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Jeffry Rothermel
,
Cathy Kessinger
, and
Darien L. Davis

Abstract

Two 10.6 μm coherent Doppler lidars participated in the Joint Airport Weather Studies (JAWS) Project field experiment, conducted in summer 1982 near Denver's Stapleton International Airport. One was operated by NOAA/ERL, Wave Propagation Laboratory (WPL), the other by NASA, Marshall Space Flight Center (MSFC). Periodic coordinated scans were made with the two lidars spared 15 km apart. This permitted the calculation of Cartesian winds. This paper presents 1) a brief comparison of radar and lidar system and performance characteristics, 2) results of the first dual-Doppler analyses to be based upon lidar measurements and 3) a comparison of radial wind estimates between the MSFC lidar and a 5.5 cm Doppler radar operated by the National Center for Atmospheric Research (NCAR).

Dual-Doppler analyses were made for the flow behind gust fronts, with the desired flow fields consistent with both surface winds measured by the NCAR Portable Automated Mesonet (PAM) and models derived from previous studies of Great Plains thunderstorm outflows. A comparison of low elevation scans made by the MSFC lidar and the NCAR CP-4 Doppler radar revealed distinct differences which could be explained by a bias in the radar estimates (toward weaker velocities) due to ground clutter contamination. Root-mean-square (rms) difference between radar- and lidar-measured radial velocities was 3.1 m s−1 which could be explained by other causes; however, the mean of the radar data set was 1–2 m s−1 lower than that of the lidar. These findings are consistent with a recent previous study comparing the WPL lidar with the NCAR CP-3 5.5 cm radar.

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Roy M. Rasmussen
,
Andrew Crook
, and
Cathy Kessinger

Abstract

The formation and evolution of convective rain and snow bands prior to and during the crash of Continental Airlines flight 1713 on 15 November 1987 at Denver Stapleton Airport are discussed. Convective rain occurred during the early stages of the storm in association with the approach of an upper-level trough from the west. Snow bands were observed following the passage of a shallow Canadian cold front from the north. These bands formed above the cold front and moved from southeast to northwest at 7 m s−1 with a horizontal spacing of 10–30 km. The winds within the cloud layer were southeasterly from 5 to 10 m s−1, suggesting that the bands were advected by the mean, cloud-layer flow. The most likely mechanism producing these bands was a convective instability in the shear layer above the cold front.

As the upper-level trough moved to the east, the winds in the cloud layer shifted to northerly, causing the bands to move southward with the major axis of the band oriented north–south. The high snowfall rate just prior to the takeoff of flight 1713 occurred as a result of one of these north–south–oriented bands moving over Denver Stapleton Airport from the north during the latter stages of the storm.

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Roger M. Wakimoto
,
Cathy J. Kessinger
, and
David E. Kingsmill

Abstract

On 9 July 1987, a series of low-reflectivity microbursts were studied over Colorado using dual-Doppler analyses, cloud photogrammetry, and in situ measurements collected by aircraft. These types of wind-shear events are particularly hazardous to the aviation community since the parent cloud and pendant virga shafts appear innocuous. The microburst downdrafts are shown to develop at the location where the virga shafts are, visually, the lowest and opaque. As the downdraft intensifies, sublimation and evaporation (to a smaller extent) rapidly deplete the hydrometeors and result in a shift of the axis of maximum negative vertical velocities into a relatively low reflectivity and transparent region of the virga shafts. Comparisons with weak downdrafts or null null cases reveal that the maximum radar reflectivities within the parent clouds for the two cases are comparable; however, the microburst storm consistently exhibits a larger horizontal area encompassed by the 10-dBZ contour at midlevels prior to downdraft formation.

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James W. Wilson
,
Rita D. Roberts
,
Cathy Kessinger
, and
John McCarthy

Abstract

Doppler weather radar data from the Joint Airport Weather Studies (JAWS) Project are used to determine the horizontal and vertical structure of airflow within microbursts. Typically, the associated downdraft is about 1 km wide and begins to spread horizontally at a height below 1 km. The median time from initial divergence at the surface to maximum differential wind velocity across the microburst is 5 min. The height of maximum differential velocity is ∼75 m. The median velocity differential is 22 m s−1 over an average distance of 3.1 km. The outflow is asymmetric, averaging twice as strong along the maximum shear axis compared to the minimum axis.

