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Mizanur M. Rahman
,
Raymond G. Jacquot
,
Edmund A. Quincy
, and
Ronald E. Stewart

Abstract

The investigation reported here involves the automatic classification of binary (black and white) images of hydrometeors (ice particles and raindrops) taken from cloud samples. The goal is to classify such images (both complete and fractional) into the seven most common classes of hydrometeors by statistical pattern recognition techniques. Detailed investigation about the data acquisition system and preprocessing is made. Four moment invariants which yield good class separation were used as features for the classification process. A Bayes decision function which minimizes the probability of misclassification is used for classification.

Bayes theorem is employed to update mean vectors and covariance matrices involved in the decision function. A discrete Kalman filtering algorithm is developed for the on-line estimation of the probability of occurrence of each class. For such estimation a discrete adaptive Kalman filtering algorithm is also developed which adjusts the filter gain matrix such as to whiten the innovations sequence. These techniques were shown to work well but the adaptive algorithm was found to converge to the correct probability more rapidly.

The classification algorithm was modified to classify incomplete or fractional images and two metrics were successfully developed to detect the unclassifiable images. The adaptive Kalman filter with the Bayes decision function was employed to classify about 2000 images per minute of CPU time with about 10% error.

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John M. Hanesiak
,
Ronald E. Stewart
,
Kit K. Szeto
,
David R. Hudak
, and
Henry G. Leighton

Abstract

On 30 September 1994 an Arctic low pressure system passed over the southern Beaufort Sea area of northern Canada and research aircraft observations were made within and around the warm front of the storm. This study is unique in that the warm front contained subzero centigrade temperatures across the entire frontal region. The overall structure of the warm front and surrounding region was similar to midlatitude storms; however, the precipitation rates, liquid water content magnitudes, horizontal and vertical winds, vertical wind shear, turbulence, and thermal advection were very weak. In addition, a low-level jet and cloud bands were aligned parallel to the warm front, near-neutral stability occurred within and around the front, and conditional symmetric instability was likely occurring. A steep frontal region resulted from strong Coriolis influences that in turn limited the amount of cloud and precipitation ahead of the system. The precipitation efficiency of the storm was high (60%) but is believed to be highly dependent on the stage of development. The mesoscale frontogenetic forcing was primarily controlled by the tilting of isentropic surfaces with confluence/convergence being the secondary influence. Sublimation contributions may have been large in the earlier stages of storm development. Satellite and aircraft radiometers underestimated cloud top heights by as much as 4 km and this was mostly due to the near transparency of the lofted ice layer in the upper portion of the storm. Maximum surface solar radiation deficits ranged between 91 W m−2 and 187 W m−2 at two surface observing sites. This common type of cloud system must have a major impact on the water and energy cycles of northern Canada in the autumn and therefore must be well accounted for within climate models.

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Yanping Li
,
Kit Szeto
,
Ronald E. Stewart
,
Julie M. Thériault
,
Liang Chen
,
Bohdan Kochtubajda
,
Anthony Liu
,
Sudesh Boodoo
,
Ron Goodson
,
Curtis Mooney
, and
Sopan Kurkute

Abstract

A devastating, flood-producing rainstorm occurred over southern Alberta, Canada, from 19 to 22 June 2013. The long-lived, heavy rainfall event was a result of complex interplays between topographic, synoptic, and convective processes that rendered an accurate simulation of this event a challenging task. In this study, the Weather Research and Forecasting (WRF) Model was used to simulate this event and was validated against several observation datasets. Both the timing and location of the model precipitation agree closely with the observations, indicating that the WRF Model is capable of reproducing this type of severe event. Sensitivity tests with different microphysics schemes were conducted and evaluated using equitable threat and bias frequency scores. The WRF double-moment 6-class microphysics scheme (WDM6) generally performed better when compared with other schemes. The application of a conventional convective/stratiform separation algorithm shows that convective activity was dominant during the early stages, then evolved into predominantly stratiform precipitation later in the event. The HYSPLIT back-trajectory analysis and regional water budget assessments using WRF simulation output suggest that the moisture for the precipitation was mainly from recycling antecedent soil moisture through evaporation and evapotranspiration over the Canadian Prairies and the U.S. Great Plains. This analysis also shows that a small fraction of the moisture can be traced back to the northeastern Pacific, and direct uptake from the Gulf of Mexico was not a significant source in this event.

