Nolan T. Atkins

The Bow Echo and MCV Experiment: Observations and Opportunities

The Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) is a research investigation using highly mobile platforms to examine the life cycles of mesoscale convective systems. It represents a combination of two related investigations to study (a) bow echoes, principally those that produce damaging surface winds and last at least 4 h, and (b) larger convective systems that produce long-lived mesoscale convective vortices (MCVs). The field phase of BAMEX utilized three instrumented research aircraft and an array of mobile ground-based instruments. Two long-range turboprop aircraft were equipped with pseudo-dual-Doppler radar capability, the third aircraft was a jet equipped with dropsondes. The aircraft documented the environmental structure of mesoscale convective systems (MCSs), observed the kinematic and thermodynamic structure of the convective line and stratiform regions (where rear-inflow jets and MCVs reside), and captured the structure of mature MCVs. The ground-based instruments augmented sou...

Observations of the Sea-Breeze Front during CaPE. Part I: Single-Doppler, Satellite, and Cloud Photogrammetry Analysis

Abstract An analysis of two sea-breeze events on 6 August (an onshore flow event) and 12 August (an offshore flow event) 1991 is presented using single-Doppler observations, satellite images, and cloud pictures collected during the Convection and Precipitation/Electrification (CaPE) Experiment. Documentation of the alongfrontal variability at the leading edge of the sea-breeze circulation is presented for the first time. The horizontal structure of the front was strongly modulated by the near-perpendicular intersections of horizontal convective rolls developing in the ambient air out ahead of the sea breeze on 12 August. These intersection points also appeared to be preferred locations for cloud development along the front. Horizontal convective rolls were also documented on 6 August; however, their orientation was nearly parallel to the sea-breeze front. As a result, extended sections of these rolls appeared to have merged with the front as it propagated inland rather than having distinct intersection po...

Wet Microburst Activity over the Southeastern United States: Implications for Forecasting

Abstract The thermodynamic properties of wet-microburst-producing days, as observed during the 1986 MIST (MIcroburst and Severe Thunderstorm) field project, conducted in northern Alabama, have been examined and are shown to exhibit common characteristics. The parent storms and environment for this microburst type are substantially different than those documented over the High Plains in that the cloud bases are warmer, the subcloud layer is shallower, the radar reflectivities are greater, and the thermal environment is more moist and stable. Analyses of the rawinsonde data, launched in the morning and afternoon, show that low-level moisture is present and is capped by a midlevel dry layer. This midlevel dry air is generally advected from the northwest, where a large area of dry air exists over the central United States. In addition, it appears to be possible to differentiate between microburst days and thunderstorm days producing no wet microbursts by plotting the vertical profile of the equivalent potenti...

Buyer Beware: Some Words of Caution on the Use of Severe Wind Reports in Postevent Assessment and Research

Abstract Postevent damage surveys conducted during the Bow Echo and Mesoscale Convective Vortex Experiment demonstrate that the severe thunderstorm wind reports in Storm Data served as a poor characterization of the actual scope and magnitude of the surveyed damage. Contrasting examples are presented in which a few reports grossly underrepresented a significant event (in terms of property damage and actual areal coverage of damage), while a large number of reports overrepresented a relatively less significant event. Explanations and further discussion of this problem are provided, as are some of the implications, which may include a skewed understanding of how and when systems of thunderstorms cause damage. A number of recommendations pertaining to severe wind reporting are offered.

Influence of the Synoptic-Scale Flow on Sea Breezes Observed during CaPE

Abstract Mean sea-breeze characteristics were determined by analyzing a number of sea-breeze events during offshore, parallel, and onshore flow regimes during the Convection and Precipitation/Electrification Experiment (CaPE). It was observed that offshore flow cases exhibited the widest, and relatively strongest, radar-detected thin lines. The thin-line reflectivity values steadily increased during the day. In contrast, a thin line was detected only during late afternoon on parallel flow days while no easily identifiable thin line was observed during onshore flow days. The gradients of temperature and moisture, as measured by a surface meteorological station during sea-breeze passage, were strongest and weakest during offshore and onshore flow days, respectively. In addition, the moisture and temperature gradients across the leading edge of the sea breeze steadily increased during the day and were strongest during late afternoon. Using dual-Doppler techniques, the detailed kinematic structure of the sea-...

