Roger M. Wakimoto

Non-supercell Tornadoes

Abstract Analyses of tornadoes that are not associated with supercells are presented. The database for this study was collected during CINDE (Convention INitiation and Downburst Experiment), a field project operated during the summer of 1987 in Colorado. A total of 27 visual vortices were studied. They appeared to form as shear instabilities along radar detected convergence lines. The circulations initiated at low levels generally in the absence of precipitation echo. Subsequently as these vortices propagated along the convergence line they appeared to strengthen to tornadic intensity when they became colocated with the updraft of a rapidly developing storm. It is hypothesized that vortex stretching is responsible for intensifying the initial rotation. Although these tornadoes were weaker than those accompanied by strong midlevel mesocyclones, estimates of their strength suggest damage capability as high as F2. The implications for operational radars to detect these types of phenomena were identified. The...

The Life Cycle of Thunderstorm Gust Fronts as Viewed with Doppler Radar and Rawinsonde Data

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 t...

Boundary Layer Clear-Air Radar Echoes: Origin of Echoes and Accuracy of Derived Winds

Abstract Boundary layer clear-air echoes are routinely observed with sensitive, microwave, Doppler radars similar to the WSR-88D. Operational and research meteorologists are using these Doppler velocities to derive winds. The accuracy of the winds derived from clear-air Doppler velocities depends on the nature of the scatterers. This paper uses dual-wavelength and dual-polarization radars to examine the cause of these echoes and the use of Doppler velocities from the clear-air return to estimate winds. The origin of these echoes has been an ongoing controversy in radar meteorology. These echoes have been attributed to refractive-index gradient (Bragg scattering) and insects and birds (particulate scattering). These echoes are most commonly observed over land from spring through autumn. Seldom do they occur over large bodies of water. Widespread clear-air echoes have also been observed in winter when temperatures are above 10°C. Radar reflectivity comparisons of clear-air echoes in Florida and Colorado wer...

Horizontal Convective Rolls: Determining the Environmental Conditions Supporting their Existence and Characteristics

Abstract Data from the Convection and Precipitation/Electrification (CaPE) project, as well as results from numerical simulations, are used to study horizontal convective rolls. The environmental conditions necessary for sustaining rolls and for influencing the aspect ratio, ratio of roll wavelength to convective boundary layer (CBL) depth, and orientation are examined. Observations and numerical model simulations both suggest that a moderate surface sensible heat flux and some vertical wind shear are necessary for roll existence. Unlike some previous studies, however, it is shown that rolls occurred within very low CBL shear conditions (∼2 × 10−3 s−1). In addition, the low-level (i.e., ∼200 m) shear seems to be more important than the shear through the depth of the CBL in roll sustenance. The aspect ratio is shown to be proportional to the CBL instability, measured in terms of the Monin–Obukhov length. The roll orientation is similar to the wind direction at 10 m AGL, the CBL wind direction, the inversio...

The Fronts and Atlantic Storm-Track Experiment (FASTEX): Scientific Objectives and Experimental Design

The Fronts and Atlantic Storm-Track Experiment (FASTEX) will address the life cycle of cyclones evolving over the North Atlantic Ocean in January and February 1997. The objectives of FASTEX are to improve the forecasts of end-of-storm-track cyclogenesis (primarily in the eastern Atlantic but with applicability to the Pacific) in the range 24 to 72 h, to enable the testing of theoretical ideas on cyclone formation and development, and to document the vertical and the mesoscale structure of cloud systems in mature cyclones and their relation to the dynamics. The observing system includes ships that will remain in the vicinity of the main baroclinic zone in the central Atlantic Ocean, jet aircraft that will fly and drop sondes off the east coast of North America or over the central Atlantic Ocean, turboprop aircraft that will survey mature cyclones off Ireland with dropsondes, and airborne Doppler radars, including ASTRAIA/ELDORA. Radiosounding frequency around the North Atlantic basin will be increased, as ...

The Bow Echo and MCV Experiment: Observations and Opportunities

The Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) is a research investigation using highly mobile platforms to examine the life cycles of mesoscale convective systems. It represents a combination of two related investigations to study (a) bow echoes, principally those that produce damaging surface winds and last at least 4 h, and (b) larger convective systems that produce long-lived mesoscale convective vortices (MCVs). The field phase of BAMEX utilized three instrumented research aircraft and an array of mobile ground-based instruments. Two long-range turboprop aircraft were equipped with pseudo-dual-Doppler radar capability, the third aircraft was a jet equipped with dropsondes. The aircraft documented the environmental structure of mesoscale convective systems (MCSs), observed the kinematic and thermodynamic structure of the convective line and stratiform regions (where rear-inflow jets and MCVs reside), and captured the structure of mature MCVs. The ground-based instruments augmented sou...

Observations of the Sea-Breeze Front during CaPE. Part I: Single-Doppler, Satellite, and Cloud Photogrammetry Analysis

Abstract An analysis of two sea-breeze events on 6 August (an onshore flow event) and 12 August (an offshore flow event) 1991 is presented using single-Doppler observations, satellite images, and cloud pictures collected during the Convection and Precipitation/Electrification (CaPE) Experiment. Documentation of the alongfrontal variability at the leading edge of the sea-breeze circulation is presented for the first time. The horizontal structure of the front was strongly modulated by the near-perpendicular intersections of horizontal convective rolls developing in the ambient air out ahead of the sea breeze on 12 August. These intersection points also appeared to be preferred locations for cloud development along the front. Horizontal convective rolls were also documented on 6 August; however, their orientation was nearly parallel to the sea-breeze front. As a result, extended sections of these rolls appeared to have merged with the front as it propagated inland rather than having distinct intersection po...

Five Scales of Airflow Associated with a Series of Downbursts on 16 July 1980

Abstract A series of destructive windstorms on 16 July 1980 in a 50 km (30 mi) wide zone from Chicago to Detroit was surveyed both from the air and the ground. In spite of the initial suspicion of 10–20 tornadoes in the area, the nature of the windstorms was confirmed to be downbursts and microbursts characterized by multiple scales of airflows with their horizontal dimensions extending tens of meters to hundreds of kilometers. An attempt was made to estimate the wind speed based on three types of airborne objects: a 180 kg (390 lb) chimney, a 1000 kg (one ton) corn storage bin, and lumber from damaged roofs found inside downburst areas, obtaining the maximum wind speed of 63 ± 10 m -1(140 ± 25 mph). A total of

Damage Survey of Hurricane Andrew and Its Relationship to the Eyewall

500 million damage reported was caused by thunderstrom-induced non-tornadic storms which affected very large areas. SMS/GOES pictures showed that the parent cloud was oval-shaped with its lifetime in excess of 12 h. The overshooting areas enclosed by the −66°C isotherms shrunk rapidly at the onset...

A Concentrated Outbreak of Tornadoes, Downbursts and Microbursts, and Implications Regarding Vortex Classification

Abstract A damage map documenting Hurricane Andrews destructive land fall over southern Florida is presented. Vectors that represent the direction of winds causing damage to trees and structures are shown along with an F-scale rating in order to assess the strength of the near-surface winds. It is hypothesized that increased surface roughness once the hurricane made landfall may have contributed to a surface wind enhancement resulting in the strongest winds ever estimated (F3) for a landfall hurricane. This intense damage occurred primarily during the “second” period of strong winds associated with the east side of the eyewall. For the first time, a well-defined circulation in the damage pattern by the second wind was documented. A superposition of radar data from Miami and Key West on top of the damage map provides the first detailed examination of the relationship between the eyewall and the surface flow field as estimated from the damage vectors.