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Todd M. Crawford
and
Howard B. Bluestein

Abstract

The characteristics of dryline passage are documented through an analysis of data from an instrumented surface mesonetwork in the Texas panhandle, and western and central Oklahoma during the Cooperative Oklahoma Profiler Studies field program. Some eastward-moving drylines at the surface during the day were characterized by monotonic drops in dewpoint after dryline passage; others were marked by a series of rapid drops punctuated by periods of no change after dryline passage, which suggests that the dryline often progresses in discrete steps, rather than continuously. The dryline during the daytime was not always collocated with a pressure trough, although the strongest dryline observed was. Analyses of surface pressure traces indicated that westward-moving drylines during the evening did not display behavior characteristic of strong, intense density currents, as had been found in other studies. Evidence is presented, in one case, of 90-min oscillations in water vapor and wind behind the dryline, which may have been associated with the downward transport of momentum associated with gravity waves aloft.

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Howard B. Bluestein
and
Todd M. Crawford

Abstract

A technique is introduced using surface data from triangular networks of adjacent mesonet stations to estimate the terms in the horizontal equation of motion at anemometer level (10 m) on either side of the dryline in the southern plains of the United States. Data from an instrumented surface mesonetwork in the Texas panhandle and western and central Oklahoma during the Cooperative Oklahoma Profiler Studies (COPS-91) field program were employed for this purpose. East of the dryline and surface pressure trough the vertical-mixing term had a component normal and to the right of the surface wind in accord with Ekman theory; west of the dryline and surface pressure trough the vertical-mixing term had a component normal and to the left of the surface wind in disagreement with Ekman theory. It is suggested that disagreements with Ekman theory may be due to baroclinic effects in the boundary layer. It is also shown that during the day both the westward component of the pressure gradient force and the easterly component of the surface wind increased east of the dryline, in accord with the “inland sea breeze” hypothesis, and that the maximum easterly wind component usually lagged the maximum westward component of the pressure gradient force by several hours.

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Carl E. Hane
,
Howard B. Bluestein
,
Todd M. Crawford
,
Michael E. Baldwin
, and
Robert M. Rabin

Abstract

Long-lived thunderstorms were initiated during the afternoon of 26 May 1991 ahead of a dryline in northwestern Oklahoma. Various reasons for initiation in this particular along-dryline location are investigated through analysis of observations collected during the Cooperative Oklahoma Profiler Studies—1991 field program. Observing systems included in situ and radar instrumentation aboard a research aircraft, soundings from mobile laboratories, a mesonetwork of surface stations, meteorological satellites, and operational networks of surface and upper-air stations.

Elevated moistening east of the dryline revealed by soundings and aircraft observations in combination with thermal plume activity was apparently insufficient to promote sustained convection on this day without aid from an additional lifting mechanism. Satellite observations reveal scattered convection along the dryline by midafternoon and a convective cloud line intersecting the dryline at an angle in the area of most pronounced storm initiation, extending southwestward into the dry air. Another prominent feature on this day was a mesoscale bulge along the dryline extending northeastward into southwest Kansas. Deep convection was initiated along this bulge, but was in general short-lived.

Potential causes of the lifting associated with the cloud line that was apparently key to the preferred location for storm development in northwest Oklahoma were investigated: (a) a mesoscale circulation resulting from horizontal differences in radiative (temperature) properties of the underlying surface and (b) upward motion induced by an upper-level mesoscale disturbance. Analysis of vegetative and surface temperature distributions from satellite observations suggests a potential (more research is needed) link between surface characteristics and the development of the dryline bulge and observed cloud line through horizontal differences in vertical momentum transport. A run of the currently operational eta model indicates some skill in predicting dryline location and motion and predicts upward motion in the northern part of the region that was generally more convectively active, but shows no indication of upper-level support in the vicinity of the observed cloud line.

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Carl E. Hane
,
Robert M. Rabin
,
Todd M. Crawford
,
Howard B. Bluestein
, and
Michael E. Baldwin

Abstract

A dryline that occurred on 16 May 1991 within a synoptically active environment is examined in detail using research aircraft, radar, surface, satellite, and upper air observations. The work focuses on multiple boundaries in the dryline environment and initiation of tornadic storms in two along-line areas.

Aircraft measurements in the boundary layer reveal that both the east–west extent of moisture gradients and the number of regions containing large moisture gradients vary in the along-dryline direction. Aircraft penetrations of thinlines observed in clear air return from radar reveal that all thinlines are associated with convergence and a moisture gradient, and that more distinct thinlines are associated with stronger convergence. However, significant moisture gradients are not always associated with either thinlines or convergent signatures.

