled, the comprehensive coverage, and the multifaceted nature of the measurements. This unique dataset forms the basis for future case studies and detailed evaluations of weather and climate predictions to improve our understanding of diabatic influences on Rossby waves and the downstream impacts of weather systems affecting Europe.

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