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M. I. Biggerstaff and R. A. Houze Jr.

Abstract

High-frequency (90 min) rawinsonde data from a special mesoscale network (26 sites) have been combined with wind profiler, dense automated surface network data (80 stations spaced 50 km apart), and a series of high-resolution dual-Doppler radar analyses in a common framework attached to a moving squall-line system to form a comprehensive dataset describing the mature phase of the 10–11 June 1985 squall line observed during PRE-STORM. The dual-Doppler radar analyses covered a 200 × 300 km2 area, from the leading edge of the convective line to the back edge of the trailing stratiform precipitation region, thus, providing high-resolution wind information over a very broad portion of the storm system.

The comprehensive analysis is used to resolve several aspects of the trailing stratiform region that had remained unclear from previous studies. First, a difference in the horizontal scale was found between the mesoscale updraft, which at upper levels was on the scale of the trailing stratiform cloud, and the strong mesoscale downdraft, which at mid-to-lower levels was on the scale of the trailing stratiform precipitation. Second, the region of heaviest stratiform precipitation (the secondary band) was found to be immediately downwind of the most intense portions of the convective line, and the width of the trailing stratiform precipitation region was controlled by a combination of the wind velocity and microphysical fall-speed scales. Third, the radar reflectivity minimum observed at mid-to-lower levels in the region just behind the convective line was found to coincide with deep subsidence from mid-to-upper levels, which may have reduced the mass of the hydrometeors through sublimation and evaporation. However, precipitation trajectories computed from the comprehensive analysis indicate another contributing factor; namely, the source region of hydrometeors at low levels just behind the convective line was at a lower altitude than the source region of low-level hydrometeors in the heavy stratiform precipitation farther behind the convective line. Thus, even if all other factors had been the same, the hydrometeors in the heavy stratiform rain would have had more time to grow than those found in the region of the radar reflectivity minimum just behind the convective line. Moreover, hydrometeor detrainment may have been greater near cloud top than at lower levels.

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M. I. Biggerstaff and R. A. Houze, Jr.

Abstract

The comprehensive analysis of the kinematic structure of the mature phase of the 10–11 June 1985 squall-line system was used to examine the midlevel vertical-vorticity structure of the storm to show that relative vertical vorticity in the stratiform region at midlevels was organized into bands (both cyclonic and anticyclonic) oriented parallel to the convective line, with anticyclonic vorticity between the rear of the convective line and the heaviest stratiform precipitation and cyclonic vorticity farther back. Since previous studies have not found anticyclonic vorticity over such a large portion of the stratiform region at midlevels and since the concentration of anticyclonic vorticity may have been detrimental to the longevity of the storm by limiting the development of an inertially stable cyclonic circulation, the tilting and stretching terms of the vertical-vorticity equation were computed to determine how the observed vertical-vorticity pattern was maintained. Tilting of horizontal vorticity into vertical vorticity by gradients of vertical motion was a factor of 2–10 greater than the stretching of vertical vorticity. Below the melting level and at the rear of the stratiform region, tilting was associated with gradients of mesoscale vertical motion. Above the melting level, near the convective line, tilting was associated with gradients of mean convective motions.

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Ryan M. May, Michael I. Biggerstaff, and Ming Xue

Abstract

A Doppler radar emulator was developed to simulate the expected mean returns from scanning radar, including pulse-to-pulse variability associated with changes in viewing angle and atmospheric structure. Based on the user’s configuration, the emulator samples the numerical simulation output to produce simulated returned power, equivalent radar reflectivity, Doppler velocity, and Doppler spectrum width. The emulator is used to evaluate the impact of azimuthal over- and undersampling, gate spacing, velocity and range aliasing, antenna beamwidth and sidelobes, nonstandard (anomalous) pulse propagation, and wavelength-dependent Rayleigh attenuation on features of interest.

As an example, the emulator is used to evaluate the detection of the circulation associated with a tornado simulated within a supercell thunderstorm by the Advanced Regional Prediction System (ARPS). Several metrics for tornado intensity are examined, including peak Doppler velocity and axisymmetric vorticity, to determine the degradation of the tornadic signature as a function of range and azimuthal sampling intervals. For the case of a 2° half-power beamwidth radar, like those deployed in the first integrated project of the Center for Collaborative Adaptive Sensing of the Atmosphere (CASA), the detection of the cyclonic shear associated with this simulated tornado will be difficult beyond the 10-km range, if standard metrics such as azimuthal gate-to-gate shear from a single radar are used for detection.