Convective clouds on this day formed at the dryline rather than significantly east of the dryline. The three thunderstorm cells that occurred in east-central Oklahoma developed along a 20-km section of the dryline south of a dryline bulge and within a 30-min period. The storms appear to have developed in this location owing to enhanced convergence resulting from backed winds in the moist air in response to lowered pressure in the warm air to the northwest. Aircraft measurements in the boundary layer and satellite-sensed surface temperature both indicate localized warming in this area to the northwest.

Farther north there was a 70–100-km segment along the dryline where few convective clouds formed during the afternoon. This coincided with a swath of cooler boundary layer air that resulted from reduced surface heating over an area that received significant thunderstorm rainfall during the previous night.

A severe thunderstorm complex that developed along the Kansas–Oklahoma border was initiated at the intersection of the dryline and a cloud line that extended into the dry air. An aircraft penetration of the cloud line about 12 km from its intersection with the dryline shows convergence and deepened low-level moisture at the cloud line. The cloud field that evolved into the cloud line over a period of several hours developed over the area that had received the heaviest rainfall during the previous night.

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Carl E. Hane
,
Michael E. Baldwin
,
Howard B. Bluestein
,
Todd M. Crawford
, and
Robert M. Rabin

Abstract

Through a case study approach the motion of a dryline (on 16 May 1991) within a synoptically active environment in the southern plains, along which severe storms ultimately developed, is examined in detail. Observations from research aircraft, surface mesonetwork stations, mobile ballooning vehicles, radar, wind profilers, and operational surface and upper air networks are examined and combined. Additionally, output from the operational mesoscale Eta Model is examined to compare predictions of dryline motion with observations and to aid in interpretation of observations.

The dryline on this day advanced rapidly eastward and included formation of a bulge; additionally, in at least two instances it exhibited redevelopment (loss of definition at one location and gain at another). Aircraft observations revealed that an eastward redevelopment occurred in the early afternoon and was characterized by a series of four “steps” along the western edge of the boundary layer moisture. The westernmost and easternmost steps coincide with the locations of the dryline before and after redevelopment, respectively. The retreat of the dryline in the central and southern portion of the analysis domain in the late afternoon included both continuous motion and redevelopment toward the west-northwest. This dual-mode retreat of the dryline was accompanied by gradual backing of the winds and moistening in low levels.

The Eta Model forecast initialized at 1200 UTC produced dryline features that were qualitatively similar to observed fields. The eastward motion of a broad area of enhanced moisture gradient agreed well with observations following an initial spinup period. A north–south moisture convergence axis preceded the rapid eastward motion of the dryline by several hours. Lack of subsidence in the air behind the modeled dryline leads to the conclusion that processes other than downward transfer of horizontal momentum by larger-scale motions (that would support eastward advection) produced the rapid dryline motion and observed eastward dryline bulge. Results of diagnosing physical processes affecting model dryline motion point toward boundary layer vertical mixing coupled with advection of dry air aloft as vital components in rapid advance of the dryline eastward in this synoptically active case.

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Howard B. Bluestein
,
Wesley P. Unruh
,
David C. Dowell
,
Todd A. Hutchinson
,
Todd M. Crawford
,
Andrew C. Wood
, and
Herbert Stein

Abstract

A large tornado was observed near Northfield, Texas, on 25 May 1994 during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX). An analysis of the tornado and its parent storm is discussed. Doppler wind velocity spectra of the tornado and its parent circulation, which were computed from data collected by a low-power, portable, FM-CW (frequency-modulated continuous-wave), 3-cm-wavelength Doppler radar, are presented at increments in the range of 78 m. The FM-CW radar data from the tornado are the first ever collected of high enough quality to analyze. The CW spectra computed from data collected by the portable radar, a pseudo-dual-Doppler analysis of airborne Doppler radar data collected by a National Oceanic and Atmospheric Administration P-3 aircraft, photogrammetric analysis of a video of the tornado, and a ground-based damage survey are discussed in the context of the FM-CW spectra. This study is unique in that both ground-based and airborne Doppler radar systems probed the tornado and its environment. Wind speeds of 60 m s−1 were indicated in the tornado in a swath 300 m across, with some smaller areas of possible wind speeds up to 75 m s−1. Circumstantial evidence is presented that the tornado originated along an elliptically shaped cyclone/shear zone along the leading edge of a large hook echo in its parent supercell storm. The tornado’s parent vortex (mesocyclone) was approximately 2 km in diameter and contained tangential wind speeds of 45–50 m s−1.

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