Search Results

You are looking at 11 - 18 of 18 items for

  • Author or Editor: John L. Schroeder x
  • Refine by Access: All Content x
Clear All Modify Search
Richard J. Krupar III
,
John L. Schroeder
,
Douglas A. Smith
,
Song-Lak Kang
, and
Sylvie Lorsolo

Abstract

A set of velocity–azimuth display (VAD) wind speed profiles derived from coastal Weather Surveillance Radar-1988 Doppler (WSR-88D) systems was paired with Automated Surface Observing System (ASOS) 10-m standardized mean and nonstandardized gust wind speeds measured within 10 km of nearby WSR-88Ds. The goal was to formulate an appropriate methodology and empirical relationships to estimate overland near-surface wind conditions in landfalling tropical cyclones (TCs) using VAD tropical cyclone boundary layer (TCBL) lower-tropospheric wind measurements. A total of 17 TCs and seven ASOS/WSR-88D sites were used to construct a unique comparative dataset. Four estimation methods including the log and power laws, mean and gust wind speed ratio (WSR) methods, and curve fitting with linear regression and polynomial fits were evaluated. Results from the evaluation show that WSR-88D site-specific linear regression equations using a VAD 0–200-m layer average wind speed and nonzero intercepts provided the most accurate predictions of the ASOS 10-m standardized mean wind speed. Results also show that a non-site-specific linear regression model using a VAD 0–500-m mean boundary layer (MBL) wind speed and nonzero intercept is 1.07% more accurate than using a single-gust WSR to predict ASOS 10-m nonstandardized gust wind speeds. Only 2.15% of the ASOS 10-m nonstandardized maximum 3-s gust wind speeds were found to exceed the VAD 0–500-m MBL wind speed, indicating that the VAD 0–500-m MBL wind speed represents a viable source of momentum available for transport to the surface in the form of a gust.

Full access
Brian D. Hirth
,
John L. Schroeder
,
Christopher C. Weiss
,
Douglas A. Smith
, and
Michael I. Biggerstaff

Abstract

The structure of the coastal internal boundary layer (IBL) during a landfalling hurricane has important ramifications on operational forecasting, structural design, and poststorm damage assessment. Despite these important issues, the mean IBL structure at the coastline during landfall is poorly understood. Knowledge of the vertical kinematic structure within tropical cyclones over water has improved greatly through aircraft reconnaissance missions and the advent of GPS dropsondes and stepped frequency microwave radiometers. Unfortunately, reconnaissance and research aircraft are limited to overwater missions, resulting in a poor understanding of vertical kinematic structure near the coastal interface, where changes in IBL structure are expected due to changes in surface roughness. Composite single- and dual-Doppler radar observations collected by the Shared Mobile Atmospheric Research and Teaching Radars during the landfall of Hurricane Frances (2004) are presented. Data analyses from the Cape Canaveral, Florida, region reveal a pronounced IBL throughout the data collection period. As a result, significant variability in the analyzed wind speed and direction are found across and near the coastal interface. IBL height is found to be suppressed when compared to an accepted empirical growth model, while multiple abrupt roughness transitions associated with the Cape Canaveral region contribute to a complex mean IBL structure.

Full access
Brian C. Zachry
,
John L. Schroeder
,
Andrew B. Kennedy
,
Joannes J. Westerink
,
Chris W. Letchford
, and
Mark E. Hope

Abstract

Over the past decade, numerous field campaigns and laboratory experiments have examined air–sea momentum exchange in the deep ocean. These studies have changed the understanding of drag coefficient behavior in hurricane force winds, with a general consensus that a limiting value is reached. Near the shore, wave conditions are markedly different than in deep water because of wave shoaling and breaking processes, but only very limited data exist to assess drag coefficient behavior. Yet, knowledge of the wind stress in this region is critical for storm surge forecasting, evaluating the low-level wind field across the coastal transition zone, and informing the wind load standard along the hurricane-prone coastline. During Hurricane Ike (2008), a Texas Tech University StickNet platform obtained wind measurements in marine exposure with a fetch across the Houston ship channel. These data were used to estimate drag coefficient dependence on wind speed. Wave conditions in the ship channel and surge level at the StickNet location were simulated using the Simulating Waves Nearshore Model coupled to the Advanced Circulation Model. The simulated waves were indicative of a fetch-limited condition with maximum significant wave heights reaching 1.5 m and peak periods of 4 s. A maximum surge depth of 0.6 m inundated the StickNet. Similar to deep water studies, findings indicate that the drag coefficient reaches a limiting value at wind speeds near hurricane force. However, at wind speeds below hurricane force, the drag coefficient is higher than that of deep water datasets, particularly at the slowest wind speeds.

