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Edward A. Brandes

Abstract

Doppler radar observations concerning the generation of vertical vorticity in tornadic thunderstorms are documented for two case studies and the role of downdrafts in intensifying vertical vorticity is examined. The observations, supporting recent numerical simulations, show that production of vertical vorticity begins at the very roots of updraft as horizontal vorticity in low-level inflow regions is tilted toward the vertical. Then, as the flow passes through the updraft, the tilted vorticity and preexisting vertical vorticity is amplified by convergence to create the tornado parental circulation or mesocyclone.

The low-level mesocyclone intensification that heralds tornadogenesis seems to result from interaction between spreading rainy downdraft air and inflow air from the storm's right flank. Amplification of vertical vorticity by convergence surges in the region of interaction. Rear downdrafts, which develop at approximately the time of tornadogenesis, do not transport significant vorticity; rather, their divergent character reduces vertical vorticity. Rear downdraft formation reverses the horizontal gradient of the vertical wind across thelow-level mesocyclone and increases vorticity generation by twisting within the mesocyclone; but the generation rate is at least a factor of 2 less than the amplification rate by convergence. Thus, tornadoes are most likely to be triggered by the vorticity amplification that follows from outflow-inflow interaction.

During dissipation, updrafts and rainy downdrafts weaken, and rear downdraft air fills the mesocyclone. Vertical vorticity rapidly dissipates toward ground via the convergence mechanism, and the association between the mesocyclone and updrafts ends.

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Edward A. Brandes

Abstract

Dual-Doppler (S-band) radar observations are used to describe both the three-dimensional flow and a fife cycle of a severe thunderstorm on 6 June 1974. The primary updraft region was near a small mesocyclonic circulation located on the storm's right (southern) flank. Rotation was first noted aloft (between 3 and 5 km elevation) and lowered with time. During the mature stage, major downdrafts were found on the storm's western edge and within the left forward (northeastern) quadrant. Outflow from both downdrafts combined near the cloud base to form a vigorous gust front. Hook echo formation is attributed to horizontal acceleration of low-level droplet-laden air as the downdrafts intensified and the outflow interacted with the inward-spiraling updraft.

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Edward A. Brandes

Abstract

A methodology for obtaining pressure perturbations and buoyancy in thunderstorms observed with Doppler radar is described and applied to two tornadic thunderstorms. Extracted thermodynamic information is combined with kinematic analyses to study observed severe storm processes.

The intensification of tornado parental circulations (mesocyclones) during tornadogenesis is found to be associated with deepening pressure deficits in lower storm levels. Upward directed perturbation pressure forces in the vicinity of the mesocyclone are reduced and can be reversed as the low-level vorticity amplifies. The sudden formation of concentrated rear downdrafts, commonly observed in tornadic thunderstorms, apparently stems from the vertical pressure gradient reversal. Reduced upward pressure forces decrease the storm's ability to lift low-level negatively buoyant air at the base of the updraft. Further, the restructured pressure forces create a flux of air parcels into the mesocyclone from higher levels on the storms' rear. In the final stages, downdrafts fill the mesocyclone, and updrafts in neighboring regions weaken.

Qualitative examination of retrieved buoyancy distributions suggests that horizontal vorticity created by buoyancy gradients in inflow regions is not essential for mesocyclone intensification or for tornadogenesis.

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Edward A. Brandes

Abstract

Observations collected during the Oklahoma–Kansas PRE-STORM experiment are used to document the evolution and structure of the mesoscale convective system (MCS) that occurred on 6–7 May 1985. The storm began when a short squall line developed in an area of preexisting thunderstorm activity. Thunderstorm updrafts along the squall line lifted warm, moist air with its southerly momentum to the upper troposphere. A broad region of convective outflow and a mesoβ-scale updraft region with a mean vertical velocity in excess of −15 × 10−3 mb s−1 were created. A stratiform rain area with an embedded mesovortex formed behind the squall line. The vortex resided beneath the deepest upper-level outflow.

The mesovortex altered the wind field and consequently became the principal organizational feature within the MCS. A descending current from the storm's rear that, depending on location, extended from 1 km to the upper troposphere was intensified and focused by the vortex. The descending rear inflow had a peak vertical velocity of 10 × 10−3 mb s−1 and concentrated into a jet that passed to the south of the vortex. The intruding flow caused the precipitation and cloud fields to develop comma-like shapes and determined the distribution of kinematic parameters within the MCS.

