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Jeffrey R. Beck
,
John L. Schroeder
, and
Joshua M. Wurman

Abstract

On 29 May 2001, Doppler on Wheels radars collected data on a supercell near Kress, Texas. The supercellular storm, cyclic in nature, produced multiple mesocyclones throughout its lifetime. Dual-Doppler syntheses were conducted using a grid spacing of 100 m, resulting in the highest-resolution observational analysis of a cyclic supercell to date. In addition, collection of data from ground-based radar allowed for the analysis of near-ground features irresolvable with airborne radar, providing another advantage over previous studies. The syntheses revealed a number of evolving low-level mesocyclones over the observation period of 900 s. While nontornadic during the synthesis period, the supercell exhibited evidence of strong (vertical vorticity greater than 10−2 s−1) low-level circulation with classic cyclic structure and multiple tornadoes beginning 3600 s later. A comparison between the current results, conceptual models, and previous lower-resolution analyses is presented. A striking similarity exists between the cyclic evolution of the Kress storm during the synthesis time period and other previous cyclic conceptual models. However, differences did exist between the Kress storm and previously studied tornadic storms. Analysis showed that the rear-flank downdraft provided the only surface boundary associated with low-level mesocyclogenesis. Other characteristics, including forward-flank gust front structure and the orientation of low-level horizontal vorticity, also differed. In addition, there was a general lack of surface convergence associated with the forward-flank reflectivity gradient, yet convergence associated with the forward-flank gust front increased with height. Finally, a large component of crosswise horizontal vorticity was found to exist throughout the supercell environment, within both the inflow and outflow. Incorporating these differences, an attempt was made to identify possible mechanisms responsible for the lack of tornadogenesis during the synthesis time period.

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Brian D. Hirth
,
John L. Schroeder
, and
Christopher C. Weiss

Abstract

The rear-flank downdraft regions of two tornadic supercells were sampled on 12 June 2004 and 9 June 2005 using four “mobile mesonet” probes. These rear-flank downdraft outflows were sampled employing two different data collection routines; therefore, each case is described from a different perspective. The data samples were examined to identify variations in measured surface equivalent potential temperature, virtual potential temperature, and kinematics. In the 12 June 2004 case, the tornadic circulation was accompanied by small equivalent potential temperature deficits within the rear-flank downdraft outflow early in its life followed by increasing deficits with time. Virtual potential temperature deficits modestly increased through the duration of the sample as well. The 9 June 2005 case was highlighted by heavy precipitation near the tornado itself and relatively small negative, or even positive, equivalent and virtual potential temperature perturbations. Large horizontal variations of surface thermodynamic properties were also noted within several regions of this rear-flank downdraft outflow.

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Sylvie Lorsolo
,
John L. Schroeder
,
Peter Dodge
, and
Frank Marks Jr.

Abstract

Data with high temporal and spatial resolution from Hurricanes Isabel (2003) and Frances (2004) were analyzed to provide a detailed study of near-surface linear structures with subkilometer wavelengths of the hurricane boundary layer (HBL). The analysis showed that the features were omnipresent throughout the data collection, displayed a horizontal and vertical coherency, and maintained an average orientation of 7° left of the low-level wind. A unique objective wavelength analysis was conducted, where wavelength was defined as the distance between two wind maxima or minima perpendicular to the features’ long axis, and revealed that although wavelengths as large as 1400 m were observed, the majority of the features had wavelengths between 200 and 650 m. The assessed wavelengths differ from those documented in a recent observational study. To evaluate the correlation between the features and the underlying near-surface wind field, time and spectral analyses were completed and ground-relative frequency distributions of the features were retrieved. High-energy regions of the power spectral density (PSD) determined from near-surface data were collocated with the features’ ground-relative frequency, illustrating that the features have an influence on the near-surface wind field. The additional energy found in the low-frequency range of the PSDs was previously identified as characteristic of the hurricane surface flow, suggesting that the features are an integral component of the HBL flow.

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Brian C. Ancell
,
Erin Kashawlic
, and
John L. Schroeder

Abstract

The U.S. Department of Energy Wind Forecast Improvement Project (WFIP) has recently been completed with the aim of 1) understanding the performance of different mesoscale data assimilation systems for lower-atmospheric wind prediction and 2) determining the observation impacts on wind forecasts within the different assimilation systems. Here an ensemble Kalman filter (EnKF) was tested against a three-dimensional variational data assimilation (3DVAR) technique. Forecasts lasting 24 hours were produced for a month-long period to determine the day-to-day performance of each system, as well as over 10 individual wind ramp cases. The observation impacts from surface mesonet and profiler/sodar wind observations aloft were also tested in each system for both the month-long run and the ramp forecasts.

It was found that EnKF forecasts verified over a domain including Texas and Oklahoma were better than those of 3DVAR for the month-long experiment throughout the forecast window, presumably from the use of flow-dependent covariances in the EnKF. The assimilation of mesonet data improved both EnKF and 3DVAR early forecasts, but sodar/profiler data showed a degradation (EnKF) or had no effect (3DVAR), with the degradation apparently resulting from a lower-atmospheric wind bias. For the wind ramp forecasts, ensemble averaging appears to overwhelm any improvements flow-dependent assimilation may have on ramp forecasts, leading to better 3DVAR ramp prediction. This suggests that best member techniques within the EnKF may be necessary for improved performance over 3DVAR for forecasts of sharp features such as wind ramps. Observation impacts from mesonet and profiler/sodar observations generally improved EnKF ramp forecasts, but either had little effect on or degraded 3DVAR forecasts.

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Patrick S. Skinner
,
Christopher C. Weiss
,
John L. Schroeder
,
Louis J. Wicker
, and
Michael I. Biggerstaff

Abstract

In situ data collected within a weakly tornadic, high-precipitation supercell occurring on 23 May 2007 near Perryton, Texas, are presented. Data were collected using a recently developed fleet of 22 durable, rapidly deployable probes dubbed “StickNet” as well as four mobile mesonet probes. Kinematic and thermodynamic observations of boundaries within the supercell are described in tandem with an analysis of data from the Shared Mobile Atmospheric Research and Teaching Radar.

Observations within the rear-flank downdraft of the storm exhibit large deficits of both virtual potential temperature and equivalent potential temperature, with a secondary rear-flank downdraft gust front trailing the mesocyclone. A primarily thermodynamic boundary resided across the forward-flank reflectivity gradient of the supercell. This boundary is characterized by small deficits in virtual potential temperature coupled with positive perturbations of equivalent potential temperature. The opposing thermodynamic perturbations appear to be representative of modified storm inflow, with a flux of water vapor responsible for the positive perturbations of the equivalent potential temperature. Air parcels exhibiting negative perturbations of virtual potential temperature and positive perturbations of equivalent potential temperature have the ability to be a source of both baroclinically generated streamwise horizontal vorticity and greater potential buoyancy if ingested by the low-level mesocyclone.

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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.

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