Search Results

You are looking at 1 - 10 of 78 items for

  • Author or Editor: Roger M. Wakimoto x
  • All content x
Clear All Modify Search
Roger M. Wakimoto
Full access
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 By evening the radiation inversion has been replaced by a superadiabatic layer at the surface.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.

Full access
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.

Full access
Roger M. Wakimoto

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.

Full access
Roger M . Wakimoto
Full access
Roger M. Wakimoto

Abstract

No Abstract available.

Full access
Roger M. Wakimoto

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.

Full access
Roger M. Wakimoto

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.

Full access
Roger M. Wakimoto and Chinghwang Liu

Abstract

On 16 May 1995, a supercell storm produced an F1 tornado near Garden City, Kansas, during VORTEX (Verification of the Origins of Rotation in Tornadoes Experiment). This event provided the first opportunity to synthesize data collected by a new airborne radar platform called ELDORA (Electra Doppler radar) developed by the National Center for Atmospheric Research. The evolution of the low- and midlevel mesocyclone was presented in a companion paper. In this paper, the sequence of events that triggered tornadogenesis within the mesocyclone circulation is shown. The occlusion downdraft, documented in Part I, appears to promote “vortex breakdown” resulting in the generation of multiple vorticity centers. It is one of these centers that intensifies into the Garden City tornado.

The structural relationship between the observed wall cloud and the radar reflectivity associated with the developing hook echo and the kinematic wind field is also presented. Although the wall cloud appears to be uniform there are pronounced asymmetries in the echo and ground-relative flow fields.

Full access
Roger M. Wakimoto and Huaqing Cai

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.

Full access