Mesovortex vertical vorticity was a maximum (25 × 10−5 s−1) at middle-storm levels where environmental air converged into the mesoscale downdraft but was also strong at lower levels where the mesoscale downdraft dominated. Stretching of preexisting vorticity seems the primary amplification mechanism at middle levels. Tilting of horizontal vorticity generated by baroclinicity in the rear inflow is given as an explanation for the low-level vorticity.

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Edward A. Brandes

Abstract

Estimates of precipitation are improved when raingage observations are used to calibrate quantitative radar data as well as to estimate precipitation in areas without radar data.

Estimated areal precipitation depth errors for nine rainfalls over a 3000 km2 watershed averaged 13 and 14% (1.5 and 1.8 mm) when the radar was calibrated by networks of raingages having densities of one gage per 900 and 1600 km2. Areal precipitation estimates derived from rainfalls observed at the gages alone produced errors of 21 and 24% (2.5 and 3.0 mm). Adjusting the radar data by a single calibration factor (the simple average ratio of gage-observed and radar-inferred rainfall at all input gages without regard to the spatial variation among ratios) resulted in error reduction to 18% (2.1 mm). Radar data added to gage observations also increased the explained variance in point rainfall estimates above that from gages alone, from 53 to 77% and 46 to 72% for the above gage densities.

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Edward A. Brandes

Abstract

Interrelationships among severe thunderstorm wind flow, tornado parent circulation and tornado are examined with dense single- and dual-Doppler radar observations from the Del City-Edmond, Oklahoma, storm of 20 May 1977. Beginning with tornadogenesis, rising inflow air from right-hand quadrants and subsiding air from a developing downdraft on the storm's rear are wrapped in a large spiral pattern about the tornado. The updraft appears to drive the tornado while the downdraft appears to form as entrained environmental air, concentrated by the intensifying parent circulation, is cooled evaporatively.

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Edward A. Brandes

Abstract

Doppler radar observations are utilized to describe the evolution of the severe thunderstorm updraft mesocyclone and its associated gust front. Tornadoes form within the elliptical mesocyclonic circulation, apparently along the major axis, and may denote a critical development stage.

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Edward A. Brandes

Abstract

Updraft mesocyclones in tornado-producing thunderstorms form along convergent and cyclonically sheared boundaries that separate inflow air entering forward and rear storm quadrants. Maximum tangential flow resides in girding wind bands, averaging usually less than 3 km in radius (depending upon the development stage), that strengthen near ground during toradogenesis. Strong inflow, concentrated at or below cloud base, is maintained throughout the intensification period. During the tornadic phase the parent mesocirculation exhibits an apparent “breakdown", i.e., a transition from single-cell to two-cell axial flow structure. At this critical period small eddies may appear within an elongated mesocyclone core and some may become tornado and/or grow to large proportions.

Tornado dissipation may result from cyclonic rotation of the major mesocyclone horizontal axis which chokes the supply of inflow air and detaches the tornado from the principal updraft. Another apparent mechanism for dissipation involves an advanced breakdown stage where downdrafts fill the parent circulation.

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Edward A. Brandes and Jerome Spar

Abstract

An investigation into the synoptic conditions necessary for the occurrence of heavy snow on the east coastal plain of the United States has revealed no obvious characteristic antecedent patterns either 12 or 24 hr prior to the onset of snow. The contemporaneous association of heavy snow with east coastal secondary cyclogenesis and with certain synoptic features at the 850-mb level is found to be of limited predictive value.

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Edward A. Brandes and Kyoko Ikeda

Abstract

A simple empirical procedure for determining freezing levels with polarimetric radar measurements is described. The algorithm takes advantage of the strong melting-layer signatures and the redundancy provided by the suite of polarimetric radar measurements—in particular, radar reflectivity, linear depolarization ratio, and cross-correlation coefficient. Freezing-level designations can be made with all volumetric scanning strategies. Application to uniform (stratiform) precipitation within 60 km of the radar and with brightband reflectivity maxima of greater than 25 dBZ suggests an accuracy of 100–200 m.

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