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Julie M. Thériault
,
Nicolas R. Leroux
,
Ronald E. Stewart
,
André Bertoncini
,
Stephen J. Déry
,
John W. Pomeroy
,
Hadleigh D. Thompson
,
Hilary Smith
,
Zen Mariani
,
Aurélie Desroches-Lapointe
,
Selina Mitchell
, and
Juris Almonte

Abstract

The Canadian Rockies are a triple-continental divide, whose high mountains are drained by major snow-fed and rain-fed rivers flowing to the Pacific, Atlantic, and Arctic Oceans. The objective of the April–June 2019 Storms and Precipitation Across the continental Divide Experiment (SPADE) was to determine the atmospheric processes producing precipitation on the eastern and western sides of the Canadian Rockies during springtime, a period when upslope events of variable phase dominate precipitation on the eastern slopes. To do so, three observing sites across the divide were instrumented with advanced meteorological sensors. During the 13 observed events, the western side recorded only 25% of the eastern side’s precipitation accumulation, rainfall occurred rather than snowfall, and skies were mainly clear. Moisture sources and amounts varied markedly between events. An atmospheric river landfall in California led to moisture flowing persistently northward and producing the longest duration of precipitation on both sides of the divide. Moisture from the continental interior always produced precipitation on the eastern side but only in specific conditions on the western side. Mainly slow-falling ice crystals, sometimes rimed, formed at higher elevations on the eastern side (>3 km MSL), were lifted, and subsequently drifted westward over the divide during nonconvective storms to produce rain at the surface on the western side. Overall, precipitation generally crossed the divide in the Canadian Rockies during specific spring-storm atmospheric conditions although amounts at the surface varied with elevation, condensate type, and local and large-scale flow fields.

Free access
Siegfried D. Schubert
,
Ronald E. Stewart
,
Hailan Wang
,
Mathew Barlow
,
Ernesto H. Berbery
,
Wenju Cai
,
Martin P. Hoerling
,
Krishna K. Kanikicharla
,
Randal D. Koster
,
Bradfield Lyon
,
Annarita Mariotti
,
Carlos R. Mechoso
,
Omar V. Müller
,
Belen Rodriguez-Fonseca
,
Richard Seager
,
Sonia I. Seneviratne
,
Lixia Zhang
, and
Tianjun Zhou

Abstract

Drought affects virtually every region of the world, and potential shifts in its character in a changing climate are a major concern. This article presents a synthesis of current understanding of meteorological drought, with a focus on the large-scale controls on precipitation afforded by sea surface temperature (SST) anomalies, land surface feedbacks, and radiative forcings. The synthesis is primarily based on regionally focused articles submitted to the Global Drought Information System (GDIS) collection together with new results from a suite of atmospheric general circulation model experiments intended to integrate those studies into a coherent view of drought worldwide. On interannual time scales, the preeminence of ENSO as a driver of meteorological drought throughout much of the Americas, eastern Asia, Australia, and the Maritime Continent is now well established, whereas in other regions (e.g., Europe, Africa, and India), the response to ENSO is more ephemeral or nonexistent. Northern Eurasia, central Europe, and central and eastern Canada stand out as regions with few SST-forced impacts on precipitation on interannual time scales. Decadal changes in SST appear to be a major factor in the occurrence of long-term drought, as highlighted by apparent impacts on precipitation of the late 1990s “climate shifts” in the Pacific and Atlantic SST. Key remaining research challenges include (i) better quantification of unforced and forced atmospheric variability as well as land–atmosphere feedbacks, (ii) better understanding of the physical basis for the leading modes of climate variability and their predictability, and (iii) quantification of the relative contributions of internal decadal SST variability and forced climate change to long-term drought.

