Robert B. Wilhelmson

The Simulation of Three-Dimensional Convective Storm Dynamics

Abstract A new three-dimensional cloud model has been developed for investigating the dynamic character of convective storms. This model solves the compressible equations of motion using a splitting procedure which provides numerical efficiency by treating the sound wave modes separately. For the subgrid turbulence processes, a time-dependent turbulence energy equation is solved which depends on local buoyancy, shear and dissipation. First-order closure is applied to nearly conservative variables with eddy coefficients based on the computed turbulence energy. Open lateral boundaries are incorporated in the model that respond to internal forcing and permit gravity waves to propagate out of the integration domain with little apparent reflection. Microphysical processes are included in the model using a Kessler-type parameterization. Simulations conducted for an unsheared environment reveal that the updraft temperatures follow a moist adiabatic lapse rate and that the convection is dissipated by water loadin...

Simulation and Analysis of Tornado Development and Decay within a Three-Dimensional Supercell Thunderstorm

Abstract A three-dimensional numerical simulation using a two-way interactive nested grid is to study tornado-genesis within a supercell. During a 40-minute period, two tornadoes grow and decay within the storms mesocyclone. The tornadoes have life spans of approximately 10 minutes. Maximum ground-relative surface wind speeds exceed 60 m s−1 during both tornadoes, and horizontal pressure gradients reach 18 hPa km−1 during the second tornado. Comparison of the simulated storm evolution with Doppler and field observations of supercells and tornadoes shows many similar features. Vertical vorticity in the mesocyclone and the tornado vortex at low levels is initially created by the tilting of the environmental vorticity and baroclinically generated vorticity along the forward gland gust front of the storm. Tornadogenesis is initiated when mesocyclone rotation increase above cloud base. The increased rotation generates lower pressure in the mesocyclone, increasing the upward pressure gradient forces. The upwar...

Simulations of Right- and Left-Moving Storms Produced Through Storm Splitting

Abstract Using a three-dimensional numerical cloud model, self-sustaining right- and left-moving storms are simulated which arise through splitting of the original storm. The right-moving storm develops a structure which bears strong resemblance to Brownings (1964) conceptual model, while the left-moving storm has mirror image characteristics. By altering the direction of the environmental shear at low and middle levels, either the right- or the left-moving storm can be selectively enhanced. Specifically, if the wind hodograph turns clockwise with height, a single right-moving storm envolves from the splitting process. Conversely, counterclockwise turning of the hodograph favors development of the left-moving storm.

Numerical Simulation of Thunderstorm Outflow Dynamics. Part I: Outflow Sensitivity Experiments and Turbulence Dynamics

Abstract In this first paper of a two-part series, a two-dimensional numerical model is developed and used to investigate the dynamics of thunderstorm outflows. By focusing only on the outflow and using essentially inviscid equations and high spatial resolution, we are able to explicitly represent important physical processes such as turbulent mixing. To simplify interpretation of the results, the model atmosphere used in all experiments is calm and dry adiabatic. This approach allows us to establish basic characteristics of modeled outflows in simple physical settings, and provides a foundation for future studies using more realistic environments. All simulated outflows are initialized by prescribing a (controlled) horizontal flux of cold air into the model domain through a lateral boundary. In a series of sensitivity tests, we examine three parameters of the cold air source region: 1) the vertical temperature deficit profile, 2) the magnitude of the temperature deficit, and 3) the cold-air depth. By hol...

Observed and Numerically Simulated Structure of a Mature Supercell Thunderstorm

Abstract Through the interactive use of Doppler-radar analyses and a three-dimensional numerical storm simulation the detailed structure of a supercell, tornadic storm is analyzed. This storm, named the Del City storm, occurred in central Oklahoma on 20 May 1977. The storm exhibits certain important features which are essential to maintaining its longevity and which promote the storms transition to its tornadic phase. These features are strongly influenced by the rotational character of the storm separates the precipitation from the updraft and which orients the resulting downdrafts to which reinforce low-level convergence along the gust front and sustain the storm. Analyses of air parcel and rain trajectories within the storm provide a detailed visualization of this internal structure. These trajectories reveal that. air parcels rising through the cyclonically rotating updraft actually turn anticyclonically with height owing to the influence of the storm relative environmental wind field. Downdraft traj...