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Robert A. Houze Jr., S. A. Rutledge, M. I. Biggerstaff, and B. F. Smull

The utility of color displays of Doppler-radar data in revealing real-time kinematic information has been demonstrated in past studies, especially for extratropical cyclones and severe thunderstorms. Such displays can also indicate aspects of the circulation within a certain type of mesoscale convective system—the squall line with trailing “stratiform” rain. Displays from a single Doppler radar collected in two squall-line storms observed during the Oklahoma-Kansas PRE-STORM project conducted in May and June 1985 reveal mesoscale-flow patterns in the stratiform rain region of the squall line, such as front-to-rear storm-relative flow at upper levels, a subsiding storm-relative rear inflow at middle and low levels, and low-level divergent flow associated with strong mesoscale subsidence. “Dual-Doppler” analysis further illustrates these mesoscale-flow features and, in addition, shows the structure of the convective region within the squall line and a mesoscale vortex in the “stratiform” region trailing the line. A refined conceptual model of this type of mesoscale convective system is presented based on previous studies and observations reported here.

Recognition of “single-Doppler-radar” patterns of the type described in this paper, together with awareness of the conceptual model, should aid in the identification and interpretation of this type of mesoscale system at future NEXRAD sites. The dual-Doppler results presented here further indicate the utility of multiple-Doppler observations of mesoscale convective systems in the STORM program.

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Kristin M. Calhoun, Donald R. MacGorman, Conrad L. Ziegler, and Michael I. Biggerstaff

Abstract

A high-precipitation tornadic supercell storm was observed on 29–30 May 2004 during the Thunderstorm Electrification and Lightning Experiment. Observational systems included the Oklahoma Lightning Mapping Array, mobile balloon-borne soundings, and two mobile C-band radars. The spatial distribution and evolution of lightning are related to storm kinematics and microphysics, specifically through regions of microphysical charging and the location and geometry of those charge regions. Lightning flashes near the core of this storm were extraordinarily frequent, but tended to be of shorter duration and smaller horizontal extent than typical flashes elsewhere. This is hypothesized to be due to the charge being in many small pockets, with opposite polarities of charge close together in adjoining pockets. Thus, each polarity of lightning leader could propagate only a relatively short distance before reaching regions of unfavorable electric potential. In the anvil, however, lightning extended tens of kilometers from the reflectivity cores in roughly horizontal layers, consistent with the charge spreading through the anvil in broad sheets. The strong, consistent updraft of this high-precipitation supercell storm combined with the large hydrometeor concentrations to produce the extremely high flash rates observed during the analysis period. The strength and size of the updraft also contributed to unique lightning characteristics such as the transient hole of reduced lightning density and discharges in the overshooting top.

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J. Sun, S. Braun, M. I. Biggerstaff, R. G. Fovell, and R. A. Houze Jr.

Abstract

Thermodynamic retrieval analysis applied to a composite of dual-Doppler radar data obtained in the 10–11 June 1985 PRE-STORM (Preliminary Regional Experiment for STORM-Central) squall line and a model simulation of a similar squall line show that the upper-level downdrafts located ahead of and behind the main convective updraft zone were generally positively buoyant. As a result, the upper-level downdrafts contributed negatively to the system heat flux.

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Michael I. Biggerstaff, Eun-Kyoung Seo, Svetla M. Hristova-Veleva, and Kwang-Yul Kim

Abstract

The impact of model microphysics on the relationships among hydrometeor profiles, latent heating, and derived satellite microwave brightness temperatures TB have been examined using a nonhydrostatic, adaptive-grid cloud model to simulate a mesoscale convective system over water. Two microphysical schemes (each employing three-ice bulk parameterizations) were tested for two different assumptions in the number of ice crystals assumed to be activated at 0°C to produce simulations with differing amounts of supercooled cloud water. The model output was examined using empirical orthogonal function (EOF) analysis, which provided a quantitative framework in which to compare the simulations. Differences in the structure of the vertical anomaly patterns were related to physical processes and attributed to different approaches in cloud microphysical parameterizations in the two schemes. Correlations between the first EOF coefficients of cloud properties and TB at frequencies associated with the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) showed additional differences between the two parameterization schemes that affected the relationship between hydrometeors and TB. Classified in terms of TB, the microphysical schemes produced significantly different mean vertical profiles of cloud water, cloud ice, snow, vertical velocity, and latent heating. The impact of supercooled cloud water on the 85-GHz TB led to a 15% variation in mean convective rain mass at the surface. The variability in mean profiles produced by the four simulations indicates that the retrievals of cloud properties, especially latent heating, based on TMI frequencies are dependent on the particular microphysical parameterizations used to construct the retrieval database.

