Search Results

You are looking at 1 - 4 of 4 items for :

  • Author or Editor: Sean Waugh x
  • Monthly Weather Review x
  • Refine by Access: All Content x
Clear All Modify Search
Matthew D. Flournoy
,
Anthony W. Lyza
,
Martin A. Satrio
,
Madeline R. Diedrichsen
,
Michael C. Coniglio
, and
Sean Waugh

Abstract

In this study, we present a climatology of observed cell mergers along the paths of 342 discrete, right-moving supercells and their association with temporal changes in low-level mesocyclone strength (measured using azimuthal shear). Nearly one-half of the examined supercells experience at least one cell merger. The frequency of cell merger occurrence varies somewhat by geographical region and the time of day. No general relationship exists between cell merger occurrence and temporal changes in low-level azimuthal shear; this corroborates prior studies in showing that the outcome of a merger is probably sensitive to storm-scale and environmental details not captured in this study. Interestingly, we find a significant inverse relationship between premerger azimuthal shear and the subsequent temporal evolution of azimuthal shear. In other words, stronger low-level mesocyclones are more likely to weaken after cell mergers and weaker low-level mesocyclones are more likely to strengthen. We also show that shorter-duration cell merger “events” (comprising multiple individual mergers) are more likely to be associated with a steady or weakening low-level mesocyclone whereas longer-duration cell merger events (3–4 individual mergers) are more likely to be associated with a strengthening low-level mesocyclone. These findings suggest what physical processes may influence the outcome of a merger in different scenarios and that the impact of these processes on low-level mesocyclone strength may change depending on storm maturity. We establish a baseline understanding of the supercell–cell merger climatology and highlight areas for future research in how to better anticipate the outcomes of cell mergers.

Significance Statement

A common assumption in idealized supercell simulations is that the background environment is homogeneous. Cells merging into a primary supercell represent one of many ways in which the environment might be significantly inhomogeneous. This study analyzes the paths of 342 supercells with a particular focus on how cell merger occurrence influences the strength of the low-level mesocyclone. Almost one-half of all supercells experience at least one cell merger. Supercells are more likely to weaken after a cell merger event if the premerger mesocyclone was strong or if the merger event is relatively short, and vice versa for the likelihood for a supercell to strengthen. These findings are important for those interested in short-term predictions of supercell evolution in response to cell mergers and suggest what dynamic processes may play a role in governing these relationships.

Full access
A. Addison Alford
,
Benjamin Schenkel
,
Samuel Hernandez
,
Jun A. Zhang
,
Michael I. Biggerstaff
,
Emily Blumenauer
,
Thea N. Sandmæl
, and
Sean M. Waugh

Abstract

Supercells in landfalling tropical cyclones (TCs) often produce tornadoes within 50 km of the coastline. The prevalence of TC tornadoes near the coast is not explained by the synoptic environments of the TC, suggesting a mesoscale influence is likely. Past case studies point to thermodynamic contrasts between ocean and land or convergence along the coast as a possible mechanism for enhancing supercell mesoscyclones and storm intensity. This study augments past work by examining the changes in the hurricane boundary layer over land in context of vertical wind shear. Using ground-based single- and dual-Doppler radar analyses, we show that the reduction of the boundary layer wind results in a increase in vertical wind shear/storm relative helicity inland of the coast. We also show that convergence along the coast may be impactful to supercells as they cross the coastal boundary. Finally, we briefly document the changes in mesocyclone vertical vorticity to assess how the environmental changes may impact individual supercells.

Restricted access
Anders A. Jensen
,
James O. Pinto
,
Sean C. C. Bailey
,
Ryan A. Sobash
,
Glen Romine
,
Gijs de Boer
,
Adam L. Houston
,
Suzanne W. Smith
,
Dale A. Lawrence
,
Cory Dixon
,
Julie K. Lundquist
,
Jamey D. Jacob
,
Jack Elston
,
Sean Waugh
,
David Brus
, and
Matthias Steiner

Abstract

Uncrewed aircraft system (UAS) observations from the Lower Atmospheric Profiling Studies at Elevation–A Remotely-Piloted Aircraft Team Experiment (LAPSE-RATE) field campaign were assimilated into a high-resolution configuration of the Weather Research and Forecasting (WRF) Model. The impact of assimilating targeted UAS observations in addition to surface observations was compared to that obtained when assimilating surface observations alone using observing system experiments (OSEs) for a terrain-driven flow case and a convection initiation (CI) case observed within Colorado’s San Luis Valley (SLV). The assimilation of UAS observations in addition to surface observations results in a clear increase in skill for both flow regimes over that obtained when assimilating surface observations alone. For the terrain-driven flow case, the UAS observations improved the representation of thermal stratification across the northern SLV, which produced stronger upvalley flow over the eastern half of the SLV that better matched the observations. For the CI case, the UAS observations improved the representation of the pre-convective environment by reducing dry biases across the SLV and over the surrounding terrain. This led to earlier CI and more organized convection over the foothills that spilled outflows into the SLV, ultimately helping to increase low-level convergence and CI there. In addition, the importance of UAS capturing an outflow that originated over the Sangre de Cristo Mountains and triggered CI is discussed. These outflows and subsequent CI were not well captured in the simulation that assimilated surface observations alone. Observations obtained with a fleet of UAS are shown to notably improve high-resolution analyses and short-term predictions of two very different mesogamma-scale weather events.

Free access
Anders A. Jensen
,
James O. Pinto
,
Sean C. C. Bailey
,
Ryan A. Sobash
,
Gijs de Boer
,
Adam L. Houston
,
Phillip B. Chilson
,
Tyler Bell
,
Glen Romine
,
Suzanne W. Smith
,
Dale A. Lawrence
,
Cory Dixon
,
Julie K. Lundquist
,
Jamey D. Jacob
,
Jack Elston
,
Sean Waugh
, and
Matthias Steiner

Abstract

Uncrewed aircraft system (UAS) observations collected during the 2018 Lower Atmospheric Process Studies at Elevation—a Remotely Piloted Aircraft Team Experiment (LAPSE-RATE) field campaign were assimilated into a high-resolution configuration of the Weather Research and Forecasting Model using an ensemble Kalman filter. The benefit of UAS observations was assessed for a terrain-driven (drainage and upvalley) flow event that occurred within Colorado’s San Luis Valley (SLV) using independent observations. The analysis and prediction of the strength, depth, and horizontal extent of drainage flow from the Saguache Canyon and the subsequent transition to upvalley and up-canyon flow were improved relative to that obtained both without data assimilation (benchmark) and when only surface observations were assimilated. Assimilation of UAS observations greatly improved the analyses of vertical variations in temperature, relative humidity, and winds at multiple locations in the northern portion of the SLV, with reductions in both bias and the root-mean-square error of roughly 40% for each variable relative to the benchmark run. Despite these noted improvements, some biases remain that were tied to measurement error and/or the impact of the boundary layer parameterization on vertically spreading the observations, both of which require further exploration. The results presented here highlight how observations obtained with a fleet of profiling UAS improve limited-area, high-resolution analyses and short-term forecasts in complex terrain.

Full access