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David M. Schultz
and
Geraint Vaughan

Traditionally, the formation of an occluded front during the occlusion process in extratropical cyclones has been viewed as the catch-up of a faster-moving cold front to a slower-moving warm front separating the warm-sector air from the low center, as first described in the Norwegian cyclone model over 90 yr ago. In this article, the conventional wisdom, or the commonly held beliefs originating from the Norwegian cyclone model, about occluded fronts and the occlusion process are critically examined. The following four tenets of this conventional wisdom are addressed. First, the occlusion process is better described not by catch-up, but by the wrapping up and lengthening of the warm-air tongue as a result of deformation and rotation around the low center. Second, the merger of the cold front and warm front does not result in the frontal zone with the warmer air ascending over the other frontal zone. Instead, the occluded frontal zone tilts over the more statically stable frontal zone. Because a warmfrontal zone tends to be more stable than a cold-frontal zone, this process usually produces a warm-type occlusion, confirming that cold-type occlusions are less common than warm-type occlusions. Third, occlusion does not mean that the cyclone has stopped deepening, because many cyclones continue to deepen 10–30 mb for 12–36 h after the formation of the occluded front. Fourth, clouds and precipitation associated with occluded fronts differ from their widespread stratiform depiction in textbooks. Embedded precipitation bands may be parallel to the front, and little relationship may exist between the fronts and the cloud mass. These four tenets help to explain anomalies in the Norwegian cyclone model, such as how occluded fronts that spiral around the low center do not require catch-up to form, how Shapiro–Keyser cyclones undergo occlusion, why some cyclones do not form occluded fronts, how some cyclones deepen after occlusion, why few cold-type occlusions have been observed, and why occluded cyclones are often associated with heavy precipitation. This reexamination of conventional wisdom leads to a new paradigm for occluded fronts and occluded cyclones.

A supplement to this article is available online:

DOI: 10.1175/2010BAMS3057.2

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Peter T. May
,
James H. Mather
,
Geraint Vaughan
,
Christian Jakob
,
Greg M. McFarquhar
,
Keith N. Bower
, and
Gerald G. Mace

A comprehensive dataset describing tropical cloud systems and their environmental setting and impacts has been collected during the Tropical Warm Pool International Cloud Experiment (TWPICE) and Aerosol and Chemical Transport in Tropical Convection (ACTIVE) campaign in the area around Darwin, Northern Australia, in January and February 2006. The aim of the experiment was to observe the evolution of tropical cloud systems and their interaction with the environment within an observational framework optimized for a range of modeling activities with the goal of improving the representation of cloud and aerosol process in a range of models. The experiment design utilized permanent observational facilities in Darwin, including a polarimetric weather radar and a suite of cloud remote-sensing instruments. This was augmented by a dense network of soundings, together with radiation, flux, lightning, and remote-sensing measurements, as well as oceanographic observations. A fleet of five research aircraft, including two high-altitude aircraft, were taking measurements of fluxes, cloud microphysics, and chemistry; cloud radar and lidar were carried on a third aircraft. Highlights of the experiment include an intense mesoscale convective system (MCS) developed within the network, observations used to analyze the impacts of aerosol on convective systems, and observations used to relate cirrus properties to the parent storm properties.

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Peter T. May
,
James H. Mather
,
Geraint Vaughan
,
Keith N. Bower
,
Christian Jakob
,
Greg M. McFarquhar
, and
Gerald G. Mace
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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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Keith A. Browning
,
Alan M. Blyth
,
Peter A. Clark
,
Ulrich Corsmeier
,
Cyril J. Morcrette
,
Judith L. Agnew
,
Sue P. Ballard
,
Dave Bamber
,
Christian Barthlott
,
Lindsay J. Bennett
,
Karl M. Beswick
,
Mark Bitter
,
Karen E. Bozier
,
Barbara J. Brooks
,
Chris G. Collier
,
Fay Davies
,
Bernhard Deny
,
Mark A. Dixon
,
Thomas Feuerle
,
Richard M. Forbes
,
Catherine Gaffard
,
Malcolm D. Gray
,
Rolf Hankers
,
Tim J. Hewison
,
Norbert Kalthoff
,
Samiro Khodayar
,
Martin Kohler
,
Christoph Kottmeier
,
Stephan Kraut
,
Michael Kunz
,
Darcy N. Ladd
,
Humphrey W. Lean
,
Jürgen Lenfant
,
Zhihong Li
,
John Marsham
,
James McGregor
,
Stephan D. Mobbs
,
John Nicol
,
Emily Norton
,
Douglas J. Parker
,
Felicity Perry
,
Markus Ramatschi
,
Hugo M. A. Ricketts
,
Nigel M. Roberts
,
Andrew Russell
,
Helmut Schulz
,
Elizabeth C. Slack
,
Geraint Vaughan
,
Joe Waight
,
David P. Wareing
,
Robert J. Watson
,
Ann R. Webb
, and
Andreas Wieser

The Convective Storm Initiation Project (CSIP) is an international project to understand precisely where, when, and how convective clouds form and develop into showers in the mainly maritime environment of southern England. A major aim of CSIP is to compare the results of the very high resolution Met Office weather forecasting model with detailed observations of the early stages of convective clouds and to use the newly gained understanding to improve the predictions of the model.