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Robert J. H. Dunn
,
Freya Aldred
,
Nadine Gobron
,
John B. Miller
,
Kate M. Willett
,
Melanie Ades
,
Robert Adler
,
R. P. Allan
,
John Anderson
,
Orlane Anneville
,
Yasuyuki Aono
,
Anthony Argüez
,
Carlo Arosio
,
John A. Augustine
,
Cesar Azorin-Molina
,
Jonathan Barichivich
,
Aman Basu
,
Hylke E. Beck
,
Nicolas Bellouin
,
Angela Benedetti
,
Kevin Blagrave
,
Stephen Blenkinsop
,
Olivier Bock
,
Xavier Bodin
,
Michael G. Bosilovich
,
Olivier Boucher
,
Gerald Bove
,
Dennis Buechler
,
Stefan A. Buehler
,
Laura Carrea
,
Kai-Lan Chang
,
Hanne H. Christiansen
,
John R. Christy
,
Eui-Seok Chung
,
Laura M. Ciasto
,
Melanie Coldewey-Egbers
,
Owen R. Cooper
,
Richard C. Cornes
,
Curt Covey
,
Thomas Cropper
,
Molly Crotwell
,
Diego Cusicanqui
,
Sean M. Davis
,
Richard A. M. de Jeu
,
Doug Degenstein
,
Reynald Delaloye
,
Markus G. Donat
,
Wouter A. Dorigo
,
Imke Durre
,
Geoff S. Dutton
,
Gregory Duveiller
,
James W. Elkins
,
Thomas W. Estilow
,
Nava Fedaeff
,
David Fereday
,
Vitali E. Fioletov
,
Johannes Flemming
,
Michael J. Foster
,
Stacey M. Frith
,
Lucien Froidevaux
,
Martin Füllekrug
,
Judith Garforth
,
Jay Garg
,
Matthew Gentry
,
Steven Goodman
,
Qiqi Gou
,
Nikolay Granin
,
Mauro Guglielmin
,
Sebastian Hahn
,
Leopold Haimberger
,
Brad D. Hall
,
Ian Harris
,
Debbie L. Hemming
,
Martin Hirschi
,
Shu-pen (Ben) Ho
,
Robert Holzworth
,
Filip Hrbáček
,
Daan Hubert
,
Petra Hulsman
,
Dale F. Hurst
,
Antje Inness
,
Ketil Isaksen
,
Viju O. John
,
Philip D. Jones
,
Robert Junod
,
Andreas Kääb
,
Johannes W. Kaiser
,
Viktor Kaufmann
,
Andreas Kellerer-Pirklbauer
,
Elizabeth C. Kent
,
Richard Kidd
,
Hyungiun Kim
,
Zak Kipling
,
Akash Koppa
,
Jan Henning L’Abée-Lund
,
Xin Lan
,
Kathleen O. Lantz
,
David Lavers
,
Norman G. Loeb
,
Diego Loyola
,
Remi Madelon
,
Hilmar J. Malmquist
,
Wlodzimierz Marszelewski
,
Michael Mayer
,
Matthew F. McCabe
,
Tim R. McVicar
,
Carl A. Mears
,
Annette Menzel
,
Christopher J. Merchant
,
Diego G. Miralles
,
Stephen A. Montzka
,
Colin Morice
,
Leander Mösinger
,
Jens Mühle
,
Julien P. Nicolas
,
Jeannette Noetzli
,
Tiina Nõges
,
Ben Noll
,
John O’Keefe
,
Tim J. Osborn
,
Taejin Park
,
Cecile Pellet
,
Maury S. Pelto
,
Sarah E. Perkins-Kirkpatrick
,
Coda Phillips
,
Stephen Po-Chedley
,
Lorenzo Polvani
,
Wolfgang Preimesberger
,
Colin Price
,
Merja Pulkkanen
,
Dominik G. Rains
,
William J. Randel
,
Samuel Rémy
,
Lucrezia Ricciardulli
,
Andrew D. Richardson
,
David A. Robinson
,
Matthew Rodell
,
Nemesio J. Rodríguez-Fernández
,
Karen H. Rosenlof
,
Chris Roth
,
Alexei Rozanov
,
This Rutishäuser
,
Ahira Sánchez-Lugo
,
Parnchai Sawaengphokhai
,
Verena Schenzinger
,
Robert W. Schlegel
,
Udo Schneider
,
Sapna Sharma
,
Lei Shi
,
Adrian J. Simmons
,
Carolina Siso
,
Sharon L. Smith
,
Brian J. Soden
,
Viktoria Sofieva
,
Tim H. Sparks
,
Paul W. Stackhouse Jr.
,
Ryan Stauffer
,
Wolfgang Steinbrecht
,
Andrea K. Steiner
,
Kenton Stewart
,
Pietro Stradiotti
,
Dimitri A. Streletskiy
,
Hagen Telg
,
Stephen J. Thackeray
,
Emmanuel Thibert
,
Michael Todt
,
Daisuke Tokuda
,
Kleareti Tourpali
,
Mari R. Tye
,
Ronald van der A
,
Robin van der Schalie
,
Gerard van der Schrier
,
Mendy van der Vliet
,
Guido R. van der Werf
,
Arnold. van Vliet
,
Jean-Paul Vernier
,
Isaac J. Vimont
,
Katrina Virts
,
Sebastiàn Vivero
,
Holger Vömel
,
Russell S. Vose
,
Ray H. J. Wang
,
Markus Weber
,
David Wiese
,
Jeanette D. Wild
,
Earle Williams
,
Takmeng Wong
,
R. I. Woolway
,
Xungang Yin
,
Ye Yuan
,
Lin Zhao
,
Xinjia Zhou
,
Jerry R. Ziemke
,
Markus Ziese
, and
Ruxandra M. Zotta
Free access