The Role of Midtropospheric Winds in the Evolution and Maintenance of Low-Level Mesocyclones

Abstract Using a three-dimensional numerical model, supercell simulations initialized in environments characterized by hodographs with large curvature in the lowest 3 km and a range of linear midlevel shears are investigated. For low values of the midlevel shear (0.005 s−1), the storm develops a mesocyclone at the lowest model level within the first hour of the simulation. The gust front starts to move ahead of the main updraft and cuts off the inflow to the storm by approximately 2 h, resulting in decay of the initial storm and growth of a new rotating storm on the outflow. As the midlevel shear increases to approximately 0.010 s−1, the initial development of the low-level mesocyclone is delayed, but the mesocyclone that develops is more persistent, lasting for over 2 h. Further increases of the shear to 0.015 s−1 result in the suppression of any low-level mesocyclone, despite the presence of intense rotation at midlevels of the storm. We hypothesize that differences in the distribution of precipitation ...

The Numerical Simulation of Non-Supercell Tornadogenesis. Part I: Initiation and Evolution of Pretornadic Misocyclone Circulations along a Dry Outflow Boundary

Abstract High-resolution three-dimensional simulations are used to study misocyclone initiation and development along the leading edge of an outflow boundary. Model conditions were designed such that this development could be simulated independent of moist processes. The outflow boundary is allowed to propagate into a region of southerly low-level flow which results in a vertical vortex sheet along the outflow’s leading edge. Lobe and cleft instabilities follow and provide perturbations for the subsequent development of horizontal shearing instabilities. These growing instabilities are the inaugural circulations of leading edge misocyclones with wavelengths ranging from 1.6 to 3.2 km. The structure of these modeled misocyclones compares favorably to observed pretornadic misocyclones along outflow boundaries in northeast Colorado. Vortex sheet dynamics are observed to exert substantial control over the structure of the evolving outflow leading edge. The vertical vortex sheet passes through discrete evoluti...

The Morphology of Several Tornadic Storms on 20 May 1977

Abstract Sixteen tornadic storms occurred in the afternoon and evening hours of 20 May 1977 and the early morning hours of 21 May 1977. Three storms, two of them tornadic, have been selected for detailed study. One of the tornadic storms was observed for 2 h during its growth stage prior to becoming tornadic. Radar displays indicated that it merged with a more mature but intensifying storm to its north. After this apparent merger, the storm to the north dissipated and the southern storm continued to become more vigorous and eventually produced a tornado. These storms displayed common structural similarities to another tornadic storm which was observed during its mature stage several hours earlier. Despite differences in the environments in which these storms grew, they share a characteristic structure and evolution as can be determined by this sample of their lifetimes.

CASA and LEAD: adaptive cyberinfrastructure for real-time multiscale weather forecasting

Two closely linked projects aim to dramatically improve storm forecasting speed and accuracy. CASA is creating a distributed, collaborative, adaptive sensor network of low-power, high-resolution radars that respond to user needs. LEAD offers dynamic workflow orchestration and data management in a Web services framework designed to support on-demand, real-time, dynamically adaptive systems

A Numerical Study of Storm Splitting that Leads to Long-Lived Storms

Abstract We have used a three-dimensional cloud model to investigate the splitting of an initially isolated storm in a one-directional east-west shear. The simulated evolution of storm splitting in some cases follows all four stages suggested by Achtemeier (1969) after analysis of radar data, including the development of two self-sustaining storm. One of these storms moves to the right of the mean wind vector and the other to the left. In the right-moving storm the updraft rotates cyclonically and the downdraft anticyclonically, forming a vortex pair, as depicted in the schematic model of Fankhauser (1971). The vortex pair structure is also similar to that observed with Doppler radar and analyzed by Ray (1976). The downdraft-induced gust front interacts with the low-level environmental wind to produce the convergence necessary to sustain the storm. This convergence extends to the south and west of the storm, and if enough low-level moisture is available a flanking line develops. The distribution of rainwa...