A large array of ground-based instruments plus two instrumented aircraft, from the U.K. National Centre for Atmospheric Science (NCAS) and the German Institute for Meteorology and Climate Research (IMK), Karlsruhe, were deployed in southern England, over an area centered on the meteorological radars at Chilbolton, during the summers of 2004 and 2005. In addition to a variety of ground-based remote-sensing instruments, numerous rawinsondes were released at one- to two-hourly intervals from six closely spaced sites. The Met Office weather radar network and Meteosat satellite imagery were used to provide context for the observations made by the instruments deployed during CSIP.

This article presents an overview of the CSIP field campaign and examples from CSIP of the types of convective initiation phenomena that are typical in the United Kingdom. It shows the way in which certain kinds of observational data are able to reveal these phenomena and gives an explanation of how the analyses of data from the field campaign will be used in the development of an improved very high resolution NWP model for operational use.

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Paolo Laj
,
Cathrine Lund Myhre
,
Véronique Riffault
,
Vassilis Amiridis
,
Hendrik Fuchs
,
Konstantinos Eleftheriadis
,
Tuukka Petäjä
,
Thérèse Salameh
,
Niku Kivekäs
,
Eija Juurola
,
Giulia Saponaro
,
Sabine Philippin
,
Carmela Cornacchia
,
Lucas Alados Arboledas
,
Holger Baars
,
Anja Claude
,
Martine De Mazière
,
Bart Dils
,
Marvin Dufresne
,
Nikolaos Evangeliou
,
Olivier Favez
,
Markus Fiebig
,
Martial Haeffelin
,
Hartmut Herrmann
,
Kristina Höhler
,
Niklas Illmann
,
Axel Kreuter
,
Elke Ludewig
,
Eleni Marinou
,
Ottmar Möhler
,
Lucia Mona
,
Lise Eder Murberg
,
Doina Nicolae
,
Anna Novelli
,
Ewan O’Connor
,
Kevin Ohneiser
,
Rosa Maria Petracca Altieri
,
Bénédicte Picquet-Varrault
,
Dominik van Pinxteren
,
Bernhard Pospichal
,
Jean-Philippe Putaud
,
Stefan Reimann
,
Nikolaos Siomos
,
Iwona Stachlewska
,
Ralf Tillmann
,
Kalliopi Artemis Voudouri
,
Ulla Wandinger
,
Alfred Wiedensohler
,
Arnoud Apituley
,
Adolfo Comerón
,
Martin Gysel-Beer
,
Nikolaos Mihalopoulos
,
Nina Nikolova
,
Aleksander Pietruczuk
,
Stéphane Sauvage
,
Jean Sciare
,
Henrik Skov
,
Tove Svendby
,
Erik Swietlicki
,
Dimitar Tonev
,
Geraint Vaughan
,
Vladimir Zdimal
,
Urs Baltensperger
,
Jean-François Doussin
,
Markku Kulmala
,
Gelsomina Pappalardo
,
Sanna Sorvari Sundet
, and
Milan Vana

Abstract

The Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS) officially became the 33rd European Research Infrastructure Consortium (ERIC) on 25 April 2023 with the support of 17 founding member and observer countries. As a pan-European legal organization, ACTRIS ERIC will coordinate the provision of data and data products on short-lived atmospheric constituents and clouds relevant to climate and air pollution over the next 15–20 years. ACTRIS was designed more than a decade ago, and its development was funded at national and European levels. It was included in the European Strategy Forum on Research Infrastructures (ESFRI) roadmap in 2016 and, subsequently, in the national infrastructure roadmaps of European countries. It became a landmark of the ESFRI roadmap in 2021. The purpose of this paper is to describe the mission of ACTRIS, its added value to the community of atmospheric scientists, providing services to academia as well as the public and private sectors, and to summarize its main achievements. The present publication serves as a reference document for ACTRIS, its users, and the scientific community as a whole. It provides the reader with relevant information and an overview on ACTRIS governance and services, as well as a summary of the main scientific achievements of the last 20 years. The paper concludes with an outlook on the upcoming challenges for ACTRIS and the strategy for its future evolution.

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