Full access
Ian M. Giammanco
,
John L. Schroeder
,
Forrest J. Masters
,
Peter J. Vickery
,
Richard J. Krupar III
, and
Juan-Antonio Balderrama

Abstract

The deployment of ruggedized surface observing platforms by university research programs in the path of landfalling tropical cyclones has yielded a wealth of information regarding the near-surface wind flow characteristics. Data records collected by Texas Tech University’s Wind Engineering Mobile Instrument Tower Experiment and StickNet probes and by the Florida Coastal Monitoring Program along the Gulf Coast of the United States from 2004 to 2008 were compiled to examine influences on near-surface gust factors. Archived composite reflectivity data from coastal WSR-88D instruments were also merged with the tower records to investigate the influence of precipitation structure. Wind records were partitioned into 10-min segments, and the ratio of the peak moving-average 3-s-gust wind speed to the segment mean was used to define a gust factor. Observations were objectively stratified into terrain exposure categories to determine if factors beyond those associated with surface frictional effects can be extracted from the observations. Wind flow characteristics within exposure classes were weakly influenced by storm-relative position and precipitation structure. Eyewall observations showed little difference in mean gust factors when compared with other regions. In convective precipitation, only peak gust factors were slightly larger than those found in stratiform conditions, with little differences in the mean. Gust factors decreased slightly with decreasing radial distance in rougher terrain exposures and did not respond to radar-observed changes in precipitation structure. In two limited comparisons, near-surface gusts did not exceed the magnitude of the wind maximum aloft detected through wind profiles that were derived from WSR-88D velocity–azimuth displays.

Full access
Christopher C. Weiss
,
David C. Dowell
,
John L. Schroeder
,
Patrick S. Skinner
,
Anthony E. Reinhart
,
Paul M. Markowski
, and
Yvette P. Richardson

Abstract

Observations obtained during the second Verification of the Origin of Rotation in Tornadoes Experiment (VORTEX2) are analyzed for three supercell intercepts. These intercepts used a fleet of deployable “StickNet” probes, complemented by mobile radars and a mobile mesonet, to map state quantities over the expanse of target storms.

Two of the deployments occurred for different stages of a supercell storm near and east of Dumas, Texas, on 18 May 2010. A comparison of the thermodynamic and kinematic characteristics of the storm provides a possible explanation for why one phase was weakly tornadic and the other nontornadic. The weakly tornadic phase features a stronger horizontal virtual temperature gradient antiparallel to the forward-flank reflectivity gradient and perpendicular to the near-surface flow direction, suggesting that air parcels could acquire more significant baroclinic vorticity as they approach the low-level mesocyclone.

The strongly tornadic 10 May 2010 case near Seminole, Oklahoma, features comparatively small virtual and equivalent potential temperature deficits, suggesting the strength of baroclinic zones may be less useful than the buoyancy near the mesocyclone for assessing tornado potential. The distribution of positive pressure perturbations and backed ground-relative winds within the forward flank are consistent with the notion of a “starburst” pattern of diverging winds associated with the forward-flank downdraft.

Narrow (~1 km wide) zones of intense baroclinic vorticity generation of O(~10−4) s−2 are shown to exist within precipitation on the forward and left sides of the mesocyclone in the Dumas intercepts, not dissimilar from such zones identified in recent high-resolution numerical studies.

Full access
Michael I. Biggerstaff
,
Louis J. Wicker
,
Jerry Guynes
,
Conrad Ziegler
,
Jerry M. Straka
,
Erik N. Rasmussen
,
Arthur Doggett IV
,
Larry D. Carey
,
John L. Schroeder
, and
Chris Weiss

A group of scientists from three universities across two different states and from one federal research laboratory joined together to build and deploy two mobile C-band Doppler weather radars to enhance research and promote meteorological education. This 5-yr project led to the development of the Shared Mobile Atmospheric Research and Teaching (SMART) radar coalition that built the first mobile C-band Doppler weather radar in the United States and also successfully deployed the first mobile C-band dual-Doppler network in a landfalling hurricane. This accomplishment marked the beginning of an era in which high temporal and spatial resolution precipitation and dual-Doppler wind data over mesoscale (~100 km) regions can be acquired from mobile ground-based platforms during extreme heavy rain and high-wind events.