Doppler radar could be an effective means for identifying microbursts and warning aircraft of wind shear hazards. For microburst detection such a radar must be able to measure wind velocities in clear air as well as in heavy rain and hail. Scan update rates should be approximately every 2 min and the lowest few hundred meters of the atmosphere must be observed. Ground clutter must be considerably reduced from levels typically obtained with present Doppler radars. New antenna technology and signal processing techniques may solve this problem. Automated range and velocity unfolding is required, as well as automated identification and dissemination techniques.

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Cathy J. Kessinger
,
Peter S. Ray
, and
Carl E. Hane

Abstract

On 19 May 1977, a severe squall line formed and moved through the National Severe Storms Laboratory observing network in Oklahoma, producing heavy rain, hail, strong winds, and tornadoes. The squall line is examined at two times: 1434 and 1502 CST. Doppler analysis of part of the squall line reveals four convective cells in the line, developing cells ahead of the line, a trailing precipitation region, and a convective rainband at the western edge of the system. The updrafts within the convective cells on the leading edge tilt westward in the lower levels and eastward near the tropopause. Convective updrafts and downdrafts are fed by low-level air entering the squall line from the front. Surface network analysis and gust front penetration by an instrumented aircraft indicated strong convergence along the leading edge of one of the stronger cells in the line. Horizontal, line-relative flow perpendicular to the squall line and within the trailing precipitation area is from east to west (front to back) at all levels, weakening with height. An exception to this is an area of weak (≤3 m s−1) rear inflow into the stratiform precipitation region in the midlevels. Flow parallel to the squall line is stronger, in general, than the perpendicular flow. A composite rawinsonde analysis shows ascending motion within the troposphere over most of the squall line region. A conceptual model is developed for 19 May 1977 and is compared to conceptual models of tropical squall lines and of the 22 May 1976 Oklahoma squall line.

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Carl E. Hane
,
Cathy J. Kessinger
, and
Peter,S. Ray

Abstract

Mechanisms for maintenance of the strong convection along the leading edge of a broad squall line that occurred in Oklahoma on 19 May 1977 are investigated. The findings are based upon analysis of data from a surveillance radar, a surface mesonetwork, Doppler radars, proximity soundings and aircraft data, and upon the results of a two-dimensional, cloud-scale numerical simulation. The detailed results of the multiple Doppler analysis are contained in the Part I paper reporting results of research on this squall line.

It is found that at a preferred location along the squall line, an area of intense convection is maintained over a long time period. A meso-β scale organized structure, which includes an area of low pressure near the southeast edge of the intense convection and an associated area of convergence extending to the east, promotes the formation of small showers in short line segments. These showers, due to their differing motion from elements within the main line, merge with the line to the north of the mesolow, resulting in maintenance of the strong area of convection. The observed meso-β structure on this day is believed to be made possible by a deep low-level layer of weak vertical wind shear and high water-vapor content.

At other locations along the line, the numerical simulation indicates an unsteady behavior in the maintenance of squall line convection by gust frontal convergence. Perturbations in the vertical motion field are periodically initiated by either (i) enhanced convergence at the gust front resulting from diverging downdrafts at locations farther to the west, or (ii) Kelvin-Helmholtz instability produced at the gust front head. These perturbations move westward relative to the gust front above the low-level cold air and periodically invigorate the main region of updrafts located a few tens of kilometers west of the gust front. Low-level updrafts, forced by diverging surface outflow from weak downdrafts, occasionally interact with the translating perturbations to increase their amplitude. The existence of the westward-moving perturbations is tentatively substantiated by the presence of similar structures in the analyzed Doppler wind fields. Greater time resolution in Doppler data, in combination with more comprehensive surface and upper air data ahead of squall lines of this type, would aid in confirming the reported structures.

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Cathy J. Kessinger
,
David B. Parsons
, and
James W. Wilson

Abstract

On 30 June 1982, a multicellular storm in Colorado produced four downbursts, three misocyclones, a miso-anticyclone, and horizontal vortex circulations within a relatively small area of the storm. Weather events associated with this storm included hail, heavy rain, and strong winds. A sounding taken two hours before storm formation showed the mixed layer was characterized by a nearly dry adiabatic lapse rate to ∼2 km and was relatively moist for eastern Colorado. A hodograph showed the environment had weak to moderate vertical shear of the horizontal wind, a condition conducive to the formation of downdraft misocyclones. The four-dimensional structure of this storm is documented below cloud base using winds, reflectivity, and thermodynamic data derived front multiple Doppler analysis.

