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Richard E. Carbone

Abstract

A narrow cold frontal band of intense precipitation is examined by means of triple Doppler radar and supporting observations. As the band passed through the Central Valley of California, it was accompanied by strong gusty winds, electrical activity, tornadoes and pressure jumps.

Part I delineates the stormwide kinematic and thermodynamic structure. A highly two-dimensional pre-frontal updraft of 15–20 m s−1 results primarily from intense planetary boundary layer forcing of a low-level jet by the gravity-current propagation mechanism. Maximum updraft speed occurs at 2.1 km and the maximum radar echo depth is 6.6 km. Diabatic cooling, due to melting hydrometers, is proposed as a likely mechanism for control of gravity current depth and maintenance of density contrast together with synoptic-scale cold air advection.

Available convective potential energy is shown to be small and kinetic energy of the environmental vertical wind shear is proposed as a likely source of energy on the updraft scale. Tornadoes develop at Helmholtz type inflectional instabilities along the surface front. Frontal zone horizontal shear, convergence and relative vorticity average 10−2 s−1. The results represent a particularly intense case in a class of storms previously studied by several other investigators.

Part II will present the stormwide vorticity structure together with a detailed kinematic analysis of a small-scale vortex which spawns a tornado. Vorticity production terms will be discussed in view of the larger scale context.

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Richard E. Carbone

Abstract

By means of multiple Doppler radar analysis, Part I established the stormwide hydrodynamic structure as an intense gravity current which advances on a prefrontal low-level jet. Part II examines the initiation and evolution of a tornado parent vortex circulation in the context of stormwide vorticity production. Tilting of the prefrontal inflow below the low-level jet axis accounts for production of observed vorticity (10−2 s−1) at the surface front. The two-dimensional flow pattern is modified by the development of an inflectional instability with a wavelength of ∼13 km. Breaking (or roll-up) of this instability occurs on a time scale of roughly 500 s and results in small-scale vortices along the surface front. One such vortex is examined in detail. This vortex is the parent circulation of a small tornado.

The parent vortex structure is determined to be quite similar to those observed in summertime deep convection. This suggests a generality to the findings concerning parent vortex initiation and tornadogenesis. Net vorticity production within the parent vortex is small but various terms force substantial redistribution and concentration of vorticity subsequent to inflow tilting. Stretching at the gust front inflection and along trailing portions of the gust front is shown to be constructive at mid-levels. Stretching at the apex of the downdraft is also an important constructive feature aloft. Vertical transport of vorticity in the surrounding updrafts as well as in the intervening downdraft provides positive forcing which heretofore has not been reported.

Downdraft initiation within the parent vortex is examined with respect to dynamical and microphysical forcing mechanisms. Spreading of downdraft air at the surface is determined to be temporally coincident with the tornadic phase.

Interpretation of tornadogenesis in supercells is made in the perspective of these observations and the findings associated with fine-scale numerical simulations. A concluding hypothesis is that the major features of tornadogenesis are not particularly sensitive to many aspects of storm-scale circulations but rather they require creation of specific localized conditions along the storm outflow boundary. One such condition is the existence of a low-level inflow “jet” which is a direct consequence of powerful storm-scale updrafts in buoyantly-driven storms.

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Richard E. Carbone

Abstract

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David Atlas, Keikichi Naito, and Richard E. Carbone

Abstract

Comprehensive calculations are carried out to predict the signal levels to be received by powerful 5- and 10-cm wavelength bistatic scatter systems from vertically thin but horizontally extensive layers of turbulently perturbed refractivity in the clear atmosphere. The results indicate that the model systems can detect regions having only modest refractivity perturbations throughout the entire depth of the troposphere. Although the horizontal coverage and the maximum signal levels decrease with increasing altitude of the scatter layer, the signals are sufficiently strong in the vicinity of the mid-path point to assure the reliable detection of extremely weak scattering layers. In particular, the tropopause should be detected routinely. Hazardous clear air turbulence should also be readily detected, at least in the vicinity of the mid-path. Vertical polarization is superior to horizontal polarization for large horizontal coverage. The shorter wavelength is also slightly superior to the longer, other things being equal. Comparison of the signals received by the bistatic system to the echo intensities received by a radar of equal characteristics shows the distinct advantage of the former. The results are also pertinent to trans-horizon scatter propagation.