In this paper, we discuss the rationale for building the mobile observing systems, highlight some of the challenges that were encountered in creating a unique multiagency coalition, provide examples of how the SMART radars have contributed to research and education, and discuss future plans for continued development and management of the radar facility, including how others may use the radars for their own research and teaching programs.

The capability of the SMART radars to measure winds in nonprecipitating environments, to capture rapidly evolving, short-lived, small-scale tornadic circulations, and to sample mesoscale regions with high spatial resolution over broad regions of heavy rainfall is demonstrated. Repeated successful intercepts provide evidence that these radars are capable of being used to study a wide range of atmospheric phenomena.

Full access
Pedro L. Fernández-Cabán
,
A. Addison Alford
,
Martin J. Bell
,
Michael I. Biggerstaff
,
Gordon D. Carrie
,
Brian Hirth
,
Karen Kosiba
,
Brian M. Phillips
,
John L. Schroeder
,
Sean M. Waugh
,
Eric Williford
,
Joshua Wurman
, and
Forrest J. Masters

Abstract

While Hurricane Harvey will best be remembered for record rainfall that led to widespread flooding in southeastern Texas and western Louisiana, the storm also produced some of the most extreme wind speeds ever to be captured by an adaptive mesonet at landfall. This paper describes the unique tools and the strategy used by the Digital Hurricane Consortium (DHC), an ad hoc group of atmospheric scientists and wind engineers, to intercept and collect high-resolution measurements of Harvey’s inner core and eyewall as it passed over Aransas Bay into mainland Texas. The DHC successfully deployed more than 25 observational assets, leading to an unprecedented view of the boundary layer and winds aloft in the eyewall of a major hurricane at landfall. Analysis of anemometric measurements and mobile radar data during heavy convection shows the kinematic structure of the hurricane at landfall and the suspected influence of circulations aloft on surface winds and extreme surface gusts. Evidence of mesoscale vortices in the interior of the eyewall is also presented. Finally, the paper reports on an atmospheric sounding in the inner eyewall that produced an exceptionally large and potentially record value of precipitable water content for observed soundings in the continental United States.

Full access
Julie K. Lundquist
,
James M. Wilczak
,
Ryan Ashton
,
Laura Bianco
,
W. Alan Brewer
,
Aditya Choukulkar
,
Andrew Clifton
,
Mithu Debnath
,
Ruben Delgado
,
Katja Friedrich
,
Scott Gunter
,
Armita Hamidi
,
Giacomo Valerio Iungo
,
Aleya Kaushik
,
Branko Kosović
,
Patrick Langan
,
Adam Lass
,
Evan Lavin
,
Joseph C.-Y. Lee
,
Katherine L. McCaffrey
,
Rob K. Newsom
,
David C. Noone
,
Steven P. Oncley
,
Paul T. Quelet
,
Scott P. Sandberg
,
John L. Schroeder
,
William J. Shaw
,
Lynn Sparling
,
Clara St. Martin
,
Alexandra St. Pe
,
Edward Strobach
,
Ken Tay
,
Brian J. Vanderwende
,
Ann Weickmann
,
Daniel Wolfe
, and
Rochelle Worsnop

Abstract

To assess current capabilities for measuring flow within the atmospheric boundary layer, including within wind farms, the U.S. Department of Energy sponsored the eXperimental Planetary boundary layer Instrumentation Assessment (XPIA) campaign at the Boulder Atmospheric Observatory (BAO) in spring 2015. Herein, we summarize the XPIA field experiment, highlight novel measurement approaches, and quantify uncertainties associated with these measurement methods. Line-of-sight velocities measured by scanning lidars and radars exhibit close agreement with tower measurements, despite differences in measurement volumes. Virtual towers of wind measurements, from multiple lidars or radars, also agree well with tower and profiling lidar measurements. Estimates of winds over volumes from scanning lidars and radars are in close agreement, enabling the assessment of spatial variability. Strengths of the radar systems used here include high scan rates, large domain coverage, and availability during most precipitation events, but they struggle at times to provide data during periods with limited atmospheric scatterers. In contrast, for the deployment geometry tested here, the lidars have slower scan rates and less range but provide more data during nonprecipitating atmospheric conditions. Microwave radiometers provide temperature profiles with approximately the same uncertainty as radio acoustic sounding systems (RASS). Using a motion platform, we assess motion-compensation algorithms for lidars to be mounted on offshore platforms. Finally, we highlight cases for validation of mesoscale or large-eddy simulations, providing information on accessing the archived dataset. We conclude that modern remote sensing systems provide a generational improvement in observational capabilities, enabling the resolution of finescale processes critical to understanding inhomogeneous boundary layer flows.

Full access