One misocyclone (<4 km scale) is particularly intense with a peak vorticity of ≈100 × 10−3 s−1 near cloud base. Despite the intense rotation, no tornadoes or funnels were observed and no damage was reported. Radar characteristics of this misocyclone are similar to those of mesocyclones that produce tornadoes or funnels except that vorticity is a maximum near cloud base and the low-level divergence created by the downbursts weakens the low-level, positive vorticity. While the misocyclone is initially separated from the downdraft, the two features evolve to become collocated. Each misocyclone becomes associated with a local downdraft maximum, suggesting that the misocyclones are important to downdraft development.

Pressure perturbation analysis does not show any evidence for strong, downward-directed pressure gradient forces below cloud base that would act to accelerate a downdraft. Since the downdraft is observed to accelerate below cloud base, other forces must be important. Observations and buoyancy estimates calculated from radar reflectivity show negative buoyancy is playing a role in downdraft intensification. Despite the lack of dynamical forcing of the downdraft by the misocyclone below cloud base, dynamical forces may be playing a role in accelerating the downdraft above cloud base.

Horizontal vortex circulations, or rotors, form along the edge of the misocyclone and downdraft and propagate away from their source region. Strongest surface winds are associated with the rotors. Pressure perturbation analysis shows that a low forms at the center of the circulation that may cause an acceleration of the low-level outflow into the rotor and may explain the strong winds. Rotors may be an integral part of downburst outflows and perhaps multiple rotors are created by pulsating downdrafts. An explanation of these circulations is important since they seem to have been involved in the Dallas-Fort Worth Regional Airport crash of an L-1011 jet.

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John McCarthy
,
Robert Serafin
,
James Wilson
,
James Evans
,
Cathy Kessinger
, and
William P. Mahoney III

Abstract

Microburst wind shear has caused or contributed to a significant number of aviation accidents. Since 1943, wind shear accidents have been responsible for more than 1,400 fatalities worldwide, including over 400 deaths in the United States between 1973 and 1985. In this paper, we describe one of the more successful and societally impactful research-to-operations (R2O) programs in atmospheric science history. The remarkable R2O journey included the discovery of microburst wind shear in the late 1970s and early 1980s, the scientific efforts to understand this phenomenon and its impact on aircraft operations, the development of a wind shear training program for pilots, and the rapid development, testing, and implementation of wind shear detection systems that successfully saved lives and property. The article includes a chronological description of the wind shear research and development program, key milestones toward implementation, and the research-to-operations best practices employed for successful technology transfer.

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Katelyn A. Barber
,
Wiebke Deierling
,
Gretchen Mullendore
,
Cathy Kessinger
,
Robert Sharman
, and
Domingo Muñoz-Esparza

Abstract

Convectively induced turbulence (CIT) is an aviation hazard that continues to be a forecasting challenge as operational forecast models are too coarse to resolve turbulence affecting aircraft. In particular, little is known about tropical maritime CIT. In this study, a numerical simulation of a tropical oceanic CIT case where severe turbulence was encountered by a commercial aircraft is performed. The Richardson number (Ri), subgrid-scale eddy dissipation rate (EDR), and second-order structure functions (SF) are used as diagnostics to determine which may be used for CIT related to developing and mature convection. Model-derived subgrid-scale EDR in past studies of midlatitude continental CIT was shown to be a good diagnostic of turbulence but underpredicted turbulence intensity and areal coverage in this tropical simulation. SF diagnosed turbulence with moderate to severe intensity near convection and agreed most with observations. Further, SF were used to diagnose turbulence for developing convection. Results show that the areal coverage of turbulence associated with developing convection is less than mature convection. However, the intensity of turbulence in the vicinity of developing convection is greater than the turbulence intensity in the vicinity of mature convection highlighting developing convection as an additional concern to aviation.

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Michael F. Donovan
,
Earle R. Williams
,
Cathy Kessinger
,
Gary Blackburn
,
Paul H. Herzegh
,
Richard L. Bankert
,
Steve Miller
, and
Frederick R. Mosher

Abstract

Three algorithms based on geostationary visible and infrared (IR) observations are used to identify convective cells that do (or may) present a hazard to aviation over the oceans. The performance of these algorithms in detecting potentially hazardous cells is determined through verification with Tropical Rainfall Measuring Mission (TRMM) satellite observations of lightning and radar reflectivity, which provide internal information about the convective cells. The probability of detection of hazardous cells using the satellite algorithms can exceed 90% when lightning is used as a criterion for hazard, but the false-alarm ratio with all three algorithms is consistently large (∼40%), thereby exaggerating the presence of hazardous conditions. This shortcoming results in part from the algorithms’ dependence upon visible and IR observations, and can be traced to the widespread prevalence of deep cumulonimbi with weak updrafts but without lightning over tropical oceans, whose origin is attributed to significant entrainment during ascent.

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