Conrad L. Ziegler

Simulated three‐dimensional branched lightning in a numerical thunderstorm model

Received 8 December 2000; revised 27 August 2001; accepted 7 September 2001; published 2 May 2002 [1] Lightning discharges are simulated by using a stochastic dielectric breakdown model within a numerical thunderstorm model with extensive parameterizations of electrification mechanisms. The lightning model simulates the macroscopic bidirectional extension of discharges as a step-by-step stochastic process. Discharge channels are propagated on a uniform grid, and the direction of propagation (including diagonals) for a particular step is chosen randomly, with the probability for choosing a particular direction depending on the net electric field. After each propagation step the electric fields are recomputed via Poisson’s equation to account for the effect of the conducting channel. The lightning parameterization produces realistic looking, three-dimensional, branched lightning discharges. A variety of lightning types have been produced, including intracloud discharges, negative cloud-to-ground (CG) lightning, and positive CG lightning. The model simulations support the hypothesis that negative CG flashes occur only when a region of positive charge exists below the main negative charge region. Similarly, simulated positive CG flashes were found to occur only in regions of storms where the two significant charge layers closest to ground had roughly a ‘‘normal dipole’’ structure (i.e., positive charge above negative). INDEX TERMS: 3300 Meteorology and Atmospheric Dynamics; 3304 Meteorology and Atmospheric Dynamics: Atmospheric electricity; 3324 Meteorology and Atmospheric Dynamics: Lightning; 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; KEYWORDS: lightning, thunderstorm electrification, numerical thunderstorm model

Single- and Multiple-Doppler Radar Observations of Tornadic Storms

Abstract The use of one, two, three or more Doppler radars has become increasingly common in research programs. The advantage in increasing the number of radars is in the increased area covered and the accuracy with which wind estimates may be obtained. Although multiple-radar systems can yield special quantitative insight, a great deal of information can still be determined in real time from a single radar. It should be noted that the interpretation of radial velocity estimates from a single radar are not always unambiguous. Color displays of single-Doppler radial velocity patterns aid in the real-time interpretation of the associated reflectivity fields and can reveal important features not evident in the reflectivity structures alone. Such a capability is of particular interest in the identification and study of severe storms. A display utilizing a 5 cm Doppler radar is used to illustrate the patterns seen from several tornadic storms that occurred in central Oklahoma on 20 May 1977. Interpretation of ...

Simulated Electrification of a Small Thunderstorm with Two-Moment Bulk Microphysics

Abstract Electrification and lightning are simulated for a small continental multicell storm. The results are consistent with observations and thus provide additional understanding of the charging processes and evolution of this storm. The first six observed lightning flashes were all negative cloud-to-ground (CG) flashes, after which intracloud (IC) flashes also occurred between middle and upper levels of the storm. The model simulation reproduces the basic evolution of lightning from low and middle levels to upper levels. The observed lightning indicated an initial charge structure of at least an inverted dipole (negative charge above positive). The simulations show that noninductive charge separation higher in the storm can enhance the main negative charge sufficiently to produce negative CG flashes before upper-level IC flashes commence. The result is a “bottom-heavy” tripole charge structure with midlevel negative charge and a lower positive charge region that is more significant than the upper posit...

TELEX The Thunderstorm Electrification and Lightning Experiment

Measurements during TELEX by a lightning mapping array, polarimetric and mobile Doppler radars, and balloon-borne electric-field meters and radiosondes show how lightning and other electrical properties depend on storm structure, updrafts, and precipitation formation.

Retrieval of Thermal and Microphysical Variables in Observed Convective Storms. Part 1: Model Development and Preliminary Testing

Abstract The air flow in convective storms, the force that regulate the flow, and the processes that produce hydrometeors of various kinds are all being studied intensively by meteorologists using Doppler radar observations. The research reported here proceeds from the observed motion through accompanying thermodynamic and micro-physical processes to the analysis of hydrometer content and thermal fields in thunderstorms. A three-dimensional numerical kinematic cloud model employing Doppler wind fields is developed and used to diagnose temperature and mixing ratios within a thunderstorm. The microphysical parameterization includes stochastic coalescence effects in warm clouds as well as well- and variable-density dry hail growth. Known fields from a dynamically simulated cloud are used to establish the accuracy of the retrieval scheme. Real data tests indicate good agreement between retrieved and observed radar reflectivities, qualitative dynamic consistency between observed winds and retrieved buoyancies,...

