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Gerald M. Heymsfield

Abstract

This paper discusses multiple-Doppler radar observations of a non-severe Illinois thunderstorm occurring on 29 May 1978. The vertical wind shear was fairly rectilinear on this day with the storm motion being related to the wind at a height of 2.5 km. The cell examined had a radar top of 10 km, and high reflectivities [60 dB(Z)]. Emphasis is placed on the evolution of the updraft, downdrafts, and rotation in this cell. Typical magnitudes of the updraft, downdrafts and vorticity associated with the cell were 12 and 8 m s−1, and 5 × 10−3 s−1, respectively. Four downdrafts were identified in the cell: a downdraft upshear of the updraft, downdrafts on the left and right flanks, and a downshear downdraft. The main downdrafts were the upshear downdraft during the growth period of the cell, and the left flank downdraft during the mature and dissipation periods of the cell. The location of upshear downdraft at mid to upper levels resembles Lemon and Doswell's conceptual model (1979) and three-dimensional cloud model results of severe storms. A vorticity couplet in mid-levels intensified as the downdrafts developed along the flanks of the cell, primarily through the tilting term in the vorticity equation. This vorticity couplet advected downwind of the updraft. The structure of this nonsevere cell is compared with that of severe storms.

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Gerald M. Heymsfield

Abstract

The paper deals with a diagnostic study of the three-dimensional kinematic and dynamic structure of the Harrah tornadic storm. Wind fields were computed from data collected over a 28 min interval in four dual-Doppler radar volume scans during a tornado occurrence. Associated with this storm, there was a strong low-level inflow jet supplying warm, moist air to a tilted updraft, a mid-level vortex doublet, and downdrafts on both storm flanks. The left flank downdraft intensified, undercut the tilted updraft, and formed a gust front along the right rear flank. The gust front propagated cyclonically around the mesocyclone. Calculations of divergence and vorticity showed that in the middle troposphere, the updraft nearly coincided with cyclonic vorticity approximately 10−2 s−1. The low-level tornado cyclone was between the horizontally sheared inflow-outflow region. A mechanism for producing and intensifying this vorticity and downdraft structure is presented on the basis of calculations of the tilting and divergence terms of the vorticity equation. The gross features of the mid-level vortex doublet were simulated by a potential flow model. The storm translational motion is discussed in terms of this model and a balance of drag, momentum and rotational forces. The force due to vertical transport of low-level momentum in the updraft is important in counteracting the large rightward force due to rotation.

In an appendix, sources of errors in the wind computations are discussed in terms of the assumptions of the statistical interpolation and vertical motion calculation. The scales of motion resolved in the analysis are larger than approximately 4 km due to the interpolation and grid-filtering used.

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Gerald M. Heymsfield

Abstract

The horizontal wind and reflectivity fields in a tornadic storm are investigated with dual-Doppler radar. Emphasis is placed on a statistical objective analysis technique of the Eddy-Gandin type for determining these fields at grid-points. A space-time correction is first made to the observations. Weighting of data to grid-points with a linear regression model required computation of a spatial autocorrelation function for different storm regions. This function was found to fall off faster vertically than horizontally, and is quite dependent on storm structure. The analyzed fields were reasonably smooth. The low-level storm structure revealed an intense tornado cyclone with a corresponding weak echo region, barrier flow of environmental winds, and several other interesting features.

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Gerald M. Heymsfield

Abstract

Vertical incidence Doppler radar observations of an evaporating ice crystal cloud revealed many multimodal Doppler spectra, with modes often separated by more than 1 m s−1, and unusually large variances. The observations are explained in terms of a vertical velocity field consisting of a plane wave motion on sub-beam scales, and a larger scale horizontal shear of the vertical wind. Doppler spectra and variances of the proposed model of the vertical velocity are presented.

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Gerald M. Heymsfield

Abstract

A simple nomogram technique to facilitate dual Doppler radar data collection is presented. Equations and nomograms are developed for relating the position of a given target relative to two radars. The results are general, but application is made to the National Severe Storms Laboratory radars at Norman and the Cimarron site 40 km northwest of Norman.

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Gerald M. Heymsfield

Abstract

This paper presents an analysis of the uncertainties expected in vertical velocities using a vertically pointing airborne Doppler radar which has a nadir or zenith-pointing beam. To examine the expected uncertainty, the Doppler velocity equation for a moving platform is derived and it is applied to cases of nadir-fixed and stabilized beams. The main emphasis of the paper is on the effect of platform stability on the deduced vertical air motions and it is shown that the antenna must be stabilized to obtain desired accuracy in the vertical velocity measurements.