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John D. Tuttle and Richard E. Carbone

Abstract

In a recent radar-based climatological study of warm-season precipitation over the continental United States, Carbone et al. found a high frequency of long-lived coherent rainfall episodes. Many of the events were of longer duration than normally associated with mesoscale convective complexes and exhibited phase speeds ∼10 m s−1 in excess of the phase speed associated with synoptic systems. The observations led to the speculation that cold pool dynamics and wavelike propagation mechanisms were responsible for the longevity of the systems. One of the long-lived episodes included in the statistics of the Carbone et al. study is described here. Occurring on 14–15 July 1998, the system lasted ∼50 h and traveled over 2800 km. At its peak intensity the system was a bow echo producing damaging wind, large hail, and local flash flooding. An interesting aspect of the event was an abrupt 90° turn in the storm's orientation and propagation vector midway through its life. The environmental factors that led to the observed behavior are investigated.

The episode consisted of two mesoscale convective systems (MCSs) and owed its longevity to a coherent regeneration process, with favorable cold pool–wind shear interactions playing a dominant role. Synoptic-scale forcing indirectly played a role by moistening the environment and creating a favorable wind shear region. An important observation is an E–W spatial displacement (∼200 km) between the N–S corridor of maximum low-level moisture/CAPE and the maximum low-level wind shear/system relative inflow, and the fact that the storm followed the most favorable wind shear corridor. High moisture/instability alone were not enough to ensure the longevity of this system.

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Cynthia K. Mueller and Richard E. Carbone

Abstract

The kinematic and thermodynamic structures of a thunderstorm outflow are examined by means of dual Doppler radar analysis, mesonet, lower, and sounding data. The data were collected in the Denver, Colorado area during June 1984.

The dual-Doppler analysis shows that the cold outflow is ducted beneath the PBL inversion. Along the gust front there is a narrow quasi-two-dimensional updraft. Kelvin-Helmholtz instability (KHI) developed along the top of the gust front head near the surface front, and propagated backwards, dissipating in the wake of the head region. An isothermal layer aloft appears to have limited billow amplification to the quasi-neutral layer below.

The gust front's leading edge had numerous inflections which are believed to result from barotrophic instabilities. Small vortices develop at some of the inflection points. Detailed analysis of one such circulation shows evidence of the formation of two enhanced updrafts separated by an occlusion downdraft. These observations are the first to confirm the existence of such vertical velocity features within a circulation that is absent from a storm and the associated precipitation loading and evaporation processes. It lends credence to the notion that small tornadoes can be produced purely by mechanical shearing forces and induced nonhydrostatic vertical pressure gradients.

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Christopher W. Fairall and Richard E. Carbone
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Peter H. Hildebrand, Richard A. Oye, and Richard E. Carbone

Abstract

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John D. Tuttle, Richard E. Carbone, and J. J. Wang

Abstract

This note documents the effects of a radar calibration error that affected 5% of the data used in an earlier study by Carbone et al. While the original intent of this correction was merely to set the record straight, a significant finding emerged from the revised time series.

The main impact is to reveal a more rapid spatial decorrelation between island Froude number (Fr) and oceanic rainfall, a measure of the dynamical blocking effects upstream of Hawaii. There no longer exists a residual correlation between Fr and cumulative rainfall over the remote ocean, defined in this instance as distances >50 km from shore. This result both simplifies and strengthens the conclusions of the original work, which was based on a very short time series. The strong dependence of rainfall on Fr over the coastal lowlands and near shore remains unchanged from the results presented by Carbone et al. To the extent that remote oceanic rainfall is expected to be uncorrelated with Fr, there is increased confidence with respect to the causes and effects of island blocking on rainfall production over and near the island.

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Bruce W. Johnston, John D. Marwitz, and Richard E. Carbone

Abstract

A method for the analysis of single-Doppler radar data called band-velocity-processing (BVP) is presented. The BVP method was designed to exploit the two-dimensional nature of banded precipitation systems with crossband length scales of 5–50 km. The BVP method offers improved horizontal (crossband) resolution over existing volume scan methods (VAD/VVP), plus the ability to extract band-perpendicular vertical cross sections from a single volume scan. The BVP analysis yields a more complete kinematic structure than RHI analyses, providing estimates for both horizontal wind components, both horizontal divergence components, the shearing deformation, and the vertical shear vector. The bias and variance errors for the BVP method are shown to be acceptably small.

The application of the BVP method to a narrow cold frontal rainband reveals several important kinematic features. The mesoscale structure of the low-level jet ahead of the narrow cold frontal rainband is examined. The band relative flow field exhibits a closed helical circulation cell at the front of the band.

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