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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

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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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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

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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 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

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 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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Gerald M. Heymsfield and Richard Fulton

Abstract

Passive microwave observations using the Special Sensor Microwave/Imager are presented for severe tornadic storms in the lower midwestern United States on 16 November 1987. These measurements are compared with Geostationary Operational Environmental Satellite infrared (IR) measurements for the same case. The IR observations had a classic “V” cold feature commonly associated with severe Midwest thunderstorms. The minimum microwave brightness temperatures at 86 GHz, which primarily respond to ice scattering by larger ice particles, were located in the convective region and the warm interior region of the anvil top, between the arms of the IR V feature. The interior warm region was the only portion of the entire anvil region that had high 86-GHz polarization difference temperatures. Microphysical implications of these multispectral observations are discussed. The observations suggest that there are large variations of ice microphysical characteristics spatially and vertically in the anvil region. These observations are discussed in the context of previous dynamical and microphysical hypotheses on the IR V feature.

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Richard Fulton and Gerald M. Heymsfield

Abstract

The use of passive remote microwave radiance measurements above cloud tops for rainrate estimation is complicated by the complex nature of cloud microphysics. The knowledge of the microphysical structure of clouds, specifically the hydrometeor types, shapes, sizes, and their vertical distribution, is important because radiative emission and scattering effects are dependent upon the hydrometeor distribution. This paper has two purposes: first, to document the structure and evolution of two strong thunderstorms in Alabama using radar multiparameter data; and second, to relate the inferred microphysics to the resulting upwelling microwave radiance observed concurrently by high altitude aircraft. These measurements were collected during the COHMEX field program in the summer of 1986. The radar analysis includes a description of the parameters reflectivity Z, differential reflectivity ZDR, linear depolarization ratio LDR, and hail signal HS for two thunderstorm cases on 11 July 1986. The simultaneous aircraft data includes passive microwave brightness temperature (TB) measurements at four frequencies ranging from 18 to 183 GHz as well as visible and infrared data.

The remote radar observations reveal the existence of large ice particles within the storms which is likely to have caused the observed low microwave brightness temperatures. By relating the evolution of the radar measureables to the microwave TB's it has been found that knowledge of the storm microphysics and its evolution is important to adequately understand the microwave TB's.

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