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Corey K. Potvin, Alan Shapiro, Michael I. Biggerstaff, and Joshua M. Wurman

Abstract

The vortex detection and characterization (VDAC) technique is designed to identify tornadoes, mesocyclones, and other convective vortices in multiple-Doppler radar data and retrieve their size, strength, and translational velocity. The technique consists of fitting radial wind data from two or more radars to a simple analytical model of a vortex and its near environment. The model combines a uniform flow, linear shear flow, linear divergence flow (all of which comprise a broad-scale flow), and modified combined Rankine vortex. The vortex and its environmental flow are allowed to translate. A cost function accounting for the discrepancy between the model and observed radial winds is evaluated over space and time so that observations can be used at the actual times and locations they were acquired. The model parameters are determined by minimizing this cost function.

Tests of the technique using analytically generated, numerically simulated, and one observed tornadic wind field were presented by Potvin et al. in an earlier study. In the present study, an improved version of the technique is applied to additional real radar observations of tornadoes and other substorm-scale vortices. The technique exhibits skill in detecting such vortices and characterizing their size and strength. Single-Doppler experiments suggest that the technique may reliably detect and characterize larger (>1-km diameter) vortices even in the absence of overlapping radar coverage.

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Clark D. Payne, Terry J. Schuur, Donald R. MacGorman, Michael I. Biggerstaff, Kristin M. Kuhlman, and W. David Rust

Abstract

On 30 May 2004, a supercell storm was sampled by a suite of instrumentation that had been deployed as part of the Thunderstorm Electrification and Lightning Experiment (TELEX). The instrumentation included the Oklahoma Lightning Mapping Array (OK-LMA), the National Severe Storms Laboratory S-band Weather Surveillance Radar-1988 Doppler (WSR-88D) polarimetric radar at Norman, Oklahoma, and two mobile C-band, Shared Mobile Atmospheric Research and Teaching Radars (SMART-R). Combined, datasets collected by these instruments provided a unique opportunity to investigate the possible relationships among the supercell’s kinematic, microphysical, and electrical characteristics. This study focuses on the evolution of a ring of lightning activity that formed near the main updraft at approximately 0012 UTC, matured near 0039 UTC, and collapsed near 0050 UTC. During this time period, an F2-intensity tornado occurred near the lightning-ring region. Lightning density contours computed over 1-km layers are overlaid on polarimetric and dual-Doppler data to assess the low- and midlevel kinematic and microphysical characteristics within the lightning-ring region. Results indicate that the lightning ring begins in the middle and upper levels of the precipitation-cascade region, which is characterized by inferred graupel. The second time period shows that the lightning source densities take on a horizontal u-shaped pattern that is collocated with midlevel differential reflectivity and correlation coefficient rings and with the strong cyclonic vertical vorticity noted in the dual-Doppler data. The final time period shows dissipation of the u-shaped pattern and the polarimetric signatures as well as an increase in the lightning activity at the lower levels associated with the development of the rear-flank downdraft (RFD) and the envelopment of the vertical vorticity maximum by the RFD.

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Michael I. Biggerstaff, Louis J. Wicker, Jerry Guynes, Conrad Ziegler, Jerry M. Straka, Erik N. Rasmussen, Arthur Doggett IV, Larry D. Carey, John L. Schroeder, and Chris Weiss

A group of scientists from three universities across two different states and from one federal research laboratory joined together to build and deploy two mobile C-band Doppler weather radars to enhance research and promote meteorological education. This 5-yr project led to the development of the Shared Mobile Atmospheric Research and Teaching (SMART) radar coalition that built the first mobile C-band Doppler weather radar in the United States and also successfully deployed the first mobile C-band dual-Doppler network in a landfalling hurricane. This accomplishment marked the beginning of an era in which high temporal and spatial resolution precipitation and dual-Doppler wind data over mesoscale (~100 km) regions can be acquired from mobile ground-based platforms during extreme heavy rain and high-wind events.

In this paper, we discuss the rationale for building the mobile observing systems, highlight some of the challenges that were encountered in creating a unique multiagency coalition, provide examples of how the SMART radars have contributed to research and education, and discuss future plans for continued development and management of the radar facility, including how others may use the radars for their own research and teaching programs.

The capability of the SMART radars to measure winds in nonprecipitating environments, to capture rapidly evolving, short-lived, small-scale tornadic circulations, and to sample mesoscale regions with high spatial resolution over broad regions of heavy rainfall is demonstrated. Repeated successful intercepts provide evidence that these radars are capable of being used to study a wide range of atmospheric phenomena.

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