Observations of the Finescale Structure of a Dryline during VORTEX 95

Abstract The three-dimensional finescale structure of a dryline observed over the Texas panhandle during the Verification of the Origins in Tornadoes Experiment (VORTEX) on 6 May 1995 is presented. High-resolution observations documenting dryline evolution were collected from midafternoon while it moved slowly eastward until late afternoon as it retrogressed back to the west. Dryline variability in the horizontal along-line direction was revealed by airborne Doppler radar observations from the National Center for Atmospheric Research Electra Doppler radar and ground-based measurements from the nearby WSR-88D at Amarillo, Texas. This variability was created as the dryline interacted with horizontal convective rolls forming west of the dryline. The rolls intersected the dryline at periodic locations creating radar reflectivity and vertical velocity maxima. More importantly, clouds initiated at these intersection points. Additional dryline variability was generated through the interaction of the low-level fl...

High winds generated by bow echoes. Part II: The relationship between the mesovortices and damaging straight-line winds

Abstract Airborne radar analysis of a mesovortex that developed near the apex of a bow echo is presented. The mesovortex was shown to play a critical role in determining the location of intense “straight-line” wind damage at the surface. The perturbation pressure gradient force (in natural coordinates) along the parcel path accelerated the horizontal winds; however, intense mesovortices modified the low-level outflow and largely determined the locations where the strongest winds occurred. Regions of maximum winds are accounted for as a superposition of the vortex and the flow in which it is embedded. The strongest winds occur on the side of the vortex where translation and rotation effects are in the same direction. This model explains the observed tongue of high wind speeds that were confined to the periphery of the mesovortex. The origin of the mesovortex is also examined. Similarities and differences of this bow echo event with recent modeling studies are presented.

The LaGrange Tornado during VORTEX2. Part I: Photogrammetric Analysis of the Tornado Combined with Single-Doppler Radar Data

AbstractThis study presents a single-Doppler radar analysis combined with cloud photography of the LaGrange, Wyoming, tornado on 5 June 2009 in an attempt to relate the radar-observed hook echo, weak-echo hole (WEH), and rotational couplet to the visual characteristics of the tornado. The tornado was rated EF2. The circulation at low levels went through two intensification periods based on azimuthal shear measurements. The first intensification was followed by the appearance of a brief funnel cloud. The second intensification was coincident with the appearance of a second funnel cloud that remained in contact with the ground until the tornado dissipated.A deep WEH rapidly formed within the hook echo after damaging wind was identified at the ground and before the appearance of a funnel cloud. The echo pattern through the hook echo on 5 June undergoes a dramatic evolution. Initially, the minimum radar reflectivities are near the surface (<15 dBZ) and the WEH does not suggest a tapered structure near the gro...

Radar and Damage Analysis of Severe Bow Echoes Observed during BAMEX

Abstract This study examines damaging-wind production by bow-shaped convective systems, commonly referred to as bow echoes. Recent idealized numerical simulations suggest that, in addition to descending rear inflow at the bow echo apex, low-level mesovortices within bow echoes can induce damaging straight-line surface winds. In light of these findings, detailed aerial and ground surveys of wind damage were conducted immediately following five bow echo events observed during the Bow Echo and Mesoscale Convective Vortex (MCV) Experiment (BAMEX) field phase. These damage locations were overlaid directly onto Weather Surveillance Radar-1988 Doppler (WSR-88D) images to (i) elucidate where damaging surface winds occurred within the bow-shaped convective system (in proximity to the apex, north of the apex, etc.), and then (ii) explain the existence of these winds in the context of the possible damaging-wind mechanisms. The results of this study provide clear observational evidence that low-level mesovortices wit...

Damaging surface wind mechanisms within the 10 June 2003 Saint Louis bow echo during bamex

Abstract Detailed radar and damage survey analyses of a severe bow echo event that occurred on 10 June 2003 during the Bow Echo and Mesoscale Convective Vortex (MCV) Experiment are presented. A bow echo formed just east of Saint Louis, Missouri, and produced a continuous straight-line wind damage swath approximately 8 km in width and 50 km in length along with five F0–F1 tornadoes. Careful superposition of the damage survey analysis and Weather Surveillance Radar-1988 Doppler (WSR-88D) data from Saint Charles, Missouri (KLSX), showed that the primary straight-line wind damage swath was not collocated with the bow echo apex as has been suggested in previous studies. Rather, the primary damage swath was found north of the bow apex, collocated with a low-level vortex that formed on the leading edge of the bow echo. Much of the primary damage swath appeared to have been created by the low-level vortex. Moreover, most of the surface straight-line wind damage was generated during the early stages of bow echo mo...