The Bow Echo and MCV Experiment: Observations and Opportunities

The Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) is a research investigation using highly mobile platforms to examine the life cycles of mesoscale convective systems. It represents a combination of two related investigations to study (a) bow echoes, principally those that produce damaging surface winds and last at least 4 h, and (b) larger convective systems that produce long-lived mesoscale convective vortices (MCVs). The field phase of BAMEX utilized three instrumented research aircraft and an array of mobile ground-based instruments. Two long-range turboprop aircraft were equipped with pseudo-dual-Doppler radar capability, the third aircraft was a jet equipped with dropsondes. The aircraft documented the environmental structure of mesoscale convective systems (MCSs), observed the kinematic and thermodynamic structure of the convective line and stratiform regions (where rear-inflow jets and MCVs reside), and captured the structure of mature MCVs. The ground-based instruments augmented sou...

The Initiation of Moist Convection at the Dryline: Forecasting Issues from aCase Study Perspective

Abstract The processes that force the initiation of deep convection along the dryline are inferred from special mesoscale observations obtained during the 1991 Central Oklahoma Profiler Studies project, the Verification of the Origins of Rotation in Tornadoes Experiment 1994 (VORTEX-94), and the VORTEX-95 field projects. Observations from aircraft, mobile CLASS soundings, and mobile mesonets define the fields of airflow, absolute humidity, and virtual temperature in the boundary layer across the dryline on the 15 May 1991, 7 June 1994, and 6 May 1995 case days. Film and video cloud images obtained by time-lapse cameras on the NOAA P-3 are used to reconstruct the mesoscale distribution of cumulus clouds by photogrammetric methods, permitting inferences concerning the environmental conditions accompanying cloud formation or suppression. The results of the present study confirm the classical notion that the dryline is a favored zone for cumulus cloud formation. The combined cloud distributions for the three ...

The Deep Convective Clouds and Chemistry (DC3) Field Campaign

AbstractThe Deep Convective Clouds and Chemistry (DC3) field experiment produced an exceptional dataset on thunderstorms, including their dynamical, physical, and electrical structures and their impact on the chemical composition of the troposphere. The field experiment gathered detailed information on the chemical composition of the inflow and outflow regions of midlatitude thunderstorms in northeast Colorado, west Texas to central Oklahoma, and northern Alabama. A unique aspect of the DC3 strategy was to locate and sample the convective outflow a day after active convection in order to measure the chemical transformations within the upper-tropospheric convective plume. These data are being analyzed to investigate transport and dynamics of the storms, scavenging of soluble trace gases and aerosols, production of nitrogen oxides by lightning, relationships between lightning flash rates and storm parameters, chemistry in the upper troposphere that is affected by the convection, and related source character...

Convective Initiation at the Dryline: A Modeling Study

Abstract A nonhydrostatic, three-dimensional version of the Colorado State University Regional Atmospheric Modeling System (CSU-RAMS) is used to deduce the processes responsible for the formation of drylines and the subsequent initiation of deep, moist dryline convection. A range of cumuliform cloud types are explicitly simulated along drylines on 15, 16, and 26 May 1991 in accordance with observations. In the simulations, narrow convergence bands along the dryline provide the lift to initiate deep moist convection. The thermally direct secondary convective boundary layer (CBL) circulations along the dryline are frontogenetic and solenoidally forced. Maximum updrafts reach 5 m s−1 and the bands are 3–9 km wide and 10–100 km or more in length. The updrafts penetrate and are decelerated by the overlying stable air above the CBL, reaching depths of about 2000 m in the cases studied. Moisture convergence along the mesoscale updraft bands destabilizes the local sounding to deep convection, while simultaneously...

THE SHARED MOBILE ATMOSPHERIC RESEARCH AND TEACHING RADAR A Collaboration to Enhance Research and Teaching

Abstract 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 multia-gency coalition, provid...