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Jeremiah O. Piersante
,
Kristen L. Rasmussen
,
Russ S. Schumacher
,
Angela K. Rowe
, and
Lynn A. McMurdie

Abstract

Subtropical South America (SSA) east of the Andes Mountains is a global hotspot for mesoscale convective systems (MCSs). Wide convective cores (WCCs) are typically embedded within mature MCSs, contribute over 40% of SSA’s warm-season rainfall, and are often associated with severe weather. Prior analysis of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data identified WCCs in SSA and associated synoptic conditions during austral summer. As WCCs also occur during the austral spring, this study uses the 16-yr TRMM PR and ERA5 datasets to compare anomalies in environmental conditions between austral spring (SON) and summer (DJF) for the largest and smallest WCCs in SSA. During both seasons, large WCCs are associated with an anomalous midlevel trough that slowly crosses the Andes Mountains and a northerly South American low-level jet (SALLJ) over SSA, though the SON trough and SALLJ anomalies are stronger and located farther northeastward than in DJF. A synoptic pattern evolution resembling large WCC environments is illustrated through a multiday case during the RELAMPAGO field campaign (10–13 November 2018). Unique high-temporal-resolution soundings showed strong midlevel vertical wind shear associated with this event, induced by the juxtaposition of the northerly SALLJ and southerly near-surface flow. It is hypothesized that the Andes help create a quasi-stationary trough–ridge pattern such that favorable synoptic conditions for deep convection persist for multiple days. For the smallest WCCs, anomalously weaker synoptic-scale forcing was present compared to the largest events, especially for DJF, pointing to future work exploring MCS formation under weaker synoptic conditions.

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Hannah C. Barnes
,
Joseph P. Zagrodnik
,
Lynn A. McMurdie
,
Angela K. Rowe
, and
Robert A. Houze Jr.

Abstract

This study examines Kelvin–Helmholtz (KH) waves observed by dual-polarization radar in several precipitating midlatitude cyclones during the Olympic Mountains Experiment (OLYMPEX) field campaign along the windward side of the Olympic Mountains in Washington State and in a strong stationary frontal zone in Iowa during the Iowa Flood Studies (IFloodS) field campaign. While KH waves develop regardless of the presence or absence of mountainous terrain, this study indicates that the large-scale flow can be modified when encountering a mountain range in such a way as to promote development of KH waves on the windward side and to alter their physical structure (i.e., orientation and amplitude). OLYMPEX sampled numerous instances of KH waves in precipitating clouds, and this study examines their effects on microphysical processes above, near, and below the melting layer. The dual-polarization radar data indicate that KH waves above the melting layer promote aggregation. KH waves centered in the melting layer produce the most notable signatures in dual-polarization variables, with the patterns suggesting that the KH waves promote both riming and aggregation. Both above and near the melting layer ice particles show no preferred orientation likely because of tumbling in turbulent air motions. KH waves below the melting layer facilitate the generation of large drops via coalescence and/or vapor deposition, increasing mean drop size and rain rate by only slight amounts in the OLYMPEX storms.

Open access
Robert A. Houze Jr.
,
Lynn A. McMurdie
,
Walter A. Petersen
,
Mathew R. Schwaller
,
William Baccus
,
Jessica D. Lundquist
,
Clifford F. Mass
,
Bart Nijssen
,
Steven A. Rutledge
,
David R. Hudak
,
Simone Tanelli
,
Gerald G. Mace
,
Michael R. Poellot
,
Dennis P. Lettenmaier
,
Joseph P. Zagrodnik
,
Angela K. Rowe
,
Jennifer C. DeHart
,
Luke E. Madaus
,
Hannah C. Barnes
, and
V. Chandrasekar

Abstract

The Olympic Mountains Experiment (OLYMPEX) took place during the 2015/16 fall–winter season in the vicinity of the mountainous Olympic Peninsula of Washington State. The goals of OLYMPEX were to provide physical and hydrologic ground validation for the U.S.–Japan Global Precipitation Measurement (GPM) satellite mission and, more specifically, to study how precipitation in Pacific frontal systems is modified by passage over coastal mountains. Four transportable scanning dual-polarization Doppler radars of various wavelengths were installed. Surface stations were placed at various altitudes to measure precipitation rates, particle size distributions, and fall velocities. Autonomous recording cameras monitored and recorded snow accumulation. Four research aircraft supplied by NASA investigated precipitation processes and snow cover, and supplemental rawinsondes and dropsondes were deployed during precipitation events. Numerous Pacific frontal systems were sampled, including several reaching “atmospheric river” status, warm- and cold-frontal systems, and postfrontal convection.

Open access
Stephen W. Nesbitt
,
Paola V. Salio
,
Eldo Ávila
,
Phillip Bitzer
,
Lawrence Carey
,
V. Chandrasekar
,
Wiebke Deierling
,
Francina Dominguez
,
Maria Eugenia Dillon
,
C. Marcelo Garcia
,
David Gochis
,
Steven Goodman
,
Deanna A. Hence
,
Karen A. Kosiba
,
Matthew R. Kumjian
,
Timothy Lang
,
Lorena Medina Luna
,
James Marquis
,
Robert Marshall
,
Lynn A. McMurdie
,
Ernani de Lima Nascimento
,
Kristen L. Rasmussen
,
Rita Roberts
,
Angela K. Rowe
,
Juan José Ruiz
,
Eliah F.M.T. São Sabbas
,
A. Celeste Saulo
,
Russ S. Schumacher
,
Yanina Garcia Skabar
,
Luiz Augusto Toledo Machado
,
Robert J. Trapp
,
Adam C. Varble
,
James Wilson
,
Joshua Wurman
,
Edward J. Zipser
,
Ivan Arias
,
Hernán Bechis
, and
Maxwell A. Grover

Abstract

This article provides an overview of the experimental design, execution, education and public outreach, data collection, and initial scientific results from the Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations (RELAMPAGO) field campaign. RELAMPAGO was a major field campaign conducted in the Córdoba and Mendoza provinces in Argentina and western Rio Grande do Sul State in Brazil in 2018–19 that involved more than 200 scientists and students from the United States, Argentina, and Brazil. This campaign was motivated by the physical processes and societal impacts of deep convection that frequently initiates in this region, often along the complex terrain of the Sierras de Córdoba and Andes, and often grows rapidly upscale into dangerous storms that impact society. Observed storms during the experiment produced copious hail, intense flash flooding, extreme lightning flash rates, and other unusual lightning phenomena, but few tornadoes. The five distinct scientific foci of RELAMPAGO—convection initiation, severe weather, upscale growth, hydrometeorology, and lightning and electrification—are described, as are the deployment strategies to observe physical processes relevant to these foci. The campaign’s international cooperation, forecasting efforts, and mission planning strategies enabled a successful data collection effort. In addition, the legacy of RELAMPAGO in South America, including extensive multinational education, public outreach, and social media data gathering associated with the campaign, is summarized.

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