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Roger M . Wakimoto
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Roger M. Wakimoto

Abstract

On 5 June 1982, a clear-air mesocyclone translated through the observational network of the Joint Airport Weather Studies (JAWS) Project new Denver, Colorado. Mesoscale analysis revealed both a mesocirculation defined by the wind field, and a mesolow defined by the pressure field. The mesocyclone appeared to be a result of a foehn-type wind descending the slopes of the Front Range of the Rocky Mountains. It is believed that this feature is the “shallow orographic low” that has been shown in past studies to be an important stage in the development of cyclones in the lee of large mountain ranges.

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Roger M. Wakimoto

Abstract

The active dry microburst days during the 1982 JAWS (Joint Airport Weather Studies) Project in Colorado are examined for common characteristics. The environments on these days are shown to have similar thermodynamic structures in the vertical. In the morning, a shallow radiation inversion is capped by a deep, dry-adiabatic boundary layer. Moisture is present at midlevels. By evening the radiation inversion has been replaced by a superadiabatic layer at the surface. Solar heating of the boundary layer is shown to be important for producing an environment favorable for dry microbursts. A model is proposed that can be used by forecasters to issue a “wind shear watch” to the general public and aviation community.

Peak downdraft speeds associated with dry microbursts appear to be a result of negative buoyancy, owing to the evaporation of precipitation during the descent below cloud base. These downward velocities are of the same magnitude as the horizontal wind speeds. Entrainment of subcloud air into the downdraft is considered minimal.

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Roger M. Wakimoto

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This paper presents the time-dependent analysis of the thunderstorm gust front with the use of Project NIMROD data. RHI cross sections of reflectivity and Doppler velocity are constructed to determine the entire vertical structure. The life cycle of the gust front is divided into four stages: 1) the formative stage; 2) the early mature stage; 3) the late mature stage; and 4) the dissipation stage. A new finding is a horizontal roll detected in the reflectivity pattern resulting from airflow that is deflected upward by the ground, while carrying some of the smaller precipitation ahead of the main echo core of the squall line. This feature is called a “precipitation roll”. As determined from rawinsonde data, the cold air behind the gust front accounts for the observed surface pressure rise. Calculations confirm that the collision of two fluids produce a nonhydrostatic pressure at the leading edge of the outflow. The equation governing the propagation speed of a density current accurately predicts the movement of the gust front.

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Roger M. Wakimoto

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A case study of a Catalina Eddy during Project BASIN is presented. There appears to he a topographic influence in the generation of this eddy. Detailed surface and upper-air data over Los Angeles illustrate the effect of the eddy on the boundary layer and on the transport of ozone out of the basin. Isentropic analyses are consistent with visual satellite images of the phenomena. The Catalina Eddy was shown to extend throughout the entire depth of the strong temperature inversion that exist over Los Angeles, with maximum wind speeds within the inversion. Surface ozone levels downwind of the eddy are shown to vary depending on the local circulations.

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Roger M. Wakimoto

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No Abstract available.

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Roger M. Wakimoto
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Roger M. Wakimoto

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This paper presents an analysis of a thunderstorm system that spawned a downburst and an F4 anticyclonictornado in the West Bend, Wisconsin area in the early morning of 4 April 1981. The tornado caused threefatalities and was one of the strongest occurring in the United States during 1981. A weakly defined bowecho of level 3 intensity was observed by the radar located at Neenah, Wisconsin during the storm. Thethunderstorm tops were measured to only 7900 m (26 000 ft) and no apparent severe weather signature inthe infrared satellite imagery could be detected. The occurrence of this severe storm event is seen to haveimplications on present and future short-term operational forecasting technology.

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Warren Blier and Roger M. Wakimoto

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The structural evolution of the extratropical cyclone that occurred during intensive observation period 5 (IOP 5) of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) is presented. In the present paper, the focus will be on the synoptic-scale development of the cyclone and the mesoscale frontal structure in the vicinity of the surface low center at a time early in its period of rapid deepening. Within this context the much higher resolution analyses of the three-dimensional wind field of the circulation center are discussed in the companion paper.

The surface low center appeared initially as one of at least two mesoscale circulation centers along a preexisting low-level frontal zone approximately aligned with the Gulf Stream. Development occurred in association with the approach of a synoptic-scale upper-level trough from the west. As in the ERICA IOP 4 cyclone, a bent-back warm front-representing the extension of the warm front to the west of its point of intersection with the cold front-developed early in the period of rapid deepening. The surface low center was found to lie along this front and was associated with a significant meso-β-scale circulation. A secondary cold front developed out of the bent-back front to the south of the low center, while the antecedent primary cold front showed evidence of fracture near its juncture with the warm front. This frontal weakening appeared to be associated with a diffluent low-level wind field and with the intrusion of a swath of warm and dry air.

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Roger M. Wakimoto and Huaqing Cai

Abstract

Analysis of a supercell storm that did not produce a tornado near Hays, Kansas, is presented. A well-defined midlevel mesocyclone was apparent throughout most of the storm’s life cycle. Numerous shallow circulations were observed along the rear-flank gust front during the data collection period. Six of these circulations strengthened into intense low-level mesocyclones. Each of these mesocyclones failed to produce a tornado. The strongest low-level mesocyclone, referred to as vortex #4, underwent a life cycle that was consistent with other tornadic mesocyclones documented in the literature. These results illustrate that the presence of a long-lived mesocyclone at low levels is not sufficient for tornadogenesis to occur.

The kinematic structure of the low-level mesocyclone that did not produce a tornado is compared with a tornadic mesocyclone from another storm in order to understand the characteristic differences between these circulations. The results lead to the conclusion that the presence of a low-level mesocyclone, occlusion downdraft, and updraft/downdraft structure that spirals cyclonically around the circulation are not sufficient conditions for tornadogenesis. Retrieved perturbation pressure and buoyancy fields are used to examine the forcing mechanism of the occlusion downdraft. A downward-directed pressure gradient appears to be the primary forcing mechanism of this downdraft. Perturbation temperature retrievals suggest that the occlusion downdraft is accompanied by a warm core.

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