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- Author or Editor: Roger M. Wakimoto x
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Abstract
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Abstract
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.
Abstract
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.
Abstract
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.
Abstract
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.
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.
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.
Abstract
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.
Abstract
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.
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.
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.
Abstract
On 13 July 1986 a cold-air outflow from thunderstorms over Illinois and Missouri propagated through the MIST (Microburst and Severe Thunderstorm) network over northern Alabama. The study of this outflow is important since the gust front was solely responsible for the initiation of numerous convective cells. Previous studies have documented the initiation of convection due to colliding gust fronts. In addition, there was a pronounced mesoscale organization of the cells atop the outflow boundary. This was most likely due to a combination of Kelvin–Helmholtz (K–H) and internal gravity (IG) wave activity. In contrast to previous cases, the K–H wave crests were oriented nearly perpendicular to the gust front within the analysis area. The resulting intersections between the circulations associated with the K–H waves and the gust front produced favorable locations for the initiation of convection. Subsequently, the convective cells remained along the updraft side of the K–H wave circulations as they propagated back relative to the gust front. In addition, the gust front induced IG waves that were oriented parallel to the gust front. The enhanced upward motions associated with the IG waves resulted in a periodic arrangement of the convective cells along the updraft side of the K–H waves. The combined motion of the K–H and IG waves was consistent with the cell movement atop the cold-air outflow.
Abstract
On 13 July 1986 a cold-air outflow from thunderstorms over Illinois and Missouri propagated through the MIST (Microburst and Severe Thunderstorm) network over northern Alabama. The study of this outflow is important since the gust front was solely responsible for the initiation of numerous convective cells. Previous studies have documented the initiation of convection due to colliding gust fronts. In addition, there was a pronounced mesoscale organization of the cells atop the outflow boundary. This was most likely due to a combination of Kelvin–Helmholtz (K–H) and internal gravity (IG) wave activity. In contrast to previous cases, the K–H wave crests were oriented nearly perpendicular to the gust front within the analysis area. The resulting intersections between the circulations associated with the K–H waves and the gust front produced favorable locations for the initiation of convection. Subsequently, the convective cells remained along the updraft side of the K–H wave circulations as they propagated back relative to the gust front. In addition, the gust front induced IG waves that were oriented parallel to the gust front. The enhanced upward motions associated with the IG waves resulted in a periodic arrangement of the convective cells along the updraft side of the K–H waves. The combined motion of the K–H and IG waves was consistent with the cell movement atop the cold-air outflow.
Abstract
An analysis of an oceanic front situated near a col defined by the surface pressure field is presented. There have been few observational examples of this type of front presented in the literature. The primary source of information for this study was data recorded by an aircraft equipped with a Doppler radar. The front was approximately two-dimensional and the cross-frontal scale at low levels was 30–40 km. A prefrontal low-level jet was identified in the high-resolution analyses and was shown to be supergeostrophic. Surface pressure measurements and the horizontal temperature gradients were used to calculate the geostrophic wind and the thermal wind imbalance (TWI) in the alongfront direction. Large negative values of TWI (the vertical shear is less than predicted for the given horizontal temperature gradient) were located near a region of frontogenesis. The strong ageostrophic component of the wind parallel to the front suggests that the alongfrontal component of the wind may not have been in geostrophic balance at the time of the observations.
Abstract
An analysis of an oceanic front situated near a col defined by the surface pressure field is presented. There have been few observational examples of this type of front presented in the literature. The primary source of information for this study was data recorded by an aircraft equipped with a Doppler radar. The front was approximately two-dimensional and the cross-frontal scale at low levels was 30–40 km. A prefrontal low-level jet was identified in the high-resolution analyses and was shown to be supergeostrophic. Surface pressure measurements and the horizontal temperature gradients were used to calculate the geostrophic wind and the thermal wind imbalance (TWI) in the alongfront direction. Large negative values of TWI (the vertical shear is less than predicted for the given horizontal temperature gradient) were located near a region of frontogenesis. The strong ageostrophic component of the wind parallel to the front suggests that the alongfrontal component of the wind may not have been in geostrophic balance at the time of the observations.