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Gerald M. Heymsfield

Abstract

This paper presents a case study of the structure of a warm frontal region as deduced from Doppler radar observations. The precipitation occurring ahead of the surface warm front was banded. The dominant precipitation bands were oriented transverse to the mid-level winds, and they were spaced ∼110 km apart. It is suggested that these bands were formed by highly organized vertical circulations in a 2.5 km thick layer just above the warm frontal zone. The precipitation bands extended from this layer down to the surface. Near the surface additional circulations were produced by pressure perturbations resulting from cooling associated with melting snow. Some diagnostic calculations of ageostrophic winds, frontogenesis and vorticity production are presented. The frontogenesis calculation gives approximately a 2–4 h doubling time of the horizontal potential temperature gradient associated with the warm front, at mid-levels. The highly organized band-associated circulations suggest the importance of their inclusion in diagnostic calculations.

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Stan L. Ulanski
and
Gerald M. Heymsfield

Abstract

Analysis of data from the high density storm-scale rawinsonde network of SESAME during the storm events on 2 May 1979 showed the existence of persistent and strong regions of tropospheric convergence and divergence which were detectable on the meso-β scale. In particular, mid- and upper-tropospheric divergence was superimposed over low-level convergence. The divergence, which had a maximum value of 4 × 10−4 s−1, occurred well upwind (75–100 km) as well as over the tornadic cells. To the south of the storm cells, the kinematic pattern is reversed with upper-level convergence superimposed over low-level divergence. Calculations indicate a vertical motion doublet with ascent (−40 µb s−1) over the squall line and descent (+40 µb s−1) approximately 70 km south of the squall line.

It is hypothesized that the above flow fields resulted from a combination of 1) blocking of tropospheric environmental flow by the storm cells, 2) anvil outflows, particularly from the tornadic cells; and 3) divergence from the exit region of the jet stream.

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Gerald M. Heymsfield
and
Steven Schotz

Abstract

A squall line on 2 May 1979 developed in Oklahoma in proximity to a synoptic-scale cold front. This line is analyzed during its growth and mature periods using radar, satellite, sounding and surface data. Some of the cells produced hail, and many of the cell tops reached 16 km. However, there were no reports of tornadoes. Three main topics are addressed in the paper: 1) examination of squall line and cell propagation mechanisms; 2) the three-dimensional structure of the squall line and individual cells during the mature period and 3) mass and moisture fluxes and precipitation efficiency. Comparison is made between the 2 May case and other tropical and Midwest squall line cases. The 2 May case does not exhibit a “trailing stratiform” anvil during the period mechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cell motion is eventually governed by the moisture convergence and lifting provided the convergence line.

The motion of the squall line (defined by centroids of cells along the line) follows closely that of a convergence line found to be associated with a synoptic scale cold front. Initially, cells move along the low- to midlevel shear vector, which is directed ∼45° clockwise from the line orientation; then the cells turn to the right (nearly normal to the line). It is postulated that two mechanisms are responsible for this rightward turn of the cells: 1) mechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cell motion is eventually governed by the moisture convergence and lifting provided the convergence line.

Triple Doppler analysis of a cell along the line indicates maximum updrafts of ∼35 m s−1, and strongest downdrafts at middle to upper levels located between cells along the line. The structure of the squall line is somewhat different from that in the case presented by Newton and other documented squall line studies in that there are not well-organized downdrafts on the rear side at low to midlevels. In addition, low-to midlevel inflow on the rear side of the squall line is apparently absent.

Man and moisture fluxes computed from sounding and radar data indicate magnitudes comparable to previous squall line cases. However, the precipitation efficiency of the squall line is estimated to fall in the range 25–40%, which is somewhat lower than other reported values (>50%). The low precipitation efficiency is suggested to be due in part to large moisture losses at upper levels.

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Gerald M. Heymsfield
and
Richard Fulton

Abstract

Observations of an isolated group of Oklahoma thunderstorms from NASA's high altitude ER-2 aircraft are presented. These observations include passive radiometric measurements at frequencies in the microwave (92, 183 GHz), infrared (10.7 μm) and visible portion of the spectrum from a perspective above the storm top. Direct measurements of cloud top height were also collected using a pulsed lidar instrument. These remote observations are discussed and compared with coincident radar data from the National Severe Storms Laboratory's two Doppler radars and in situ cloud top particle data from the University of North Dakota's Citation aircraft.

Reflectivity cores are nearly colocated with cold anomalies in the microwave brightness temperature field. Coldest infrared brightness temperatures however, are displaced downshear of the convective region in association with the cirrus anvil. Radar and in situ microphysical comparisons support previous theoretical and numerical modeling results which suggest that microwave frequencies are sensitive to the deeper layer of large ice particles in the storm's convective region. The trailing anvil which is comprised of smaller ice particles is transparent at 92 GHz and nearly transparent at 183 GHz. This observation has relevance to spaceborne passive microwave measurements of rainfall.

Evolution of the thunderstorm complex is also discussed. The trend of the radar volumetric rain rate correlates well with the trends of minimum 92 GHz brightness temperature and area of the cold brightness temperature region at 92 GHz. The correlation at 183 GHz as well as at the infrared wavelength is not nearly as clear.

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